advances in atomic spectroscopy, volume 2

ADVANCES IN ATOMIC SPECTROSCOPY

Vo lume 2 �9 1995

This Page Intentionally Left Blank

ADVANCES IN ATOMIC SP

Editor: JOSEPH SNEDDON Department of Chemistry McNeese State University Lake Charles, Louisiana

VOLUME 2 �9 1995

@ Greenwich, Connecticut

JA! PRESS INC.

London, England

Copyright �9 1995 by JAI PRESS INC 55 Old Post Road, No. 2 Greenwich, Connecticut 06836

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ISBN: 1-55938-701-7

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CONTENTS

LIST OF CONTRIBUTORS

PREFACE Joseph Sneddon

LASER-EXCITED ATOMIC AND MOLECULAR FLUORESCENCE IN A GRAPHITE FURNACE

David J. Butcher

ELECTROTHERMAL VAPORIZATION SAMPLE INTRODUCTION INTO PLASMA SOURCES FOR ANALYTICAL EMISSION SPECTROMETRY

Henryk Matusiewicz

HYDRIDE GENERATION TECHNIQUES IN ATOMIC SPECTROSCOPY

Taketoshi Nakahara

THE EXCIMER LASER IN ATOMIC SPECTROSCOPY Terry L. Thiem, Yong-lll Lee, and Joseph Sneddon

RECENT DEVELOPMENTS IN ANALYTICAL MICROWAVE-INDUCED PLASMAS

Robbey C. Culp and Kin C. Ng

INDEX

oo

v i i

ix

63

139

179

215

285

LIST OF CONTRIBUTORS

David J. Butcher

Robbey C. Culp

Yong-III Lee

Henryk Matusiewicz

Taketoshi Nakahara

Kin C. Ng

Joseph Sneddon

Terry L. Thiem

Department of Chemistry and Physics Western Carolina University Cullowhee, North Carolina

Department of Chemistry California State University Fresno, California

Department of Chemistry Konyang University Nonsan, Chungnam, South Korea

Department of Chemistry Politechnika Poznanska Poznan, Poland

Department of Applied Chemistry University of Osaka Sakai, Osaka, Japan

Department of Chemistry California State University Fresno, California

Department of Chemistry McNeese State University Lake Charles, Louisiana

Department of Chemistry United States Air Force Academy Colorado Springs, Colorado

vii

PREFACE

The use of atomic spectroscopic techniques for trace and ultra-trace metal determination in complex matrices has led to the continuous development of the method. Volume 2 of Advances in Atomic Spectroscopy reflects this development and is a continuation and extension of Volume 1 in this series.

Chapter 1 of this volume describes laser-excited atomic and molecular spectroscopy in a graphite furnace. This method is being shown as capable of detecting femtogram to attogram levels. This chapter details the latest developments in instrumentation including the laser systems used, as well as an evaluation of current systems and how they compare to more conventional methods. A comprehensive listing of sample analyses performed by this method is presented.

Chapter 2 involves the historical, fundamental, and practical aspects of electrothermal vaporization methods as a sample introduction technique for plasma source analytical atomic emission spectrometry. The advantages and disadvantages, limitations, performance characteristics, and comparison to other sample introduction techniques is presented. Finally, the chapter describes future directions in this technique.

The ability of certain elements to form gaseous covalent hydrides has led to the powerful technique of hydride generation techniques for atomic spectroscopy. Chapter 3 gives an overview of this technique including both fundamental and practical aspects. A comparison of the method to more conventional techniques plus an update of the application is presented.

x PREFACE

Continuing the trend of the increase in the use of lasers in atomic spectroscopy, Chapter 4 deals with the use of the excimer laser. Following a brief description and theory of the excimer laser, the chapter concentrates on the application as a sample introduction system, as a source of producing a plasma, and as a pumping source for other lasers used in atomic spectroscopy. The application of the excimer laser in industry including thin film preparation and deposition is included.

Chapter 5 is concerned with recent developments, including improvements and innovations utilizing the microwave induced plasma. Specific focus is in the area of a detector for high performance chromatography and as an excitation source for atomic emission spectrometry. A discussion on microwave discharge devices is presented.

Joseph Sneddon Editor

LASER-EXCITED ATOMIC AND MOLECU LAR FLUORESCENCE IN A G RAPH ITE F U RNACE

David J. Butcher

.

II.

III.

IV.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

LEAFS Reviews 1988-1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Instrumentation and Spectroscopic Transitions for LEAFS . . . . . . . . . . . . 3

A. Laser System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

B. Graphite Furnaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

C. Detection Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

D. Spectroscopic Transitions for LEAFS . . . . . . . . . . . . . . . . . . . 12

Analytical Results for LEAFS . . . . . . . . . . . . . . . . . . . . . . . . . . 14

A. Detection Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 B. Calibration Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Background Correction for LEAFS . . . . . . . . . . . . . . . . . . . . . . . 25

A. Multichannel Background Correction Techniques . . . . . . . . . . . . . 27

B. Wavelength Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

C. Zeeman Background Correction . . . . . . . . . . . . . . . . . . . . . . 34

Advances in Atomic Spectroscopy Volume 2, pages 1-62. Copyright �9 1995 by JAI Press Inc. All rights of reproduction in any form reserved. ISBN: 1-55938-701-7

2 DAVID J. BUTCHER

D. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 VI. Real Sample Analyses by LEAFS . . . . . . . . . . . . . . . . . . . . . . . . 47

VII. Conclusion-.--LEAFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 VIII. Laser Excited Molecular Fluorescence (LEMOFS) . . . . . . . . . . . . . . . 52

A. Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 B. Choice of Molecules and Optimization Procedures . . . . . . . . . . . . 53 C. Detection Limits and Linear Dynamic Ranges . . . . . . . . . . . . . . . 57 D. Interferences with the LEMOFS Signal . . . . . . . . . . . . . . . . . . 58 E. Real Sample Analyses/Background Correction . . . . . . . . . . . . . . 58 E ConclusionmLEMOFS . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

I. I N T R O D U C T I O N

Laser-excited fluorescence in a graphite furnace atomizer is a very sensitive method of elemental analysis, with detection limits for some elements in the attogram (10 -18 g) range. Laser-excited atomic fluorescence spectrometry (LEAFS), which was developed in the mid- 1970s (Neumann and Kriese, 1974; Bolshov et al., 1976, 1978), involves the use of a laser to excite atoms produced in an electrothermal atomizer. Fluorescence from the atoms is collected by a detection system and serves as the analytical signal. More recently, molecular fluorescence has been collected for the determination of halogens. Laser-excited molecular fluorescence spectrometry (LEMOFS), which was developed by Dittrich in the late 1980s (Dittrich, 1986; Dittrich and St~irk, 1987a), involves the formation of diatomic molecules, consisting of a metal reagent and a halogen analyte, in a graphite furnace. The molecules are excited by a laser and the resulting fluorescence is used for quantitative analysis.

The purpose of this chapter is to focus upon advances in laser-excited fluorescence since my last major review (Butcher et al., 1988) through 1993. This includes a discussion of recent LEAFS review articles, as well as advances in instrumentation made over the past five years. The instrumentation section describes the laser systems and atom cells that have been used for LEAFS. A section on analytical results describes the detection limits and linear dynamic ranges obtained by this technique and compares these results to those obtained by conventional methods of elemental analysis, such as graphite furnace atomic absorption and inductively coupled plasma-mass spectrometry. The types of backgrounds and methods of background correction for LEAFS are described, and the effectiveness of each technique is evaluated. A comprehensive listing of real sample analyses is presented in order to demonstrate the versatility of LEAFS for practical use. A short section on LEMOFS introduces the technique and describes the limited results obtained to date.

Laser-Excited Fluorescence 3

ii. LEAFS REVIEWS 1988-1993

This section focuses on review articles concerning graphite furnace LEAFS published since my last review (Butcher et al., 1988). That article reviewed LEAFS in flames, plasmas, and graphite furnaces, and included sections on the choice of laser system for LEAFS, the number of atoms detected by LEAFS, sources of noise for LEAFS, and methods of background correction. More recently, in a short article, I (Butcher, 1993) reviewed basic principles of the technique and discussed recent applications. Smith et al. (1989) contributed a comprehensive list of atomic fluorescence detection limits, which includes graphite furnace LEAFS results. Sj/3str/3m (1990b) reviewed LEAFS in a graphite furnace, focusing upon the theory of the atomic fluorescence signal, experimental arrangements, detection efficiency, and signal-to-noise ratio. In a later article, Sjt~str/Sm and Mauchien (1993) reviewed a number of laser-based atomic spectroscopy techniques, including graphite furnace LEAFS.

Omenetto has reviewed a number of facets of LEAFS in a series of recent articles. The significance of several laser-based atomic techniques, including LEAFS, was reviewed (Omenetto, 1988). Omenetto (1989a) also reviewed LEAFS and laser- enhanced ionization as techniques for elemental analysis. In the same year, Ome- netto (1989b) projected future research in and applications of LEAFS. More recently, Omenetto (1991) reviewed the feasibility of absolute analysis by LEAFS.

I!1. INSTRUMENTATION AND SPECTROSCOPIC TRANSITIONS FOR LEAFS

This section discusses advances in LEAFS instrumentation made between 1988 and 1993. The purpose of this section is to describe the optimal setup for graphite furnace LEAFS at this time.

A. Laser System

The ideal laser system for LEAFS would operate continuously or at a high repetition rate (10,000 Hz) to sample most accurately the transient furnace signal and to provide suitable pulse energies to saturate atomic and molecular transitions (5-100 ktJ); it would be tunable between 180 and 800 nm, and be capable of switching wavelengths conveniently. In addition, for several methods of background correction, it is desirable that the width of the laser line be less than 0.005 nm. Although improvements have been made in laser technology, currently we are far away from approaching the ideal system. Lasers are currently available that operate at these high repetition rates, but they cannot cover the entire ultraviolet visible region. Laser systems are currently available that provide acceptable power,

4 DAVID J. BUTCHER

tunability, and a repetition rate up to 1,000 Hz. However, none of these systems is truly reliable and easy to use.

A laser system for LEAFS is composed of three major components: a pump laser, a dye laser, and a frequency converter (Butcher et al., 1988). Each of these components will be discussed in detail.

Pump Laser

The pump laser provides a high energy pulse of light that is used to produce tunable radiation from the dye laser. Pump lasers employed for graphite furnace LEAFS include nitrogen, Nd:YAG, copper vapor, and excimer lasers. Nitrogen lasers provide insufficient energy for most elements, and Nd:YAG lasers operate at low repetition rates (less than 50 Hz). Copper vapor lasers operate at repetition rates up to 10,000 Hz, but provide relatively low pulse energies and their output is tunable only over a limited range of the UV-visible region. At present, excimer lasers provide the best compromise of laser characteristics. They have relatively high repetition rates (up to 1000 Hz), the light emitted from the system is tunable between 205 and 800 nm with energies between 50 and 200 ktJ/pulse, and they have adequate linewidths. However, all of these pump lasers are difficult to operate, and require much maintenance. For example, with daily use, the electrodes in an excimer laser need to be rebuilt every six months to a year.

Vera et al. (1989a, b) evaluated three different laser systems for graphite furnace LEAFS with front surface illumination. These included a nitrogen-pumped laser system (repetition rate: 20 Hz; pulse energy of 25 l.O/pulse), a Nd:YAG system (30 Hz; 250 mJ/pulse), and a copper vapor system (6 kHz; average power, 20 W). Although identical detection systems were not used with each laser system, their results indicate the improved sensitivity obtained with a high repetition rate (Table 1). In the case of lead, the detection limit was improved by a factor of six by use of a copper vapor laser. A higher detection limit was obtained for gallium with the

Table 1. Absolute Detection Limits Obtained by Graphite Furnace LEAFS with Three Laser Systems

Element Laser System ~ex; )~let (nm) Detection Limit ~g)

Ga Cu vapor* 287.4; 294.4 2000

Nd:YAG 403.3; 417.2 25

Pb Cu vapor 283.3; 405.8 0.5

Nd:YAG 3

Nitrogen 3

Notes: *Laser energy was insufficient for optical saturation. Taken with permission from Vera et al. (1989a,b).


DYE LASER OUTPUT

,/n Li

OUTPUT MIRROR

TUNING MIRROR

PUMP LIGHT

111

PUSHER

! .... i \ \

DYE DIFFRACTION CELL GRATING J

OPTICAL STAGE

Figure 1. Schematic diagram ofthe grazing incidence dye laser oscillator constructed for wavelength modulation. Taken with permission from Suet al. (1992).

copper vapor laser, compared to the Nd:YAG system, that was attributed to its lower pulse energy and the use of a less sensitive transition.

Dye tasers

Dye lasers serve to convert the pump laser light into tunable laser radiation (Butcher et al., 1988). A dye, dissolved in a suitable solvent and placed in a cell, is excited with light from the pump laser. A grating serves as one of the mirrors in the laser cavity, and movement of the grating allows variation of the wavelength of the output light. A series of dyes is required to obtain the wavelengths between 320 and 900 nm, and consequently it is usually necessary to change dyes whenever a different element is to be determined. Changing dyes is a messy and time-consuming task (one hour) that limits the ease of use of LEAFS compared to other techniques, such as inductively coupled plasma-mass spectrometry (ICP-MS).

Two cavity designs for dye lasers have been commonly used for LEAFS: Hansch and grazing incidence (Butcher et al., 1988). Both designs are available for commercial systems, and both appear adequate for graphite furnace LEAFS, but the grazing incidence design is simpler to construct in the laboratory. Su et al. (1992) constructed a grazing incidence dye laser that employed a piezoelectric pusher to drive the wavelength-tuning mirror and allow the use of wavelength modulation as

6 DAVID J. BUTCHER

a method of background correction (Figure 1). This dye laser allowed modulation of laser wavelength over an interval between 0.0 and 0.2 nm with a spectral linewidth of 0.003 nm.

Frequency Conversion Techniques

Frequency conversion of visible dye laser radiation is necessary to obtain the ultraviolet wavelengths between 180 and 320 nm that are required to determine most elements. Frequency conversion techniques were described in my previous review (Butcher et al., 1988) and are not discussed here. However, it should be noted that frequency doubling, which involves halving the wavelength of light with a crystal, has been employed for all graphite furnace LEAFS work to date.

B. Graphite Furnaces

Three basic types of graphite furnaces have been employed for LEAFS: (1) open atomizers, such as graphite cups, rods, and filaments; (2) graphite tube atomizers into which additional ports were incorporated to allow passage of the laser beam; and, (3) enclosed graphite tube atomizers, like those used in atomic absorption. Open atomizers were widely used in LEAFS work in the 1970s and 1980s (Bolshov et al., 1976, 1978, 1981a,b, 1986a,b; Falk et al., 1988; Goforth and Winefordner, 1986) because of the ease of fluorescence collection at 90 ~ to the direction of the beam (Figure 2), but they are prone to vapor interferences and hence are not practical for real sample analysis. Bolshov et al. (1986a,b, 1988) reported that cobalt signals in agricultural samples were suppressed by a factor of ten owing to matrix effects. It was possible to reduce the matrix interferences by the use of vacuum atomization, but detection limits were degraded by a factor of one hundred.

In the middle and late 1980s, graphite tube atomizers were modified by the addition of ports to allow passage of laser radiation at 90 ~ to the direction at which

o

L J Figure 2. Collection of fluorescence from a graphite cup furnace with transverse illumination. Taken with permission from Butcher et al. (1988).


To HONOCHROMATOR

LASER BEAH

LENS

TUBE ~ /I FURNACE I / II

I / , r l _ . .

II 11 VOLUME

Figure 3. Collection of fluorescence from a graphite tube furnace using transverse illumination. Taken with permission from Butcher et al. (1991).

fluorescence is collected (Figure 3) (Dittrich and St~k, 1986, 1987b; Dougherty et al., 1987a, b, 1988, 1989, 1990; Preli et al., 1987, 1988). This geometry is called transverse illumination. Although this system has not been extensively studied for real sample analysis, the additional holes in the furnace would be expected to increase chemical interferences compared to an enclosed furnace. In addition, the modification of the graphite furnace alters its heating characteristics, and hence this design is not commonly employed in current LEAFS work.

The preferred collection geometry for LEAFS is the use of front surface illumination, first described by Goforth and Winefordner (1987), in which the laser beam passes through ahole ina mirror oriented at 45 ~ to the beam and through the furnace (Figure 4). Fluorescence is collected at 180 ~ to the beam by the mirror. This arrangement allows the use of conventional atomic absorption graphite furnaces, which should allow minimization of matrix interferences. A number of analyses were performed with this configuration, including work using solid and slurry sampling.

Wei et al. (1990) compared transverse and front surface illumination on the basis of the signal to noise ratio (S/N) (Table 2). Front surface illumination was shown to provide superior detection limits for elements whose primary source of noise is blackbody emission, such as lead and manganese. Elements (e.g., thallium) for which laser stray radiation was the dominant source of noise had comparable detection limits by the two geometries. The same article also compared dispersive

8 DAVID J. BUTCHER

LENS

I To HONOCHROHATOR

FLUORESCENCE PROBE VOLUME

LASER BEAM l ~ I>

_ J MIRROR

TUBE FURNACE

Figure 4. Collection of fluorescence from a graphite tube furnace using front surface illumination. Taken with permission from Butcher et al. (1991).

and nondispersive detection on the basis of S/N (Table 2). For thallium and lead, nondispersive detection with a 1-nm bandpass filter gave an improved detection limit compared to dispersive detection with a 4-nm bandpass. Detection limits for iron were the same with dispersive and nondispersive detection because the bandpasses were nearly the same. In the case of aluminum, a 1-nm bandpass filter

Table 2. Summary of Graphite Furnace LEAFS Detection Limits with Transverse and Front-Surface Illumination with Dispersive and Nondispersive Detection

Detection Limit (fg)

Front Surface

Element Z,ex: kdet ( n m ) T r a n s v e r s e D i s p e r s i v e Nondispersive

T1 277" 353 6 3 0.3 (1 nm)

Pb 282; 405 7 1 0.2 (1 nm)*

Fe 297; 373 - - 70 100 (3.6 nm)

A1 308; 394, 396 m 100 100 (1 nm)

Mn 280; 403 600 100 u

Notes: *Bandpass of filter. Taken with permission from Wei et al. (1990).


was used for the nondispersive work, but the detection limits were the same with the 4-nm bandpass dispersive system because it collected light from both the 394- and 396-nm fluorescence transitions, whereas the nondispersive system collected only the light from the latter.

Farnsworth et al. (1990) investigated the collection efficiency of laser-excited atomic fluorescence from a graphite furnace with front surface illumination by use of a computer program that employed a ray-tracing algorithm. Three lens combinations were investigated in this work: a symmetric biconvex lens, a pair of matched piano-convex lenses, and a pair of matched achromats. The efficiency of light collected from a single point was highest for the achromats, with an efficiency of 97%, lower for the piano-convex lenses (36.2%), and lowest for the biconvex lens (11.5%). The use of the achromats or piano-convex lenses was shown to improve the S/N by between two and three times. The use of a pair of matched piano-convex lenses gave a detection limit of 0.1 fg for thallium, which is the best graphite furnace LEAFS detection limit for any element (Smith et al., 1990).

Liang et al. (1993) compared the use of two piano-convex lenses, a single biconvex lens, and two biconvex lenses for the determination of tellurium and selenium by graphite furnace LEAFS. The lowest detection limits were obtained with the piano-convex lenses, which were a factor of two lower than those obtained with the single biconvex lens, and a factor of four lower than those with two biconvex lenses.

lk TO MONOCHROMATOR

LENS

FLUORESCENCE

FURNACE

LASER BEAM

SAMPLE

BURNER

Figure 5. Schematic diagram of the transverse illumination method used in a continuous flow atomizer. Fluorescence is collected from a point just beyond the furnace exit aperture. Taken with permission from Womack et al. (1989).

10 DAVID J. BUTCHER

Womack et al. (1989) evaluated a ceramic continuous flow-atomizer for the determination of lead by LEAFS with both a nitrogen (20 Hz; 50 ktJ/pulse) and a copper laser system (7.5 kHz; 20 W average power). Two optical schemes were used in this work: (1) collection of fluorescence at a fight angle to the laser beam just beyond the tip of the furnace (Figure 5), and (2) front surface illumination. The best detection limit of 1.3 ng/mL was obtained with the copper vapor laser system and front surface illumination. This is five orders of magnitude higher than the best LEAFS detection limit for lead (Wei et al., 1990). The addition of easily ionized elements such as sodium was shown to increase the fluorescence signal; the presence of less easily ionized elements such as calcium was shown to suppress the analytical signal.

C. Detection Systems

Few changes have been made in the detection systems used for LEAFS since my last review. Most graphite furnace LEAFS work has employed a monochromator, photomultiplier tube, and a boxcar integrator. As discussed previously, Wei et al. (1990) compared dispersive and nondispersive LEAFS on the basis of S/N.

Wei et al. (1990) investigated the effect of slitwidth upon the S/N. The two principal types of background noise that were considered were blackbody emission from the furnace and laser stray-light noise. In order to characterize each of these noise sources as flicker noise limited or shot noise limited, the effects of slitwidth upon s/N were investigated for a stray-light noise-dominated element (thallium)

"0 a . _

:3 O"

Z

v

20! 16

12

8

4 .>_ .--4 0 | I I I I

0.0 0.5 1 0 1 5 2.0 " ,

rr Slit Width, mm Figure 6. Effect of slitwidths on the S/N of nondispersive detection for TI in LEAFS. The excitation wavelength was 277 nm and detection wavelength was 353 nm. The bandpass of the filter was 10 nm. Taken with permission from Wei et al. (1990).


t~

. m

))

t~ (9

20-

10-

0 0.0

O

! I I I

0.5 1.0 1.5 2.0 Slit Width, mm

Figure 7. Effect ofslitwidths on the magnitude ofthe signal and noise ofnondispersive detection for Pb in LEAFS. The excitation wavelength was 283 and the detection wavelength 405 nm. The bandpass of the filter was 10 nm. A. Relative fluorescence signal with respect to a 0.1 mm slitwidth. B. Relative magnitude of the noise with respect to a 0.1 mm slitwidth. Taken with permission from Wei et al. (1990).

and a blackbody-noise-dominated element (lead). When an instrument is shot- noise-dominated, according to S/N theory, there is no improvement of the limit of

detection with increasing slitwidth. The flicker-dominated case should display a decrease in detection limit with an increase in slitwidth. For thallium, with nondispersive detection, a plot of the relative (S/N) 2 versus slitwidth was linear (Figure 6), indicating that the limiting source of noise under these conditions was shot noise, and that laser stray-light noise is shot-noise-dominated. In the case of lead, the effects of slitwidth upon S/N were investigated for nondispersive detection with a

10-nm bandpass filter and with a 1-nm bandpass filter. The use ofa 10-nm bandpass filter showed that both the fluorescence signal and the noise were linear with respect to the slitwidth, indicating that the blackbody emission was flicker noise limited

(Figure 7). However, when the 1-nm filter was employed, the square of the S/N

ratio was linear with respect to the slitwidth, indicating that the instrument was shot

noise limited (Figure 8). These results demonstrate that when the bandpass decreases, flicker noise decreases faster than shot noise.

Liang et al. (1993) investigated the effect of slitwidth upon S/N for antimony and

tellurium (Table 3). Both elements were shot noise limited with slitwidths between

0.5 and 2.0 mm; this was confirmed by the measurement of detection limits, which

remained the same for each element over this range of slitwidths (Table 3). A

slightly worse detection limit was obtained at a slitwidth of 0.25 mm because the

noise did not decrease as much as did the signal size.

12 DAVID J. BUTCHER

"0 x. . .

t~

i1} m.m..

0 Z I1)

m

rr" V

10

8

6

4

2 ! 0

0.0 0.5 1.0 Slit Width, mm

Figure 8. Effect of slitwidths on the noise magnitude of nondispersive detection for Pb in LEAFS. The excitation wavelength was 283 and the detection wavelength 405 nm. The bandpass of the filter was 1 nm. The noises were relative values with respect to a 0.25-mm slitwidth. Taken with permission from Wei et al. (1990).

D. Spectroscopic Transitions for LEAFS

Various transitions used for LEAFS are illustrated in Figure 9 (Butcher et al., 1988; Omenetto and Winefordner, 1979). The simplest scheme is called resonance fluorescence (Figure 9a), in which one photon is used to excite the atoms, and the wavelength of the collected fluorescence is equal to that of the exciting radiation. Resonance fluorescence has the disadvantage that scattered laser radiation cannot be discriminated against by the detection system, which degrades detection limits. The use of resonance fluorescence is a particular disadvantage for front Surface illumination because the levels of stray light are very high.

Table 3. The Effect of Slitwidth upon Detection Limits of Tellurium and Antimony

Detection Limit (fg)

Slitwidth (mm) Tellurium Antimony

0.25 40 17

0.5 30 13

1.0 29 10

2.0 30 11

Note: Reproduced with permission from Liang et al. (1993).


(a)

Excited state

Absorption

G r o u n d , state

Excited state 2

Excited ,,, state 1

Absorpt ion

Grou nd ,,,, state

(b)

Fluorescence

Fluorescence

: Nonradiat ive i d e a c t i v a t i o n o o o

(c)

Excited state 3

Photon 2

Photon 1

Excited state 2

Excited state 1

Ground state

Figure 9. Transitions for atomic fluorescence: (a) resonance fluorescence, (b) nonresonance fluorescence, (c) double resonance fluorescence.

Nonresonance LEAFS (Figure 9b) involves the use of one wavelength of light for excitation and a second wavelength for the collection of fluorescence. Nonreso- nance detection is particularly advantageous for front surface illumination because the detection system can discriminate against stray laser light (Butcher et al., 1988). However, some elements (e.g., sodium, cadmium) do not have strong nonresonance transitions, which limits the sensitivity of these elements when this method is used.

Double resonance LEAFS (Figure 9c) involves the use of two dye lasers to simultaneously excite the atoms in a furnace with two wavelengths of light, followed by collection of fluorescence of a third wavelength (Leong et al., 1988; Omenetto et al, 1988a; Vera et al., 1989c). This scheme has the advantage that, for

14 DAVID J. BUTCHER

every element, different fluorescence and excitation wavelengths can be employed, but double resonance LEAFS requires the use of two dye lasers and is difficult to align. Omenetto et al. (1988) reported the use of double resonance fluorescence for cadmium. A detection limit of 18 fg was obtained.

Leong et al. (1988) compared the use of double resonance transitions for lead (excitation at 283.306 and 600.193 nm; fluorescence at 261.418, 239.379, 216.999 nm) to the common nonresonance transition (excitation at 283.306 nm; fluorescence at 405.783 nm). The double resonance technique provided detection limits between 130 and 270 fg, which was one to two orders of magnitude worse than the nonresonance detection limit of 5 fg. The poor sensitivity of the double resonance transitions was attributed to ionization processes which depleted the population of lead atoms.

Vera et al. (1989c) reported the use of double resonance fluorescence with a solar blind photomultiplier tube (PMT). For this instrument, the ideal set of transitions would involve the use of excitation wavelengths longer than 320 nm, which allows high spectral energy (no frequency doubling) and eliminates noise due to stray laser radiation because the solar blind PMT does not respond to these wavelengths. The ideal fluorescence wavelength would be less than 280 nm, because the PMT response is high and the effects of blackbody radiation at these wavelengths is relatively low. Detection limits of 2, 1, and 220 fg were obtained for indium, gallium, and ytterbium, respectively. For indium, this represents an improvement by a factor of five compared to the best nonresonance detection limit; for gallium, the double resonance result was twenty-five times lower than the best nonresonance result. This was the first reported detection limit for ytterbium.

IV. ANALYTICAL RESULTS FOR LEAFS

A. Detection Limits

Table 4 is a list of all detection limits that have been obtained by graphite furnace LEAFS. Three conclusions can be drawn from these data. First, detection limits between 0.1 and 50 fg can be obtained for most elements of medium to high volatility (Ag, Au, Cd, Co, Ga, In, Pb, Sb, Sn, Te, T1) by the use of front surface illumination and a commercial atomic absorption (AA) furnace. Second, cup furnaces can be used to obtain low detection limits with volatile elements (e.g., Cd, Pb, T1), but these atomizers cannot efficiently vaporize involatile elements (Ga, Mn, Ni, Pd, Rh, V), and hence considerably higher detection limits are obtained. In addition, these atomizers have been shown to cause vapor phase interferences for real sample analysis (Bolshov et al., 1981b, 1986a, b, 1988). Third, the use of front surface illumination and modern furnace technology for LEAFS allows detection limits between 50 and 500 fg for involatile elements (A1, Fe, Mn, Yb). More research needs to be done to achieve similar detection limits with other

Table 4, Summary of LEAFS Detection Limits

Reference Element ~kex/%fluor LOD (fg) Excitation Geometry [front surface Laser: Type; Rep Rate; Pulse Energy;

(FS)/transverse (T)]; Furnace Saturated (Y/N)

. _ a

La--I

Anzano et al. (1991 )

Apatin et al. (1989) Bolshov et al. (1976)

Bolshov et al. (1978)

Bolshov et al. (1981a)

Bolshov et al. (1984) Bolshov et al. (1986a, b, 1988)

Bolshov et al. (1989)

Bolshov et al. (1991 )

Bolshov et al. (1992b)

Bolshov et al. (1992a)

T1

Pb Fe

Pb Eu Ir

Pt Ag

Co

Cu Eu

Fe Ir

Mn Na Pb Pt

Ir

Co

Co Pb

Cd

Cd

Cd Pb

276.8/352.7 100

283.3/405.8 4

296.7/373.5 750 283.3/405.8 75

287.9/536.1 360,000

295.1/322 480,000 293.0/299.7 120,000 328.1/338.3 100 304.4/340.5 60

324.7/510.5 150 287.9/536.4 300,000

296.7/373.5 100 295.1/322.1 6,000

279.5/279.5 200

589.6/589.6 600 283.3/405.8 1.5

293.0/299.7 120,000

295.1/322.1 100,000 304.4/340.5 60

304.4/340.5 6000

283.3/405.8 5

228.8/228.8 0.5

228.8/228.8 4 326.1/326.1 16

228.8/228.8 5

283.3/405.8 15

FS; AA tube

T; graphite cup

T; graphite cup

T; graphite cup

T; graphite cup

T; graphite cup T; graphite cup

T; graphite cup; vacuum

T; graphite cup

T; graphite cup

T; graphite cup

T; graphite cup T; graphite cup

N2; 20Hz; 151xJ; Y Excimer; 25 Hz; ?; Y Nd:YAG; 25 Hz; ?; N?

?; Y?

Nd:YAG; 12.5 Hz; ?; ?

Nd:YAG; 25 Hz; ?; N? ?; N?

?; Y? ?; ?

?; N? ?; ?

?; ?

?;Y

?;Y

?;Y

Nd:YAG; 12.5 Hz; ?; ?

Nd:YAG; 25 Hz; ?; N

Excimer; 25 Hz;

Excimer; 25 Hz;

Excimer; 25 Hz;

Excimer; 25 Hz;

25 rtJ;V 2 l.tJ; Y

2~; x 60 laJ; Y 2~J;X 25 laJ; V

(continued)

Table 4. (continued)

Reference Element J~.ex/~fluor LOD (fg) Excitation Geometry [front surface Laser: Type; Rep Rate; Pulse Energy;


. . . . a

Butcher et al. (1990)

Cheam et al. (1992) Denisov et al. (1985)

Dittrich and St~k (1986)

Dittrich and St~rk (1987b)

Dougherty et al. (1987a)

Dougherty et al. (1987b)

Mn Pb TI Pb Ba

Na Pb

AI

Ga

In

Ir

V

Co

Ag

Co Cu

279/403 283.3/405.8 276.8/353 283.3/405.8 597.2/597 589.0/589 283.3/405.8

308.2/308.2, 309.3

287.4/294.4

303.9/325.6

284.9/357.4

264.7/354.4

304/341

328/338

304/341 325/510 325/325

100 1

3 10

80,000,000 55

1,450 68

45,000 4,000

70,000 10,000 5,500

140 6• 109

475,000 5.3 x 109

2,200,000 300 700

10

8

500 6,000

40O

FS; AA furnace Excimer; 80 Hz; 9 IxJ; Y

6BJ;Y 10 !s Y

FS; AA furnace Cu vapor; 6000 Hz; ?; N T; graphite cup N2; 35 Hz; ?; Y

T; graphite rod N2; 5 Hz; 10 laJ; Y T; AA furnace

T; graphite rod N2; 5 Hz; ?; ? T; AA furnace T; graphite rod N2; 5 Hz; ?; ? T; AA furnace

T; graphite rod N2; 5 Hz; ?; ? T; AA furnace

T; graphite rod N2; 5 Hz; ?; ? T; AA furnace

T; graphite rod N2; 5 Hz; ?; ? T; AA furnace T; tube furnace Excimer; 80Hz; 10 l.tJ; N T; tube furnace; Zeeman T; AA furnace; wall Excimer; 80Hz; 2 IxJ; N T; AA furnace; platform

T; AA furnace; wall 5 BJ; N T; AA furnace; wall 1.5 lxl; Y

T; AA furnace; wall 5 laJ; Y

(continued)


Reference Element LOD (fg) Excitation Geometry ffront surface Laser: Type; Rep Rate; Pulse Energy;


. _ _ x

",4 Dougherty et al. (1988)

Dougherty et al. (1989)

In

Mn

Pb

T1

Tl

Ag

Co

Cu In

Mn

Pb

Tl

4101451

279/403

2791279 2791403

279/279

2831405

3771535

2771353

328/338

3041341

3251325

410/451

2791279

279/279

283/405

3771535

80 90

600

90

600

80

7 10

100 100

6 20

200

300

700 600

20

40 100

1000

10

10

100 100

T; AA furnace; wall

T; AA furnace; platform T; AA furnace; wall

T; AA furnace; platform

T; T; T; I'; T; T; T; T; T; T; T; I"; T; I"; T; I,, 1"; I";

AA furnace; wall AA furnace; platform

AA furnace; wall

furnace; platform AA furnace; platform

tube furnace tube furnace; Zeeman

tube furnace

tube furnace; Zeeman tube furnace

tube furnace tube furnace; Zeeman

tube furnace

tube furnace; Zeeman

tube furnace


tube furnace


Excimer; 80 Hz;

Excimer; 80 Hz;

3 l.tJ; N

2 I.t; Y

6 I.tJ; Y

lo l.tJ; Y 2gJ ;N

5 I,tJ; N

1 .5~; Y 5 r a ; v

3 ~ ; N

2 l.t; Y

6gJ ;Y

(continued)

Table 4, (continued)

Reference Element ~/Znuor LOD O~g) Excitation Geometry ~ront surface Laser: Type; Rep Rate; Pulse Energy;


_ . a

Falk and Tilch (1987), Falk et al. (1988)

Gorforth and Winefordner (1986)

Ag

Bi

Co Cu In

Ir

Na Ni

Pb Pd

Rh Sn TI V A1

Cu

?/?

306.8/472.2

308.3/345.4 324.8/510.6 304.0/325.9 .9/?

589?/589? 322.2/361.9

283.3/405.8 .9/? ?/?

286.3/317.5

276.8/352.9 .9/?

394.4/396.2

324.8/327.4

100

800 200 800

100

300,000 600

1000

5 700

2000 500,000

0.7 1,700,000

500,000 300,000

100,000 7,000

60,000

50,000

7,000

2,000 200,000

T; cup fumace N2; 20 Hz; ?/?

T; cup furnace; pyro

T; cup furnace; Ta foil

T; cup furnace; Ta carbide T; cup furnace; pyro

T; cup furnace; Ta foil

T; cup furnace; Ta carbide

T; cup furnace; H2/Ar atm

T; cup furnace; Ar atm

T; cup furnace; vacuum

N2; 20 Hz;

l o ~ ; Y 101aJ; Y 10 laJ; Y 10 IJJ; Y ?/?

?/Y

10 IJJ; N 10 [uJ: Y ?; ?

?; ?

lO~; ? 10/JJ; Y ?; ? ?; ?

?; ?

(continued)


Reference Element ~x/~fluor LOD (fg) Excitation Geometry [front surface Laser: Type; Rep Rate; Pulse Energy;


_ . , a

t.D

Goforth and Winefordner (1987)

Gonchakov et al. (1979)

Hohimer and Hargis (1978)

Human et al. (1984)

|n

Li

Mn

Pb

Pt

Sn

A1

Cu Mo

V

Na TI

Pb

TI

303.9/325.6

670.8/670.8

279.8/280.1

283.3/405.8

265.9/270.2

286.3/317.5

394.4/396.2

324.8/327.4

313.3/317.0

385.6/411.2

589/589 276.6/351.9,

352.9 283.3/405.8

276.8/352.9

50 300 700

400,000 400,000

4,000,000

7,000 100,000

10,000 7,000 1,000 7,000

200

60,000 1,000

2O,000

200 10,000

100,000

8,000 100,000

2 x 109

60

25

6 100

T; Cup furnace; H2/Ar atm ?; ?

T; cup furnace; Ar atm T; cup furnace; vacuum

T; cup furnace; H2/Ar atm ?; ? T; cup furnace; Ar atm

T; cup furnace; vacuum

T; cup furnace; pyro ?; ? T; cup furnace; Ta foil

T; cup furnace; Ta carbide

T; cup furnace; H2/Ar atm T; cup furnace; Ar atm

T; cup furnace; vacuum T; cup furnace; H2/Ar atm ?; ?

T; cup furnace; H2/Ar atm ?; ? T; cup furnace; Ar atm T; cup furnace; vacuum

T; cup furnace; H2/Ar atm ?; ? T; cup furnace; Ar atm

FS, tube furnace N2; 20 Hz; .9/? .9/?

.9/?

.9/?

T; cup furnace N2; 10 Hz; 9./?

T: graphite boat N2; 50 Hz; 0.15 laJ; ?

T; graphite rod Excimer; 50 Hz; .9/?

?/? (continued)


Reference Element ~Lex/~fluor LOD (fg) Excitation Geometry ffrontsurface Laser: Type; Rep Rate; Pulse Energy;


bO O

Irwin et al. (1992)

Leong et al. (1988)

Liang et al. (1993)

Miziolek and Willis (1981)

Pb

Co Pb

Co

Pb

Sb Te Pb

283.3/405.8 304/341 283.3/405.8

304/341

283.3/405.8

283.3, 600.2/261.4

283.3, 600.2/239.4

283.3, 600.2/217.0

212.7/259.8 214.3/238

283.3, 600.2/261.4

4

500 2

200

30

3 200

130

270

10 20

1

FS; AA furnace; Zeeman Excimer; 240 Hz; 3 ILl; Y

3 laJ; Y FS; AA furnace

FS; AA furnace; piano-convex lenses FS; tube furnace

Excimer; 500 Hz;

Nd:YAG; 30 Hz; l m J ; Y

1, 10m J; Y

FS; AA furnace Excimer; 500 Hz; 1 ILI; Y 1 ILl; Y

T; AA Nd:YAG; ? ?; 200 ILl; Y

Neumann and Kriese (1974)

Omenetto et al. (1988a)

Preli et al. (1987)

Pb

Cd

Pb

TI

Ag

Co

Cu In

Mn

283.3/405.8

228.8, 643.8/361.0

283.3/405.8

276.8/353

328/338 304/341

325/510 410/451

279/279

200 18

5

2

20

300

600

20

100

FS; carbon rod Flash; 10 Hz; ?; ? FS; AA furnace Excimer; 50 Hz; .9/?

T; AA furnace Excimer; 80 Hz;

200 g J; Y

200/ttJ; Y

4 laJ; N 13 l.tJ; N

6 ILI; Y 19 p.l; Y

3BJ;N (continued)


Reference Element ~e~nuor LOD (fg) Excitation Geometry [front surface Laser: Type; Rep Rate; Pulse Energy;


Remy et al. (1990)

Sj6str6m et al. (1990)

Smith et al. (1990) Vera et al. (1989c)

Vera et al. (1989a, b)

Pb

TI Au

Co

Ga

TI

Ga

In

Yb

Fe

Ga

In

Ir

Pb

T1

283/405

377/535

308.3/345.3 242.8/312.3 294.4/287.4

276.8/351.9 403.3, 641.4/250

410.1, 571.0/271

398.8, 666.7/246.4

296.7/373.5

287.4/294.4 403.4/417.2

410.1/451.1

295.1/322.1

283.3/405.8

276.8/352.9

10

100 10

4

60

50

0.1 1

220

500

2,000 25

10

10,000

3

3

0.5

7

FS; AA furnace Nd:YAG; 10 Hz;

FS; AA furnace Excimer; 50 Hz;

FS; AA furnace; multi-channel background correction

FS; AA furnace FS; AA furnace; solar blind PMT

FS; tube furnace

Excimer; 35 Hz; Nd:YAG; 30 Hz;

Cu vapor; 6000 Hz;

Nd:YAG; 30 Hz;

N2; 20 Hz;

Nd:YAG; 30 Hz;

Cu vapor; 6000 Hz;

N2; 20 Hz;

7gJ ;Y

250 gJ; Y

30gJ; Y

30gJ; Y

20 IXJ; Y

l m J ; Y 2, 10 mJ; Y

2, 10m J; Y

2, 10 mJ; Y

o.2~; N 0.2 IM; N 25 g J; Y

25 I.tJ; Y 25 ~tJ; Y

25gJ; Y

25 gJ; Y

0.2 laJ; N

25 gJ; Y

(continued)


Reference Element ~x/Lf]uor LOD (fg) Excitation Geometry [front surface Laser: Type; Rep Rate; Pulse Energy;


Wei et al. (1990) A1

Fe

Mn Pb

Sn TI

308/394, 396 100 100

297/373 70

279/403 100 283/405 1

0.2 286/318 30 277/353 3

0.3 Wittman and Winefordner (1984) Mn 403.1/403.1 20,000

Na 589.6/589.6 3,000 Sn 300.9/317.5 5,000

Womack et al. (1989) Pb 283.3/405.8

FS; AA furnace; dispersive FS; AA furnace; nondispersive FS; AA furnace; dispersive FS; AA furnace; nondispersive FS; AA furnace; dispersive FS; AA furnace; dispersive FS; AA furnace; nondispersive FS; AA furnace; dispersive FS; AA furnace; dispersive FS; AA furnace; nondispersive T; graphite filament

1.3 ng/mL FS; continuous flow fum. 2 ng/mL 3.5 ng/mL T; continuous flow fum.

Excimer; 80 Hz; ?; ?

N2; 16 Hz;

Cu vapor; 7500 Hz;

N2; 20 Hz; Cu vapor; 7500 Hz;

?; ?

?/Y

?/?

?/Y

?/? ?/? 0.6 BJ; N 50 laJ; Y 0.6 laJ; N


involatile elements (Ba, Bi, Eu, Ir, Li, Mo, Ni, Pd, Pt, Rh, V) that to this time have only been investigated using cup furnaces.

Table 5 is a comparison of the best graphite furnace LEAFS detection limits to the most sensitive, commercially available atomic spectrometry techniques: graphite furnace AAS and inductively coupled plasma-mass spectrometry (ICP-MS). The LEAFS detection limits obtained with modem furnace technology are one to five orders of magnitude lower than the AAS detection limits. For several involatile elements, very poor LEAFS detection limits were obtained with cup furnace atomization. These detection limits can be improved with the use of better instrumentation. In comparison to ICP-MS, the modern LEAFS results are lower by several orders of magnitude for the most sensitive elements (Cd, In, Pb, and T1), but for most other elements the detection limits are the same or within one order of magnitude. Hence, at the present time, ICP-MS is a more attractive method for most analyses because of its multielemental capability. In addition, as discussed previously, laser systems for LEAFS continue to be difficult to operate and unreliable, but the development of a better laser system would make LEAFS a viable option for some applications, for example, when the available amount of sample is very small.

B. Calibration Curves

A number of graphite furnace LEAFS publications have reported the linear dynamic ranges (LDRs) of LEAFS calibration curves. For most elements, LDRs are between four and seven orders of magnitude. Dougherty et al. (1990) investi-

(9 0 C

0 U) L. 0 N U.

tll

1 0 '-1

1~ ,urnayJ 1 0 " f u r n a c

4 t J / 28 mrn

10 10 1 0 ~ , , , , , , , , , 1 021 031 041 051 061 071 081 091 0101011

A m o u n t o f I n d i u m , f g

Figure 10. Calibration curves for nonresonance LEAFS of indium: (o) in an 8-mm laboratory-constructed furnace, and (o) in a 28-mm commercial furnace. Taken with permission from Dougherty et al. (1990).

Table 5. Comparison of Best Graphite Furnace LEAFS Detection Limits to those of Graphite Furnace Atomic Absorption Spectrometry (AAS) and Inductively Coupled

Plasma-Mass Spectrometry (ICP-MS) Limits of Detection

Graphite Furnace LEAFS

Graphite Concentration Furnace AAS ICP-MS

Element Reference Absolute (fg) (pg/mL) (fg)a (pg/mL)b

Ag Dougherty et al. 10 0.5 500 5 (1987b)

A1 Wei (1990) 100 5 4,000 15

Au Remy (1990) 10 0.2 10,000 5

Ba Denisov et al. (1985) 80,000,000 4,000,000 10,000 6

Bi Falk et al. (1988) 800 16 10,000 4

Cd Bolshov et al. (1991) 0.5 0.01 300 12

Co Remy et al. (1990) 4 0.08 2,000 5

Cu Bolshov et al. (1981 a) 150 3 1,000

Eu Bolshov et al. (1981a) 300,000 6,000 10,000

Fe Wei et al. (1990) 70 3.5 2,000 580

Ga Vera et al. (1989c) 1 0.05 40,000 4

In Vera et al. (1989c) 2 0.01 9,000 2

Ir Bolshov et al. (1981a) 6,000 120 300,000

Li Goforth and 400,000 80,000 2,000 27 Winefordner (1986)

Mn Wei et al. (1990) 100 5 1,000 6

Mo Goforth and 100,000 20,000 4,000 6 Winefordner (1987)

Na Denisov et al. (1985) 55 2.8 1,000 110

Ni Falk et al. (1988) 1,000 20 4,000 13

Pb Wet et al. (1990) 0.2 0.01 5,000 50 c

Pd Falk and Tilch (1987) 700 14 25,000 9

Pt Goforth and 1,000 200 50,000 Winefordner (1986)

Rh Falk and Tilch (1987) 2,000 40 8,000 2

Sb Liang et al. (1993) 10 0.5 20,000 12

Sn Wei et al. (1990) 30 1.5 20,000 10

Te Liang et al. (1993) 20 1 10,000 32

TI Smith et al. (1990) 0.1 0.005 10,000 3

V Falk and Tilch (1987) 1,700,000 34,000 20,000 8

Yb Vera et al. (1989c) 220 11 4,000

Notes: aTaken from Slavin (1984). ~l'aken from Takahashi and Hara (1988). eraken from Gray (1986).

24


0 C

0 r 11) L .

0 :3

m I L

11) >

= m

m

rr'

O m

, , .

6-

4-

" i I

-3 -2 -1

A B

C

I I I I I I

0 1 2 3 4 5 log(Concentration, l g/mL)

Figure 11. Calibration curves of LEAFS for TI in graphite tube atomizers. A, front surface approach; B, transverse approach; C, the same conditions as graph A except that laser power was attenuated to 1% of that of A. Taken with permission from Wei et al. (1990).

gated factors that affect the shapes of LEAFS calibration curves using transverse illumination with a commercial furnace (28 mm in length) and with a laboratory- constructed furnace (8 mm in length). Under conditions of optical saturation, the nonlinear behavior at high concentrations was attributed to postfilter effects. The postfilter effects were approximately the same for the longer commercial furnace and the laboratory-constructed furnace. Figure 10 shows calibration graphs for indium in the two furnaces that become nonlinear at the same concentration, indicating that the atom population is the same in each furnace.

Wei et al. (1990) compared calibration graphs for thallium obtained with transverse and front surface illumination (Figure 11). Both geometries gave approximately the same LDR, but the front surface graph curves at a slightly lower concentration because more atoms are present in the probe volume and hence the atoms drop out of saturation at a lower concentration than they do in the transverse geometry. Reduction of the laser power by a factor of one hundred caused a reduction in the LDR by one order of magnitude for the same reason.

V. BACKGROUND CORRECTION FOR LEAFS

Butcher et al. (1988) reviewed the types of background present in graphite furnace LEAFS. These include blackbody emission, laser scatter, molecular fluorescence, and nonanalyte atomic fluorescence. Blackbody emission and laser scatter can be

26 DAVID J. BUTCHER

described as broadband background signals, whose magnitude does not vary near the atomic line. Molecular fluorescence and nonanalyte atomic fluorescence may give a structured background whose magnitude changes with wavelength.

Until recently, only broadband background signals had been observed for LEAFS, which allowed the following simple method of background correction. Background levels were measured by averaging the signal obtained from several furnace atomizations while the dye laser was tuned 0.1 to 0.3 nm away from the analytical wavelength (background), and subtracting this value from the measurement made at the analytical wavelength (fluorescence signal plus background). Bolshov et al. (1981 b) used this method to determine cobalt in agricultural standard reference materials (SRMs) using cup furnace atomization. The LEAFS results were a factor of ten lower than the certified value, which was attributed to smoking of the sample that produced vapor phase interferences.

Dougherty et al. (1988) determined thallium in bovine liver by graphite furnace LEAFS with transverse illumination. A scatter signal was observed due to the bovine liver matrix with a char temperature of 700 ~ but this background signal was eliminated by the use of a 900 ~ char temperature.

LLI (.D 1.00 Z LLI C.) 0.75 Lf) LLI rY: 0 0 . 5 0 ZD __1 LL 0.25--

_J LLI

0.00----4 -0.08

0

O 0 /o 0 0

o/g%o __0 .'0 I

0 _ 0 _ 0 . - 0 �9 /" \ O

O �9 \

%

00~0-_0.-0-~0__ 0

- 0 . 04 0.00 0.04 0.08

WAVELENOTH, i q m

Figure 12. LEAFS wavelength profiles of thallium: (.) aqueous thallium (0.1 pg) and (o) dissolved bovine liver (50 lag), containing 0.15 pg thallium. Excitation wavelength 276.8 nm; detection wavelength, 353 nm; atomization temperature, 1700 ~ The analytical precision was between 5 and 10%. Measured or convoluted (laser + atomic) iinewidth for thallium: 0.018 nm; laser linewidth: 0.003 nm; atomic iinewidth: 0.018 nm. Taken with permission from Butcher et al. (1990).


Butcher et al. (1990) investigated the use of LEAFS with front surface illumination for the determination of manganese, lead, and thallium with slurry sampling in SRMs. For thallium, a broad band scatter signal was observed for a dissolved bovine liver sample (Figure 12) that was attributed to scatter caused by the sample matrix. For manganese, a small background signal was present for both standards and samples that was attributed to blackbody emission from the graphite furnace (Figure 13). Correction for these backgrounds was performed by measuring the background signal 0.1 nm away from the analytical wavelength.

Although this manual correction technique works for relatively low background levels, there are several drawbacks. It is time-consuming, and would not be expected to work well with relatively large background levels or with structured backgrounds. Liang et al. (1993) reported the presence of structured backgrounds in their work involving the determination of antimony and tellurium in nickel-based alloys. An excitation spectral scan of tellurium in an aqueous standard and in the dissolved sample revealed the presence of a structured background signal in the sample (Figure 14). This signal, at 214.243 nm, did not match exactly any known atomic line and its origin could not be determined. Excitation spectral scans of antimony in the samples revealed the presence of three structured background signals: a line at 212.714 nm due to cobalt, a shoulder at 212.733 nm that could not be identified, and a line at 212.791 nm due to nickel (Figure 15).

Liang et al. (1993) also reported large background signals near the antimony excitation wavelength that were present in deionized water. Tap water was used as a sample because it gave a larger signal. Figure 16 shows excitation spectra for 20 pg antimony and the background present in the tap water. The background signals were tentatively attributed to the formation of silicon monoxide, which was produced from sodium silicate.

This recent work indicates that background correction for LEAFS may require measurement of the background signal with a more accurate and precise method than off-line measurements. Several instrumental approaches to background correction for graphite furnace LEAFS have been investigated that include (1) multichannel correction techniques, (2) wavelength background correction and, (3) Zeeman background correction.

A. Multichannel Background Correction Techniques

Sjtistrt~m (1990a) described a multichannel background correction technique for graphite furnace LEAFS that used three optical fibers located at the exit slit of the monochromator. The center fiber was coupled to a photomultiplier tube (PMT1) to measure the fluorescence and background signals, and background light collected from the other two fibers was sent to a second PMT (PMT2). Subtraction of the signal from PMT1 from that of PMT2 gave a background-corrected signal. This technique was used to correct for a background signal generated by 100 lag of sodium chloride. Like other multichannel techniques, SjtistrOm's method had the

LLI C) Z W C) GO W n I G

_J LL

LLI > I--- 3 m 13s LLI C) Z Sl C) O9 ILl

0 E3 __1 LL_

LLI > I--- 3 Sl rY

4000-

3000

2000

1000

0 -0.10

4000

3000

2000

1000

0 - 0 . 1 0

(a)

O

- Io o/ /',o

6, i~.

. o.._._o__o_o_o.~---~ n'~ ~176 ~==,,.. o___._, o ~ o I / I I

- 0 . 0 5 0.00 0.05 O. 1 0

WAVELENGTH (nm) ( b )

0

.oo o o ~

OO

o- -o - - -o -o -o -~ 0~ ~ : l l ' -v~o- - -o - -o I i i

- 0 . 0 5 0.00 0.05 0.10

WAVELENGTH (n m) Figure 13. LEAFS manganese wavelength profiles for NIST citrus leaves (SRM 1572): (a) (o) aqueous manganese, 400 pg and, (o) dissolved citrus leaves (100 gg), which contained 860 pg manganese; (b) (o) aqueous manganese, 400 pg, and (o) slurried citrus leaves (100 gg), which contained 860 pg manganese. Excitation wavelength 279.5 nm; detection wavelength, 403 nm; atomization temperature, 2000 ~ The analytical precision was between 7 and 10%. Measured or convoluted (laser + atomic) linewidth for manganese: 0.012 nm; laser linewidth: 0.003 nm; atomic linewidth: 0.011 nm. Taken with permission from Butcher et al. (1990).

28

(9 o 80 I::: (9 0

60 (9 L .

0 2 40

~ 20

0 " r - , , , (9 214 225 214 245 214 265 214.285 21 .305214.325 rr

Wavelength, nm

Figure 14. Excitation spectrum of tellurium" (.) 20 pg aqueous Te; (o) NIST SRM 898 nickel-based alloy sample, 0.1125 g per 100 mL (12.2 pg Te). The laser linewidth (FWHM) of 0.2 cm -1, i.e., 0.0036 nm at 426 nm, was quoted by the manufacturer. Frequency doubled linewidth" 0.002 nm at 213 nm calculated from the fundamental. Taken with permission from Liang et al. (1993).

(1) 0300 I::: (9 0

~200 0

IJ., 100 (9 m m

(9

(a)

0 2 1 2 . 6 8 212 .71 2 1 2 . 7 4 2 1 2 . 7 7 2 1 2 . 8 0

Excitation Wavelength, nm

Figure 15. Excitation spectra of antimony using a detection wavelength of 259.8 nm. (a) Antimony aqueous standard, 20 pg, Xfl = 259.8 nm. (b) P&W 1A nickel alloy (0.100 g per 100 mL), monochromator slitwidth: 0.1 mm (0.8-nm bandpass). (c) P&W 1A nickel alloy (0.100 g per 100 ml), monochromator slitwidth: 1 mm (8-nm bandpass). Taken with permission from Liang et al. (1993). (continued)

29

30 DAVID J. BUTCHER

o 80- C

0 60 11)

0 :: 40

m

i,1,,, 11) >

= m

m

mr'

20

(b)

0 . . . . i - - i . . . . I . . . . . . . �9

2 1 2 . 6 8 212 .71 2 1 2 . 7 4 2 1 2 . 7 7 2 1 2 . 8 0

Excitation wavelength, nm

o 2500

2000

1500 0 ~ 1000 r > 500

= m

~ 0

(c)

Shoulder

212.68 212.71 212.74 212.77 212.80

Excitation wavelength, nm

Figure 15. (continued)

advantage that background correction was made simultaneously with the signal measurement, both signal and background being measured with every laser pulse. Second, this technique can be employed with any atom cell because it involves modification of the detection system. Third, this multichannel method can be used with any laser repetition rate. The principal disadvantage of this technique is that the background correction measurement was made 3-4 nm away from the analytical wavelength. This technique will not be able to accurately correct for background signals whose size varies between the analytical and background wavelengths (structured background). For example, this method will not correct for the background signals observed by Liang et al. (1993) in their antimony and tellurium


I1) o300 C

0

200 ! . _

0

u'lO 0

0

(a)

i i I I = = = =

l l ) n -

I

212 .68 212.71 212 .74 212.77 212 .80

Excitation Wavelength, nm

o 600 i,-,

o 500

400 x ._

0 = 300

i m m m

i,1,,, 200

�9 P- 100

(b)

212.68 212.72 212.76 212.80 Excitation wavelength, nm

Figure 16. Excitation spectra for antimony. Monochromator slitwidth" 0.5 mm (4-nm bandpass). (a) Antimony (20 pg) in sub-boiled distilled water; (b) tap water, ;Lfl = 259.8 nm. Taken with permission from Liang et al. (1993).

work. The other disadvantage is that approximately 25% of the light was lost in the use of the optical fibers.

Remy et al. (1990) employed a slightly different method of multichannel background correction for graphite furnace LEAFS. They used a beamsplitter to produce two beams of fluorescent light that were collected by two monochromators. One of the monochromators was set at the analyte fluorescence wavelength and used to measure the fluorescence plus background signals. The other was set 8 nm away from the fluorescence wavelength and used to measure the background signal. Subtraction of the latter from the former gave a background-corrected signal.

32 DAVID J. BUTCHER

Advantages and disadvantages of Remy et al.'s technique are similar to those of SjOstr6m's, except Remy's background measurement is made even farther away from the fluorescence wavelength (8 nm) than is Sjt~strtim's (3--4 nm). Multichannel techniques would probably be adequate to correct for broadband background signals, such as scatter and blackbody emission, but could not correct for the spectral interferences described by Liang et al. (1993).

~ 120 (a) 0

r ~100 o 8O ,'7 60 > �9 40 ~ _ ~ , = 20 "6 0 . . . . . .

I I I I

n- -0.05-0.03-0.01 0.01 0.03 0.05 Relative Wavelength of Laser Excitation, nm

(1) ~ C

o100 80

0 = 60

It_ 40

�9 > 20

(b)

0 , n- -0.050-0.025 0.000 0.025 0.050

Relative Wavelength of Laser Excitation, nm Figure 17. Excitation spectral scan of 0.1 ppb thallium in an AICI3 matrix (1 mg/mL as AI) with wavelength modulation. Each point represents the average signal from three or more furnace firings at each wavelength: (a) spectral profile ofthe total signal; (b) spectral profile of the background-corrected signal. The fwhm was 0.012 nm. The 0.0 point in the X-axis indicates the peak wavelength position of thallium at 276.787 nm. Taken with permission from Su et al. (1992).


B. Wavelength Modulation

Suet al. (1992) constructed a grazing incidence dye laser to employ wavelength modulation for background correction in graphite furnace LEAFS (Figure 1). A piezoelectric pusher was used to move the wavelength-tuning mirror so that alternately the laser was tuned on the analytical wavelength (fluorescence signal plus background) and 0.1--0.2 nm away from the analytical wavelength (background). Subtraction of the second measurement from the first provided a background-corrected signal.

The effectiveness of wavelength modulation to correct for background signals produced by an aluminum chloride matrix in the determination of thallium was investigated (Figure 17). Figure 17a shows an excitation profile of 0.1 ppb thallium in the presence of 1 mg/mL aluminum chloride without background correction, and indicates the presence of a broadband background caused by scatter from the matrix. Figure 17b is an excitation profile with wavelength modulation. The background correction clearly eliminated this background signal.

Table 6 shows quantitative measurements demonstrating the ability of wavelength modulation to correct for background signals produced by an aluminum chloride matrix. A comparison of background-corrected signals from 0.1 ppb thallium and from 0.1 ppb thallium in the aluminum chloride matrix showed that their values were statistically the same.

Wavelength modulation has many of the same advantages as the multichannel techniques: it can be used with any atom cell and it can be used at any laser repetition rate. It has the disadvantage that the fluorescence signal is measured only every other laser pulse, which leads to a small loss of sensitivity. Its most significant disadvantage is that it measures background away from the analytical wavelength, and will not accurately correct for the types of background signals reported by Liang et al. (1993). However, compared to the multichannel correction methods, the spectral resolution is greatly improved. Su et al. (1992) measured background 0.1-0.2 nm away from the excitation wavelength, while the multichannel techniques measured background 2-8 nm away from the emission wavelength.

Table 6. Background Correction for 0.1 ppb TI in the Aluminum Chloride Matrix (1 mg/mL as AI) for Graphite Furnace LEAFS with Wavelength Modulation

Total Signal Background-Corrected Sample (Arbitrary Units) Background Signal Signal

0.1 ppb TI 12.4 + 0.9 AIC13 matrix 5.9 + 0.5 0.1 ppb TI in A1C13 matrix 15.5 + 0.7

.

Note: Taken with permission from Suet al. (1992).

0.22 + 0.29 12.2 + 1.0 (n = 4) 5.2 + 0.4 0.68 + 0.8 (n = 5) 3.8+0.9 11.8+2.1 (n= 12)

34 DAVID J. BUTCHER

Apatin et al. (1989) reported a method of background correction for graphite furnace LEAFS that is related to wavelength modulation. A computer was used to adjust the laser wavelength on the analytical wavelength for a fixed number of pulses and then away from the analytical wavelength for the same number of pulses. A computer program reconstructed the temporal measurements of signal plus background and background alone. These measurements were then subtracted to give a background-corrected signal. There are a number of disadvantages associated with this technique. First, the laser repetition rate was limited to 20 Hz because 40 ms were required to change the laser wavelength. Second, the authors reported that reconstruction of the signals was most effective for furnace transients whose width exceeded 4-5 s. A significant distortion of the signal would be expected for fast transients, for example, the furnace signals for volatile elements.

C. Zeeman Background Correction

Zeeman background correction has been widely studied for graphite furnace AAS because the background measurement is made at the analytical wavelength, rather than away from the analytical wavelength like other methods of background correction (e.g., wavelength modulation). Two configurations are possible for the inverse Zeeman effect: the longitudinal Zeeman effect, in which the laser beam is parallel (longitudinal) to the ac electromagnetic field, and the transverse Zeeman effect, in which the laser beam is perpendicular (transverse) to the magnetic field.

Energy level diagrams for longitudinal and transverse Zeeman are shown in Figure 18. When the magnetic field is off ("field off") for either configuration, absorption of laser light by analyte atoms is possible, and hence both fluorescence and background signals are measured. With the magnetic field on ("field on"), using longitudinal excitation (Figure 18a), the atomic energy levels are split into two sigma components whose wavelengths are displaced away from the analyte wavelength, and hence no absorption (and fluorescence) by the analyte is possible. In the presence of the magnetic field, only background is measured. Subtraction of the "field on" measurement from the "field off" measurement gives a background- corrected signal. In the case of transverse excitation with the magnetic field on, the atomic energy levels are split into three linearly polarized components, two sigma and one pi (Figure 18b). The pi component is at the same wavelength as the atomic transition, and it is necessary to prevent absorption of the pi component in order to do background correction. Polarization of the laser beam to provide perpendicularly polarized light prevents analyte absorption by the pi component. The difference of the "field off" from the "field on" measurement provides a background corrected signal.

Zeeman background correction has been employed with both transverse and front surface illumination, and the four possible Zeeman/illumination configurations are shown in Figure 19. Longitudinal Zeeman has the advantage compared to transverse Zeeman that no polarizer is required. It is also necessary to be able to collect

(a)

En~ergy

A~

v

/ \

Laser-Excited Fluorescence

Field off Field on

Sigma

Sigma

35

(b)

EnTergy

u

i i

i

Sigma

Pi

Sigma

Field off Field on

Figure 18. Energy diagrams for ZETA LEAFS: (a) longitudinal Zeeman background correction, and (b) transverse Zeeman background correction. Taken with permission from Irwin et al. (1992).

fluorescence over a relatively wide solid angle. Collection of fluorescence along the axis of the magnet is undesirable because large holes in the magnet are required that may reduce the magnetic field. With transverse illumination, it is relatively easy to use longitudinal Zeeman and collect fluorescence down the bore of the tube (Figure 19a). Tranverse Zeeman with transverse illumination has the disadvantages of requiting a polarizer and large holes in the magnet to collect fluorescence (Figure 19b). However, with front surface illumination, the use of longitudinal Zeeman requires large holes in the magnet (Figure 19c), and hence transverse Zeeman is easier to set up experimentally, even though it requires a polarizer (Figure 19d). At present, two instruments have been constructed for Zeeman background correction: one that uses longitudinal Zeeman in a laboratory-constructed furnace with transverse illumination, and one that uses transverse Zeeman in an atomic absorption furnace with front surface illumination.

36 DAVID J. BUTCHER

nace

Laser Fluorescence

7 Fluorescence ///~ Laser

(c)

I "N Fluorescence ! t ~ ' / ' / M a g net piec eL " "~

M ' , ', I Jrror ~- ~ Furnace Laser

(d)

Magnet pole

/

Laser Mirror ~ ~ ' Furnace / / '

Figure 19. Instrumental configurations for ZETA LEAFS: (a)longitudinal Zeeman with transverse illumination, (b) transverse Zeeman with transverse illumination, (c) longitudinal Zeeman with front surface illumination, and (d) transverse Zeeman with front surface illumination. Taken with permission from Preli et al. (1988).

Longitudinal Zeeman in a Laboratory Constructed Furnace with Transverse Illumination

Dougherty et al. (1987a, 1989) and Preli (1988) described the first application of Zeeman background correction to graphite furnace LEAFS. This technique was named Zeeman electrothermal atomizer LEAFS (ZETA LEAFS). They employed a laboratory-constructed furnace and magnet, an excimer-pumped dye laser system

Laser-Excited Fluorescence 3 7

Table 7. Comparison of LEAFS and ZETA LEAFS Detection Limits with a Laboratory-Constructed Furnace System, Longitudinal Zeeman, and Transverse

illumination Limits of Detection ~g)

LEAFS ZETA LEAFS

Element On-Line Off-Line On-Line Off-Line

A g 20 - - 200

Co 300 - - 700

Cu 600 ~ *

In 20 - - 40

M n 400 100 1000

Pb 200 10 200 10

TI 100 ~ 500

Note: *No signal was observed.

Taken with permission from Dougherty et al. (1989).

at 80 Hz, transverse illumination, and the longitudinal Zeeman effect in their experiments (Figure 19a). The magnet was powered by the 60 Hz line frequency, and the magnetic field was variable up to 12 kG. No polarizer was required with this instrument.

The Relative Sizes of the LEAFS to ZETA LEAFS Signal. Detection limits comparing LEAFS to ZETA LEAFS are listed in Table 7. Preli et al. (1988) summarized the factors that influence the relative size of the LEAFS signal to the ZETA LEAFS signal. These include the magnitude of Zeeman splitting, the width of the atomic line, and the laser linewidth. The size of the Zeeman splitting is dependent upon the quantum states of the energy levels involved in the excitation process and the magnetic field strength. The atomic linewidth is dependent upon the number of isotopes present and the number of hyperfine states. The laser linewidth is dependent upon the excitation wavelengths employed, which are determined by the characteristics of the laser system. A summary of these parameters for each of the elements determined is given in Table 8.

In the case of cobalt, the ZETA LEAFS detection limit was within a factor of two of the LEAFS detection limit, indicating relatively little signal loss due to the background correction. Although the laser profile was relatively wide, the Zeeman splitting factor was large and the atomic linewidth was relatively narrow, leading to only a small loss of sensitivity. Identical LEAFS and ZETA LEAFS detection limits were obtained for lead because of a relatively narrow atomic linewidth and a very narrow laser linewidth. Indium and thallium show an increase in detection limit for ZETA LEAFS compared to LEAFS by factors of two and five, respectively. The loss of sensitivity for these elements was attributed to a very wide atomic

38 DAVID J. BUTCHER

Table 8. Summary of Factors that Affect the Relative Sizes of the LEAFS and ZETA LEAFS Signals

Laser Linewidth [nm Atomic linewidth [nm Experimental Zeeman Element (FWHM)] (FWHM)] Splitting (nm) *

Ag 0.0042 0.0087 0.0090

Co 0.0040 0.011 0.0066

Cu 0.0042 0.011 0.0075

In 0.0031 0.014

Pb 0.0027 0.0067 0.0065

T1 0.0035 0.024 0.012

Notes: Longitudinal Zeeman was Employed with a Laboratory-Constructed Furnace and Transverse Illumination. Taken with permission from Preli et al. (1988). *Using a field strength of 11.7 kG.

profile. The sensitivity loss was greater for thallium than for indium because of its wider atomic linewidth (Table 8). The ZETA LEAFS detection limit for silver was a factor of ten worse than the LEAFS detection limit, and no ZETA LEAFS signal at all was observed for copper (Table 7, Figure 20). The laser linewidth was extremely wide for these elements, and the sigma components were very wide. No

0 r

0 m z . _

0 , . , = . . ,

LI.

> , . m

= , = , =

(D n-

12 10-

8 6 4 2

r

0 ' ' -0.050 -0.025 0.000

!

0.025 0.050

a)

�9 ! ~ I "

Relative Wavelength, nm Figure 20. ZETA LEAFS spectral profiles for a magnetic field strength of 11.7 kG using transverse Zeeman, a laboratory-constructed furnace, and transverse illumination. (a) "field off", analyte + background channel for 100 pg silver; (b) "field-on", background channel for 100 pg silver; (c) "field off", analyte + background channel for 10 ng copper, (d) "field on", background channel for 10 ng copper. Taken with permission from Preli et al. (1988). (continued)


0 " 12 0 m 10

~ 8 0

2 6 U.

>

t~ m

r r

4 2 0

-0 �9 I ' I ' I '

. 050 - 0 . 0 2 5 0 . 0 0 0 0 . 0 2 5 0 . 0 5 0

Relative Wavelength, nm

0

0

x . _

0

I.!.. 11) >

l u

11) n..,

2

0 8 6 4 2

0 -0

(c)

, 1 . i " I " I

�9 0 4 8 - 0 . 0 2 4 0 . 0 0 0 0 . 0 2 4

�9 !

0 . 0 4 8

Relative Wavelength, nm Figure 20. (continued)

detection limit could be measured for copper because the sigma components were so broad that the "field on" and "field off' signal sizes were nearly identical (Figure 20c, d).

Calibration Curves forZETA LEAFS. Preli et al. (1988)reported that the linear dynamic range of ZETA LEAFS is approximately equal to that of LEAFS, and is generally between four and seven orders of magnitude. However, as comparison of the thallium curves indicates (Figure 21), the ZETA LEAFS calibration curve exhibits more bending at high concentrations. An explanation for this behavior is

40 DAVID J. BUTCHER

O e" tl) O U) L .

O ::3

m

LI.

> = m

m m

n-

10-

8

6

4

0 -0 .048 -0 .024 0.000

)

' I ' I ' I '

0.024 i

0.048

Relative Wavelength, nm Figure 20. (continued)

provided in Figures 22 and 23. At relatively low thallium (1 ng) concentrations (Figure 22), the sigma components are split away from the analytical wavelength, although incompletely, which leads to some loss of signal. However, at relatively high thallium concentrations (10 lag), the sigma components are sufficiently broadened to cause complete overlap with the unshifted atomic line (Figure 23).

O C

O U) (l) L .

O :3

m

U..

> , . m

m m @) rr

O ,.J

8 7 6 5 4- 3 2 1 lOO 0

0 1

LEAFS ZETA LEAFS

g"" 500 fg I I I I I I

2 3 4 5 6 7 Log Mass, fg

I I I

8 9 1 0

Figure 21. LEAFS and ZETA LEAFS calibration curves for thallium using transverse Zeeman, a laboratory-constructed furnace, and transverse illumination. Taken with permission from Preli et al. (1988).

0 C

0 o~ G) L .

0 :3

i m m

14.

.> r

m

(U n -

.

0 x

-2

- - 4 I I I I

-0 .06 -0 .03 0 .00 0 . 0 3 0 .06


Figure 22. ZETA LEAFS spectral profiles for 1 ng thallium, for a magnetic field strength of 11.7 kG using transverse Zeeman, a laboratory-constructed furnace, and transverse illumination. The upper profile (o) is for the "field off", analyte + background channel. The lower (inverted) profile (x)is for the "field on", background channel. Taken with permission from Preli et al. (1988).

{P o 6 I:::

o 4 {n

2 l . _

0 = 0

m

LI.. -2 �9 -> -4 ,4,,='

-~ -6

x x

! I I I I I

- 0 . 0 9 - 0 . 0 6 - 0 . 0 3 - 0 . 0 0 0.03 0.06 0.09


Figure 23. ZETA LEAFS spectral profile for 10 Fg thallium, for a magnetic field strength of 11.7 kG using transverse Zeeman, a laboratory-constructed furnace, and transverse illumination. The upper profile is for the "field off", analyte + background channel. The lower (inverted) profile (x)is for the "field on" background channel. Taken with permission from Preli et al. (1988).

41

42 DAVID J. BUTCHER

Table 9. Determination of Manganese by ZETA LEAFS and Zeeman AAS

Technique Calibration Method Sample Matrix Mn (ng/mL) RSD (%)

Zeeman AAS Aqueous calibration Zinc chloride 4.8 + 0.1 2

Zeeman AAS Standard additions Zinc chloride 5.1 + 0.1 2

ZETA LEAFS Aqueous calibration Zinc chloride 5.0 + 1.5 30

Zeeman AAS Aqueous calibration Mouse brain 1.6 + 0.1 6

ZETA LEAFS Standard additions Mouse brain 1.9 + 0.4 20

Notes: Longitudinal Zeeman was employed with a laboratory constructed furnace and transverse illumination. Taken with permission from Dougherty et al. (1989).

Subtraction of the "field on" from the "field off" measure gives no ZETA LEAFS signal, and causes rollover of the calibration curve.

Applications of ZETA LEAFS. Dougherty et al. (1987a) reported the use of ZETA LEAFS to correct for blackbody emission in the determination of cobalt. Accurate correction for this continuum background was achieved. Dougherty et al. (1989) reported the use of ZETA LEAFS for the determination of manganese in a zinc chloride matrix and in mouse brains (Table 9). The ZETA LEAFS results were compared to those obtained by Zeeman AAS. For the zinc chloride matrix, which produced a scatter signal, good accuracy was obtained by ZETA LEAFS with either standard addition or aqueous calibration compared to the Zeeman AAS results, but the precision of the aqueous calibration method was very low (30%). Accurate analyses could not be performed using the mouse brain sample with ZETA LEAFS and aqueous calibration because of the relatively unsophisticated design of the laboratory-constructed furnace (e.g., no temperature feedback or platform). Accu- rate results were obtained for this determination with the method of standard additions, but with poor precision (20%). The high precision was attributed to the furnace design, low laser repetition rate, and wide variation in laser pulse energies.

Inverse ZETA LEAFS. Dougherty et al. (1989) reported that, with the magnetic field on, analyte atomic fluorescence could be measured by tuning the laser to the absorption maximum of a sigma component, and measuring signal plus background. At this wavelength, the field off measurement corresponded to background only, because the laser was tuned away from the analytical wavelength. Dougherty et al. (1989) named this technique "inverse ZETA LEAFS", and demonstrated its application to extend the linear dynamic range of calibration curve for silver (Figure 24). Detailed experiments were not performed to study the analytical utility of this technique.

Summary--Longitudinal Zeeman. Although the longitudinal Zeeman instrument with a laboratory-constructed furnace was not widely used for real sample


o 1 0 ~ -

0 t,/) �9 10 ~- I,=, ,

0

I,,I,. 10 o. W >

lI~ -1 -610 r 10 0

I I I I I

101 10 2 10 a 10 4 10 s A m o u n t of Ag, pg

Figure 24. The upper range of ZETA LEAFS calibration curves for Ag (o) measured with the laser tuned to the resonance wavelength (328.1 nm), and (.) with the laser tuned 9 pm away from the resonance wavelength using transverse Zeeman, a laboratory constructed furnace, and transverse illumination. Taken with permission from Dougherty et al. (1989).

analyses, several conclusions were drawn from this research (Dougherty et al., 1989). First, modem furnace technology, which includes the use of a rapidly heated furnace, matrix modifiers, and platform atomization, is required in order to determine successfully the elements in complex sample matrices. The laboratory-constructed furnace lacked these features and hence would be difficult to use for these determinations. Second, a higher magnetic field strength than the 12 kG used for this work would be needed to increase the splitting of the sigma components away from the analytical wavelength, and to improve the ZETA LEAFS sensitivity of elements such as silver and copper. A field strength of 16 kG was reported to be adequate to achieve this goal. Third, the use of a laser with a narrower linewidth would also improve the sensitivity of ZETA LEAFS by reducing absorption of laser radiation by the sigma components. Fourth, the use of a higher repetition rate laser with better pulse-to-pulse reproducibility would improve the precision of ZETA LEAFS. These conclusions were used to produce a more modem ZETA LEAFS instrument (Irwin et al., 1992) that was an improvement upon the first-generation instrument.

Transverse Zeeman in an Atomic Absorption Furnace with Front Surface Illumination

Irwin et al. (1992) reported the use of transverse Zeeman background correction in a Perkin-Elmer HGA-500 graphite furnace with front surface illumination. A

44 DAVID J. BUTCHER

Table 10. Laser Linewidths and Sensitivity Ratios (R) of ZETA LEAFS to LEAFS

Linewidth (nm) Element (Irwin et ai., 1992) R (Irwin et al., 1992) R (Preli et al., 1988)

Lead 0.0038 0.39 0.70

(etalon narrowed) 0.0008 0.82 m

Cobalt 0.0044 0.89 0.98

(etalon narrowed) 0.0009 0.98 m

Notes: *Field strength = 10.5 kG

**Field strength = I 1.7 kG; linewidth = 0.0016 nm.

***Field strength = 11.7 kG; linewidth = 0.0040 nm.

Transverse Zeeman was employed with front surface illumination in a commercial graphite furnace.

Taken with permission from Irwin et al. (1992).

Perkin-Elmer 5100 electromagnet was modified to provide an average field strength of 10.5 kG. Transverse Zeeman was used for this work because of the relative ease of fluorescence collection with this configuration (Figure 19c,d). The use of longitudinal Zeeman would require very large holes in the magnet to collect fluorescence. An excimer-pumped dye laser with a maximum repetition rate of 500 Hz was employed as the light source, and a polarizer was used to produce perpendicularly polarized light to minimize absorption by the rt component when the magnetic field was on. The electromagnet was powered by the 60 Hz-line frequency, and the laser operated at a repetition rate of 240 Hz to make a "field off" measurement at each zero crossing and a "field on" measurement at each maximum. Intercavity etalons were used to narrow the laser linewidth.

Irwin et al. (1992) used this instrument for the determination of lead and cobalt. The sensitivity ratio (R) was defined to be the ratio of the ZETA LEAFS signal to the LEAFS signal, and their results for these elements are summarized in Table 10. The sensitivity ratios for these elements were approximately the same as those obtained by Preli et al. (1988), because, although an etalon was used to narrow the laser linewidth, a lower magnetic field was used than in the previous work. The

Table 11. LEAFS and ZETA LEAFS Detection Limits

LEAFS LOD (fg) ZETA LEAFS LOD Oeg)

Element 200 Hz 500 Hz* Conventional Etaton narrowed

P b 2 - - 8 4

Co 200 30 500 500

Notes: *Using piano-convex lens system.




relative precision using ZETA LEAFS with this instrument was 5%, which is a factor of six better than the results obtained by Preli et al. (1988). The improved precision was attributed to a higher laser repetition rate, better pulse-to-pulse reproducibility, and the use of a commercial furnace.

Detection limits obtained for cobalt and lead are listed in Table 11. For both elements, with the etalon-narrowed laser, the ZETA LEAFS detection limits were within a factor of two of the LEAFS detection limits, indicating that absorption of light by the ~ components with the field on has been minimized. Figure 25 shows calibration curves for cobalt by LEAFS and ZETA LEAFS. The curves were nearly identical, indicating that t~ component broadening was inconsequential owing to the favorable o component splittings.

The effectiveness of Zeeman background correction for the correction of blackbody emission and scatter due to aluminum chloride in the determination of cobalt was also investigated. The background signal measured by the Zeeman technique was compared to the background measured by tuning the laser 0.05 nm away from the cobalt wavelength. The background and background-corrected signals were the same size as expected.

Transverse Zeeman was also employed for the determination of lead with slurry sampling in four NIST SRMs: estuarine sediment, coal fly ash, citrus leaves, and pine needles (Table 12). Good agreement was obtained between the certified values and the ZETA LEAFS results. The precision was relatively high, between 8-14%

> ,,===, ,I1,.,,I

m

0 ,,,,,J

1 I " I I I I I I I I I

2 3 4 5 6 7 8 9 1 0 1 1 Log mass, fg

_

6- 5- 4- 3- 2- 1- O' '

0 1

0 C

0

0

t _ . .

0

Figure 25. LEAFS (.) and ZETA LEAFS (o) calibration curves for cobalt with front surface illumination, a commercial graphite furnace, and transverse Zeeman background correction. Taken with permission from Irwin et al. (1992).

46 DAVID J. BUTCHER

Table 12. Comparison of Certified and Experimental Results for the Determination of Lead in Various NIST SRMs by Slurry Sampling and Aqueous Calibration

NIST SRM Certified Value ( lag/g) Experimental value (lag/g)

Estuarine sediment (1646) 28.2 + 1.8 28.4 + 3.2

Coal fly ash (1633a) 72.4 + 0 . 4 74.4 + 7.9

Citrus leaves (1572) 13.3 + 2.4 13.2 + 1.7

Pine needles (1575) 10.8 + 0.5 10.3 + 0.8

Notes: Results are reported as + one standard deviation.



RSD, which was attributed to sample inhomogeneity. No sample-generated backgrounds were reported either away from or at the analytical wavelength.

Cobalt was determined by ZETA LEAFS in esturaine sediment and coal fly ash, but slurry sampling was ineffective for these analyses, and consequently the samples were dissolved by a microwave technique (Table 13). The use of palladium as a matrix modifier was necessary to obtain accurate results. The precision for these measurements was between 5 and 7% RSD, which is comparable to the precision obtained for aqueous standards. Both of these samples produced a broadband background signal that was attributed to scatter or molecular fluorescence and was approximately 10% of the cobalt signal size.

D. Conclusion

Several background correction techniques have been developed for graphite furnace LEAFS. For most of the samples analyzed to date, the manual method of measuring background a short distance away from the analytical wavelength is adequate. However, this technique cannot correct for large background signals, or

Table 13. Comparison of Certified Values to Experimental Values for the Determination of Cobalt in NIST SRMS with Dissolved Sampling

NIST SRM* Certified Value ( lag/g) Experimental Value (lag/g)

Estuarine sediment (1646) 10.5 + 1.3 10.1 + 0.7

Coal fly ash (1633a) 46** 44.1 + 2.5

Notes: *All dissolved samples diluted 1:5.

**Non-certified value.

Results are reported as + one standard deviation. 2 mg of palladium were used as a matrix modifier.




for structured background, and requires additional furnace measurements. Mul- tichannel techniques (Sjrstrrm, 1990a; Remy et al, 1990) are relatively easy to implement and allow simultaneous measurement of signal and background, but they cannot correct for structured background and the background measurement is made 4-8 nm away from the fluorescence wavelength, resulting in poor spectral resolution. Wavelength modulation allows measurement of background 0.1-0.2 nm away from the excitation wavelength, but this may not be adequate to correct for the structured backgrounds observed by Liang et al. (1993). Zeeman background correction requires the use of an electromagnet and also limits the available laser repetition rates, but it is the only method reported to date that allows measurement of background at the analytical wavelength. More work needs to be done to investigate whether it can correct for structured backgrounds.

Vi. REAL SAMPLE ANALYSES BY LEAFS

A list of real sample analyses done by graphite fumace LEAFS is presented in Table 14. These data show that poor accuracy was obtained with the use of cup furnaces for analyses in complex matrices (Bolshov et al., 1981 b; Goforth and Winefordner, 1986), probably because of the presence of vapor phase interferences. Bolshov et al. (1981b) were able to improve the accuracy of cup furnace LEAFS by use of atomization into a vacuum, but this led to an increase in detection limits by a factor of 100. Bolshov and co-workers (Apatin et al., 1989; Bolshov et al., 1989, 1992a; Boutron et al., 1990) reported the use of cup furnace LEAFS for the determination of cadmium and lead in water, but no assessment of accuracy was performed. In addition, one would not expect many matrix interferences in these samples.

Several recent publications have reported the use of graphite furnace LEAFS with solid or slurry sampling using front surface illumination. Anzano et al. ( 1991) accurately determined thallium in bovine liver and tomato leaves with solid sampling. Butcher et al. (1990) determined manganese, lead, and thallium in agricultural and food standard reference materials (SRMs) by graphite furnace LEAFS and AAS with dissolved and slurry sampling. Good agreement with the certified values was obtained for all the samples investigated. Thallium was determined by LEAFS at levels 10 to 100 times below the atomic absorption detection limit. The LEAFS backgrounds were relatively small compared to the fluorescence signal. The background signals that were present were broadband and were corrected for by measuring their size 0.1 nm away from the excitation wavelength.

Irwin et al. (1990) determined lead and thallium in NIST nickel-based alloy SRMs by graphite furnace LEAFS with solid sampling. Good agreement between the LEAFS results and the certified values were obtained. The background signals were relatively small and were corrected for by measurement of the background 0.1 nm away from the excitation wavelength.

Table 14. Real Sample Analyses Done by Graphite Furnace LEAFS

Reference Graphite

Element ~x/Lfluor Samples Calib. a BC b Furnace c Accuracy RSD (%)

Anzano et al. (1991 )

Apatin et al. (1989) Bolshov et al. (1989, 1991, 1992a)

Boutron et al. (1990) Bolshov et al. (1981b)

Bolshov et al. (1984) Bolshov et al. (1986a, b, 1988)

Butcher et al. (1990)

Cheam et al. (1992)

T1 276/353

Cd 228/228 Pb 283/405

Co 304/341 Co 304/341 Cu 324/510 Fe 296/373 Ir 295/322

Co 304/341

Co

Mn

Pb T1

Pb

279/403

283/405

276/353 283/405

Bovine liver, tomato AC None FS, ? leaves SRM; solid sampling

Water AC PWM T, GC

Agricult. SRMs AC None Soil extracts

H2SO4 and HCI AC None Agricult. SRMs, AC None

quartz, tin Agricult. SRMs,

quartz, tin

Biolog. SRMs; slurry AC OW and dissolved sampling

Water SRMs, water AC OW

T, GC

T, GC T, GC

T, GC

FS, AA tube

FS, AA tube

Good

Not assessed

low by 10 x not assessed not assessed not assessed

Good compared to AAS low by l0 x

vacuum within 30% of certified

Good

Good

12-16

50 18

15 ?

9 o

?

7-25 10-30

10-30

3-10

(continued)

Table 14. (continued) Graphite

Reference Element ~x/A, fluor Samples Calib. a BC b Furnace c Accuracy RSD (%)

Dougherty et al. (1988) T1 276/353 Liver SRM; mouse SA OW T, tube good 7 brains

Dougherty et al. (1989) Mn 279/279 Mouse brains SA Zeem T, AA tube good compared to AAS 20

Goforth and Winefordner (1986) Cu 324/327 Steel, agricult. SRMs AC None T, GC within 30% of certified 10-15

Mn 279/280

Irwin et al. (1990) Pb 283/405 Ni-based alloys; solid AC OW FS, AA good 7-20 sampling tube

TI 276/353 Irwin et al. (1992) Co 304/341 NIST geol. SRMS AC Zeem. FS, AA good 5-7

tube Pb 283/405 Geol and agricult AC 6-14

SRMs Slurry sampling

Liang et al. (1990) Pb 283/405 Air by impaction AC OW FS, AA not assessed 13-34 Tube

Liang et al. (1993) T! 276/353

Sb 212/260 Ni-based alloys; solid AC OW FS, AA good 9-13 and dissolved tube sampling

Te 214/238 good-dissol.

poor-solids Remy et al. (1990) Au 242/312 Water SA MC FS, AA not assessed 15

tube

Notes: *Method of Calibration: AC = aqueous calibration; SA = standard addition. bBackground correction: Zeem = Zeeman; MC, multichannel; PWM = pulsed wavelength modulation; OW = off wavelength background measurement: the laser was tuned away from the excitation wavelength and the background was measured. ':Furnace: T = transverse illumination; FS = front surface illumination; GC = graphite cup; AA = atomic absorption.

50 DAVID J. BUTCHER

Liang et al. (1993) determined antimony and tellurium in nickel-based alloys with dissolved and solid sampling (Table 14). Good accuracy for tellurium was

obtained with both methods of sample preparation; accurate determination of

antimony was achieved only with dissolved sampling. Background signals

caused by nitric oxide were observed for the determination of tellurium in

nickel-based alloys with dissolution in the absence of a char step. A char step

at 800 ~ with a 30-s duration was used to remove this background signal. Other

background signals observed during this work were described previously. Liang et al. (1990) described the determination of lead and thallium in

laboratory air by graphite furnace LEAFS and AAS, with use of an impaction

method. Air was drawn by a vacuum pump into a single stage impactor and onto

a conventional graphite furnace (Figure 26). The LEAFS detection limit for lead

(1 • 10 -4 ng/m 3) was two orders of magnitude lower than the AAS limit of

detection. Reasonable agreement was achieved between the AAS and LEAFS

values for lead. The levels of thallium were too low to be measured by AAS;

by LEAFS a detection limit of 1 x 10 -5 ng/m 3 was obtained. The accuracy of

the LEAFS measurements was not assessed because of the low concentrations involved.

Cheam et al. (1992) determined lead in water and water SRMs with front surface illumination in an AA furnace. Good agreement with the certified values was

obtained with aqueous calibration. The RSD of the measurements was 5%. Remy et al. (1990) determined gold in water by graphite furnace LEAFS using standard addition and multichannel background correction. Front surface illumination was employed with an AA tube furnace. The accuracy of the measurements was not confirmed by other techniques. Dougherty et al. (1989) determined manganese in mouse brains using transverse illumination, a laboratory-constructed furnace, and Zeeman background correction. Calibration was performed by the method of standard additions, and good agreement was obtained with results obtained by

atomic absorption (Table 9). Irwin et al. (1992) determined cobalt and lead in NIST SRMs with front surface

illumination, a commercial tube furnace, and transverse Zeeman background

correction. Lead was accurately determined in agricultural SRMs with slurry sampling with an RSD between 6 and 14% (Table 12). Cobalt could not be accurately determined using slurry sampling, but good agreement with the certified

values was obtained with a dissolution procedure (Table 13).

In summary, the potential of graphite furnace LEAFS for accurate analyses has

been demonstrated, but much more work needs to be done in this area. Much of the

work up to now has concentrated on "easy" elements, such as lead and thallium.

The determination of involatile elements by LEAFS needs to be further investi-

gated, as well as work concerning the types of backgrounds present.

(a)

GRAPHITE TUBE TO PUMP

IMPACTOR TUBE WITH / 1.0ram TANTALUM JET

TO PUMP

(b) HOLDER

GRAPHITE TUBE / ~I ~

! JET WITH lmm DIAMETER NOZZLE

(c) RUBBER O-RING

TU

~ MAIN CHAMBER

GRAPHITE TUBE HOLDER

Figure 26. Three views of the impaction chamber used for graphite furnace LEAFS: (a) general view; (b) impaction device mounted into the graphite furnace; (c) close up of the nozzle inside the graphite tube. Taken with permission from Liang (1990).

51

52 DAVID J. BUTCHER

VII. CONCLUSION--LEAFS

Graphite furnace LEAFS has been shown to be a very sensitive method of analysis with detection limits between 0.1-500 fg and a linear dynamic range between four and seven orders of magnitude. At present, its greatest disadvantage is the unreli- ability of the laser systems used as excitation sources. The development of more reliable laser systems, such as titanium sapphire systems, may allow the wider use of LEAFS. Several background correction techniques have been developed for LEAFS, and Zeeman background correction is probably the best choice because of its ability to correct for structured background, which has recently been demonstrated. The use of graphite LEAFS for real sample analysis has been limited, with most determinations involving elements such as lead and thallium. Atomic absorption graphite tube furnaces with front surface illumination have been shown to be useful for real sample analyses. With the development of better lasers and more research of background signals, graphite furnace LEAFS has the potential to be a practical method for ultratrace analysis.

VIII. LASER EXCITED MOLECULAR FLUORESCENCE (LEMOFS)

The determination of halogens by atomic spectroscopy is difficult because these electronegative elements do not readily form free atoms and their most sensitive transitions are in the vacuum ultraviolet region, requiring a spectrometer that can be reduced to low pressure. An alternative method for their determination involves the formation of diatomic molecules consisting of a metal reagent and the halide as the analyte. In order to take advantage of the high sensitivity of laser excitation, Dittrich and co-workers (Dittrich, 1986; Dittrich and Stfirk, 1987a) developed laser-excited molecular fluorescence spectrometry (LEMOFS), in which diatomic molecules are formed in a graphite furnace and excited by a laser. Fluorescence from the molecule serves as the analytical signal. In general, the same instrumentation for LEAFS has been used for LEMOFS, with appropriate excitation and fluorescence wavelengths. At present, only a handful of papers have been published, without any reviews, in this area. Here will be presented some preliminary conclusions regarding this relatively new technique.

A. Instrumentation

In general, the same instrumentation has been used for LEMOFS as for LEAFS. Laser systems for LEMOFS have similar requirements to those for LEAFS, with the exception that higher laser energies (100 ja.l) are required to saturate some transitions. The initial work by Dittrich and co-workers (Dittrich, 1986; Dittrich and St~irk, 1987a; Garden et al, 1988) employed transverse illumination with a


graphite tube furnace that was modified by the addition of laser ports. Recent LEMOFS work (Butcher et al., 1991; Anwar et al., 1991a, b, c) has Used front surface illumination with an unmodified atomic absorption furnace.

B. Choice of Molecules and Optimization Procedures

Dittrich and co-workers (Dittrich, 1986; Dittrich and St~k, 1987a) did the first reported work on LEMOFS. Fluorine was determined using the MgF molecule; chlorine, with the InC1 molecule; and bromine, with the A1Br molecule. These molecules were probably chosen because their excitation wavelengths were accessible with the laser equipment available to these researchers. All LEMOFS work done since this original study used the same diatomic molecules.

Conditions to be optimized for LEMOFS include the amounts of reagents added, the furnace conditions, and the laser energy. Chemical optimization involves the investigation of the amount of reagents that provides the largest signal. Dittrich and co-workers (Dittrich, 1986; Dittrich and St~irk, 1987a) and Butcher et al. (1991) added an optimized quantity of metallic reagent to every standard and sample. These quantities ranged from 0.3 lag aluminum for A1Br (Dittrich) to 20 ktg magnesium for MgF (Butcher) (Table 15). Anwar et al. (199 la, b, c) added 300-fold excess of metal to each sample.

For some elements, a second metallic element, called a chemical modifier, was added to increase formation of the diatomic molecule. Butcher and co-workers (1991) reported that an increase in fluorescence signal size was obtained by the addition of barium or strontium (Figures 27 and 28). The maximum signal was obtained with the addition of 1.65 lag of barium. Dittrich (1986) and Dittrich and Stark (1987a) reported that the addition of barium hydroxide promoted formation of AIBr in the determination of bromine. The enhancement was attributed to barium's ability to ensure that vaporization of aluminum and bromine occurs at the same time, which is necessary for diatomic molecule formation. Anwar et al. (1991 c) also employed barium as a chemical modifier for their AIBr work.

Most workers have reported the use of relatively high atomization temperatures for LEMOFS, with temperatures between 2500 and 2800 ~ Butcher et al. (1991) did an atomization optimization for the determination of fluorine with MgF (Figure 29a), and determined that the optimum temperature was 1800 ~ This indicates that very high atomization temperatures may not be needed for all elements. The use of lower atomization temperatures was also reported by Anwar et al. (1991 b), who used 1800 ~ for the InC1 work.

The use of a char step was reported to enhance the formation of diatomic molecules (Dittrich and St~k, 1987a), and hence char temperatures between 600 and 1500 ~ were used in most work. Butcher et al. (1991 ) did a char optimization for the determination of fluorine by MgF (Figure 29b), and reported a maximum fluorescence signal between 800 and 1100 ~

Table 15. Optimized Chemical, Furnace, and Laser Conditions, Detection Limits, and Linear Dynamic Ranges Obtained by LEMOFS

Reference Elem. a Mol. b ~exc~fluor Atom Temp; Char Laser Type; Rep. EGe; Pulse Energy;

Reagents Temp (~ LODc pg LDR ~ Rate, (Hz) Sat'd (Y/N)

t.rl

Anwar et al. (1991 a) F

Anwar et al. (1991 b) CI

Anwar et al. (1991 c) Br

Butcher et al. (1991) F

Dittrich (1986) F

Dittrich and St~rk C1 (1987a) Br

MgF 268/358 [Mg] = 300 x [F] 2500; 800

InC1 267/358 [In] = 300 x [CI] 1800; 600

AIBr 279/284 [AI] = 200 x [Br] 2600; 600

[Ba] = 1700 x [Br]

MgF 268/358 20 lag AI 1800; 800

1.65 lag Ba

MgF 268/268 5 lag Mg 2700; ?

268/358

InCl 267/267 1 lag In 2800; 900

A1Br 279/279 0.3 lag A1 2700; 1500

14 lag Ba

7 4 N2; 20 FS; 20 laJ; N

17 3 N2; 20 FS; 20 laJ; N

45 3.5 N2; 20 FS; 20 laJ; N

0.3 5

2

11 2.5

45 1.5

15 2

70 2

Excimer; FS; 100 laJ; Y

Excimer; 80 FS; 10 laJ; Y

N2; 7 T; 6 laJ; N

Notes: aElem. = Element. bMol. = Diatomic molecule. CLOD = Limit of detection. dLDR = Linear dynamic range. eEG = Excitation geometry.

U 4000 �9

3000 �9

2000 ..

,ooo / l-Y" 0 - ~ - o

i I

0.001 0.010 0.100 I .C~O0 "0

i !

10.600 I00.000 I000.000

Ba, /.sg Figure 27. Optimization of amount of barium upon the MgF fluorescence signal. Experimental conditions: NaF (10 ng as F); Mg(NO3)2 (20 pg as Mg); atomization temperature, 1800 ~ char temperature, 800 ~ and laser power, 10 ld/pulse. Taken with permission from Butcher et al. (1991).

w 2000

I T O---Q

U iooo TI' ~

BOO

0 I 0.010 O. I O0 1.000 I 0.000 100.000 1000.000

Sr , /zg Figure 28. Optimization of amount of strontium upon the MgF fluorescence signal. Experimental conditions: NaF (10 ng as F); Mg(NO3)2 (20 gg as Mg); atomization temperature, 1800 ~ char temperature, 800 *C; and laser power, 10 gl/pulse. Taken with permission from Butcher et al. (1991).

55

12oo (a)

IZI IOOO

8oo

j 400

j 200

rY

"" "\; T i'\, _{__§ t 1

/ /

0 , ...,...I'!', ! 1200 1400 1600

! I ! ! I I 1BOO 2000 2200 2400 2600 2800

ATOMIZATION TEMPERATURE, oc ! , ! (.D Z i l l (.3 O9 LL_I rh ~ 0

__1 LL

1400

1200

1000

BOO

600

400

] 200 I~1 0::::: o

(b)

IX, , ... J. 1 X

11. T

! ! ! I I I 0 200 400 600 800 1000 1200

,! 1400 1600

CHAR TEM PERATU RE, oc figure 29. Optimization of furnace parameters for LEMOFS. (a) Effect of atomization temperature on the magnesium fluoride fluorescence signal. Experimental conditions: NaF (10 ng as F); Mg(NO3)2 (20 l~g as Mg); Ba(NO3)2 (1.65 pg as Ba); char temperature, 800 ~ and laser power, 10 l.tJ/pulse. (b) Effect of char temperature on the magnesium fluoride fluorescence signal. Experimental conditions: NaF (10 ng as F); Mg(NO3)2 (20 l.tg as Mg); Ba(NO3)2 (1.65 lug as Ba); atomization temperature, 1800 ~ and laser power, 10 luJ/pulse. Taken with permission from Butcher et al. (1991).

56


Ld (D Z W (D O0 LO r~ 0

__J LL

_J m rY

2500 00

2000

1500

1000

5OO

0 268.00

1,1

2,2

i i

268.25 268.50 268.75 269.00 |

269.25

WAVELENGTH, nm Figure 30. Spectral scan for magnesium fluoride, which is a plot of MgF signal versus the laser wavelength. Experimental conditions: NaF (10 ng as F); Mg(NO3)2 (20 lag as Mg); Ba(NO3)2 (1.65 lag as Ba); atomization temperature, 1800 ~ char temperature, 800 ~ and laser power, 10 laJ/pulse. Taken with permission from Butcher et al. (1991 ).

Laser conditions to be optimized included the choice of excitation wavelength and the laser power. A spectral scan of the excitation wavelength was used to locate the most sensitive transition. Butcher et al. (1991) did a spectral scan of MgF (Figure 30), and reported the presence of four vibrational bands. Analytical work was performed at the peak of the most intense (0,0) vibrational transition. Based on the limited amount of work done to date, diatomic molecules require higher pulse energies in order to achieve saturation. Butcher et al. (1991) reported that a pulse energy of 1 O0 l.l.J was needed to saturate the MgF molecule. Pulse energies up to 20 l.tJ were insufficient to saturate InC1 and A1Br (Anwar et al, 199 lb, c).

C. Detection Limits and Linear Dynamic Ranges

Detection limits for LEMOFS are listed in Table 15. In general, when comparable laser pulse energies and repetition rates are considered, little difference in sensitivity is observed between Dittrich's work using transverse illumination and the more recent work using front surface illumination (Anwar et al., 1991 a, b, c; Butcher et al., 1991 ), with detection limits between 2 and 50 pg. However, Butcher et al. (1991) demonstrated that the use of higher laser powers and repetition rates allowed detection limits in the high femtogram range (0.3 pg) to be obtained for fluorine.

58 DAVID J. BUTCHER

This result is two to six orders of magnitude more sensitive than other methods for the determination of fluorine, such as the ion-selective electrode and photometric techniques. Similar sensitivity could probably be obtained for chlorine and bromine with the use of a modern laser system. Reported linear dynamic ranges for LEMOFS are between 1.5 and 5 orders of magnitude.

D. Interferences with the LEMOFS Signal

Most of the reports concerning LEMOFS have discussed the effects that other halides have upon the fluorescence signal. Dittrich and St~k (1987a) reported typical results for these studies (Figure 31), in which the molecular fluorescence signal was depressed by the presence of 10- to 1000-fold excess of other halides. These interferences are believed to be caused by a reduction in the formation of diatomic molecules containing the analyte (Dittrich and Still&, 1987a).

E. Real Sample Analyses/Background Correction

A summary of real sample analyses done by LEMOFS is given in Table 16. All of the analyses were done with front surface illumination, an atomic absorption graphite furnace, and aqueous calibration. Anwar et al. (1991 b) determined chlorine

(a) 100-

> , |, , i .

r-

50

0 t-

O d} 0

m

" I I I i

10 10 -1 10 0 10 10 Amount of Chloride, pg

Figure 31. Effects of other halides upon the molecular fluorescence signal: (a) effect of amount of chloride added on the MgF signal. Experimental conditions: nonresonance fluorescence (268/358 nm); 10 ng F; 5 lug Mg. (b) Effect of amount of bromide added upon the InCI signal. Experimental conditions: resonance fluorescence (267/267 nm); 1 ng Ci; 1 lug In. (c) Effect of amount of chloride added upon the AIBr signal. Experimental conditions: resonance fluorescence (279/279 nm); 1 ng Br; 0.3 lug AI; 14 lug Ba. Taken with permission from Dittrich and St~rk (1987a). (continued)

(b) .,100

"~ 80 t "

�9 ,-, 60 __= ..

40- 0 t"

20 O .

~ 0 L. -2 o 10

m

I I I

10 -1 100 101 Amount of Bromide, pg

o~ ~,120

"~ 100 I=

80 E - 60 (I,) o 40 C

o 20 0 I,==

0 =3 m

LI.

(c)

~ o

i I i i

O-a 10 ~ 10 -~ 10~ 10 Amount of Chloride, gg

Figure 31. (continued)

Table 16. Real Sample Analyses Done by LEMOFS

Reference Element Molecule kexc/~nuor Sample Accuracy RSD (%)

Anwar et al. CI InCI 267/359 Orchard leaves Good 1 (1991b) SRMs

Anwar et al. Br Wheat flour SRM (1991c)

Butcher et al. F MgF (1991)

A1Br 279/284 Good

268/358 Urine SRM Good

Not reported.

9-11

Tap water Good compared to 7 an ion-selective

electrode

Note: All analyses employed front surface illumination with an AA furnace and aqueous calibration.

59

60 DAVID J. BUTCHER

Table 17. Determination of Chloride in Orchard Leaves SRMs by LEMOFS

Sample Noncertified Value (lag~g) LEMOFS value (lag~g)*

Orchard leaves 1571a 700 713 + 8

Orchard leaves 157 lb 700 722 + 8.5

Notes: *n = 5; the data are -!- one standard deviation.

Taken with permission from Anwar et al. (1991b).

in orchard leaves SRMs by LEMOFS, and their results are summarized in Table 17. Good accuracy was obtained for these analyses with an RSD of 1%. Anwar et al. (1991c) analyzed wheat flour for its bromide content. The LEMOFS result of 8.85 ktg/g agreed well with the certified value of 9.00 ~g/g. Butcher et al. (1991) determined fluorine in a urine SRM and in tap water (Table 18). Good accuracy was obtained by dilution of all samples one hundred times before analysis. The RSD of these analyses was between 7 and 11%.

At present, no background signals have been reported for LEMOFS, and no methods of background correction have been described. Clearly more work needs to be done to evaluate the ability of this technique to determine nonmetals in samples.

F. Conclusion--LEMOFS

LEMOFS has been shown to provide subpicogram detection limits for fluorine with the use of a high power, high repetition rate laser system. Similar sensitivity

Table i8. Determination of Fluorine by LEMOFS in NIST SRM 2671a, Freeze-Dried Urine and in University of Connecticut Tap Water

Fluorine Concentration (mg/L)

LEMOFS RSD Sample Certified Value ISE* LEMOFS** (%)

SRM 2671 a, Freeze-dried urine, control level

SRM 2671 a, Freeze-dried urine, elevated level

U. Connecticut tap water

0.55 + 0.03 - - 0.54 + 0.05 9

5.7 + 0.3 M 5.6 + 0.6 11

0.77 + 0.09 0.70 + 0.05 7

Notes: *Fluoride ISE with an acetate total ionic strength adjustment buffer. The data are + one standard deviation.

**Molecular fluorescence of MgF: excitation at 268.94 nm; detection at 359 nm. Experimental conditions: NaF (10 ng as F); Mg(NO3)2 (20 lag as Mg); Ba(NO3)2 (1.65 lag as Ba); atomization temperature, 1800 ~ char temperature, 800 ~ and laser power, 10 laJ/pulse.

Taken with permission from Butcher et al. (1991).


would probably be obtained for chlorine and bromine with the use of a modern

laser. Chemical interferences in the form of other ions have been observed. A few

real sample analyses have been performed with this technique, but more work is

required to assess its analytical suitability.

REFERENCES

Anwar, J., Anzano, J.M., Petrucci, G., Winefordner, J.D. Analyst 1991a, 116, 1025. Anwar, J., Anzano, J.M., Petrucci, G., Winefordner, J.D. Microchem. J. 1991b, 43, 77. Anwar, J., Anzano, J.M., Winefordner, J.D. Talanta 1991e, 38, 1071. Anzano, J.M., Anwar, J., Smith, B.W., Winefordner, J.D. Spectrosc. Letr 1991, 24, 837. Apatin, V.M., Arkhangel'skii, B.V., Bolshov, M.A., Ermolov, V.V., Koloshnikov, V.G., Kompanetz,

O.N., Kuzetsov, N.I., Mikhailov, E.L., Shishkovski, V.S., Boutron, C.E Spectrochim. Acta 1989, 44B, 253.

Bolshov, M.A., Boutron, C.E, Ducroz, EM., G/Srlach, U., Kompanetz, O.N., Rudinev, S.N., Hutch, B. Anal. Chim. Acta 1991, 251, 169.

Bolshov, M.A., Boutron, C.F., Zybin, A.V. Anal. Chem. 1989, 61, 1758. Bolshov, M.A., Koloshnikov, V.G., Rudnev, S.N., Boutron, C.F., Gorl~ich, U., Patterson, C.C.J. Anal.

Atom. Spectrom. 1992a, 7, 99. Bolshov, M.A., Rudnev, S.N., Huetsch, B. J. Anal. Atom. Spectrom. 1992b, 7, 1. Bolshov, M.A., Zybin, A.V., Kolonina, L.N., Maiorov, I.A., Smirenkina, I.I., Shiryaeva, O.A. Zh. Anal.

Khim. 1984, 39, 320. Bolshov, M.A., Zybin, A.V., Koloshnikov, V.G., Mayorov, I., Smirenkina, I.I. Spectrochim. Acta 1986a,

41B, 487. Bolshov, M.A., Zybin, A.V., Koloshnikov, V.G., Pisarkii, A.V., Smirnov, A.N.Zh. Prikl. Spectrosk. 1978,

28, 45. Bolshov, M.A., Zybin, A.V., Koloshnikov, V.G., Smirenkina, I.I. Spectrochim. Acta 1988, 43B, 519. Bolshov, M.A., Zybin, A.V., Koloshnikov, V.G., Vasnetsov, M.M. Spectrochim. Acta 1981a, 36B, 345. Bolshov, M.A., Zybin, A.V., Lolomiiskii, Y.R., Koloshinikov, V.G., Loginov, Y.M., Smirenkina, I.I. Zh.

Anal. Khim. 1986b, 41,402. Bolshov, M.A., Zybin, A.V., Smirenkina, I.I. Spectrochim. Acta 1981b, 36B, 1143. Bolshov, M.A., Zybin, A.V., Zybina, L.A., Koloshnikov, V.G., Majorov, I.A. Spectrochim. Acta 1976,

31B, 493. Boutron, C.E, Bolshov, M.A., Koloshnikov, V.G., Patterson, C.C., Barkov, N.I. Atmospheric Environ-

ment 1990, 24A, 1797. Butcher, D.J. Spectroscopy 1993, 8(2), 14. Butcher, D.J., Dougherty, J.P., Preli, ER., Walton, A.P., Wei, G.T., Irwin, R.L., Michel, R.G.J. Anal.

Atom. Spectrom. 1988, 3, 1059. Butcher, D.J., Irwin, R.L., Takahashi, J., Michel, R.G.J. Anal. Atom. Spectrom. 1991, 6, 9. Butcher, D.J., Irwin, R.L., Takahashi, J., Su, G., Wei, G.T., Michel, R.G. Appl. Spectrosc. 1990, 44,

1521. Cheam, V., Lechner, J., Sekera, I., Desrosiers, R., Nriagu, J., Lawson, G. Anal. Chim. Acta 1992, 269,

129. Denisov, L.K., Loshin, A.E, Kozlov, N.A., Nikiforov, V.G. Zh. Priki. Spectrosk. 1985, 43, 566. Dittrich, K. CRC Crir Rev. Anal. Chem. 1986, 16, 223. Dittrich, K., Hanisch, B., Stfirk, H.J. Fresenius Z. Anal. Chem. 1986, 324, 497. Dittrich, K., Stfirk, H.J.J. Anal. Atom. Spectrom. 1986, 1,237. Dittrich, K., Stfirk, H.J.J. Anal. Chhn. Acta 1987a, 200, 581. Dittrich, K., St~k, H.J.J. Anal. Atom. Spectrom. 1987b, 2, 63.

62 DAVID J. BUTCHER

Dougherty, J.P., Costello, J.A., Michel, R.G. Anal. Chem. 1988, 60, 336. Dougherty, J.P., Preli, ER., McCaffrey, J.T., Seltzer, M.D., Michel, R.G.Anal. Chem. 1987a, 59, 1112. Dougherty, J.E, Preli, ER., Michel, R.G.J. Anal. Atom. Spectrom. 1987b, 2, 429. Dougherty, J.E, Preli, ER., Michel, R.G. Talanta 1989, 36, 151. Dougherty, J.E, Preli, ER., Wei, G.T., Michel, R.G. Appl. Spectrosc. 1990, 44, 934. Falk, H., Tilch, J. J. Anal. Atom. Spectrom. 1987, 2, 527. Falk, H., Paetzold, H.J., Schmidt, K.E, Tilch, J. Spectrochim. Acta 1988, 43B, 1101. Farnsworth, EB., Smith, B.W., Omenetto, N. Spectrochim. Acta 1990, 45B, ll51. Garden, L.M., Littlejohn, D., Dittrich, K., St~irk, H.J. Anal. Proc. 1988, 25, 230. Goforth, D., Winefordner, J.D. Anal. Chem. 1986, 58, 2598. Goforth, D., Winefordner, J.D. Talanta 1987, 34, 290. Gonchakov, A.S., Zorov, N.B., Kuzyakov, Y.Y., Matveev, O.I. Zh. Anal. Khim. 1979, 34, 2312. Gray, A.L. Spectrochim. Acta 1986, 41, 151. Hohimer, J.P., Hargis, EJ. Anal. Chim. Acta 1978, 97, 43. Human, H.G.C., Omenetto, N., Cavalli, P., Rossi, G. Spectrochim. Acta 1984, 39B, 1345. Irwin, R.L., Butcher, D.J., Takahashi, J., Wei, G.T., Michel, R.G.J. Anal. Atom. Spectrom. 1990, 5, 603. Irwin, R.L., Wei, G.T., Butcher, D.J., Liang, Z., Su, E.G., Takahashi, J., Walton, A.E, Michel, R.G.

Spectrochim. Acta 1992, 47B, 1497. Leong, M., Vera., J., Smith, B.W., Omenetto, N., Winefordner, J.D. Anal. Chem. 1988, 60, 1605. Liang, Z., Wei, G.T., Irwin, R.L., Walton, A.P., Michel, R.G., Sneddon, J. Anal. Chem. 1990, 62, 1452. Liang, Z., Lonardo, R.E, Michel, R.G. Spectrochim. Acta 1993, 48B, 7. Miziolek, A.W., Willis, R.J. Opt. Lett. 1981, 6, 528. Neumann, S., Kriese, M. Spectrochim. Acta 1974, 29B, 127. Omenetto, N., Winefordner, J.D. Prog. Anal. Atom. Spectrosc. 1979, 2, 1. Omenetto, N. Appl. Phys. B 1988, 46, 209. Omenetto, N., Cavalli, E, Broglia, M., Qi, E, Rossi, G. J. Anal. Atom. Spectrom. 1988a, 3, 231. Omenetto, N., Smith, B.W., Winefordner, J.D. Spectrochim. Acta 1988b, 43B, 1111. Omenetto, N. Quim. Anal. 1989a, 8, 247. Omenetto, N. Spectrochim. Acta 1989b, 44B, 131. Omenetto, N. Mikrochim. Acta [Wehl] 1991, II, 277. Preli, ER., Dougherty, J.P., Michel, R.G.Anal. Chem. 1987, 59, 1784. Preli, ER., Dougherty, J.E, Michel, R.G. Spectrochim. Acta 1988, 43B, 501. Remy, B., Verhaeghe, I., Mauchien, E Appi. Spectrosc. 1990, 44, 1633. Sj6str6m, S. J. Anal. Atom. Spectrom. 1990a, 5, 261. Sj6str6m, S. Spectrochim. Acta Rev. 1990b, 13, 407. Sj6str6m, S., Mauchien, P. Spectrochim. Acta Rev. 1993, 16, 153. Slavin, W. Graphite Furnace AAS: A Source Book, Perkin-Elmer: Ridgefield, CT, 1984. Smith, B.W., Farnsworth, P.B., Cavalli, P., Omenetto, N. Spectrochim. Acta 1990, 45B, 1369. Smith, B.W., Glick, M.R., Spears, K.N., Winefordner, J.D. Appl. Spectrosc. 1989, 43, 376. Su, E.G., Irwin, R.L., Liang, Z., Michel, R.G.Anal. Chem. 1992, 64, 1710. Takahashi, J., Hara., R. Anal. Sci. 1988, 4, 331. Vera, J.A., Leong, M.B., Omenetto, N., Smith, B.W., Womack, B., Winefordner, J.D. Spectrochim. Acta

1989a, 44B, 939. Vera, J.A., Leong, M.B., Stevenson, C.L., Petrucci, G., Winefordner, J.D. Talanta 1989b, 36, 1291. Vera, J.A., Stevenson, C.L., Smith, B.W., Omenetto, N., Winefordner, J.D.J. Anal. Atom. Spectrom.

1989c, 4, 619. Wei, G.T., Dougherty, J.P., Preli, ER., Michel, R.G.J. Anal. Atom. Spectrom. 1990, 5, 249. Wittman, P., Winefordner, J.D. Can. J. Spectrosc. 1984, 29, 75. Womack, J.B., Ricard, C.A., Smith, B.W., Winefordner, J.D. Spectrosc. Lett. 1989, 22, 1333.

ELECTROTHERMAL VAPORIZATION SAMPLE INTRODUCTION INTO PLASMA SOURCES FOR ANALYTICAL EMISSION SPECTROMETRY

Henryk Matusiewicz

Io II.

III.

IV.

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Introduct ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Nomencla ture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Electrothermal Vaporization Sample Introduction . . . . . . . . . . . . . . . . 66

A. Induct ively Coupled Plasma . . . . . . . . . . . . . . . . . . . . . . . . 68

B. Induct ively Coupled P la sma-Mass Spect rometry . . . . . . . . . . . . . 78

C. M i c r o w a v e - I n d u c e d Plasma . . . . . . . . . . . . . . . . . . . . . . . . 81

D. Mic rowave- Induced P la sma-Mass Spect rometry . . . . . . . . . . . . . 89

E. Direct Current Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

E Capaci t ively Coupled Microwave Plasma . . . . . . . . . . . . . . . . . 91

Preconcentrat ion Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A. Electrodeposi t ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92


63

64 HENRYK MATUSIEWICZ

B. In Situ Preconcentration . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 C. Other Preconcentration Techniques . . . . . . . . . . . . . . . . . . . . . 95

V. Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 VI. Summary of Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

VII. Commercial Availability of Electrothermal Vaporization Plasma Source Emission Spectrometric Systems . . . . . . . . . 98

VIII. Detection Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 IX. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 X. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

XI. Suggestions for Future Studies . . . . . . . . . . . . . . . . . . . . . . . . . . 131 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

ABSTRACT

A review is presented on historical, fundamental, and practical aspects of electrothermal vaporization (ETV) as a sample introduction technique for plasma sources in analytical emission spectrometry. Methods for ETV sample introduction into inductively coupled plasmas (for atomic emission and mass spectrometry-AES and MS, respectively) microwave-induced plasmas (for MIP-AES and MIP-MS), direct current plasmas (DCP), and capacitively coupled microwave plasmas (CCMP) are reviewed and evaluated critically and the performance of plasma sources for real sample determinations is evaluated. The advantages and disadvantages, limitations, figures of merit and ease of operation, as compared to conventional pneumatic nebulization and other sample introduction techniques, are described. It will be shown that progress in the field of ETV sample introduction has occurred through a series of steps which involve the addition of new techniques and the occasional displacement of established ones. Overall, this chapter discusses the state of the art of electrothermal sample introduction into plasma sources for analytical emission spectrometry and possible future directions of the technique.

!. I N T R O D U C T I O N

Analytical emission spectrometry with plasma excitation sources has gained importance for major, minor, trace, and ultratrace analysis. The main thrust is the development of the plasma as a stable, versatile emission source. The plasma produces intense elemental emission from essentially all elements of the periodic table. The field of emission spectrometry is very wide and some limits had to be set for the scope of this chapter. The title includes the word "plasma" to clearly exclude flames; this review will therefore cover plasma emission spectrometry" inductively coupled plasma (ICP), inductively coupled plasma-mass spectrometry (ICP-MS), microwave-induced plasma (MIP), microwave-induced plasma-mass spectrometry (MIP-MS), direct current plasma (DCP) and capacitively coupled microwave plasma (CCMP) techniques.

Electrothermal Vaporization Sample Introduction 65

Over the last decades, plasma excitation sourceshave been well characterized as sensitive sources for optical emission spectrometry. After considering all of the characteristics of plasma sources that make the technique as popular as it is, one problem area that remains unresolved is the sample introduction process, particularly with regard to microsampling, which is crucial whenever sample size is limited. There is general agreement that the various aerosol generation techniques constitute the weakest link in atomic emission spectrometry. Fortunately, sample introduction to excitation sources is one of the most fertile fields of research in analytical atomic emission spectrometry [e.g., the books edited by Sneddon (1990, 1992) have reviewed this topic].

Traditionally, sample introduction has been effected by using pneumatic nebulization. The popularity of nebulization owes much to its simplicity, rapid sample changeover, relatively good stability, and low cost. Negative aspects of the technique include low sample introduction efficiency, nebulizer blockage problems, and the requirement for sample volumes of greater than 1 mL. Overall, the most important disadvantage of pneumatic nebulization as used for plasma source optical emission spectrometry is poor sample transport efficiency, typically about 1% compared with 5-10% efficiency with flame atomic absorption spectrometry (FAAS). Although this causes no problems in routine work, in certain applications related to biochemical, clinical, forensic, environmental, toxicological and solid state determinations, micro- or ultra-trace analyses are required. In these situations, conventional pneumatic nebulization is inappropriate.

Many of the problems of sample introduction can be circumvented if the analyte can be presented in the form of a vapor to the plasma. One such means of sample introduction is electrothermal vaporization (ETV). Hyphenated techniques, and the one addressed in this review, are becoming more common in analytical atomic spectrometry. In such cases, independent atomic spectrometric techniques are combined together, with the aim of these couplings being to use the properties of one method to supplement the properties of the second. It is for this reason that an increasing number of researchers have developed and used electrothermal devices to vaporize microvolumes of solutions, and to some extent solid and powder samples, into the plasmas. Kantor (1983), Ng and Caruso (1985, 1990), Zimnik and Sneddon (1987), and Carey and Caruso (1992) have recently reviewed the important aspects and techniques of electrothermal vaporization for sample introduction in atomic emission spectrometry in general.

This review will consider the current state, basic properties, advantages, limitations, and historical development of ETV for sample introduction into the plasma sources. Possible future trends and analytical applications of this relatively new method will be summarized. Other thermal vaporization introduction systems such as a direct sample insertion technique (reviewed by Karanassios and Horlick, 1990), arc and spark nebulization/vaporization, laser ablation/vaporization, specialized techniques using electrothermal atomizers (reviewed by Sturgeon, 1995), and other techniques are beyond the scope of this review and will not be discussed here.


Sample ._f" -L

I Emission A n a t y t e j r Source 2 > Source 1~ species ~l~

Va por i z (1 tion Ex cit ct t ion

Figure 1. Graphical representation of a combined source.

I i . N O M E N C L A T U R E

Throughout this review the term electrothermal vaporization will be used where the electrothermal vaporizer is the single high-temperature source used to produce the analytical vapor for spectrometric detection. The term ETV is widely accepted in the literature. This author can see no ambiguity in the use of the expression, although electrothermal vaporization may be considered by some to be a more specific term. It should be pointed out that terms such as electrothermal atomization, thermal vaporization, evaporation technique, plasma volatilization, and electrothermal volatilization have been used by different authors. Thus, when preparing this review chapter on this particular subject it was necessary to choose arbitrarily from the terms used by other authors. This author prefers to see the term electrothermal vaporization confined to situations where two separate systems are used, one source to vaporize the sample and form the analyte species and the other source as the observation cell, to produce the greatest amount of chemical information. Although the second source might provide additional fragmentation or atomization, or serve to prevent recombination, its main function is generally to excite or ionize the analyte species that are formed in the first source. Figure 1 graphically illustrates this concept.

The choice of the term vaporizer rather than atomizer throughout this paper is deliberate, as the main requirement for the electrothermal device in conjunction with the plasma is to produce a vapor or dry aerosol suitable for transport at atmospheric pressure by the carrier gas to the plasma source. For these reasons, the expression electrothermal vaporizer suggested by several authors will be used here. This requirement is in marked contrast to that for graphite furnace atomic absorption spectrometry (GFAAS), where the graphite furnace must produce a transient cloud of atoms as a precursor to absorption and is consequently described by the generally accepted term electrothermal atomizer.

I I I . E L E C T R O T H E R M A L V A P O R I Z A T I O N SAMPLE I N T R O D U C T I O N

Several approaches to thermal vaporization sample introduction for the plasma sources have been reported. These include electrothermal vaporization, direct


gos

somple

jl .. �9 - I ~ to p t a ~ a

metal filament/coil

sampte

gos .------ to plasma

graphite furnace

t0 ptQsmQ

I I sample

II gcIs

car ban cup / rod or

metal furnace

Figure 2. Three favored designs for electrothermal vaporization devices.

sample insertion, and solid sampling systems. One of the most common and nearly optimal techniques for converting liquid microsamples into dry aerosols and introducing them into a plasma source is that of the ETV, a technique that has become well developed for GFAAS. This approach involves the direct production of a dry aerosol from microvolumes of solution (e.g., 0.1-50 ~tL) or from solid samples of as little as 1 mg of material deposited on a metal loop, ribbon, boat, filament or on a graphite yarn, rod, cup, filament or electrode. Modified commercial electrothermal atomizers manufactured for atomic absorption spectroscopy (AAS) and laboratory-built electrothermal vaporizers (the furnace is viewed principally as a microsample vaporizer) have been used with plasma sources. Typical ETV devices for sample introduction are shown in Figure 2. The heating programme may include solvent evaporation, ashing, and vaporization steps, requiring the plasma only to excite or ionize the analyte atoms. In contrast to GFAAS, ashing may be unnecessary and the final temperature may be lower if the sample is completely volatilized in molecular form. Normally, such a sample would need to be acid digested with methods requiting up to several hours. The resulting aerosol/vapor is transported into the plasma in a carrier gas stream, generally through a minimum length of connecting plastic or glass tubing, or even directly. In contrast to continuous sample nebulization, the atomization and excitation steps are separated from the desolvation process with the ETV-plasma method, thus resulting in a much less disturbed plasma. As a result, the functions are well separated and the two sources can be optimized independently.


In the literature survey presented here, the main characteristics of the ETV sample introduction method will be emphasized. A brief discussion of subgroups of these methods, identified by the vaporization unit, will be presented.

A. Inductively Coupled Plasma

The ICP has developed into a highly efficient source for atomic emission spectrometry (AES) and is especially amenable for use with an ETV because it is very energetic. The historical development, basic properties and limitations, and recent development of ETV-ICP spectrometry have been reviewed to some extent by Sneddon and Bet-Pera (1986), Matusiewicz (1986, 1991), Plsko (1988), Broekaert and Boumans (1987), Routh and Tikkanen (1987), and McLeod et al. (1992).

ETV: Metal Heating Devices

The first reported work combining electrothermal sample vaporization with ICP atomization and excitation was by Nixon et al. (1974), who used a tantalum filament. The sample was introduced into a small depression in a tantalum strip contained in a quartz dome with a volume of ca. 120 mL. The dome was provided with an inlet for argon (1.2 L/min) at the base and an outlet port at the top that was connected to the sample introduction orifice of the plasma torch. The length and material of the transfer tube were not detailed, and the possibility of deposition of elements on the tube walls was not examined. Detection limits in the mg/L to fractional mg/L range were obtained for 16 elements from 100-1aL samples. These workers considered the main advantage of this type of introduction system to be the fact that a single set of parameters was sufficient for the vaporization of many types of samples; therefore, the potential for multi-element trace analysis in real samples was considered promising.

Smythe (1980) briefly described a multicoil tungsten filament passing through a small graphite furnace for the introduction of aqueous microsamples (1-5 laL) into the ICP. Interelement effects, matrix effects, and real sample applications were not examined nor discussed.

Kitazume (1983) reported filament (primarily platinum and tungsten) vaporization of 10-1aL samples for ICP. In this technique, the sample solution was vaporized from a filament that was heated by a momentary condenser discharge in a small quartz evaporation chamber (ca. 4.5 mL in volume). The vaporized specimen was introduced into the ICP torch through polypropylene tubing (20 cm) and a three- way stopcock. Detection limits for boron, germanium, phosphorus, lead, tin, and zinc were measured and the effects of sodium, potassium, and lithium on analyte emissions were briefly studied.

Tikkanen and Niemczyk (1984) have described the incorporation of an ETV system into a commercial ICP direct reader based on the concept of rapid vapori-


zation of the sample solution (5 ktL) from a tantalum boat. The distance the sample must travel from the tantalum boat to the plasma was kept rather short, ca. 30 cm. The system could readily be switched between the ETV mode and the conventional pneumatic nebulization mode of sample introduction. However, they were limited to monitoring a single analyte channel for each firing of the ETV.

A further extension of Tikkanen and Niemczyk's work (1984) has recently been reported (Tikkanen and Niemczyk, 1985, 1986). In the first paper (Tikkanen and Niemczyk, 1985), they detailed the manner in which simultaneous signal versus time profiles could be obtained. When a multielement solution was introduced into an ETV-ICP system, the retardation of appearance time in the plasma with decreasing volatility for the various components was demonstrated. However, they used a multielement solution of the trace elements at a rather high concentration, 1 ppm of each element, which is an artificial situation and does not reflect the real trace element content of different materials. In the latter paper (Tikkanen and Niemczyk, 1986), the use of an ETV system to sequence in time the arrival of various components of a sample into an ICP direct-reader system is discussed. By using a multichannel, time-gated detection sequence, they demonstrated the elimination of some well-documented spectral interferences noted for aluminum on arsenic determinations and the easily ionized element sodium (as the chloride or the sulphate salt) on iron, manganese, and lead determinations. In this reviewer's opinion, it is a good start and one approach that can be utilized to eliminate some interferences, as long as the introduction of the analyte and the interferant can be separated in time. However, in these results, the ratios of analyte to interferant, for example, iron in the presence of sodium, range from comparable to at most 200-fold higher, and one must therefore question the value of this approach, especially as the ratios of sodium to analyte in natural matrices (e.g., biological or clinical samples) are much higher, being, for example, 1:3500 for iron and sodium in serum, respectively. These workers did not take into account natural ratios and did not present any actual matrix effects on real sample applications to demonstrate the practical utility of the time-gating concept.

A design similar to that of Kitazume (1983) was described by Kawaguchi et al. (1986). A V-shaped tungsten wire was used for the thermal vaporization of 10-ktL sample solutions. A glass evaporation chamber (volume ca. 1 mL) was connected to a torch with a Teflon tube of 1-mm i.d. This system involved the electrothermal vaporization of the sample using a constant voltage dc source and high-capacity condenser in a tungsten wire, followed by atomization and excitation of the vapor cloud in an argon-ICP. Detection limits for various elements were measured and the effects of calcium and potassium on analyte emissions were examined. The method was successfully applied to the direct determination of lead in urine.

Dittrich et al. (1988, 1990a,b) have described the use of a tungsten coil electrothermal vaporization system with a 3-kW argon-nitrogen ICP. Normal conditions, as used for continuous nebulization, were employed. Normal tungsten coils, as are fitted in halogen lamps, were used for the introduction of liquid microsamples


(10-50 laL). The system was closed in a quartz chamber. In the two-step ETV-ICP, the ETV system is used only to vaporize the dry solution residue. Detection limits for rare earth elements were measured.

Okamoto et al. (1990) modified a Seiko I Model SAS-705V metal furnace atomizer for AAS that was equipped with a tungsten boat for rapid and easy switching between ICP and AAS systems. (Essentially, a quartz dome is switched for the window holder.) The modification retained as much of the original design of the atomizer as possible. This system was confirmed to be applicable to the determination of vanadium and titanium in steel and to have excellent powers of detection with small sample volumes, for example, 20 laL. Hydrogen was mixed with the argon carrier during the vaporization cycle to prevent deterioration of the tungsten boat furnace.

A laboratory-made device (vaporization chamber made of glass, i.d. 1 cm) with tungsten coil ETV for liquid sample introduction (10 ktL) into an argon ICP was proposed by Mei et al. (1992a, b). The tungsten filament was wound into four-turn- coils and the ETV device was connected to the central tube of the ICP torch by a soft poly(tetrafluorothylene) (PTFE) tube of 15 cm length. The formation of refractory carbides was avoided and detection limits for rare earth and other elements were reported.

A refractor plate has been incorporated in an ICP-AES system to perform background correction on analyses utilizing ETV sample introduction (Verrept et al., 1991). This background correction technique eliminates the need for attempting to matrix match the standards to the sample and seems to be useful for transient signal sample introduction methods, such as ETV.

At this stage of development, compared with conventional nebulization, a limited to considerable improvement in detection limit has been reported for the vaporization of practical samples from the metal surfaces of electrothermal devices into the ICP.

ETV: Graphite Heating Devices

Although tantalum filaments were initially employed, the most widely used and preferred material for constructing vaporization cells is carbon, for example graphite or pyrolytically coated graphite. An early approach (Dahlquist, 1974; Fassel, 1977) used graphite yarn as the substrate for the vaporization of liquid microsam- pies (5-50 laL) into an ICP. No investigations were reported concerning matrix and residue effects resulting from real samples or interelement effects and other interferences.

Although early attempts to use ETV for ICP sample introduction showed promise, the efforts of Kirkbright and co-workers clearly established the features and limitations of this combination. A graphite rod ETV device, contained in a I-L cylindrical glass manifold, has been used (Gunn et al., 1978; Kirkbright et al., 1980; Kirkbright, 1982; Long et al., 1985) for the introduction ofmicrolitre (10-ktL) liquid


samples into an ICP via a polyethylene connecting tube (0.5 m). They examined the following parameters: sample transport, effect of viewing height and plasma operating power, and detection limits and precision. They concluded that the transport systems had good capabilities for distances up to 20 m. In spite of the dilution of picogram amounts of vaporized sample in the large-volume manifold, adequate signals were produced in the plasma. In later papers (Millard et al., 1980; Kirkbright, 1981; Kirkbright and Snook, 1983; Kirkbright et al., 1983), they continued the investigation of matrix, interelement, and sample transport effects. The studies mainly concerned the effects of matrix and concomitant elements on the determination of arsenic and cadmium, variations in associated prevaporization loss, and transport efficiency of the analyte. These workers concluded that the primary interference was actually caused by prevaporization losses of analyte, deposition of analyte during transport to the plasma, and the formation of refractory carbides with the graphite rod. They (Kirkbright and Snook, 1979) circumvented this latter problem by the addition of a halocarbon (0.1% trifluoromethane) to the argon carrier gas to preferentially form the volatile halides. This resulted in an improvement in detection limits to the subnanogram level for elements such as boron, chromium, molybdenum, tungsten, and zirconium. It was proposed that this approach would be suitable for multielement investigation. It should be noted that the studies (Kirkbright and Snook, 1979) were carried out with aqueous solutions and there were none made using real samples.

A further extension of their work, volatilization of refractory elements, and compounds via their more volatile metal halides, has recently been reported. Bootes et al. (1987), Satumba et al. (1987), and Matousek et al. (1989b) have used a chlorine-argon gas mixture as the carrier gas for volatilization of elements that form refractory carbides. Nickel et al. (1989, 1993) and Reisch et al. (1989) clarified the principal importance of the thermochemical processes in ETV-ICP analysis by studying the role of chemical modifiers on the vaporization of difficult-to-vaporize carbide-forming elements in powdered ceramic material. PTFE was used as a slurry fluorinating reagent in order to avoid the formation of refractory carbides by converting the analytes into their corresponding fluorides and to facilitate a direct analysis of refractory elements in different matrices by ETV-ICP. Some factors which affected fluorinating vaporization, such as amount of PTFE, were optimized experimentally (Hu et al., 1991a,b,c; Huang et al., 1991a, b, 1992). The analytical capability of high-temperatUre halogenation with carbon tetrachloride vapor in a graphite furnace was investigated for silicon carbide powder (Zaray et al., 1992) and silicon nitride samples (Kantor and Zaray, 1992).

Dean et al. (1985) have assessed the accuracy of graphite rod vaporization (GRV), the same device that was used by Kirkbright and co-workers (Gunn et al., 1978; Kirkbright and Snook, 1983), for sample introduction into an ICP. Based on the results for the concentrations of silver, cadmium, copper, manganese, and lead in Bowen's kale, they judged that ETV for sample introduction into an axially oriented plasma was an accurate technique. The method of standard additions was used to


compensate for evident matrix effects. In a subsequent paper (Dean and Snook, 1986), they reported atomic absorption measurements above a graphite rod used as an ETV device for sample introduction into an ICP. These measurements show that volatile elements such as cadmium persist as atoms for a considerable distance (90 mm) above the rod when atomized, whereas nonvolatile elements (e.g., silver and copper) persist for relatively short distances of up to 20 mm. As atoms are highly reactive, this fact has considerable implications in the design of enclosures for the apparatus.

A design similar to that of Nixon et al. (1974) and Gunn et al. (1978) was described by Ng and Caruso (1982). This system involved the vaporizing of the sample electrothermally in a carbon cup followed by atomization and excitation of the vapor cloud in an ICP. Compromise conditions were used for the ICP but the furnace conditions were varied from element to element. The electrothermal vaporizer assembly (a glass dome with an inner volume of ca. 280 mL was positioned directly underneath the ICP torch. The two were connected via 18 cm of PTFE tubing. The detection limits reported for 21 elements in 10 ktL of aqueous sample are at the ng/mL and sub-ng/mL level. Pyrolytic graphite and impregnation of the graphite sample holder with tantalum salts was found to be advantageous for certain elements. In later papers they continued the investigation of matrix effects in synthetic ocean water (Ng and Caruso, 1983a) and reported on improvements obtained by the preferential formation of the halides of refractory elements (chromium, uranium, vanadium, and zirconium) in the electrothermal carbon cup (Ng and Caruso, 1983c). Their system was utilized for the introduction of organic solvents into a low-power ICP (Ng and Caruso, 1983b). In the latter paper, for example, the addition of iodine to spiked gasoline samples allowed tetramethyllead and tetraethyllead to be determined without difficulty.

Efforts to improve ETV devices for introducing samples into an ICP in recent years have been directed toward increasing the efficiency of sample transport into the plasma and, as a consequence, improving detection limits. Many versions of ETV devices have been explored for use with the ICP. Several analytical researchers have modified Perkin-Elmer (PE) graphite furnaces for microvolume sample introduction.

Crabi et al. (1982) modified a PE HGA-500 graphite furnace and used it with a L'vov platform to introduce samples into an ICP. They stated that background correction is necessary. In a later paper, Casetta et al. (1985) continued this line of investigation using similar instrumentation (PE HGA-500) for the determination of sulfur in solid rubber. They used graphite tubes without holes and therefore overcame the need (Crabi et al., 1982) to enclose the furnace head for collecting sample vapors (the dead volume of the vaporization cell is also reduced).

Recently, Christian and co-workers (Swaidan and Christian, 1983; Hartenstein et al., 1983; Swaidan and Christian, 1984) modified a PE HGA-2000 graphite furnace and transported the sample aerosol through 20 cm of tubing to a spray chamber, which was connected to an ICP torch. They used this system for single

Electrothermal Vaporization 5ample Introduction 73

element (Swaidan and Christian, 1983) and simultaneous multielement (Harten- stein et al., 1983; Swaidan and Christian, 1984) analyses of aqueous solutions, and obtained rather poor detection limits, probably caused by the loss of vapors in the spray chamber. It should be recognized that internal standards were used (Harten- stein et al., 1983) to correct for errors due to changes in the flowrate of the argon carrier gas, the observation zone, the observation period, and the sample volume and to graphite tube deterioration.

Nimjee et al. (1984) have reported on the interfacing of a PE HGA-2100 graphite furnace to an ICP with the former replacing the nebulizer. The furnace was connected to the torch by means of a concentric quartz tube secured in a gas-tight graphite bushing at one end of the furnace. Gas flows to the plasma torch and through the furnace to the torch were metered and controlled by means of high precision valves. A stopcock and a T-connection in the quartz tube allowed the gas (argon/Freon) flow through the furnace to be interrupted while the furnace was opened. The flow of argon carrier gas through the bypass line maintained the plasma. They used this system for single element analysis of solid samples (air filters).

Aziz et al. (1982a) used a PE HGA-74 graphite furnace with a special aerosol transport system that transports the sample aerosol through a 30-cm glass tube to the base of an ICP torch. They examined the matrix effects from biological samples on analyte emission and generally found them to be significant, necessitating use of a standard additions technique. Van Berkel and Maessen (1988) also used a PE HGA-74 graphite furnace for the pyrolysis of a poly(dithiocarbamate) resin and subsequent vaporization of the analyte (the total distance of the furnace to the ICP was about 60 cm). The quartz sampling boat with a maximum capacity of 40 mg of resin was easily introduced into the furnace via the gas inlet side. Special attention was paid to matrix effects which occur when analytes and pyrolysis products of the resin enter the plasma simultaneously. In a later paper (Van Berkel et al., 1990) they continued the investigation of the properties of plasmas fed with dry aerosols compared with those of plasmas fed with aqueous aerosols. It was observed that excitation conditions in dry plasmas differ noticeably from those in aqueous plasmas.

Evaluation of a novel configuration and a new furnace design for ETV-ICP was reported by Matusiewicz et al. (1986). Modification of a PE HGA-500 furnace, which allowed vertical mounting of the graphite tube, insertion of a graphite cuvette, and a direct, shortest practical connection to the base of an ICP torch, was described. The operational characteristics, including the effect of transport tube length to the ICP torch, vaporization temperature, carrier argon flowrate, observation height above the coil, and plasma power, were investigated. The effects of major matrix constituents (calcium, iron, potassium, magnesium, sodium, and phosphorus) on the determination of trace elements (beryllium, cadmium, cobalt, copper, manganese, lead, and zinc) by ETV-ICP were also investigated. It was found that significant enhancement or suppression of the analyte emission occurs in the


presence of these major (matrix) elements. Unfortunately, no results for actual analyses were presented to demonstrate the practical uses of the system. Also, even though matrix effects have been characterized, there was no suggestion about how to overcome them by, for example, altering furnace or plasma conditions.

Alimonti et al. (1987) modified and used a PE HGA-400 graphite furnace designed for AAS. The losses due to condensation and adhesion of the analyte species (atomic clouds) on the transfer line walls were minimized by using a short, electrically heated quartz tube. Although not studied, this should give a better transport efficiency than that found in similar ETV devices. This system was found to be suitable for the determination of platinum in biological samples (serum, urine, tissues).

Recently, Karanassios et al. (1991) slightly modified a commercially available PE HGA-2200 system by removing the graphite tube and by replacing the graphite contact rings and the left and fight observation windows with machined brass blocks (electrodes). The modified furnace assembly is enclosed within a Pyrex chamber. This ETV sample introduction system was used for the analysis of pelletized solids (powdered botanical samples) by ICP-AES. The pellet is placed between the electrodes of the modified ETV device. A current causes rapid ohmic heating of the pellet and results in analyte vaporization. Vaporized samples are routed into the plasma using Tygon tubing of ca. 45 cm in length. The system shows considerable promise for rapid screening of botanical samples of environmental concern. Subsequent work (Ren and Salin, 1993) investigated use of similar instrumentation (PE HGA-2200 graphite furnace) as a sample introduction device (microlitre volume of liquid samples, 5 or 10 ~L) for ICP-AES. The major modification made is the addition of sheath and cooling gas flows. The sheath gas provides a thin sheath layer between the analyte vapor and the wall of the transport tube (ca. 7 cm) to prevent vapor condensation, and the cooling gas cools the analyte vapor to promote aggregate formation. Experiments with liquid samples showed that the addition of these gas flows increases the analyte transport efficiency and reduces matrix effects on the transport efficiency. The results also suggest that the carrier gas flow rate changes the vaporization rate.

Other workers have modified Varian carbon rod atomizers, which were combined with glass or quartz chambers, for ICP sample introduction. Some of them have been discussed previously in this chapter (Gunn et al., 1978; Kirkbright et al., 1980; Kirkbright, 1981, 1982; Long et al., 1985; Millard et al., 1980; Kirkbright and Snook, 1979, 1983; Dean and Snook, 1986; Ng and Caruso, 1982). In another approach, commercial ETV electrodes were replaced by an optimized electrode, and a novel double-walled quartz chamber (ca. 30 mL volume) was added to prevent analyte aerosol losses owing to leakage and to reduce pressure surges produced during heating of the argon carrier gas (Barnes and Fodor, 1983). The chamber exhaust was attached to the ICP torch aerosol tube by means of a 1-m length of Tygon tubing. A number of significant modifications were made (Matusiewicz and Barnes, 1984) since the original design was described (Barnes and Fodor, 1983).


These include (a) the reduction of the chamber volume from ca. 30 to 7 mL which damps the negative background transient signal considerably, (b) the reduction of the argon flowrates, (c) the connection of the chamber to the ICP torch through a glass tube (55 cm long), and (d) the modification of the graphite electrode dimensions so that they are similar to the design of conventional graphite electrodes employed for dc arc spectrography.

More recently, a Varian carbon rod atomizer CRA-63 was modified (Hull and Horlick, 1984) to give a system that consists of a normal Varian-type carbon rodcup (the upper half of the cup wascut off and discarded) enclosed in a 50-mL quartz cell between water-cooled electrode-holding blocks, in a manner similar to that described by Barnes and Fodor (1983). A 30-cm intermediate tube was used to connect the cell to the ICP torch. This relatively simple and rather large cell (chamber) is not likely to prevent the negative background signal (peak) caused by the pressure pulse created upon rapid argon heating.

The ETV vaporizer described by Bootes et al. (1987) was powered by the standard Varian Techtron CRA-63 power supply. The design of the sample introduction system was governed mainly by the need to work in corrosive atmospheres, but other considerations included a maximum sample conversion efficiency and simplicity of operation. The choice of stainless steel as a material for the cylindrical enclosure was dictated by the precise tolerances required. A tight-fitting dome of Teflon together with a 5-cm length of tubing interfaced the vaporizer directly to the sample inlet of the plasma torch. The pyrolytic graphite-coated furnace was mounted upright so that the sample aerosol was swept into the plasma by the sample gas stream, reducing diffusion of the vaporized analyte.

A Varian graphite furnace CRA-90 was used for the external ETV introduction of powder samples, as described by Ohls and Htitsch (1986). A small graphite crucible heated between two graphite electrodes was surrounded with an argon gas flow so that the vapor generated in the crucible was injected into the torch. The lower part of the torch was formed like a funnel to collect the sample vapor surrounded by the carrier gas flow. The system was open to the atmosphere, and the argon-nitrogen discharge was stable. No vapor from the crucible appeared to be lost, because the funnel acts like a chimney in the surrounding argon flow. This system has been found useful for the analysis of highly volatile elements from a solid matrix.

As a contribution to the further development of ETV-ICP systems (Alvarado et al. 1987), a standard Varian CRA-90 system was used and was fitted with the graphite cup atomization cell. The CRA-90 was sealed into an air-tight chamber. The glass dome included a port for the injection of samples into the CRA-90 graphite cup and an 8-mm i.d. polyethylene exit tube that was connected directly to the ICP torch assembly. The length of the polyethylene transport tubing was variable. It was found that the optimum length for peak-height measurement was 0.5 m, but for peak-area measurement the optimum length was 1.5 m. The experiments reported in this paper also highlighted an interesting aspect of the


performance of electrothermal vaporization systems. The design of the chamber for the CRA-90 necessarily involved the more rigorous exclusion of oxygen from the atmosphere of the workhead than is normally possible during its use in AAS, and it appears that the removal of oxygen provides a substantial extension in tube lifetime.

Other workers preferred to modify various models of Instrumentation Laboratory (IL) controlled-temperature furnace atomizers (CTF). The ETV graphite furnace Model IL655 CTF was modified for ICP application by Matusiewicz and Barnes (1984a). Some of the existing designs have the drawback of being of large scale (Nixon et al., 1974; Gunn et al., 1978; Ng and Caruso, 1982; Barnes and Fodor, 1983; Hull and Horlick, 1984), which leads to the problem of"dead space" and to memory effects owing to the plating-out of the analyte on the cold surfaces. The CTF chamber was adapted so that an argon flow swept the furnace and carried the aerosol to the ICP. An important feature of this design is the very small internal volume (0.8 mL) of the graphite cuvette used as the vaporization chamber. A small vaporization chamber volume minimizes the volume of hot argon gas produced during vaporization as well as aerosol dilution, but the temperature must therefore be higher and the pressure pulse sharper. This reduces variations in ICP background emission from pressure pulses and broadening of the temporal analyte emission peak signal. A variety of graphite cuvette geometries with and without microboats and a new carbon tube-platform arrangement were examined (Matusiewicz and Barnes, 1985). The same workers extended their investigations to the evaluation of discrete nebulization using aerosol deposition into the furnace as a procedure for sample introduction in ETV for ICP-using commercial instruments (Matusiewicz and Barnes, 1985d). Small volumes of solution (ca. 50 laL) were introduced manually from a PTFE microsampling device or funnel (Matusiewicz, 1983), or automatically by a flame sampler system, into a pneumatic nebulizer and deposited under controlled conditions on the surface of a graphite platform. The entire system could be easily automated; however, automation was not adopted for routine use. No sample preparation was required. Matrix effects, although not completely eliminated by the above method, become more consistent from sample to sample. The multiple peaks often observed in ETV-ICP (Kitazume, 1983; Ng and Caruso, 1982; Crabi et al., 1982; Aziz et al., 1982a) can be eliminated by modification of the graphite tube. For example, use of a contoured tube provided for more even heating of the tube ends, resulting in a single pulse of sample vapor (Matusiewicz and Barnes, 1985b).

Blakemore et al. (1984) recently used a modified IL555 atomizer for sample introduction with the ICP in multichannel mode. A carbon rod was used in place of the graphite tube and samples were weighed or injected in IL pyrolytic microboats and placed on the flat portion of the carbon rod. The furnace was mounted in the ICP below the quartz torch so that only 12 cm of PTFE tubing were required to connect them. Heating of argon in the rather large cell, however, resulted in variations in background emission throughout the heating cycle with a consequent


loss of sensitivity. Also, these workers stated that no ashing step was needed for the direct simultaneous determination of major and trace elements in biological materials. However, this statement was not confirmed experimentally and matrix effects caused by major elements were not studied. No detection limit data were listed and the data show relatively large uncertainties for certain analyte-matrix combinations.

Modifications to furnaces from a number of other manufacturers have also been described. These modified instruments have been used with ICPs and typical modification involved combining the two furnace gas flows, which normally enter both ends of the furnace tube in the GFAAS configuration, to enter one end of the furnace for carrying vaporized sample into a plasma source. In this modification, the furnace windows need to be replaced by tubing adaptors for cartier gas to enter and exit. Kumamaru et al. (1987) modified a Nippon Jarrell-Ash Model FLA- 100 graphite furnace. It is clear that the article described a modification of a commercial graphite furnace identical to the system described by Matusiewicz and Barnes (1984a). There are extremely close similarities in the two papers even though they used different equipment. (At this point, one thing becomes apparent during this review that I find quite alarming. Many authors do not make appropriate reference to earlier work in their area. Neither the authors of these papers nor the referees who reviewed them seem to have paid enough attention to references. Yet proper credit and honesty is an essential part of any scientific publication, and referees, who are experts in the fields of the papers they review, should also pay closer attention.) A Hitachi GA-3 graphite atomizer was used as the ETV unit, which can handle liquid samples (Ida et al., 1989) or powders (Atsuya et al., 1991). A WF-1-type heating device with matching graphite furnace (Hu et al., 1991) and a graphite furnace WF-4 (Beijing, China) (Huang et al., 199 lb), which is similar to a PE HGA-500, were employed for the slurry sample introduction and vaporization into ICP.

A graphite furnace vaporizer which is applicable to indirect coupling to an ICP torch was designed by Kantor and Zaray (1992) (it is not clear and not specified what commercial graphite furnace was modified, if any). The furnace design is characterized by the use of a horizontal graphite tube as the current-carrying element, into which a graphite sample boat can be inserted to a predetermined position. The construction makes possible the introduction and analysis of both liquid and solid samples at minimum risk of contamination.

Very recently, some parameters (observation height, carrier gas flow rate, radio frequency power) of solid-sampling ETV-ICP were investigated (Verrept et al. 1993). The study concentrates on the determination of copper, cadmium, and lead. Their ETV-system was originally designed for Zeeman AAS. It is a commercially available boat-in-tube- (graphite) type ETV from Grtin Analytische Mess-Systeme GmbH, Germany. One side of the furnace was closed with an automatic valve, and the other side was connected to the ICP by a glass tube. The results obtained in this


study suggest that dominant matrix effects occur in the processes of vaporization and transport rather than in the processes taking place in the plasma.

Matousek and Mermet (1993) studied the effect of small amounts of hydrogen added to carrier argon on atomic and ionic lines of chromium, magnesium, manganese, and lead in the ICP for ETV-produced aerosols. Similar to their earlier design (Matousek et al. 1989b), the vaporizer was based on a miniature graphite furnace held between two support rods, one of them being hollow to allow sample introduction. The improved design was based on a GBC Scientific Equipment Pty. Ltd. GF 1000 graphite furnace atomizer workhead. The pyrolytic graphite-coated furnace is mounted upright to further assist efficient aerosol/vapor transport and to prevent analyte deposition within the stainless steel cylindrical chamber, which has a tight-fitting Teflon dome. The sample introduction port in one of the support rods allows the sample droplet to be placed consistently on the system. The hydrogen introduction via the ET vaporizer caused lowering of the analyte appearance temperatures and induced analyte loss for the more refractory elements of chromium and magnesium.

B. Inductively Coupled Plasma-Mass Spectrometry

During the past decade, inductively coupled plasma-mass spectrometry (ICP- MS) has been established as one of the most powerful techniques for multielement determination. Aconsiderable amount of research effort was expended in enhancing and expanding sample introduction options for the ICP-MS technique. ETV techniques have exhibited the greatest potential for ultratrace ICP-MS determinations, with detection limits obtainable in the 1-10 fg region and freedom from water- related interferences. Applications of such techniques have already been reviewed to some extent by Baumann (1992), Carey and Caruso (1992), Williams (1992), Gregoire (1992), Gregoire et al. (1992b), and Evans and Giglio (1993).


Combining an ICP or an ETV source with mass spectrometry provides a convenient method for the introduction of liquid or solid samples and is particularly attractive for isotope ratio determinations on small samples. A system similar to that described by Gunn et al. (1978) has been used, for the first time, to introduce 5-ktL samples (Gray and Date, 1983) into a mass spectrometer. The sample was desolvated in the usual way at low temperature and then vaporized into the injector gas flow. A short pulse of ions lasting a few seconds was obtained. As scan times of as little as 20 ms may be used, this provides an ample number of scans over the changing signal to enable isotope ratio measurements to be made. Excellent agreement, according to these workers, has been obtained between isotope ratios determined on microsamples and on nebulized solutions.


Park and Hall (1986), Park et al. (1987b), and Gregoire and Park (1992) developed an electrothermal vaporizer especially designed for use with ICP-MS. This system can adopt a custom-made pyrolytic graphite platform (flat or V-shaped) as well as different metal filaments (rhenium, tantalum, or tungsten ribbon). The evaporation cell was encased in a quartz glass dome. The optimum volume (5-10 mL) in the glass dome above the graphite platform or metal filament was designed such that the distance from the vaporizer element to the glass surface is sufficient to allow for condensation of the aerosol before contact with the glass surface and, hence, loss of analyte is minimized. The argon carrier gas is introduced tangentially and carries the vaporized sample out of the glass dome, through transfer tubing made of Tygon (50-70-cm length, 5-mm i.d.), into the ICP. The transport efficiency was reported to be greater than 80%. A graphite platform was used for the determination of refractory elements such as molybdenum and tungsten, whereas metal filaments were used for the determination of arsenic, cadmium, chromium, copper, iron, nickel, lead, selenium, vanadium, and alkali metals.

An ETV device was constructed by Byrne et al., (1992) and was of a design similar to that reported by Park and Hall (1986) and Park et al. (1987b). Several modifications were made to this device, including installation of a photocell, which was used to monitor the temperature of the vaporization surface, and the use of specially designed graphite strips. The interface between the ETV unit and the ICP-MS instrument consisted of a length (1 m) of Tygon tubing (5-mm i.d.) connected directly to the plasma torch. A three-way valve was installed between the ETV unit and the argon plasma to allow venting during the drying or thermal pretreatment step, as required.

Sample introduction with ETV instead of the normal nebulizer-spray chamber arrangement (Date and Cheung, 1987) was carried out using a graphite rod system from Shandon Southern Instruments Model A3470, originally developed for AAS but modified for use with an ICP by placement of the graphite rod assembly into a specially designed bell jar, as first described by Gunn et al. (1978). The bell jar, graphite rod assembly and four-way valve were designed to maintain argon carrier flow to the ICP during rod loading and sample drying stages. The manufacturer's standard 5-~tL capacity cup-type graphite rod was used.

Voellkrpf et al. (1991, 1992) used a modified PE HGA-600 furnace as an electrothermal vaporizer. The ETV sampling cells were the flow-through designs described by Aziz et al. (1982a) and Crabi et al. (1982). It should be noted that no attempt was made to optimize the cell and transfer-tube design for minimum transport loss. Rather, the design was chosen to be representative of many of the tube cell modifications of commercial graphite furnaces reported in the ETV-ICP literature. One of the major advantages of using a standard graphite furnace as an ETV system is that a standard autosampler, normal graphite tubes, and any other standard HGA accessories can be used directly. It was also shown (Ulrich et al. 1992a) that under suitable conditions the system could be used for single element as well as for multi-element determinations.


A modified PE HGA-300 graphite furnace was investigated for sample introduction into ICP-MS (Carey et al., 1991). This system has the advantage that the atomic vapor immediately condenses into clusters of atoms prior to being transported to the plasma. Results indicated that the system successfully increases transport efficiency of the analyte from the furnace to the plasma. However, this report does not present any indication of the actual transport efficiency. The sample introduction method (10 l.tL) involved the use of a L'vov platform in a manner similar to that used in GFAAS.

The significance of chemical modification for ETV-ICP-MS has been demonstrated (Ediger and Beres, 1992; Gregoire et al. 1992a; Gregoire and Sturgeon, 1993). The importance of mass transport effects in this combination was demonstrated, which indicated that knowledge of the physical and chemical form of the analyte and matrix components is important for the practical application of ETV- ICP-MS to chemical analysis. In addition, Gregoire and Sturgeon (1993) concluded that the background spectrum in ETV-ICP-MS and the molecular ions produced when using chemical modifiers do not seriously limit the use of the ETV-ICP-MS for ultra-trace analysis.


A new merging introduction technique has been developed for osmium determination with ICP-MS (Hirata et al., 1989). The sample (20-40 laL) was placed in a miniature heater in a merging chamber, the solution was heated up to 70 ~ by a nickel-chromium filament, and osmium tetroxide was gently vaporized. The evaporated osmium tetroxide was mixed with blank spray mist in the merging chamber and was finally carried to the ICP. This method assures the effective introduction of osmium to the ICP torch.

Tsukahara and Kubota (1990) investigated the performance of an ICP-MS with a tungsten metal ribbon (50fftL capacity hole) electrothermal vaporizer. To prevent oxidation of the furnace material, argon with a small amount of added hydrogen (Shibata et al. 1990, 1992b) was supplied as the carrier gas. The ETV device was a Micro Sampling System (Seiko Instruments Inc., Tokyo, Japan). It can be resistively heated to 2700 ~ using a voltage-controlled power supply. The furnace is placed in a Pyrex glass chamber (ca. 300 mL volume). Argon gas is supplied to the glass chamber and carries the vaporized sample through a 75-cm long, 6 mm-diameter Teflon tube to the plasma. Detection limits for cobalt, iron, and lead were reported.

Matsunaga et al. (1989) used a modified PE HGA-500 graphite furnace equipped with a tantalum tube (0.5 mm thick), which was inserted into a graphite tube. A 20-1aL aliquot of solution was pipetted into the tantalum tube. The furnace can be resistively heated up to 2800 ~ by use of a power supply.

Shen et al. (1990) used a modified PE HGA-300 graphite furnace unit as an electrothermal vaporizer for the determination of lead, which was initially chosen


to characterize this ETV-ICP-MS system. The device was modified in four sections: coolant section, front adaptor, furnace tube, and rear adaptor. Sample introduction was accomplished by a tungsten wire probe which was inserted into one end of the tantalum tube (the other end of the tantalum tube was sealed). The loop was aligned at the center of the graphite tube. A 3-~L sample was manually loaded on to the tungsten wire loop with the use of a pipette. Inserting the tantalum loop reduces the possibility of forming refractory carbides that hinder volatilization; however, impurities in the tantalum can lead to increased background levels. In addition, the reproducibility of the system was found to be limited by the ability to position the loop precisely in the furnace at the same position from run to run.

C. Microwave-Induced Plasma

The source that has gained considerable attention as an alternative to the ICP is the microwave-induced plasma (MIP), a rapidly developing plasma system. The characteristics of the MIP as a relatively low-cost excitation source have been well-documented; however, its low operating power limits the capacity of the plasma to vaporize and atomize solid or liquid samples. This fact, together with the sensitivity of the plasma to changes in impedance when small amounts of foreign material are introduced, causes fundamental problems with sample introduction.

Many of these problems can be circumvented if the analyte can be presented in the form of vapor to the plasma. One such means of sample introduction is ETV. This aspect of MIP application has been reviewed (Zander and Hieftje, 1981; Carnahan, 1983; Skogerboe and Coleman, 1976; Kantor, 1983; Matousek et al., 1984b; Ng and Caruso, 1985; R6hl, 1985, 1986; Barnett, 1989; Matusiewicz, 1990; Sneddon, 1990; and Carey and Caruso, 1992) but not exhaustively and often in a fragmentary manner. It has become a rather generally accepted opinion that separate vaporization of samples is necessary for low-power MIP excitation to obtain high analytical performance.


The most widely used and preferred material for constructing vaporization cells is metal. To the author's best knowledge, the first reported work combining electrothermal sample vaporization with argon MIP atomization and excitation was by Runnels and Gibson (1967), who used a platinum filament. The sample was introduced onto a 0.4-mm platinum wire filament contained in a small glass vaporization chamber. Microwave power levels of approximately 25 W gave maximum emission intensities when metals were introduced as volatile metal chelate (acetylacetonates) or volatile inorganic salts such as halides. Very low detection limits (due to efficient vapor transfer), in the range of 10 -11 to 10 -12 g, on a sample size of 10 -5 to 10 -6 g were obtained for five elements (Ag, Co, Cr, Cu,


Fe), but unusual interelement effects were found and log-log calibration plots were nonlinear, a result of the low heat capacity of the metal atomizer.

Aldous et al. (1971) described a platinum wire or tungsten loop for the introduction of aqueous microsamples (ca. 0.12 BL) into the MIP. A Pyrex sample cup (minimum capacity 20 mL) was mounted in a plasma cell; this allowed the cup to be slid up and down so that the sample solution could be supplied to the wire or loop. Detection limits for 12 elements were measured and some spectral and chemical interferences on cadmium emission were studied. They also showed that under suitable conditions the system could be used not only for the determination of total concentration of an element, but also for the determination of the different species present.

A major series of studies have been carried out by Kawaguchi et al. (Kawaguchi et al., 1972, 1977; Kawaguchi and Auld, 1975; Kawaguchi and Vallee, 1975; Sakamoto et al., 1976; Atsuya et al., 1977a-c; Yanagisawa et al., 1979). A design of a sample vaporization chamber similar to that of Runnels and Gibson (1967) was described by these authors. This system involved the electrothermal vaporization of the sample using a constant voltage dc source and a high-capacity condenser in a tungsten loop filament (Kawaguchi et al., 1972; Kawaguchi and Auld, 1975; Sakamoto et al., 1976) or tantalum wire filament (Kawaguchi and Vallee, 1975), followed by atomization and excitation of the vapor cloud in an argon MIP (Kawaguchi et al., 1972; Sakamoto et al., 1976) or low-pressure helium MIP (Kawaguchi and Auld, 1975; Kawaguchi and Vallee, 1975). Detection limits reported for 11 elements in 2-BL or 5-BL aqueous sample volumes are at nanogram and subnanogram per milliliter levels. The microsample capability has been used to advantage in the characterization of trace metal functions in biological systems. In later papers they continued the investigation of interelement effects (Kawaguchi et al., 1977), matrix effects (Kawaguchi and Vallee, 1975; Sakamoto et al., 1976; Atsuya et al., 1977c), and vaporization characteristics of metal salts (Yanagisawa et al., 1979). The studies mainly concerned the pronounced enhancement of emission when alkali halide salts were present with the analyte. These workers concluded that addition of potassium chloride to the sample enhances the spectral line intensity of many elements and eliminates or suppresses interference effects. The presence of potassium chloride was also favorable for simultaneous multielement analyses because various elements showed their maximum emission at the same spot along the discharge. It was concluded that the effect was in part due to phenomena occurring during the vaporization of the analyte and in part due to altered excitation conditions in the presence of the alkali halide (Kawaguchi et al., 1977; Atsuya et al., 1977c). An excitation mechanism was proposed to account for the effects observed (Kawaguchi et al., 1977). It should be noted that the studies were completed with aqueous solutions and there were no studies made using real samples.

Fricke et al. (1976) utilized a tantalum-strip vaporization assembly for analyte introduction into an argon MIP. Its operation was similar to that described by Nixon


et al. (1974), who used an ICE Bomb digestion and chelate extraction combined with ETV-MIP permitted determination of trace metals in biological samples with satisfactory results. Interferences, however, were not investigated. Detection limits for nine elements were reported.

Van Dalen et al. (1982) reported the possibility of using tantalum strip vaporization of 20-1aL samples for the helium MIP. A tantalum furnace was coupled directly to a TM010 cavity and instrumentation was described for the direct determination of halogens (C1, Br, I) and sulfur from solution. Detection limits were in the submilligram per liter region. Serious interferences were encountered, and the effects of counter cations were removed by the addition of potassium hydroxide, which also helped to suppress interferences by large amounts of matrix constituents. A standard additions technique remained necessary to permit determinations that were reliable to within 5%. It should be noted that use of a tantalum furnace vaporizer and a silica discharge tube did not permit hydrogen, carbon, nitrogen, oxygen, fluorine, and phosphorus to be reliably determined.

Chiba et al. (1984) and Tanabe (1985) have described use of a metal vaporizer originally developed for AAS. A tungsten boat vaporization chamber was used for the introduction of discrete liquid microsamples into a helium MIP via a vinyl tube. The distance the sample must travel from the tungsten boat to the plasma was kept rather long, 4 m. Detection limits for six elements were measured.

Rait et al. (1984) reported on the feasibility of multielement determination of halogens in rock samples. Analytical measurements were made by placement of the samples on a laboratory-built tungsten boat and electrothermal vaporization of the halogen into a reduced or atmospheric pressure helium MIP. These results were considered to be preliminary because the transport losses between the tungsten boat and the MIP were high, and the manual control of current through the tungsten boat was not satisfactorily reproducible.

Another approach which has been proposed for the introduction of liquid microsamples (5-25 ktL) into an atmospheric pressure helium MIP was described by Brooks and Timmins (1985) and Timmins (1987a, b). The device is a miniature unit, based on a heated wire tantalum filament. The design is such that most dead space is eliminated and turbulence of the plasma gas kept to a minimum. Detection limits for six elements were comparable to or better than the best obtainable by other MIP techniques. A further extension of Timmins' work (Brooks and Timmins, 1985; Timmins, 1987a, b) has recently been reported (Stahl et al., 1989). As pointed out, the unit should be readily amenable to the automation of both the sample injection and the application of dc current. An autosampler was constructed to introduce microliter samples into a helium MIP-ETV system. The ETV power cycle and data acquisition were automated and placed under computer control, resulting in a faster sample repetition rate, better overall precision and greater ease of use. Fully automated sample deposition has overcome the need to open the wire filament device to the atmosphere, minimizing the possibility of contamination and plasma disturbances via air entrainment. These improvements have been demonstrated for


indium and similar results are expected for other elements previously analyzed on the manually operated system (Brooks and Timmins, 1985).

An inexpensive tantalum boat vaporization device was coupled to a helium MIP for the determination of iodine (Barnett and Kirkbright, 1986). It was fabricated from PTFE and powered by a Shandon Southern A3370 electrothermal atomizer. Efforts to improve the ETV device have been directed toward increasing the efficiency of sample transport into the plasma and, as a consequence, improving detection limits compared with the previously reported detection capability for this element by ETV-MIP (Aldous et al., 1971; Van Dalen et al., 1982; Rait et al., 1984).

A further recent development of the microwave discharge is of a very different nature. In this case a microwave discharge, produced with a resonant cavity, is used to generate a stream of active nitrogen. A vapor cloud of the analyte species is mixed with the active nitrogen downstream from the discharge where atomic emission is observed from analyte atoms due to collisional energy transfer with the metastable triplet state N2(A3u+). The method, metastable transfer emission spectrometry (MTES), is particularly useful in materials analysis (Niemczyk and Na, 1983).

The introduction of solids (Capelle and Sutton, 1977, 1978) and solutions (Melzer et al., 1980; Na and Niemczyk, 1982, 1983; Hood and Niemczyk, 1986, 1987) into the MTES system using an electrically heated metal crucible, boat, and filament was demonstrated. Sutton and co-workers (Capelle and Sutton, 1977, 1978; Melzer et al., 1980) have shown a number of important applications of the MTES technique. Applications have included the detection of bismuth vapors produced in a furnace (Capelle and Sutton, 1977, 1978) and the determination of lead in aqueous samples (Melzer et al., 1980).

Na and Niemczyk (1982, 1983) and Hood and Niemczyk (1986, 1987), on the basis of the results of the previous work, designed an experimental system to determine trace metal concentrations and sulfur- and phosphorus-containing compounds in aqueous solutions using an active nitrogen excitation system. Aqueous solutions of trace metals and sulfur- and phosphorus-containing analytes are electrothermally dried and atomized from a tantalum boat. The active nitrogen is produced in a microwave discharge and mixed with the electrothermally produced atomic vapor in a flow cell. Detection limits for 12 elements (including S and P) were reported, and a linear dynamic range of 4 to 5 orders of magnitude was seen in all cases. Results were obtained using laboratory samples, so matrix or other interferences were not discussed. In a subsequent paper (Hood and Niemczyk, 1987), interferences that can be a problem in a MTES system employing an electrothermal vaporizer were discussed. It was shown that there is a fundamental limit to the amount of material (analyte and matrix combined) that can be introduced into the MTES plasma before the intensity vs. analyte mass relationship breaks down. In addition, there can be interferences in the atomization step. These interference processes are very similar to those seen in electrothermal atomization AAS. The results obtained here were compared to those obtained in an AA system. One major difference is that the reactive nature of the nitrogen plasma can


contribute to the elimination of interferences due to the vaporization or formation of molecular species involving the analyte.

It can be concluded that the major disadvantage of the MTES system is the need to operate at low pressure, generally 1-30 torr, which causes difficulties with sample introduction and makes operation inconvenient.

Dittrich et al. (1990a) tested and used a new tungsten coil atomizer for the determination of traces of sulfur by ETV in an MIP. The tungsten coils (Osram GmbH) had 16 turns and are positioned in a quartz enclosure provided with a sampling port; the system is powered by a 24-V supply.

An interesting comparison of a Surfatron and a traditional Beenakker resonant cavity as excitation sources following ETV was presented by Richts et al. (1991). The tungsten coils used for electrothermal evaporation were produced from halogen lamps. They consist of 12 turns and were constructed for sample aliquots of 20-50 ktL. The operation was very similar to that described by Dittrich et al. (1990b) who used an MIP. They found that the Surfatron-generated plasma was more stable, exhibited a larger linear dynamic range for copper and cadmium, and was less susceptible to matrix effects than the resonant cavity.

Oki et al. (1990) reported the laser-induced fluorescence detection of sodium atoms atomized by an MIP of helium with a tungsten filament vaporization system (Kawaguchi and Vallee, 1975) that has a detection sensitivity on the order of picograms per milliliter. The filament was a looped tungsten wire of 0.1-mm diameter mounted in an acrylic chamber; 10 ktL of sample was evaporated onto the filament, flash-heated, and vaporized into the plasma.

Recently, the atmospheric pressure MIP was used as an atomizer for AAS measurements, when combined with an electrothermal device for sample introduction to determine silver, copper, and lead (Ling et al., 1990a). Sensitivities were in the tens of nanograms per milliliter. The sample solution (3 IuL) was injected using a microsyringe onto the tantalum loops of a microsample introduction system combined with a concentrated sulfuric acid desolvation apparatus. A similar desolvation unit was proposed by Que et al. (1989a, b) and it was shown that the water vapor was better removed by the new system than with the conventional desolvation system. Therefore, the stability and the excitation capability were improved.


An eady approach by Fricke et al. (1975) used a carbon cup as the substrate for the vaporization of liquid microsamples (5 ktL) into an argon MIP. They also used a tantalum strip vaporization device. These devices were similar in design to that described by Nixon et al. (1974). The system was used for single (Fricke et al., 1975) and simultaneous multielement (Fricke et al., 1975; Rose et al., 1976, 1978; Zerezghi et al., 1983) analyses of aqueous solutions. In the case of simultaneous determinations, a compromise element vaporization temperature of approximately


2000 ~ was used. These workers also concentrated on coupling the MIP to a vidicon detector (Fricke et al., 1975) and later to a rapid scanning spectrometer (Rose et al., 1976, 1978; Zerezghi et al., 1983) for simultaneous determinations. No investigations were reported concerning matrix and residue effects resulting from real samples or inter-element effects and other interferences (Fricke et al., 1975; Rose et al., 1976). In subsequent studies (Rose et al., 1978; Zerezghi et al., 1983), the authors examined the matrix effects from sodium on analyte emission and generally found them to be significant, which required tuning the MIP in sustaining a plasma. Detection limits for 16 elements were reported (Fricke et al., 1975; Rose et al., 1978; Zerezghi et al., 1983). In the latter paper (Zerezghi et al., 1983), use of a modified Varian Model 63 electrothermal atomizer system for the MIP, which was combined with a small quartz dome (minidome) was described. The graphite cups were cut to a height which accommodated a 10-~tL volume and were coated with pyrolytic graphite. A Teflon sleeve served as a coupler between the dome and the plasma containment tube (6 cm long).

Beenakker et al. (1980) described use of a 2-mm thick cord of pyrolytic graphite mounted in a Pyrex glass chamber as an electrothermal vaporizer for the helium MIP. The sample for analysis (1-3 laL) was applied to the cord by a syringe and the vaporized specimen was introduced into the MIP through teflon tubing (about 10 cm). Detection limits for nine elements, including halogens and sulfur, were measured, and the effect of 1% KC1 on analyte emissions was briefly studied.

A uniquely designed carbon rod ETV cell was operated at low pressure in conjunction with an argon MIP (Alder and Da Cunha, 1980). The matrix effect from KCI and NHaF on line intensities of some metals and of uranium was examined and the analytical performance of the system (i.e., detection limits) reported. It should be noted that pretreatment of the carbon rod with gold prior to loading mercury solution diminished analyte loss during the preheating stage. A shortcom- ing of their system was that the reproducibility of the signals was not good.

Aziz et al. (1982a), Heltai et al. (1990b), and Broakaert and Leis (1985) modified and used a PE HGA-74 graphite furnace with a special aerosol transport system that conducted the sample aerosol through a 30-cm glass tube to the base of an argon MIP discharge tube. Memory effects were avoided by elimination of the solvent vapor during the drying stage. The introduction of argon flows from both ends of the tube so that the vapors escape through the sampling hole of the graphite furnace achieves this. However, in order to get efficient analyte transport, the sampling hole was closed with the aid of a graphite stub of suitable dimensions during the evaporation step (Heltai et al., 1990b). The authors examined matrix effects on analyte emission from biological samples and generally found them to be significant, which required the use of a standard additions technique for analysis. This type of vaporizer and modified furnace was used to investigate the analysis of real samples in solution (5-50 ~tL). For solid samples (ca. 2 mg) the ETV developed by Broekaert and Leis (1985) incorporates the same basic features as the previously described system. Two types of MIP (in a Beenakker cavity) were also studied by


Heltai et al. (1990b) as excitation sources following graphite furnace vaporization. A further extension of their work has recently been reported (Heltai and Broekaert, 1991). The studies mainly concerned the possibility of coupling different sample introduction devices to atmospheric MIPs, including graphite furnace aerosol generation.

Although early attempts to use ETV for nonthermal excitation source MIP sample introduction showed promise, the efforts of Matousek and co-workers clearly established several aspects of both enhancement and suppression of the analyte emission intensity caused by an easily ionized element and the existence of spatial emission properties of an MIP. Drying and vaporization of the analyte are achieved separately using a modified Varian Techtron CRA-63 electrothermal atomizer (Matousek et al., 1984a, b). This vaporization assembly is enclosed in a Pyrex dome of ca. 10 mL internal volume. The plasma is sustained in a silica tube using argon. Subsequently, Matousek et al. (1986b) investigated use of microliter volumes of liquid samples for analysis. Their combined minifurnace-based electrothermal vaporizer and helium MIP was employed to monitor both atomic and ionic emission from non-metals (CI, I, S, P). Possible applications were evaluated by determining iodine in milk and analyzing a multicomponent mixture of sulfur compounds. A further extension of their work has recently been reported (Matousek et al., 1986a, 1989b). The studies mainly concerned the spatial emission properties of an argon MIP when the analyte elements are introduced into the plasma in a vapor form from an electrothermal vaporizer. These workers concluded that the drift of analyte ions in the inhomogeneous microwave field surrounding the discharge, followed by interaction with the silica wall, is a major source of analyte atom loss in MIP emission spectrometry (Matousek et al., 1984a, 1989b). A pronounced interference effect in the MIP system has also been found when easily ionized elements (Li, Na, K, Rb, Cs) are present. Enhancement or suppression of analyte line emission was found to depend upon the element and type of line (atomic or ionic) used. From the standpoint of practical analytical chemistry, the interfering effect of the easily ionized element may lead to an improved analytical performance; in many instances, a strong enhancement of analyte emission is found. The findings of this work suggest need for the routine addition of rubidium or cesium for example, in order to take advantage of the improved response.

An ETV device, based on the original design by Nixon et al. (1974) and modified by Wu and Carnahan (1990), was used. A carbon cup-type electrothermal vaporizer (Varian Model 63 carbon cup atomizer) in a glass dome structure was used for vaporization of aqueous samples (15 laL) into a high power 500-W helium MIP. A vapor restriction device was used to enhance the efficiency of sample transport to the plasma. The detection limits for bromine and chlorine were reportedly the best yet obtained by detection in the UV-visible region.

Introduction of solutions into a low-pressure, metastable nitrogen plasma has been accomplished using an electrically heated carbon furnace (McCaffrey and Michel, 1983). Simplex optimization of eight factors which were postulated to


affect the intensity of atomic emission signals in the plasma resulted in a detection limit of 3 mg/L (30 pg) for the determination of chromium in aqueous solution.

Very recently, Bulska et al. (1993b) used a modified CRA-63 electrothermal atomizer (Varian Techtron) for the direct vaporization of the dry sample residue from the graphite furnace into low-power MIPs (toroidal, one- or three-filament MIPs), operated in a TM010 cavity according to Beenakker. The graphite tube was connected directly to the quartz capillary of the MIP and positioned between three supporting rods for more efficient heating and uniform temperature distribution. The MIP was also coupled with hydride generation, using trapping of the analytes in the graphite furnace, which was found to be a powerful analytical technique for the determination of arsenic, antimony, and selenium.

Also very recently, Abdillahi (1993) presented examinations using a graphite rod ETV sample introduction with a low-power MIP for the sensitive determinations of some nonmetals (chlorides, bromides, iodides, sulfides, ammonium, nitrogen). A laboratory-constructed graphite rod vaporization unit, based on Kirkbright design (Kirkbright and Snook, 1979), was used as a sample introduction technique. A modified PE HGA-74 graphite rod ETV system has also been used for comparative studies of the results. The specially grooved graphite rods (platforms) showed better performance than the cups in terms of uniformity of sample vaporization and increased concentration of analyte, with correspondingly better signals.

An active nitrogen plasma was investigated as an atom reservoir for laser- enhanced ionization in an MIP (Seltzer and Green, 1989). A graphite furnace atomizer was modified to permit sample introduction. Graphite platforms, made in the laboratory, were inserted into the furnace tubes to delay sample atomization until the temperature of the entire tube was stabilized. Vaporized samples were entrained directly in the plasma gas flow (nitrogen and/or argon). Suppression of signal for laser-induced ionization in the active nitrogen plasma was similar to that encountered in flames in the presence of thermally ionized Group IA elements.

Additional Techniques

A few techniques have been investigated for introducing solution samples into the MIP, other than those described in previous sections.

Hingle et al. (1969), in their preliminary communication, proposed the introduction of sample solution via an indirect nebulizer. The sample mist is passed through a heated silica tube in an electrically heated furnace at 550 ~ and drawn into a quartz argon MIP discharge tube. In this way, the evaporated aerosol is sampled and the discharge maintained at reduced pressure. Detection limits for 13 elements were reported.

Watling (1975) developed a system which utilizes an amalgamation stage where mercury is amalgamated onto silver wool. The wool is subsequently heated with a Meker burner (a resistance furnace was used for later experiments), and the mercury thus released is flushed by argon into a plasma where it is excited.


A design similar to that of Watling (1975) was described by Nojiri et al. (1986). In this study, an amalgamation technique for mercury analysis in a natural water sample was successfully applied to the helium MIP. Modification of the usual amalgamation apparatus for cold vapor AAS was undertaken. Mercury vapor was generated from water samples by reduction and purging and was collected with a gold amalgamation trap. The mercury vapor, removed by heating (up to 900 ~ the trap electrically with a constant voltage, was introduced into the MIP quartz discharge tube.

Still another method for the analysis of natural water and tissue digests utilizes chloride generation to vaporize metals from samples into an argon MIP (Skogerboe et al., 1975). Hydrogen chloride gas was introduced into a quartz chamber (containing sample residue) to permit rapid vaporization of cadmium and lead. The furnace (quartz chamber) was powered by a variable transformer and heated to 850 ~ The chlorides, in gaseous forms, were carried by an argon stream into a plasma for the excitation measurements. Detection limits obtained via the chloride generation approach were reported for eight elements. No details on the cavity or microwave power supply used were given.

Another approach which has recently been proposed for the introduction of liquid or suspension microsamples ( 1-5 ltL) into an atmospheric pressure helium MIP has been described by Kitagawa et al. (1989). The device is a so-called separative column atomizer (SCA), based on an alumina tube heated to 755-1100 ~ and packed with activated charcoal. The sample holder is a cup made of a molybdenum sheet, mounted on the sample introduction port, and then inserted into the hot region of the SCA column (after the MIP becomes stabilized). This method is intended for use in direct analysis of trace amounts of elements in complex matrices by atomic emission spectrometry.

D. Microwave-Induced Plasma-Mass Spectrometry

Research on MIP-MS coupling is still in its infancy. While microwave plasmas have been shown to be excellent sources for both atomic emission and mass spectrometric detection, there have only been a few reports on MS detection for ETV-MIP.

In a series of papers, Satzger and co-workers (Satzger, 1989; Satzger and Brueggemeyer, 1989; Evans et al., 1991; Shen and Satzger, 1991) described an integrated ETV-MIP device for spectrochemical analysis that was originally designed and developed by Satzger. The design of the instrument is such that the analyte is vaporized at the plasma, thereby affording no opportunity for reconden- sation. Additionally, the utilization of a tantalum-tipped injection tube ensures the formation of a power-efficient, annular plasma, with the carrier gas plus analyte passing through the center. In order to accommodate the ETV mode of sample introduction, one end of the torch body is pressed into the downstream side of a furnace (modified PE HGA-500-graphite furnace with a quartz dome) electrode


block. Aqueous samples (3 or 5 IuL) are introduced onto a tungsten wire loop, which is then inserted in an injector tube (with a tantalum tip) situated in the microwave cavity; the sample is then vaporized fight at the plasma. Detection limits for some elements were measured.

E. Direct Current Plasma

The direct current plasma (DCP) has become widely used and accepted as an excitation source for trace metal determination in a variety of materials by emission spectrometry. With nebulized samples, the concentration detection limits for the DCP are similar to those of the ICP. This approach is satisfactory for routine analyses, but under special circumstances benefits may be obtained from the use of ETV sample introduction and electrothermal sample vaporizers have also been interfaced to the DCP. Work in this area has not been extensive and the detection limits obtained with these interfaces have not been impressive.

The first reported work combining electrothermal sample vaporization with DCP atomization and excitation was by Pfluger and Nessel (1984), who used a wire loop microfurnace. The sample (0.5-10 ~tL) was introduced into a small depression in an electrically heated tungsten, tantalum, or iridium wire contained in a Pyrex tube. The vaporized specimen was introduced into the miniature helium DCP. Although the microfurnace was designed specifically to couple to the miniature helium DCP, the design should also be of value in other emission methods such as MIP or ICP.

Elliott et al. (1986) explored the adaptation of a commercial ETV system for sample introduction into the three-electrode DCP. A modified controlled-temperature graphite furnace (Instrumentation Laboratory Model IL 655) was used to generate the sample vapor (Matusiewicz and Barnes, 1984b, 1985c). Samples (5 IaL) were introduced onto a pyrolytically coated microboat in a round graphite tube (Matusiewicz and Barnes, 1985c). During the high-temperature vaporization cycle, the sample vapor was transported by argon carrier gas through a 6-mm i.d. glass tube from the ETV chamber to the DCP sample introduction adapter approximately 50 cm away. The detection limit for manganese was measured, but interelement effects, matrix effects, and real sample applications were not examined or discussed.

A study of the use of ETV for introduction of milligram masses and microliter volumes into a DCP emission source was presented by Mitchell and Sneddon (1987) and Zimnik and Sneddon (1987). Use of the ETV allows in situ pretreatment of a sample before its introduction into the DCP. Sample introduction was achieved by using an Allied Instrumentation Laboratories Model 455 temperature-controlled furnace with modifications. A short (20 cm) Pyrex or Tygon tube connects the left side of the ETV to the chimney of the DCP. The required volume or mass of sample was placed in a microboat, which was then inserted into the ETV cuvette. Detection limits for various metals were measured and the system was shown to give accurate results for complex biological, nutritional, water, and geological samples.


The instrument reported by Buckley and Boss (1990) uses an electrically heated (with a microprocessor control) tungsten wire (0.1 mm) to vaporize the sample (1 laL) and a DCP to cause emission. The filament apparatus and housing were designed so that the sample could be placed on the filament easily with little disturbance to the plasma. The filament housing has a small cell volume (3 mL). The effectiveness of this interface was tested by determining detection limits for aluminum, calcium, copper, and iron.

Slinkman and Sacks (1990) have described the application of a rotating magnetron DCP with graphite furnace sample introduction to the determination of selected trace metals in microvolume aqueous solution samples. A commercial graphite tube furnace (Instrumentation Laboratory Model 555) was modified and interfaced to the rotating DCP. A 7.5-cm long ceramic tube was placed in one end of the graphite furnace, and the other end of the ceramic tube was connected to the arc assembly by a 16-cm long Tygon tube. Sample vapor from a graphite tube furnace is introduced into the arc plasma by passing the vapor through the anode tube. This ensures adequate sample-plasma interaction and results in detection limits generally in the parts per billion range. In the subsequent papers (Slinkman and Sacks, 199 la,b) they investigated the effect of introducing easily ionized elements (EIE) into a sample using this same ETV-DCP system. The presence of an EIE reportedly does not significantly alter the sample-plasma interaction; however, it has been shown that the addition of an EIE to the rotating magnetron DCP with the graphite furnace sample introduction causes an increase in both the excitation temperature of the analyte species and the electron density of the plasma.

A modified tungsten-spiral electrothermal vaporizer for sample introduction into a dc arc discharge has been designed and tested (Mei et al., 1992a). Its operation was exactly the same as that described by Mei et al. (1992b), for use with an ICP.

A graphite furnace and dc arc-combined source with a halogenating atmosphere in the furnace was described by Kantor and Zaray (1992). The design of the horizontal graphite tube furnace incorporates the construction principles of several commercial systems manufactured for use in AAS and makes possible the introduction and analysis of both liquid and solid samples with minimal risk of contamination.

F. Capacitively Coupled Microwave Plasma

In the capacitively coupled microwave plasma (CCMP), a magnetron generates microwaves that are conducted through a coaxial waveguide to the tip of a central single electrode, where a flamelike plasma is formed. CCMPs have also been used as spectrochemical sources for excitation of atomic and polyatomic species, primarily because of their high degree of excitation, their relatively low cost, and their simplicity of operation. Most previous research has been focused on the analysis of liquid samples; hence, very few studies have involved electrothermal sample introduction methods.


Introduction of solid samples into a single electrode atmospheric pressure CCMP has been accomplished using an electrically heated furnace vaporizer (Hanamura et al., 1983, 1985a,b) originally described by Hanamura (1976). The sample is held in a quartz crucible; as the sample is heated, the volatile constituents are swept by the carrier gas flowing past the indented top of the quartz furnace tube. Solid samples of 250 mg or more were used, to avoid problems with sample heterogeneity. A method for the determination of traces of oxygen and hydrogen in metals has been developed (Hanamura et al., 1985b). The metal sample (200-500 mg) is placed in a quartz crucible (a Leco Model 521-100 induction furnace), which is heated by an induction furnace under the gas at a pressure of 400 torr. The mixture of extracted gas and helium is carried into the plasma and the atomic emission line intensities of oxygen and hydrogen are sequentially measured. The method is applied to the determination of oxygen and hydrogen in titanium.

Liang and Blades (1988) described a new atmospheric pressure, capacitively coupled radio frequency plasma discharge (CCP) that was developed for atomic absorption and emission analyses of small, discrete sample volumes (1-10 ktL). Sample introduction into the plasma is accomplished by using an electrically heated tantalum strip vaporizer (Varian Model CRA-61). The plasma discharge tube and sample introduction device allows for the separate control of the vaporization and atomization environments.

IV. PRECONCENTRATION TECHNIQUES

There are numerous instances when the analyte to be determined is present in the test sample at a concentration below the capability of the available analytical techniques. Preconcentration techniques, which separate trace metals from interfering matrix components, can improve detection limits, enhance accuracy, ease calibration, and also effect more representative results owing to the increased sample size.

A. Electrodeposition

It has become quite common to apply electrochemical deposition as a preconcentration and separation method for instrumental determinations of metals in complex matrices such as biological fluids, waters, food and mineral digests, and in general, in many samples in which there is a high content of interfering matrix elements. Electrolytic preconcentration requires a single, controlled-potential electrolysis to concentrate traces of reducible metals as a deposit on an inert electrode or amalgam in mercury, leaving electrochemically inactive (i.e., complex matrices) interfering elements in solution. This approach is frequently used for improving detection limits in many instrumental methods of analysis and should be well suited for the determination of trace and ultratrace elements that are soluble in mercury.


Also, the method should provide a good simultaneous separation-preconcentration step prior to ETV-plasma source emission spectrometric analysis.

The application of controlled-potential electrolysis for the determination of mercury by deposition on copper, or by amalgamation with silver or gold in a column, followed by vaporization into an argon MIP by heating of the carder metal, has been described (Kaiser et al., 1975). The procedure was optimized using 2~ allowing an absolute detection limit of 0.1 ng in aqueous solution, air, and organic and inorganic matrices to be attained. Excluding digestion steps, one determination requires about 15 min.

Volland et al. (1981) reported a further extension to the work of Kaiser et al. (1975). Electrolytic preconcentration of trace elements in the nanogram and picogram range in a graphite tube (Kaiser et al., 1975) extended MIP capabilities. A multistage combined procedure is described for the sensitive and reliable determination of trace elements in high-purity metals. Electrolytically active elements such as the noble metals, bismuth, cadmium, cobalt, copper, iron, zinc, and others are preconcentrated from acidic solutions at concentrations >0.05 ng/mL. The electro- lyte is cycled through a small cylindrical cathode of pure graphite on the inner wall of which the elements are deposited. The graphite tube is coupled directly to the quartz capillary of a helium MIP. After electrothermal vaporization, the trace elements are determined by emission spectrometry.

The application of controlled-potential electrolysis for the determination of trace metals in biological standard reference materials and water using graphite electrodes that were both previously coated with mercury (Matusiewicz et al., 1987; Malinski et al., 1988; Fish et al., 1988) and a hanging mercury drop electrode (Matusiewicz et al., 1987) as a separation and preconcentration technique for ICP with ETV has been described. The metals are separated from the matrix by electrolysis on a hanging mercury drop electrode (Matusiewicz et al., 1987); the mercury is transferred into a crater-preformed graphite electrode (Matusiewicz, 1985) and removed by evaporation, and the metals are simultaneously determined by ICE Many elements can be determined from a single electrochemical deposition. The method has been applied to the determination of eight trace metals: cadmium, cobalt, copper, manganese, nickel, lead, antimony, and zinc.

The production and use of mercury film electrodes for matrix separation and preconcentration of trace metals from biological materials and water prior to their determination by ICP are described (Matusiewicz et al., 1987; Malinski et al., 1988; Fish et al., 1988). Separation and preconcentration are achieved by controlled- potential electrolysis on mercury-plated, glassy carbon electrodes. The electrodes with trace metal amalgam are transferred to an ETV device (PE HGA-500 graphite furnace modified by Matusiewicz et al., 1986), the mercury is removed by evaporation, and the metals are then vaporized and determined simultaneously by ICE The method was applied to the determination of eight trace elements: cadmium, cobalt, chromium, copper, manganese, nickel, lead, and zinc in water solution and in the biological standard reference materials, urine and bovine liver.


B. In Situ Preconcentration

A second technique, similar in nature to the above, is sorption of metallic hydrides in a graphite furnace. This methodology offers substantial advantages over conventional furnace or hydride generation techniques, including simplicity of operation, use of small sample volumes, high sensitivity, and a considerable increase in the relative detection power as a result of the in situ preconcentration afforded by this approach. In addition, this technique further enhances the ability of the sample introduction method to remove interfering species prior to introduction of the analyte into the plasma. This technique should also provide a useful simultaneous separation-preconcentration step prior to ETV-plasma source spectrometric analysis.

Evaluation of a novel configuration for ETV-MIP was reported by Matusiewicz et al. (1990b, 1991). Use of a PE HGA-2200 graphite furnace (Matusiewicz et al., 1990b) or a Carl Zeiss Jena EA3 graphite furnace (Matusiewicz and Kurzawa, 1991) in combination with hydride generation and in situ concentration (Ma- tusiewicz et al., 1990b) of arsine and hydrogen selenide (Matusiewicz and Kur- zawa, 1991), generated from aqueous samples, permitted vaporization of the sequestered arsenic and selenium into a helium MIP for excitation. The interface between the graphite furnace and the MIP was based on the system described by Aziz et al. (1982a). The hydride generation system used for in situ arsine concentration has been described previously (Sturgeon et al., 1986). This study suggests the feasibility of multielement analyses of other hydride-forming elements at extreme trace levels by taking advantage of in situ preconcentration in the furnace.

Evaluation of a novel configuration of an aerosol transport interface for an ETV in conjunction with an MIP was recently reported by Matusiewicz et al. (1990a). The aerosol transport line and interface between a graphite furnace and an MIP used previously for in situ analyte hydride deposition was modified (Matusiewicz et al., 1990b). A novel single valve replaced two valves required in the previous setup.

Bulska et al. (1993a, b,c) reported a further extension to the work of Matusiewicz et al. (1990b). For the determination of arsenic, antimony, and selenium, analyte introduction was accomplished with hydride generation, followed by hot-trapping in a graphite tube furnace (Bulska et al. 1993b), or with in situ preconcentration on reticulated vitreous carbon (RVC) (Bulska et al., 1993a), followed by ETV of the analytes and their detection with MIP. A comparison between the results of conventional graphite tubes and those filled with RVC shows that the use of the latter results in enhanced powers of detection and precision. Suitable conditions for multielement determination with MIP is possible due to the multielement capabilities of each step (hydride generation, in situ trapping in the graphite furnace, and determination by MIP). Detection limits for arsenic, antimony, and selenium were measured and the method has been applied satisfactorily to the determination of the above elements at the nanogram per milliliter level in various types of fresh water samples.


C. Other Preconcentration Techniques

Although ETV as a sample introduction technique for atomic emission spectrometry is generally used as a stand-alone technique, it has been combined with other methods to acquire additional analyte enrichment. These coupled techniques can enhance analyses in ways not possible by ETV-emission spectrometry alone.

One example of this is the coupling of high-performance liquid chromatography (HPLC) for preconcentration of the analyte to the ETV, as described by Nisamanee- pong and Caruso (1985). Electrothermal carbon cup vaporization (Ng and Caruso, 1982) is described for preconcentrating and vaporizing collected effluent fractions from the HPLC. This study demonstrates the applicability of this preconcentration technique by monitoring ICP emission signals from tetraphenyllead and hexaphenyldilead. The preconcentration (enrichment) factor is limited only by the length of time used to deliver samples into the desolvation-vaporization carbon cup.

The application of a graphite rod ETV-ICP arrangement for the determination of arsenic, chromium, copper, nickel, and selenium in a urine standard reference material (Barnes and Fodor, 1983) and of copper in human and animal bone (Mahanti and Barnes, 1983b) was described. Sought metals are preconcentrated and separated from matrices by means of a poly(dithiocarbamate) (PDTC) resin prior to ICP analysis. This method was examined by employing a solution of the mineralized resin as the sample. The resin-ETV-ICP technique provides the preconcentration and separation benefits and thus the measurement becomes essentially independent of the original sample matrix.

The aim of the study presented by Van Berkel and Maessen (1988) and Van Berkel et al. (1990) was to explore the analytical performance of ETV-ICP when used in combination with PDTC for the enrichment of seawater and biological materials. However, in their study, the analyte-loaded resin was directly introduced into the ETV device. Special attention was paid to the matrix effects which occur when analytes and pyrolysis products of the resin enter the plasma simultaneously. The bulk of the resin matrix can be separated from the analytes by submitting the furnace to a suitable temperature program. The experimental conditions established permit the analysis of up to 20 mg of resin. In routine operation, limits of detection for the technique are at the picogram per milliliter level.

Heltai et al. (1990a) have described the incorporation of separation of the alkali elements by sorption of the trace elements as hexamethylenedithiocarbamates (HMDC) on acetylated cellulose, with MIP coupled to ETV for multielement determination of copper, iron, and zinc in biological samples, subsequent to wet chemical sample decomposition.


V. SPECIATION

One of the more intriguing areas of application of atomic spectrometry at present is the possibility of deriving information regarding the various forms in which elements are chemically present in a sample. "Speciation" refers to the ability to discriminate between various forms in a mixture containing diverse chemical forms of the same element, or indeed several elements. A detailed consideration of the many techniques currently applied to speciation problems (i.e., GC, LC, and so forth) that can potentially be coupled to atomic emission spectrometers is beyond the scope of this review. However, in the simplest consideration, many chemical compounds may have different vaporization temperatures; by gradually raising the temperature of the ETV system, speciation of the analyte compounds becomes possible. In other words, with temperature programming of the furnace heating rate, it should be possible to distinguish different chemical forms of the trace components of a sample, on the basis of their characteristic vaporization temperatures. Controlled ETV coupled with the plasma source has the potential to extend the capabilities of emission spectrometric analysis. There is a current trend in environmental and other trace metal analyses to obtain speciation information rather than just total (bulk) element concentrations in a given sample.

The approach which shows considerable promise for speciation of metals and molecular compounds (inorganic and metalloorganic) in solid biological and environmental materials is evolved gas analysis-microwave-induced plasma emission detection.

Evolved gas analysis involves vaporization of the molecular species as a function of temperature. This technique was proposed by Mitchell et al. (1977). Their approach involved a furnace-microwave plasma system to determine organic and inorganic carbon by differential vaporization. Water or suspended particulate samples are dispensed into platinum boats and dried at 85 ~ for 10 min. Oxidant is added and the boats are inserted into a furnace at 850 ~ at which temperature organic carbon vaporizes several seconds before inorganic carbon. The resulting vapor is fed to an argon MIP, and carbon emission at 193.0 nm is measured. Quantitation was found to be possible. Bauer and Natusch (198 la,b) subsequently developed an evolved gas analysis-microwave emission spectrometer for identifying trace inorganic compounds in solid samples. In this work the helium MIP was used to monitor the evolution of vapors as a function of temperature in order to gain information on chemical form in addition to chemical composition. In the first paper (Bauer and Natusch, 1981 b) they identified and quantitatively determined alkali, alkaline earth, and ferrous carbonates in several coal fly ashes. In a later paper (Bauer and Natusch, 198 la), qualitative and quantitative analyses were attempted to monitor metal and nonmetal components, and they investigated the chemical forms of elements in solid samples. Both the cation and anion components of a compound could be monitored to make identification more certain. In both cases (Bauer and Natusch, 1981a,b), the solid samples were heated from 25 to 1000 ~


at 140 ~ in a modified Leco induction furnace, and the molecular components were vaporized into a low-power atmospheric pressure MIP. The transfer line from furnace to plasma must be kept as short as possible; it was heated to 650 ~ to minimize condensation along its length. Failure to do this can result in spurious peaks, peak-broadening, and memory effects. Neither system (Mitchell et al., 1977; Bauer and Natusch, 1981a,b) permitted the vaporization of samples of sufficient size, that is, >250 mg, to reduce problems due to sample heterogeneity. The authors suggested using this approach with the ICP because of greater freedom from chemical interference with this device.

Hanamura (1976) and Hanamura et al. (1983) have only scratched the surface of the usefulness of the evolved gas analysis-microwave plasma emission detection system. By means of ETV, inorganic, organic and metallo-organic species are separated and elemental emission in a microwave plasma is detected as a function of vaporization temperature.

By coupling ETV to a chromatography technique such as HPLC, additional speciation information is obtained (Nisamaneepong et al., 1985). In this report, the ETVoHPLC interface is similar to that utilized with GFAAS. Using the HPLC-ETV- ICP instrument the authors were able to speciate a pair of lead compounds (tetraphenyllead and hexaphenyldilead) with detection limits that were nearly four orders of magnitude better than those obtained using HPLC-ICP (nebulization sample introduction).

In order to evaluate the potential of the MIP technique for speciation studies, a preliminary investigation with some sulfur-containing compounds was made (Ma- tousek et al., 1986b). The approach adopted employs the helium MIP as an evolved-gas detector which measures the emission signal of the vaporized analyte as a function of temperature. The use of the ETV allows in situ pretreatment of a sample (e.g., solid algal cells) before its introduction into the DCP, and use of different combinations of ashing and vaporization conditions may allow differentiation between different species of more volatile elements (e.g., various mercury compounds) (Mitchell et al. 1986).

A study of the determination of inorganic and organic sulfur in aqueous solutions using the helium MIP was presented by Alvarado et al. (1992). By careful control of the atomization temperature of the ETV, multicomponent thermal analysis was accomplished. The separation and analysis of organic and inorganic sulfur-containing compounds from binary mixtures was possible without adversely affecting analytical performance.

Very recently, Richner and Wunderli (1993) described use of ICP-MS as a screening test for chlorinated organic compounds such as polychlorinated biphenyls (PCBs) in oils. ETV-ICP-MS served as an example of the advantages that tandem source MS can offer. The differentiation between organic and inorganic chlorine in waste oils is only possible because of the additional control over the vaporization process gained through the graphite furnace. Electrothermal vaporization ICP-MS achieves this using a temperature programme having one step at


400 ~ for the vaporization of the PCBs, and a second at 2650 ~ for the vaporization of the inorganic chlorine compounds.

Although ETV sample introduction alone provides, at best, a crude means of speciation through volatility programming, if electrothermal furnace-rod-cup interfaces could be designed and developed that were truly continuous and on-line, with real-time analyte determinations, then such preconcentration and speciation methods might readily provide outstanding detection limits and make practical applications possible.

VI. SUMMARY OF INSTRUMENTATION

Tables 1-3 summarize instrumentation and methodology reported to date that are used in ETV-plasma sources for atomic emission spectrometry. Several plasma source--emission spectrometric systems are also summarized.

VIi. COMMERCIAL AVAILABILITY OF ELECTROTHERMAL VAPORIZATION PLASMA SOURCE EMISSION

SPECTROMETRIC SYSTEMS

In order of market preference and associated research activity, two excitation sources, the ICP and DCP, have received commercial acceptance, whereas the MIP has been relegated to the research laboratory. However, the MIP has held and still does hold a prominent position of interest within the analytical atomic analysis research community, and probably offers great opportunity for configurational development. The technique, described by Dahlquist (1974), has been applied with a commercial spectrometer, Plasma-Spec Spectrometer, Leeman Labs. Inc. (Instrument Column, 1982). The operation of this equipment is based on the electrothermal drying of small liquid sample aliquots on a graphite filament prior to analyte vaporization and excitation in an ICP. Presently, this system does not exist and is not produced.

It is clear that an electrothermal vaporizer can be easily incorporated as an accessory into an existing ICP system, as has been suggested (Matusiewicz and Barnes, 1984a,b, 1985c), and indeed commercial accessories are available. An Allied Analytical Systems EVA (electrothermal vaporization accessory) system, a graphite furnace-aerosol deposition system combined with the ICP9000, was presented at the 1985 Pittsburgh Conference (Hull et al., 1985a,b). The design adopted suggestions made in the literature (Matusiewicz and Barnes, 1984a,b, 1985c), although the manufacturer used graphite tubes instead of platforms or contoured tubes for sample vaporization, and long tubing (ca. 80 cm) connecting the IL graphite furnace with the ICP torch, which can cause transport losses. The furnace could have been mounted in the ICP box below the quartz torch so that only a few centimeters of tubing would be required to connect them, as has been shown previously (Blakemore et al., 1984).

Table 1. Operating Parameters for ETV-ICP/ICP-MS Studies

ICP/ICP-MS System

A. ICP/ICP-MS Instrument and Operating Parameters

Observation Height Power (kW) (mm) Electrothermal Atomizer Device or Electrode Power Supply

ko

Ebert mount, 0.5 m Jarrell-Ash 82000

Hilger monospek 1000

Jarrell-Ash 975

Jarrell-Ash 965 Plasma Atom Comp

Hilger Monospek 1000

Spex Industries 0.85 m 1402

Spex Industries 1.26 m 1269

Jarrel-Ash 955 Plasma Atom Comp

0.9 m Czerny-Turner monochromator

1 m Czerny-Turner monochromator


McPherson 0.35 m Czerny-Turner system and photodiode array spectrometer




ARL 34000 Quantometer

Kontron Plasmaspec ASS-80

Spectrometer SPS- 1 1 0 0

ARL 34000

Kyoto Koken UOP- 1S

Jarrell-Ash 975 Atom Comp


1 18 General Electric transformer

1 B Shandon Southern A3370

1.1 16 Condenser

1 20 Varian-Techtron CRA-90

1 20 Shandon Southern A3370

1 18 Varian-Techtron CRA-63

1.2 14 Perkin-Elmer HGA-500

1 10-20 Perkin-Elmer HGA-2000

3 - - Perkin-Elmer HGA-74

0.35-1 13.5-18.5 Varian-Techtron CRA-90

0.55 16 Varian-Techtron CRA-90

1.5 ~ Varian-Techtron CRA-63

0.55 16 Instrumentation Laboratory IL655


1 6 - 1 0 Perkin-Elmer HGA-500

1.3 B Perkin-Elmer HGA-2100


1.1 18 Condenser

1.3 18 Varian-Techtron CRA-90

1.5 8.5-14 Nippon Jarrell-Ash FLA-100


3.0 - - Condenser

(continued)


ICP/ICP-MS System

A. ICP/ICP-MS Instrument and Operating Parameters

Observation Height Power (kW) ( r a m ) Electrothermal Atomizer Device or Electrode Power Supply

Hitachi P-5100

Labtam 2000

Kyoto Koken UOP- 1S

Nippon Jarrell-Ash P-575

Hitachi M-306

Jan'ell-Ash 90750

Monochromator WDG500-1A

Spectrograph PGS 2.2 m, Carl Zeiss Jena

Labtam 8440 Plasmalab

Jarrell-Ash 90750

SPQ-6100S, Seiko Instrum. Inc.

Plasma-Therm HFP-2500D

VG PlasmaQuad type I

VG Elemental PQ2 Plus quadrupole

Prototype ICP-MS system

Elan 250, Sciex 1

VG PlasmaQuad

VG PlasmaQuad

1.0 15 Hitachi GA-3

1.6 - - Varian-Techtron CRA-63

1.5 8.5-14 Seiko I SAS-705 V

1.2 12.5 Nippon Jarrell-Ash FAP- 1

1.3 20 Hitachi GA-3


1 15 WF-4 (Similar to PE HGA-500)

2 - - Transformer

1.2 12 Transformer

2 15 Perkin-Elmer HGA-2200

1.2 - - Seiko Micro Sampling System

1.25 5-19 Griin Analytische Mess-Systeme GmbH SM-30

1.35 - - Microheater, laboratory-built

1.3 - - Microtherm ETV

1.2 m Shandon Southern Instruments A3470

w m Instrumentation Laboratory IL555 power supply

1.1 ~ Perkin-Elmer HGA-300

1 ~ Perkin-Elmer HGA-300

(continued)


Type of Electrode

B. Electrothermal Vaporization Sample Introduction System

Vaporization Cell Sample Size

Volume (mL) Temp (~ Time(s) mL

Carrier Sample Argon Flow Transport Tube

mg Rate (L/min) Length (cm) Reference

. . . a

o . . . . a

Tantalum filament 120 1800 ~ 1-200

Tungsten ~ 2300 3 1-10

Platinum filament 4.5 1400 3 10

Tantalum ~ ~ 3.5 5

Graphite rod 1000 2400 1.5 10

Carbon cup 280 ~ 1.5 10

L'vov platform ~ 2200-2900 4 20

Graphite tube m 2400 5-10 10

Graphite tube ~ 2400 6 50

Graphite rod 30 1600-2500 1.5 5

Graphite 7 2500 1 5

Carbon rod 50 ~ ~ 15-20

Graphite platform or microboat 0.8 2500 5 5

Carbon rod and microboat ~ 2700 7 5-10

Graphite rod or platform 1 2800 4 3-5

Graphite tube and tantalum- ~ 2800 8 coated platform

Graphite crucible ~ 2600 10 10

Tungsten wire 1 1500 0.2 10

Graphite cup ~ 1900 5 10

Graphite tube ~ 2700-3000 5 20

Quartz boat ~ 2600 12

Tungsten coil ~ 3000 ~ 50

h 1.2

0.5

1.0 m

0.8 5

0.8 50

0 .4 18

1.2 90

1 -2 .5 20

4.5 30

0.8-1.6 100

m 0.6 55

2-3 0.8-1.0 30

0.5 45

0.5 0.5 12

1.65 2

0 .4

5-10 1.0

0.6 40

0.5 150

0.5 30

20 0.3 60

1.0

Nixon et al. (1974)

Smythe (1980)

Kitazume (1983)

Tikkanen and Niemczyk (1984)

Gunn et al. (1978)

Ng and Caruso (1982)

Crabi et al. (1982)

Swaidan and Christian (1983)

Aziz et al. (1982a)

Barnes and Fodor (1983)

Matusiewicz and Barnes (1984b)

Hull and Horlick (1984)

Matusiewicz and Barnes (1985c)

Blakemore et al. (1984)

Matusiewicz et al. (1986)

Nimjee et al. (1984)

Ohls and Hiitsch (1986)

Kawaguchi et al. (1986)

Alvarado et al. (1987)

Kumamaru et al. (1987)

Van Berkel and Maessen (1988)

Dittrich et al. (1988)

(continued)

Table 1. ( c o n t i n u e d )

Type of Electrode

B. Electrothermal Vaporization Sample bztroduction System

Vaporization Cell Sample Size Carrier Sample Argon Flow Transport Tube

Volume (mL) Temp (~ Time(s) mL mg Rate (L/min) Length (cm) Reference

o

Graphite tube

Graphite tube

Tungsten boat

Graphite boat

Graphite cup

Brass electrode

Graphite tube

Tungsten coil

Graphite boat

Graphite tube

Tungsten ribbon

Graphite boat-in-tube

N i -Cr filament

Graphite tube

Graphite rod

Graphite platform or Re filament

Tungsten wire loop

L'vov platform

3000 10 20 - - 0.3 14

ca. 1700 3 2 - - 1.6

2500 5 20 - - 0.32 m

m 1 0 4 0 ~ c a . 0.8 50

2500 10 ~ 10(O1500 0.3

1250 5 - - 250 0.2-1.0 45

2500 6 20 - - 0.5 50

1 - - 10 10 ~ 1.0 15

2 1 0 0 30 ~ 1 0 1 . 0 60

2400 7 5-10 ~ 1.0 22

ca. 300 ca. 2500 3 20 ~ ca. 1.0 75

ca. 2300 12 5--40 0.5-2 0.18-0.95

7 0 m 2 0 - - 0 . 8 m

2800 6 ~ D ca. 0.8

- - ~ 5 - - 0 . 8

5 1600-1800 ~ 5 ~ 1.5-1.8 50

1800 5 3 ~ 0.1

2400 10 10 ~ 0.11

Ida et al. (1989)

Matousek et al. (1989b)

Okamoto et al. (1990)

Isoyama et al. (1990)

Atsuya et al. (1991)

Karanassios et al. ( 1991)

Huang et al. (1991)

Mei et al. (1992b)

Zaray et al. (1992)

Ren and Salin (1993)

Tsukahara and Kubota (1990)

Verrept et al. (1993)

Hirata et al. (1989)

Hulmston and Hutton ( 1991)

Date and Cheung (1987)

Park et al. (1987b)

Shen et al. (1990)

Carey et al. (1991)

Table 2. Operating Parameters for ETV-MIP/MIP-MS Studies

MIP/MIP-MS System

A. MIP/MIP-MS hzstrument amt Operating Parameters

Discharge Tube Magnetron* Cavity i.d./o.d. (ram)

Electrothermal Vaporizer Device or Electrode Power Supply

_ _ . . x

o kao

0.5 m Ebert Jarrell-Ash 82000 monochromator

Unicam SP9000 spectrometer

0.5 m Ebert Nippon Jarrell-Ash50

Rank Precision Ind., D330 monochromator

0.3 m Czerny-Turner McPherson Instr. Corp., Model 218 monochromator

1 m Shimadzu GE 100 monochromator

0.5 m Ebert Jarrell-Ash 82500

0.5 m Ebert Jarrell-Ash 82500

0.5 m Ebert Nippon Jarrell-Ash

Medium Littrow-type Bausch and Lomb spectrograph

Hilger Monospek 1000 monochromator

1 m Czerny-Turner Monospek D500, MK3 Hilger Analytical monochromator

0.3 m Czerny-Turner D330 Hilger Ana- lytical monochromator

0.5 scanning monochromator or 1 m McPherson Model 216 monochromator

0.35 GCA McPherson Model EU-700 monochromator

25/ Evenson NBS type 5 quartz, 1/3

40/ Evenson, 1/4 wave quartz, 2/

TEoI 3 tapered rectangular quartz, 1.6/4.2

ca. 40/ Evenson, 1/4 wave quartz, 2/

30/ Evenson, 1/4 wave quartz, 3.2/4.3

70-100/ Evenson, NBS type 1 quartz, 1.6/4.2

45/ Evenson, No. 5 quartz, 1.3/

75/<1 TM010 quartz, 2/7

75/<10 TM010 quartz, 3/6

50-100/2-8 TM010 quartz, 4/6 alumina, 1-3/6

75/14 TM010 silica, 2.5/5.0

100/ TM010 silica, 2.7/7.0

130/17 TM010 fused-silica 2/6

70-90/ McCarroll quartz,/12

100/<2 McCarroll N

Power source, 6A, 2V

power source, 1.9A, 4.5 V

dc source, Hamamatsu TV

dc supply, 4.5 V

condenser dc current, 3.5 A, 7.5 V, 0.216 F

dc source, 3 A, condenser, 220 000 mF

laboratory-constructed

Perkin-Elmer HGA-72, tantalum furnace

Daini Seiko-SAS-714

laboratory built, 100 A, 208 VAC

dc current, 21 A, Lambda LES-F-02-OV-VI

Varian-Techtron CRA-63, 14 A

Shandon Southern Instrum. A3370 atomizer

Perkin-Elmer HGA-2100

Varian-Techtron CRA-90

(continued)


MIP/MIP-MS System

A. MIP/MIP-MS Instrument and Operating Parameters

Discharge Tube Magnetron* Cavity i.d./o.d. (mm)

Electrothermal Vaporizer Device or Electrode Power Supply

_ . a

o

0.5 m Ebert Jarrell-Ash


Czerny-Turner D330 Rank Hilger monochromator


0.9 m Czemy-Turner monochromator

0.5 m Spex Model 1870 spectrometer

Varian-Techtron AA5 spectrometer in the emission mode

Varian-Techtron AA5 spectrometer in the emission mode

0.2 m Jobin Yvon DH20A monochromator

Optica Model CF 2768 monochromator

Spectrametrics SMI III echelle grating monochromator

Carl Zeiss Jena AAS3 spectrometer in the emission mode

0.75 m echelle Spectrospan III spectrometer, Spectrametrics

0.75 Ebert monochromator

0.5 m Ebert Jarrell-Ash monochromator

ICP-MS, VG PlasmaQuad, VG Elemental

0.5 m Czerny-Turner Spex Ind. monochromator 70-100/

70/<1 TM010 quartz, 1.3/

45/ Evenson, No. 5 TM010 quartz, 1.3/quartz

10-90/ Evenson, 1/4 wave quartz, 4/6

40/ TM010 quartz, 1/6

50/ TM010 quartz, 1/6

70/ TM010 silica

TM010 or Broida EMS, 3/4 silica, 2/4 wave

50-100/<1 TM010 silica, 3/

80/<2 Evenson, 1/4 wave quartz, 6/8

130/0.4 TM010 quartz, 1.7/6.4

200/15-20 TM010 boron nitride, 3.0/6.3

200/10 TM010 boron nitride, 3.0/6.3

350/ TM010 silica

50/ TM010 Surfatron ca. 110/ca. 2.3

50-120/1-8 TM010 or Surfatron

50-70/ TM010

m T M 0 1 0

quartz, 1/5 quartz, 4/6

quartz, 4/6 quartz, 1/2

quartz


Varian-Techtron CRA-63 dc source

dc source

Perkin-Elmer HGA-74

Perkin-Elmer HGA-74

Vaxian-Techtron CRA-63


Vaxian-Techtron CRA-63


Shandon Southern A3370 atomizer


C.Z. Jena EA3 graphite furnace


power source, 250 W, 24 V




(continued)


Volume Type of Electrode (mL)


Vaporization Cell Sample Size Carrier Gas Flow Rate

Temp (~ lime (s) mL mg (L/rain)

Sample Transport

Tube Length (cm) Reference

Platinum filament

Platinum or tungsten loop

Tungsten loop filament

Platinum loop

Tantalum filament

Tungsten filament

Tantalum strip ca. 120

Tantalum strip

Tungsten boat

Tungsten boat

Tungsten rod

Tantalum loop

Tantalum boat ca. 30

Metal furnace or tantalum boat

Tantalum boat

Tantalum boat ca. 120

Carbon cup ca. 120

Graphite cord

Carbon rod

Graphite tube

- - 1 0 ~ A r , 0 . 1

~ ca. 0.12 ~ A t , 1 . 8 5

800-1200 - - 2 ~ Ar, 0.3

ca. 1200 ~ ca. O. 1 ~ Ar, 1.8

1800-2000 2 5 D He, 0.48

1600 ~ 2 u Ar, 0.4

ca. 1800 1 5 - - Ar, 0.9

ca. 1600 - - 20 - - He, 0.2

2100 5 10 ~ He, 0.5

~ 1 0 30 He, 0.6

- - 1 0 D He, 1.1

ca. 1800 1.5 5 ~ He, 1.1

3 15 ~ He, 0.25

1 1 0 0 20 20 - - Ar, 0.8

ca. 2200 ~ 5 - -

D ~ 1 0 ~ He, 0.4

ca. 2000 D 5 ~ Ar, 0.9

~ 1 - 3 m He

2400-2800 ~ 5 - - Ar, 0.28-0.52

2400 6 50 ~ Ar, 4.5

ca. 10

ca. 10

ca. 10

ca. 4

ca. 15

ca. 18

4 m

m

m

m

Runnels and Gibson (1967)

Aldous et al. (1971)


Dagnall et al. (1973)

Kawaguchi and Vallee (1975)

Sakamoto et al. (1976)

Fricke et al. (1976)

Van Dalen et al. (1982)

Chiba et al. (1984)

Rait et al. (1984)

Brooks and Timmins (1985)

Stahl et al. (1989)

Barnett and Kirkbright (1986)

Capelle and Sutton (1977, 1978), Melzer et al. (1980)

Na and Niemczyk (1982)

Carnahan and Caruso (1982)


Beenakker et al. (1980)

Alder and Da Cunha (1980)

Aziz et al. (1982a) (continued)


Type of Electrode Volume (mL)



Temp (~ Time (s) mL mg (lJmin)

Sample Transport

Tube Length (cm) Reference

Graphite tube

Graphite cup

Graphite tube

Graphite tube

Graphite cup

Graphite rod

Graphite tube

Graphite tube

Carbon cup

Tungsten coil

Graphite tube or filled with reticulated vitreous carbon

ca. 15

ca. 10

ca. I0

750

ca. 1

ca. 1

12.5

85-940 30 - - ca. 2 Ar, 4.5

ca. 2000 1.5 5 - - He, 0.4

1800-2000 2-2.5 1-2 - - Ar, 0.8-3.9

2000 2.5 1-2 - - He, 1.7-2.1

- - 7 10 - - N2, 8.6 cm3/s

1900 3 10 - - He, 0.81

2600 4 hydride - - He, 4.0 generation

2600 5 hydride - - He, 3.0 generation

- - ca. 2.5 1 5 - - He, 0.8

<2000 - - 20 - - Ar, 0.46 Ar, 0.38

- - 2 5 - - Ar, 0.5

Tantalum tip - - 970-

Tantalum tip - - 970-1470

1-2

hydride generation

cold trapping

3

3

Ar,0.5; He, 0.3

Ar,0.5

He, 0.1--0.3

He, 0.1-0.175

6

ca. 15

5O

5O

20

Broekaert and Leis (1985)

Zerezghi et al. (1983)

Matousek et al. (1984a)


McCaffrey and Michel (1983)

Abdillahi and Snook (1986)

Matusiewicz et al. (1990b)

Matusiewicz and Kurzawa (1991 )

Wu and Camahan (1990)

Richts et al. (1991)

Bulska et al. (1993a, b)

Evans et al. (1991)

Note: *Microwave frequency 2450 MHz; forward/reflected power, W

Table 3. Operating Parameters for ETV-DCP/CCMP Studies A. DCP/CCMP hzstrument and Operating Parameters

DCP/CCMP System Electrothermal Atomizer Device or Electrode Power Supply

_ _ a

o ,,,4

Spectrajet SmithKline Beckman 0.75 m Czerny-Tumer monochromator

Beckman SpectraSpan V

Beckman SpectraSpan IV

Magnetron rotating

Carl Zeiss Jena arc power supply Model UB-2, spectrograph PGS-2


Instrumentation Laboratory IL655

Instrumentation Laboratory ILA55

Filament apparatus, laboratory-built

Instrumentation Laboratory IL555

Halonarc, laboratory-built


Type of Electrode



Volume (mL) Temp (~ Time (s) mL mg (L/min)

Sample Transport Tube

Length (cm) Reference

Graphite microboat

Tungsten filament

Graphite tube

Graphite boat

Tantalum strip

0.8 2500 5 5 w Ar, ca. 2.5

2500 5 20 3 Ar, 1.0

ca. 2200 0.25 1 - - Ar, 3.5

ca. 2300 5 ca. 10 m Ar or He, 1.0

1800 60 - - ca. 35 ca. 0.05

2700 2 2-5 w Ar, 0.6

5O

20

16

Elliott et al. (1986)

Zimnik and Sneddon (1987)

Buckley and Boss (1990)

Slinkman and Sacks (1990)

Kantor and Zaray (1992)

Liang and Blades (1988)


A versatile accessory for ICPs, an electrothermal vaporizer specifically designed for use with ICPs and ICP-MS, has been announced by PSA Analytical Ltd., Orpington, Kent, England. The design was based on suggestions made by Gunn et al. (1978) and Millard et al. (1980). This microprocessor-controlled accessory offers a flexible and versatile system for linking the advantages of an ETV system with ICP and ICP-MS systems. The interface to the ICP system has been specifically designed to provide good and reliable mass transfer characteristics. The PSA atomizer design was practically evaluated as an evaporation unit for ICP by Krasilshchik (1992).

Recently, the availability of a Model ETV 100, developed under the direction of Golloch (University of Duisburg), has been announced by Fischer Labor- und Verfahrenstechnik GmbH, Meckenheim bei Bonn, Germany. The main component of the ETV unit is a graphite bridge with a ring-shaped orifice. A graphite cup can be put into the bridge. The graphite bridge and the cup can be heated up to a maximum temperature of 2800 ~ The volume of the cup is 700 }.tL and the sample to be analyzed can be directly weighed into it without sample preparation, prefer- ably in solid form.

Instrumentation aspects of using ETV with ICP-MS are straightforward, with a simple tubing connection of 30-60 cm sufficing as an interface between the two instrumental units. In the system using an Elemental Micro Therm ETV furnace interfaced to an ICP-MS instrument (both from VG Elemental Limited, Winsford, Cheshire, England), the ETV is operated in a flow-through manner rather than the "bell-jar" configuration used in other studies, ensuring efficient analyte transport and minimal transport time.

Electrothermal vaporization technology has received an enormous amount of engineering and applications development at Perkin-Elmer and holds considerable promise for ICP-MS applications. The HGA-600MS Electrothermal Vaporization System for ICP-mass spectrometers is available from PE. This device can be fully integrated with the Elan ICP-MS and samples as small as 5 ktL can be analyzed. A pneumatic device equipped with a graphite tip is used to plug the sample introduction port of the graphite tube during the vaporization step. All components, the ETV power supply, the pneumatic plug, and the bypass control system are fully computer controlled. The system can also be used to volatilize solid samples (using a slurry or cup-in-tube technique).

VIII. DETECTION LIMITS

It is usual practice to quote the detection limit as the result for a particular technique or method, and to draw comparisons between the detection limits observed using similar techniques. The detection limits for the ETV-plasma source emission spectrometric techniques are summarized in Tables 4-9 and compared with those obtained using plasma source pneumatic nebulization and GFAAS. Detection limits

Table 4. Comparison of Reported Detection Limits for ETV-ICP, Pneumatic Nebulization ICP, and GFAAS

ETV-ICP GFAAS* Ele- ICP* ment ng/mL or ng/g pg Reference ng/mL ng/mL + pg#

Ag

A1

As

Au

B

Ba

Be

Bi

Ca

Cd

Ce

Co

Cr

Cu

Dy

Er

Eu

Fe

Ga

Gd

Ge

Hg

Ho In

K

La

Li

Lu

Mg

Mn

Mo

Na

Nd

Ni

P

Pb

Pr

Pt

Sb

Sc

Se

0.1 1 Gunn et al. (1978) 6 0.12

0.15 4.5 Idaet al. (1989) 3 1.2

20 200 N g and Caruso (1982) 60 1.2

1 10 Gunn et al. (1978) 30 0.3

0.1 10 Nixon et al. (1974) 3 60

0.0003 0.03 Nixon et al. (1974) 0.15 6

0.02 2 Nixon et al. (1974) 0.15 0.12

2 200 Nixon et al. (1974) 91 1.2

0.002 0.02 Long et al. (1985) 0.015 0.9

0.000025 1 Van Berkel et al. (1990) 1.5 0.03

0.2 4 Huang et al. (1991 a)

3 30 Ng and Caruso (1982) 3 1.2

0.06 1.8 Ida et al. (1989) 1.5 0.6

0.00004 4 Van Berkel et al. (1990) 1.5 0.9

3.7 70 Huang et al. (1991)

3.0 60 Huang et al. (1991a)

1.8 36 Huang et al. (1991a)

2 20 Long et al. (1985) 6 6

1 10 Long et al. (1985) 9

8.2 160 Huang et al. (1991 a)

1 10 Ng and Caruso (1982) 15 # 0.3 #

0.8 4 Matusiewicz et al. (1985) 60 60

2.5 50 Huang et al. (1991 a) 2 20 Gunn et al. (1978) 60 6

110 550 Matusiewicz and Barnes (1985c) 60 0.3

4.0 80 Huang et al. (1991a)

0.01 0.05 Matusiewicz and Barnes (1985c) 0.6 0.9

0.2 4 Huang et al. (1991 a)

0.01 0.1 Ng and Caruso(1982) 0.15 0.02

0.002 0.01 Matusiewicz and Barnes (1985c) 0.6 0.2

0.7 7 Hu et ai. (1991b) 3 6

80 400 Matusiewicz and Barnes (1985a) 0.3 1.8

17 340 Huang et al. (1991a)

0.3 3 Kawaguchi et al. (1986) 6 3

10 100 Kirkbright (1981) 60 & 0.3 #

0.00013 4 Van Berkel et al. (1990) 6 0.6

10 200 Huang et al. (1991a)

0.25 25 Alimonti et al. (1987)

1 100 Nixon et al. (1974) 151 1.2

0.2 4 Huang et al. (1991a)

6 600 Nixon et al. (1974) 60 0.8

6

60

60

18

3000

240

6

60

45

1.8

60

30

45

300

3000

240

15

45

1

10

300

90

150

15

30

60

90

1 09 (continued)

Table 4. Comparison of Reported Detection Limits for ETV-ICP, Pneumatic Nebulization ICP, and GFAAS

ETV-ICP GFAAS* Ele- ICP* ment ng/mL or ng/g pg Reference ng/mL ng/mL + pg#

Sm 9.6 190 Huang et al. (1991a)

Sn 0.5 5 Kawaguchi et al. (1986) 30 1.8 90

Sr 1 5 Matusiewicz and Barnes (1985a) 0.06 0.6 30

Tb 8.7 170 Huang et al. (1991a)

Ti 1.15 5.7 Barnes and Fodor (1983) 1.5 1200 60000

TI 5 50 Ng and Caruso (1982) 151 3 150

Tm 1.6 30 Huang et al. (1991a)

V 0.3 3 Kawaguchi et al. (1986) 6 12 600

W 8 160 Huang et al. (1991b)

Y 0.6 12 Huang et al. (1991a)

Yb 1.2 20 Huang et al. (1991a)

Zn 0.0006 0.6 Van Berkel et al. (1990) 1.5 0.02 1

Zr 0.2 4 Huang et al. (1991b) 3 D

Notes: *Koch et al. (1982) +On the basis of a consumed sample volume of 50 laL #Authors calculation: 10/3.3 (3~i) based on given reported limit of determination (10~5) 8'Omenetto (1981)

Table 5. Detection Limits for ETV-ICP-MS ETV-ICP-MS

Element ng/mL or ng/g pg Reference

Ag 0.08 0.16

A1 0.03 0.05

As 0.0002 0.005

Au 0.0001 0.002

Ba 0.02 0.4

Bi 0.001 0.01

Ca 0.7 1.4

Cd 0.006 0.12

Ce 0.0001 0.002

Co 0.007 0.07

Cr 0.1 0.2

Cu 0.002 0.02

Dy 0.0003 0.007

Er 0.0003 0.005

Eu 0.0003 0.006

Fe 0.02 0.2

110

Park et al. (1987a)

Park et al. (1987a)

Hulmston and Hutton (1991)


Shibata et al. (1992a)

Voellk6pf et al. (1991)

Park et al. (1987a)


Shibata et al. (1991)


Park et al. (1987a)





Carey et al. (1991)

(continued)


Element

ETV-ICP-MS

ng/mL or ng/g Pg Reference

Gd

Hg

Ho

In

K

La

Li

Lu

Mg

Mn

Na

Nd

Ni

Os

P

Pb

Pd

Pr

Rb

Rh

Sb

Sc

Se

Si

Sm

Sn

Tb

Te

T1

Tm

U

Yb

Zn

0.0006

0.001

0.0001

0.00005

1.5

0.0001

0.002

0.0001

0.005

0.002

0.02

0.0004

0.005

0.0042

0.3

0.0001

0.06

0.0001

0.007

0.002

0.0005

0.03

5.7

2.7

0.0005

0.0001

0.0001

0.34

9

0.0001

0.00005

0.0004

0.008

0.012

0.03

0.001

0.001

3

0.002

0.04

0.002

0.02

0.02

0.4

0.008

0.05

0.085

15

0.001

0.6

0.002

0.14

0.02

0.001

0.6

11.4

5.4

0.010

0.002

0.002

3.4

45

0.002

0.001

0.007

0.16





Park et al. (1987a)









Hiram et al. (1989)

Marshall and Franks (1990)


Hall et al. (1988)




Hulmston and Hutton ( 1991)


Park et al. (1987a)

Park et al. (1987a)

Shibata et al. ( 1991)



Newman et al. (1989)

Park and Hall (1988)





111

Table 6. Comparison of Reported Detection Limits for ETV-MIP and Pneumatic Nebulization

Ele- ment

ETV-MIP

ng/mL or M1P ng/g pg Reference ng/mL Reference

Ag 0.2 0.6

As 0.0012 120

Au 2 10

Ba 4000 2000

Be 20 I00

Bi 0.5 100

Br 20 300

C1 8 120

Co 7 40

Cr 1 10

Fe 2 10

Ga 0.6 3

Ge 3 5000

Hg 0.01 0.5

I 500 1000

In 0.3 1.5

Mg 0.05 0.3

Mn 0.3 2

Ni 20 100 P 10 500

Pb 7 21 Rb 16 80

S 6 12000

Sb 0.35 0.35 Se 0.25 0.25 Sn 60 300 Te 5 15

T1 1 5

U 1000 5000

Evans et al. (1991) 1

Matusiewicz et al. (1990a) 30

Zerezghi et al. (1983) m



Fricke et al. (1975) 0.5

Wu and Camahan (1990)

Wu and Carnahan (1990)


Runnels and Gibson (1967)


Timmins (1987a)

Skogerboe et al. (1975)

Nojiri et al. (1986)


Stahl et al. (1989)



Fricke et al. (1975) Heltai et al. (1990)

Evans et al. (1991) Fricke et al. (1975)


Bulska et al. (1993b) Bulska et al. (1993b) Fricke et al. (1975) Yu et al. (1990)

Na and Niemczyk (1982)

Alder and Da Cunha (1980)

Zander and Hieftje (1981)



1 Zander and Hieftje (1981)




3 Zander and Hieftje ( 1981)

1 Zander and Hieftje ( 1981)

0.05 Zander and Hieftje (1981)

0.05 Zander and Hieftje (1981) 80 Zander and Hieftje (1981)


100 Zander and Hieftje (1981) 40 Zander and Hieftje (1981)


0.05 Zander and Hieftje (1981)

Table 7. Detection Limits for ETV-MIP-MS

Element

ETV-MIP-MS

ng/mL or ng/g pg Reference

Ag 0.009 0.03

Br 0.5 1.5

Cd 0.03 0.09

Pb 0.25 0.75

Evans et al. (1991)

Evans et al. (1991)

Evans et al. (1991)

Evans et al. (1991)

112

Electrothermal Vaporization (Sample Introduction 113

Table & Detection Limits for ETV-DCP

ETV-DCP


A1 0.09 0.09

Au 30 600

Ca 0.08 0.08

Cd 10 300

Cu 0.2 0.2

Fe 2 2

Hg 10 200

Mg 1 30

Mn 25 500

Pb 20 400

Zn 30 600




Slinkman and Sacks (1991b)




Slinkman and Sacks (1991b)




are presented in terms of both mass and concentration to simplify comparison with

other work (or between different works). The compiled data refer mainly to the

determination of elements in pure aqueous solutions which contain only the element in question. Because of this simplification of the procedure of data compilation, any application of the data as standard values in practical trace analysis must be

subject to some restrictions. The limit of detection is just one of the figures of merit

characterizing a technique and should not be used alone as a criterion of choice.

Nevertheless, the data compiled in this review may be usefully used as an initial survey of the effectiveness of ETV-plasma source emission spectrometry with

respect to the determination of small quantities of elements and materials. While direct comparison of detection limits is misleading owing to the use of different systems, operating conditions, and modes, it is clear that detection limits obtainable

Table 9. Detection Limits for ETV-CCMP

ETV-CCMP


Ag 2 10 Liang and Blades (1988)

As m 50000 Hanamura et al. (1983)

Cd 0.7 3.5 Liang and Blades (1988)

Cu 8 40 Liang and Blades (1988)

Hg - - 6000 Hanamura et al. (1983)

Li 4.5 23 Liang and Blades (1988) Sb 4.8 24 Liang and Blades (1988)


with, for example, ETV-ICE are one to two orders of magnitude superior to those of pneumatic nebulization, and tend to approach those obtained with electrothermal atomization AAS. For ETV-MIP they are comparable to or slightly superior to the best limits obtained with pneumatic nebulization (where data are available) and do not tend to approach those obtained with GFAAS. On the other hand, it appears that ETV-ICP-MS detection limits have been reduced to femtogram levels for most elements and are typically about 100-fold lower than ETV-ICP detection limits. It is rather surprising that ETV-MIP has not achieved the detection power available with the ETV-ICE Therefore, this technique is complementary rather than competitive to the nebulization MIP method; moreover, the number of elements amenable to ETV-MIP emission is significant. Current detection limits that can be obtained in realistic matrices under compromise operating conditions for a broad range of elements are thus of the order of 1 ng/mL. In the author's opinion, there is no indication that the sensitivity, and hence detection limits, can be further improved without additional preconcentration steps or optimization of the interface between the plasma sources and ETV devices in terms of analyte vapor transport time and efficiency, stabilization of the discharge during analyte injection, and elimination of the various interferant effects common to ICP/MIP/DCP/CCMP systems. How- ever, the ETV-MIP has the capability of determining the halogens, carbon, nitrogen, phosphorus, and sulfur well enough that they can be quantitated at the microliter per milliliter level using their UV and visible lines.

At the current stage of development, the linear dynamic range that has been demonstrated is six orders of magnitude in the case of ETV-ICP and two to four orders of magnitude in the case of ETV-MIP for single element solutions. It is not yet clear how this varies with ETV mode, plasma power, or carrier gas flow rates.

IX. APPLICATIONS

There are many applications of ETV-plasma source emission spectrometry for qualitative and quantitative measurements (elemental determinations) in a wide range of samples. For clarity, illustrative applications have been summarized in Tables 10-13, with descriptions of the elements determined, ETV instrumental modes used, and the sample matrices studied. The aim of this section is to examine representative publications, not merely to present potential users with established methods, but rather to point to the reasons why ETV-ICP/MIP/DCP/CCMP has been used to solve particular problems and to stimulate further interest in its novel application.

It should be noted that Matusiewicz (1991) has already reviewed the application of ETV-ICP/MIP to the analysis of biological materials and dealt primarily with the usefulness of ETV-ICP/MIP in determining trace elements.

Table 10. Applications of ETV-Plasma Source Systems to the Analysis of Waters

Sample Type Elements Amount of

Determined ETV Device Mode Sample Pretreatment Sample* Reference

Acidic water solution Pb

Wastewater

ETV-ICP

Ca, Cd, Cu, Fe, K, ETV-ICP, carbon rod- Mg, Na, P, Pb, microboat Zn

Direct determination with resin 5 ~tL preconcentration

Direct determination 10 I, tL



Synthetic ocean water

Drinking water

Artificial seawater

Water solution

River water, SLRS-1 reference material

Fresh and sea water

Fresh water

Water

Lake water

As, Au, Cd, Li, Sn, ETV-ICE carbon cup Zn

Hg ETV-ICP

Cd, Cu, Pb, Zn ETV-ICE graphite tube

A1, Cu, Fe, Mg, Mn, S, Se, Si, Zn

As, Au, Mn, Mo, Ni, Fe, Pd

Hg

C

ETV-ICE graphite boat

ETV-ICP-MS, graphite platform or metal filament

Vapor-MIP

Evolved gas-MIP

Pb

Hg

MTES-MIP, tantalum boat

Vapor-MIP

Direct determination 5 ~L

Direct determination with 5 IxL (NH4)2S modifier

Direct determination with resin 10 mg preconcentration

Direct determination 50 IxL

Direct determination, extraction 10 ~tL into IBMK for Au

Amalgamation onto Ag wool 1 L

Direct determination, oxidation 400 IxL with V205 and CuO

Direct determination 20 l, tL

Gold amalgamation 50 mL

Ng and Caruso (1983a)

Matusiewicz et al. (1985)

Van Berkel et al. (1990)

Manshina et al. (1990)

Hall et al. (1988)

Watling (1975)

Mitchell et al. (1977)

Melzer et al. (1980)

Nojiri et al. (1986) (continued)

Table lO. (continued)

Sample Type Elements

Determined ETV Device Mode Sample Pretreatment Amount Sample ~ Reference

_ . a

. _ . a

C7~

Coastal sea water, As CASS- 1 reference material

Potable, surface, and Hg waste water

Groundwater

River and tap water

As, Se

As, Sb, Se

Sea water, CASS-I ref- Fe erence material

Rio Grande water Mn

Water, ultrafiltrates Arctic snow

Pt

Cd, Co, Cs, Cu, Mn, Ni, Pb, TI, V

ETV-MIP, graphite tube

Vapor-MIP

MTES-MIP, tantalum boat

ETV-MIP, graphite tube filled with reticulated vitreous carbon

ETV-ICP-MS, graphite platform or metal filament

ETV-DCP, graphite microboat

ETV-ICP, graphite tube

Direct determination with in situ hydride generation preconcentration

HNO3-PTFE bomb digestion, deposition on Cu or amalgamation with Ag or Au

Dilution (1:10)

Direct determination with in situ hydride generation preconcentration

Direct determination


Direct determination ETV-ICP-MS, pyrolytic Direct determination with

graphite tube evaporating concentration

2mL

2-10 mL

2.5 ~tL

3-5 mL

5 ~tL

40 laL

100 laL 10mL

Matusiewicz et al. (1990b)

Kaiser et al. (1975)

Hood and Niemczyk (1987)

Bulska et al. (1993b)


Mitchell and Sneddon (1987)

Alimonti et al. (1987)

Sturgeon et al. (1993)

Note: *Amount of sample refers to original test portion used in the analytical procedure

Table 11. Applications of ETV-Plasma Source Systems to the Analysis of Environmental and Mineral Substances and Technical Products

Sample Type

Amount of Sample* Elements

Determined ETV Device Mode Sample Pretreatment ~tL mg Reference

_ _ . . t

_ _ a

Uranium

Cadmium mercury telluride

High-purity graphite

Pure zirconium

Solid rubber

Motor oil

Gasoline

Soil

Ambient air

Filter paper

Synthetic feld- spar, basalts

Ag, AI, Be, Cd, ETV-ICE graphite rod Cr, Cu, Fe, Zn

Ag, A1, Co, Cr, ETV-ICE graphite rod Cu, Fe, In, Mn,

Ni, Pb, Zn Cu ETV-ICP, graphite rod-cup

Cd ETV-ICE graphite rod-cup

S ETV-ICE graphite tube

Zn ETV-ICE pyrolytically coated carbon cup

Pb ETV-ICE pyrolytically coated carbon cup

Hg Vapor-MIP

Hg Vapor-MIP

Hg Vapor-MIP

CI, Br, I ETV-MIP, tungsten boat

10

Dissolution in hot aqua regia 10

Graphite containing 5% 5 m Mg(NO3)2 as ashing aid was dissolved in HCI-HNO3 (3 + 1 ), resin preconcentration

Dissolution in HF-HNO3 and 10 addition of H3BO3

Direct determination of solid ~ ca. 1 samples

Direct determination 5

Chemical stabilization with iodine 5

HNO3-HF-PTFE bomb digestion, m 500 deposition on Cu or amalgamation with Ag or Au

Deposition on Au 20 L

HNO3-PTFE bomb digestion, m 100 deposition on Cu or amalgamation with Ag or Au

Addition of B203 m 10

Kirkbright and Snook (1983)

Cope et al. (1982)

Mahanti and Barnes (1983a)

Jing and Barnes (1984)

Casetta et al. (1985)

Ng and Caruso (1983b)

Ng and Caruso (1983b)




Rait et al. (1984)

(continued)



Determined ETV Device Mode

Amount of Sample*

Sample Pretreatment lxL mg Reference

Hydrochloric acid Garment Coal fly ash

Sulfur

Air glass fibre filter

Suspended particulate matter

Semiconductor silicon

Lubricating oil

Semiconductor amorphous silicon

Silicate rock, USGS BCR-1 reference material

Semiconductor materials

1 ETV-MIP, tantalum boat

Br, CI ETV-MIP, tantalum boat CO32- Evolved gas-MIP, graphite

crucible

As, Se ETV-MIP, graphite tube

Cu, Pb, V ETV-ICP, tantalum coated graphite platform

Cr, Mn, Pb ETV-ICP, graphite microboat

P ETV-ICP, platinum filament

Ca, Cu, Fe, Mg, ETV-ICP, graphite crucible Mn B ETV-ICP, platinum filament

Pb ETV-ICP-MS, cup-type graphite rod

U, Th ETV-ICP, tungsten boat

Direct determination Direct determination Direct determination

15 10

ca. 100

HNO3-PTFE bomb microwave or PTFE bomb dissolution, in situ hydride generation preconcentration

Direct determination of solid samples

Direct determination after dissolution of samples with 2% HF

Dissolution in HF 10


Dissolution with HNO3-HF 10

HNO3-HC104-HF-PTFE bomb 5 dissolution

Dissolution with acids and separation of matrix by cation exchange method

50

Barnett and Kirkbright (1986)

Carnahan and Caruso (1982) Bauer and Natusch (1981 b)

100-500 Matusiewicz and Kurzawa (1991)

Nimjee et al. (1984)

Sugimae and Barnes (1986)

Kitazume (1986)

1-10 Ohls and Hiitsch (1986)

Kitazume et al. (1987)

B Date and Cheung (1987)

Okada and Hirate (1988)

(continued)




Amount of Sample*

Sample Pretreatment ~tL mg Reference

Alumina-based ceramic materials

Ceramic powders

Cd, Fe ETV-ICE graphite crucible Direct determination of samples containing thermochemical additives

Cd, Fe ETV-ICP, graphite crucible Direct determination of samples containing thermochemical additives

Nickel et al. (1989)

Reisch et al. (1989)

. . . a

. _ . a

Silicon Fly ash Fly ash

La203

Acetic acid, ammonia water

Silica, silicon

Vehicle exhaust particulates, pond sediment, NIES reference materials

Pond sediment, vehicle exhaust particulate, NIES reference materials

P ETV-ICE graphite tube Pb ETV-ICE graphite rod Pb ETV-ICP-MS, graphite rod Y ETV-ICP, graphite tube

AI, Ba, Ca, Cr, ETV-ICP, tungsten filament Cu, Fe, Mg, Mn, Ni, Ti

B ETV-ICE tungsten boat

HNO3-HF-HCIO4 dissolution Preparation of slurries Preparation of slurries HC1 (1 + 1) dissolution,

preparation of slurries, fluorination


HNO3-HF-H2SO4 dissolution, extraction of B

A1, Cu, Mn, Pb, ETV-ICE graphite miniature cup Direct determination of solid Zn samples

Cd, Pb ETV-ICE tungsten boat HNO3-HF-HCIO4-PTFE bomb dissolution

10-30 10 10 20

5

20

20

0.1-1.5

Ida et al. (1989) Darke et al. (1989) Darke et al. (1989) Huang et al. (1991 a)

Fujimoto et al. (1991 )

Mikasa et al. (1991)

Atsuya et al. (1991)


(continued)


Sample Type


Determined ETV Device Mode Sample Pretreatment lxL mg Reference

. . . a

o

SiC

Geological materials, standards

Rock powders

AI, Cr, Cu, Fe, ETV-ICP, graphite crucible Direct determination with Mn, Mo, V, Ti thermochemical modifiers

Mo, W ETV-ICP-MS, graphite platform Dissolution 5

Ir, Pd, Pt, Ru

Geological materi- T1 als, standards

Silicon, silicon di- U, Th oxide, quartz glass

Iridosmine Os Petroleum Hg

HCI, H2SO4 Photoresists

ETV-ICP-MS, pyrolytically Decomposition using the nickel 2 coated graphite filament sulfide fire assay method

ETV-ICP-MS, pyrolytic graphite HNO3-HF-HCIO4-HCI 5 platform or tungsten filament dissolution or fusion with

LiBO2 method ETV-ICP-MS, tantalum tube HNO3-HF-H2SO4 dissolution 20

V, Ti ETV-ICP-MS, graphite tube Cu, Fe, Mn, Ni, ETV-ICP-MS, graphite tube

Na AI, Ba, Ca, Cd, ETV-ICP-MS, tungsten ribbon Co, Fe, Li, M g, furnace Mn, Na, Pb, Rb,

Sc, Zn As, Cd, Co, Cr, ETV-ICP-MS, graphite tube

Potassium hydro- genphthalate

Coal, NIST 1635 reference mate- Cu, Fe, Mn, Pb, rial Se, Zn

Sandstone Ni, Pb

ETV-ICP-MS, graphite platform Fusion method 2-5 ETV-ICP-MS, pyrolytic graphite Direct determination m

tube

Direct determination 50 Direct determination after dilution 10

ETV-ICP-MS, graphite tube

Direct determination after ion- 20 exchange separation

Direct determination of slurries

Acid dissolution

5 Nickel et al. (1993)

n

m


Gregoire (1988)


Matsunaga et al. (1989)

Gregoire (1990) Osborne (1990)

B Marshall and Franks (1990) Q Etoh et al. (1991 )


Voellktpf et al. (1992)

Ulrich et al. (1992b)

(continued)




Amount of Sample*


Sediment and geochemical samples, GRD-46, GSD-7, GRD- 35 standard materials

NaC1

USGS Geological standards

Phosphorous trichloride

Pb ETV-MIP-AAS, tantalum wire

E l l

Mn ETV-DCP, tungsten spiral ETV-DCP, graphite microboat

AI, Cu, Fe, Mg, ETV-ICE graphite tray Si, Zn

HNO3-HCI-HCIO4 (1:3:4) dissolution

Dilution Dilution (1:20) with powdered

cellulose


10

50

m

5

Ling et al. (1990b)

Mei et al. (1992a) Mitchell and Sneddon (1987)

Manshina et al. (1991)


Table 12. Applications of ETV-Plasma Source Systems to the Analysis of Metals, Alloys and Other Metallurgical Samples Elements Amount of

Sample Determh~ed ETV Device Mode Sample Pretreatment Sample*, mg Reference

Cast iron S

Tantalum Cu

Niobium and beryllium

ETV-MIP, platinum filament

ETV-MIP, tungsten filament

Fe, Zn ETV-MIP, graphite tube

Steel Pb ETV-ICP, graphite crucible

Alloy steel reference Bi, Pb, Te, Zn ETV-ICP, graphite crucible materials

Ni-base alloys As, Se ETV-ICP, graphite cup Alloy steel reference Ag, Bi, Ca, Cd, ETV-ICP, graphite crucible

materials Mg, Pb, Se, Te, Zn Low-alloy steel, NBS V, Ti ETV-ICP, tungsten boat

362, 364 Alumina Ti ETV-ICP, graphite boat

Iron and steel Bi ETV-ICP-MS, graphite tube

Dissolution with HC1 1000

Dissolution in HF-HNO3, 10 removal of Ta by cation- exchange resin

HF-HNO3-PTFE bomb 200-500 digestion, determination with electrochemical preconcentration and separation

Direct determination 5-10


Direct determination ca. 5 Direct determination 5-10

HNO3-HC1-HF dissolution 20 lxL

Direct determination 5-20

Dissolution with aqua regia 1000

Taylor et al. (1970)

Kitazume et al. (1978)

Volland et al. (1981)

Ohls and Hiitsch (1986)

Ohls et al. (1986)

Clarke et al. (1986)

Ohls (1989)


Kantor and Zaray (1992)

Imakita et al. (1991)


Table 13. Applications of ETV-Plasma Source Systems to the Analysis of Biological and Clinical Materials


Sample Type Determined ETV Device Mode Sample Pretreatment ~tL mg Reference

Human whole blood Mn, Ni ETV-ICE graphite rod

Test serum Mn, Zn ETV-ICE graphite tube

Human serum AI, Si ETV-ICP, graphite electrode

Human blood plasma Ca, Cd, Cu, Fe, K, ETV-ICP, carbon rod and Mg, Na, P, Pb, Zn microboat

Pooled urine AI, Si ETV-ICE graphite electrode

NIOSH-NBS "Elevated" As, Cr, Cu, Ni, Se ETV-ICP, graphite Reconstituted with water, 5 freeze-dried urine electrode direct determination

except Se (resin preconcentration)

NIOSH-NBS "Normal" Cd, Cr, Cu, Mn, Ni, ETV-ICP, graphite rod Direct determination with 10 and "Elevated" freeze- Pb electrochemical dried urine preconcentration and

separation

NIOSH-NBS "Elevated" Cr, Cu, Ni ETV-ICP, graphite Reconstituted with water, 50 freeze-dried urine tube-platform direct determination

NIOSH-NBS "Elevated" Cr, Cu, Ni ETV-ICP, graphite cuvette Direct determination with 5 freeze-dried urine electrochemical

preconcentration and separation

Human milk Ni, Pb ETV-ICP, graphite rod Dissolution with Me4NOH 10

Dilution with Triton X- 10 100 (1 + 1), pyrolysis with Me4NOH

Dilution with Herrmann 50 solution (1 + 4)




Camara Rica et al. (1981 )

Aziz et al. (1982a)




Barnes and Fodor (1983)

Fish et al. (1988)

Matusiewicz and Barnes (1985d)

Matusiewicz (1987)

Camara Rica and Kirkbright (1982)

(continued)

Table13. (continued)



Amount of Sample*

Sample Pretreatment BL m g Reference

_ _ _ _ t

Human milk

Hemodialysis solution

Incubation solution Tree ring wood

NBS-SRM 1576 Rice flour

NBS-SRM 1571 Orchard leaves

NBS-SRM 1577 Bovine liver

NBS-SRM 1577 Bovine liver

Ni, Pb ETV-ICE graphite rod

A1 ETV-ICE graphite electrode

Au ETV-ICE graphite cup AI, As, Ba, Ca, Cu, ETV-ICE graphite tube- Fe, Ge, K, Mg, Mn, platform and graphite

Na, Si, Sr, V, Zn electrode Cd, Co, Cu, Mn, ETV-ICE graphite Ni, Pb, Sb, Zn

Mn, Pb, Zn

Cd, Mn. Zn

HNO3-H2SO4 digestion, dithizone extraction


Direct determination 50% H202-PTFE bomb

digestion

Direct determination with electrochemical preconcentration and separation

ETV-ICP, graphite cuvette HNO3-H202-HCIO4 digestion

ETV-ICP, graphite tube HNO3-HCIO4-H202 digestion

Cd, Co, Mn, Pb, Zn ETV-ICE graphite cuvette Direct determination with electrochemical preconcentration and separation

15

5

10 5

50

50

m

m

Barnett et al. (1983)


Tepperman et al. (1984) Matusiewicz and Barnes

(1985a)

Matusiewicz (1987)

Aziz et al. (1982a)

Aziz et al. (1982a)

Matusiewicz (1987)

NBS-SRM Bovine liver Ca, Cd, Cu, Fe, K, ETV-ICE carbon rod- Mg, Na, P, Pb, Zn microboat

Direct determination Blakemore et al. (1984)

Bowen's kale Ag, Cd, Cu, Mn, Pb ETV-ICP, graphite rod HNO3-HCIO4 (5 + 1) digestion

10 Dean et al. (1985)

IAEA H-5 Animal muscle Mn, Ni ETV-ICE graphite rod Dissolution with Me4NOH 10 Camara Rica et al. ( 1981)

(continued)




Amount of Sample*

Sample Pretreatment l, tL mg Reference

IAEA V-9 Cotton cellulose material

Human bone

Formulation Whole capsule

Urine Serum, urine, tissues

Freeze-dried urine, SERONORM standard

material Whole blood

NIES-CRMs: Pepperbush No. 1; Human Hair No. 5; Mussel No. 6; Tea Leaves No. 7

Human serum, NIST 1577 Bovine liver

NBS SRM 1573 Tomato leaves

Milk, wheat flour, garlic, spinach

AI, As, Ba, Ca, K, ETV-ICP, graphite tube- Cu, Fe, Mg, Mn, platform

Na, Pb, Si, Sr, V, Zn Cu ETV-ICP, graphite

electrode Ca, Cu, Fe, Mg, Zn ETV-ICP, graphite rod Ca, Cu, Fe, Mg, P, ETV-ICP, graphite rod

Zn Pb ETV-ICP, tungsten wire Pt ETV-ICP, graphite tube

Cd, Cu, Pb, Zn ETV-ICP, quartz boat

Pb ETV-ICE carbon rod

Cd, Co, Cu, Fe, ETV-ICE graphite boat Mn, Pb, Zn

50% H202-PTFE bomb digestion

HNO3-H202 digestion or resin preconcentration

Slurry Slurry

Direct determination Dilution of serum and

urine, mineralized tissues

Reconstituted with buffer, enrichment by resin

Dilution with Triton-X or water, direct determination

Acid digestion

Cr ETV-ICP, graphite tube Acid digestion, fluorination

B ETV-ICP, graphite tube Dry ashing, fluorination with PTFE, sample injected as a slurry

Mo ETV-ICP, graphite tube Dry ashing, fluorination, slurry

5

10 10

10 100

10

40

10

10

10

Matusiewicz and Barnes (1985a)

Mahanti and Barnes (1983b)

Long and Snook (1982) Long and Snook (1982)

m

Kawaguchi et al. (1986) Alimonti et al. (1987)

4-20 Van Berkel and Maessen (1988)

Alvarado et al. (1989)

Isoyama et al. (1990)

Hu et al. (1991c)

Hu et al. (1991a)

Hu et al. (1991b)

(continued)




Amount of Sample*

Sample Pretreatment lxL lxg Reference

Botanical samples: V85-1, white birch, Norway maple

Grain, rice

NBS SRM 1571 Orchard leaves,

NBS SRM 1566 Oyster tissue

Human plasma and urine

Blood serum Hair

Human hair

Test serum

Human serum, ceruloplas- min

Cow milk

Powdered milk, fish

Pb, Zn

Cu, Dy, Ho, La, Mn, Nd, Y, Yb

As, Ag, Cu, Mn, Na, Pb, Rb, V, Zn

ETV-ICP, brass electrodes Pellet, dilution sample: graphite (1:9)

ETV-ICP, tungsten coil Acid digestion, fluorination, slurry

ETV-ICP-MS, Re filament HNO3-HF-HC104-PTFE bomb digestion

Te ETV-ICP-MS, pyrolytic Dilution with buffer, direct ceramic tube determination

Fe

As, Cd, Cu, Pb, Zn ETV-MIP, tungsten filament

Hg Vapor-MIP

ETV-ICP-MS, graphite rod Direct determination


Cu, Fe, Zn ETV-MIP, graphite tube

Cu ETV-MIP, tantalum strip

I ETV-MIP, graphite rod

Hg Vapor-MIP

HNO3_PTFE bomb digestion, Au amalgamation

Dilution with Herrmann solution (1 + 4)


Reconstituted with water

HNO3-HF-PTFE bomb digestion, deposition on Cu or amalgamation with Ag or Au

10

10

5

50

1000

1.4

250 Karanassios et al. (1991 )

Mei et al. (1992b)

Park et al. (1987a)

Newman et al. (1989)

Whittaker et al. (1989)

250 Sakamoto et al. (1976)

10--20 Nojiri et al. (1986)

Aziz et al. (1982b)



300-500 Kaiser et al. (1975)

(continued)




Amount of Sample*

Sample Pretreatment laL mg Reference

Animal bone, IAEA H-5 reference material

Fish, lettuce, pears

Eucalyptus leaves Orchard leaves, NBS

SRM 1571

Orchard leaves NBS SRM 1571, spinach NBS SRM 1570, bovine liver NBS SRM 1577

Orchard leaves NBS SRM 1571

Bovine liver NBS SRM 1577

Bovine liver

Metalloenzymes

Viruses type C

Carboxypeptidose

Monoarsanilazocar- boxypeptidase

Ni, Pb

Cd, Cu, Pb, Se

Pb

Cu, Mn, Zn

Mn

Hg

Cu, Mn, Zn

Cu

Zn

Z n

Hg

As

ETV-MIE tantalum ribbon Dissolution with HNO3- H2SO4

ETV-MIP, tantalum strip HNO3-PTFE bomb digestion

MTES-MIP, tantalum boat Leaching with HNO3

ETV-MIP, graphite tube HNO3-HCIO4 digestion

ETV-MIE graphite tube Direct determination

SCA-MIP, molybdenum cup


ETV-MIP, tantalum strip

ETV-MIP, tantalum or tungsten filament


HNO3-HCIO4 digestion

HNO3-PTFE bomb digestion

Microgel exclusion chromatography

ETV-MIP, tantalum or tungsten filament

ETV-MIP, tantalum filament

ETV-MIP, Mo, Ta, W filaments

Microgel exclusion chromatography

Dilution with KC1

Dilution with NaC1 or KC1

m

1000 Barnett (1987)

3-5 g Fricke et al. (1976)

4.8 g Melzer et al. (1980)

10 Aziz et al. (1982b)

2 Broekaert and Leis (1985)

0.64 Kitagawa et al. (1989)

10 Aziz et al. (1982b)

280-350 Fricke et al. (1976)

0.1 lag Kawaguchi and Auld (1975), Kawaguchi and Vallee (1975), Auld et al. (1974a, b, 1976), Kumamaru et al. (1982)

Auld et al. (1975)

1.1-3.3 lag Atsuya et al. (1977b)

8-26 Atsuya et al. (1977a)

(continued)




Amount of Sample*


_..a

I ' J

Antler Cu Bovine liver NBS 1577A, Cu, Fe, Zn

orchard leaves NBS 1571, tomato leaves NBS 1573

Orchard leaves NBS 1571 Cu

Algal cells

Whole swine blood

NIST SRM materials: tomato leaves 1573, bovine liver 1577, pine needles 1575, oyster tissue 1566, citrus leaves 1572

NBS SRM 2670 Urine, bull sperm

Body fluids

ETV-MIP, tungsten wire


ETV-MIE graphite tube

Au, Hg ETV-DCP, graphite microboat

Cu ETV-DCP, tungsten filament

Cu, Fe, Mg, Mn, Zn ETV-DCP, graphite tube

HNO3-HCIO4 digestion HNO3-HF-PTFE bomb

digestion, separation by complexation and selective sorption

HNO3-HCIO4 digestion and separation by selective sorption


300-fold dilution, direct determination

Slurry suspension

Cu, Mn

AI, Au, Li, Os, Pd, Pt, Ru

ETV-DCP, graphite Dilution microboat

ETV-ICP, graphite cuvette Direct determination

3 50

50

1

30

20

Zhang et al. (1989) Heltai et al. (1990a)

Heltai and Broekaert (1991)

ca. 3 Mitchell et al. (1986), Greene et al. (1986)


Slinkman and Sacks (1991 a)

Mitchell and Sneddon (1987)

Matusiewicz and Barnes (1988)



X. CONCLUSIONS

In recent years increasing attention has been placed on the development of atomic discharge systems that segregate the sampling from the excitation step. It is evident from this review that there is considerable research being done on the problem of introducing different sample types into various plasma sources. The plasma sources are used for excitation of microsamples that are separately evaporated and atomized. The relatively low cost of the equipment and the economy of gas consumption and the wide range of elements, including nonmetals, especially make the MIP a most useful device in applications where overloading of the plasma with sample is not likely to occur, that is, in combination with electrothermal devices. Thus, this sample introduction method should complement conventional pneumatic nebulization for microliter solutions and microgram amounts of material. It is clear from this review that electrothermal atomizers (commercially available and laboratory- built), which are used extensively for atomization in AAS, perform better as a means of sample introduction than for atomization and are of potential interest for the analysis of small-volume samples and can be expected to become useful accessories to ICP/MIP/DCP/CCMP equipment (it is relatively easy to interface a commercially available electrothermal atomizer to an emission source). In addition, for nonvolatile samples, use of electrothermally heated metal and graphite devices offers the best prospects for sample introduction. However, almost all of the systems reviewed in this paper are still in the research and development stages. Although, in general, interfacing an ETV to a plasma source is mechanically straightforward, tandem operation of the two components (or sometimes three as in, e.g., electrochemical or hydride generation, in situ preconcentration, and separation) has required careful manipulation of the instrumental variables. Some practice is needed to enable a new operator to become skilled. The advantage of the method is its freedom from time-consuming steps and potential contamination associated with sample preparation (i.e., digestion). The ETV-ICP/MIP/DCP/CCMP combination should be well suited for analyses where only small volumes of sample or amounts of solids are available, such as biological, environmental and forensic applications, but where high sensitivity is desirable or necessary.

Introduction of solids directly into plasma sources is feasible using an approach which separates the evaporation of solids from their excitation. Electrothermal vaporization has been used to generate a vapor that is swept into plasma sources for consecutive excitation and detection. However, a number of limitations must be considered when applying this method of sample introduction such as reproducibility, sample heterogeneity, accuracy, relative instability of sources and plasmas, particle size limitations, memory effects, and so forth. To date, most work has been performed with solutions. Consequently, the bulk of information for solids analysis is considered to be preliminary. In the author's opinion, based on literature investigations of the electrothermal vaporization with solid sampling techniques,


it can be concluded that the analysis of dissolved samples will remain the universal procedure. However, for some special purposes, appropriate processes are offered for solids. The author is skeptical about introduction of solids into ETV-plasma sources.

In general, detection limits are better than those obtained with conventional pneumatic nebulization. The reported overall precision is satisfactory, 1-20%, and is similar to that of GFAAS, but in most instances is worse than that of solution nebulization with plasma source emission analysis. Because ETVo ICP/MIP/DCP/CCMP is used in an emission mode, simultaneous multielement detection is possible even for discrete sample analysis.

Furnace vaporization techniques are inherently slow, but have certain advantages such as in situ chemical processing. Encouraging results were obtained with in situ

electrochemical, hydride generation, and amalgamation preconcentration and separation methods. Provided this preconcentration and separation is included, plasma source emission spectrometry coupled to graphite furnace evaporation is a powerful tool for precise and accurate multielement determinations. With this method, it is possible to directly determine trace metals simultaneously as only a single electrolysis, hydride generation, or amalgamation is needed for each sample regardless of the number of metals to be determined.

Relatively few observations have been reported concerning the speciation of solid samples, thus making it difficult to draw any conclusion on this matter, although encouraging preliminary results were obtained for the speciation of solid samples. However, there is little widespread use of ETV-plasma source emission spectrometry with application to practical samples for speciation. The present approach allows direct sample analysis, that is, no sample preparation is needed. Although this technique is appealing owing to its simplicity, it seems that it is not as accurate as more conventional approaches.

The major argument in favor of the ETV-ICP/MIP/DCP/CCMP combination is that matrix interferences are not expected for the analyte transport between the vaporization and excitation unit. An exception to this general expectation would suggest that the matrix may have a strong influence on the transport efficiency of the analyte to the MIP or DCP source. A pronounced interference effect, for example, in the ETV-MIP system, has been found when the alkali and alkaline earth metal matrices (i.e., the main components of biological materials) are present. Enhancement or suppression of analyte line emission was found to depend upon the element and type of line (atomic or ionic) used. This, from the standpoint of practical analytical chemistry, may lead to improved analytical performance. However, in the reviewer's opinion, it is unlikely that absolute or complete separation of the two processes (vaporization and excitation) is possible in practice.


XI. SUGGESTIONS FOR FUTURE STUDIES

The sensitive microsample analysis capabilities of ETV-plasma sources should encourage their adoption and be consistently useful in atomic emission spectrometry.

It seems probable that most future developments will arise in instrument design. For example, an ETV system should be designed specifically for use with a plasma source (optimized for their specific functions) and not be just a modification of existing, commercially available furnaces designed for use with AAS, as has been shown by PSA Analytical Ltd. Future improvements lie in further optimization of the interface between the plasma sources and electrothermal vaporizer in terms of analyte vapor transport time and efficiency (the length of the tubing or connection and streaming parameters would be expected to affect the efficiency of transporta- tion). Ideally the operating conditions of the electrothermal vaporizer should have minimal effect on the excitation conditions of the plasma sources, and the ramp time of the temperature should be as short as possible (to ensure that the rate of sample introduction is independent of the carrier gas flow rate). Also, the tube-platform configuration (Matusiewicz and Barnes, 1985c) should act like a L' vov platform in a constant-temperature vaporizer (graphite, pyrolytically coated graphite, or even glassy carbon) and is worthy of further consideration to eliminate or reduce matrix effects in the analysis of practical samples.

As selective volatilization of different sample constituents may occur, and emission profiles are also found to be highly sensitive to changes in input power and flow rate (and, in addition, are element dependent), optimum conditions must be determined separately for each element, thus making simultaneous detection difficult. Establishment of multielement conditions and the implications of matrix effects have to be investigated in each particular case. Additional work remains to be done on the development of new or improved preconcentration and separation techniques. Controlled-potential electrodeposition on a hanging mercury drop electrode, thin-film layer of mercury on graphite or metal electrodes, or in situ hydride generation procedures should be considered in some situations.

A further area of growing interest is speciation of elements to supplement the total element figure, which will help scientists in the biomedical materials field to obtain a better understanding of the role of metals, especially toxic metals, in life processes. Of the new plasma discharges, ETV-MIP is especially suitable for speciation work. Certainly, more elemental species need to be measured to determine their analytical figures of merit, and, because helium can be used as a working gas, the halogens can also be detected, as well as other metals. The evolved gas section must be further refined to allow a considerably greater upper sample temperature, 1500 ~ or more. Finally, it is necessary to verify the identity of the evolved molecular species by an independent method, such as mass spectrometry.

It should be noted that each of the subgroups of ETV sample introduction for plasma source atomic emission spectrometry described in this review has its own

1 32 HENRYK MATUSIEWICZ

attributes and specialized applications that cannot be combined in a single, universal device (usually these two units are connected in some way). The critical factors, in the reviewer's opinion, in the construction and use of those ETV-plasma source systems (combinations) for attaining the best analytical performance are (a) the interface between the thermal vaporizer and the plasma sources; (b) vaporization temperature steps or ramp-rate; (c) capabilities for simultaneous multielement vaporization and introduction of the vapors into the plasma sources, with the possibility of detection systems that can measure the very short residence time of atoms in the plasma sources; and, (d) minimum matrix and transport effects on the excitation conditions of the plasma sources, which should be considered as the optimum design parameters.

In conclusion, there are certain texts that contain excellent discussions concerning electrothermal vaporization methods used for sample introduction in atomic spectrometry, which can be used to provide additional references and different approaches to the subject other than this one. The reviews of Kantor (1983, 1988) and Borer and Hieftje (1991) contain excellent papers on dry aerosol introduction for optical spectrometry, and the suggestions for future studies included the ETV- ICP (Matusiewicz, 1986) and ETV-MIP (Matusiewicz, 1990) techniques which could be applied and are much related to ETV-DCP/CCMP.

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HYDRIDE GENERATION TECHNIQUES IN ATOMIC SPECTROSCOPY

Taketoshi Nakahara

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

II. Hydride Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

A. Covalent Hydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

B. The Hydride Generation Reaction . . . . . . . . . . . . . . . . . . . . . 142

III. Hydride Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

A. Direct-Transfer Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

B. Collection Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

IV. Methods of Atomization, Excitation and Ionization . . . . . . . . . . . . . . . 145

A. Atomic Absorption Spectrometry (AAS) . . . . . . . . . . . . . . . . . . 145

B. Atomic Emission Spectrometry (AES) . . . . . . . . . . . . . . . . . . . 148

C. Atomic Fluorescence Spectrometry (AFS) . . . . . . . . . . . . . . . . . 154

D. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) . . . . . . . . 154

V. Analytical Figures of Merit . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

VI. Optimization of Parameters for Hydride Generation . . . . . . . . . . . . . . 157

VII. Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

A. Spectral Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158


139

140 TAKETOSHI NAKAHARA

B. Chemical Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 C. Possible Methods for Overcoming Interferences . . . . . . . . . . . . . . 159

VIII. Atomization Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 IX. Chemical Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

A. Inorganic Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 B. Organic Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

X. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 A. Sample Digestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 B. Practical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

XI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

ABSTRACT

A general overview of hydride generation techniques in atomic spectroscopy is presented, including both fundamental and practical considerations of hydride generation. Following an introduction which attempts to illustrate the development of this technique, atomization and/or excitation of the elements from their hydrides is described in detail with respect to atomic absorption, plasma atomic emission, atomic fluorescence, and inductively coupled mass spectrometry. A brief comparison of detection limits for analytical atomic spectrometric methods that utilize hydride generation as presented in most published reports is discussed, and a section of this chapter examines the interferences which take place when these techniques are used. Finally, in addition to atomization mechanisms and chemical speciation, selected results from an updated application of hydride generation are discussed and presented.

i. I N T R O D U C T I O N

In the early stages, there were a few difficulties associated with direct solution nebulization as a sample introduction method in the determination of some elements such as arsenic and selenium by conventional flame atomic absorption spectrometry (AAS). Clearly, the introduction of sample materials in the gas phase is an ideal technique for analytical atomic spectroscopy. The significant advantages of a gas-phase sample introduction over the widely used pneumatic solution nebulization are (1) the avoidance of the use of a nebulizer, (2) the potential for 100% efficiency of transport in comparison with a maximum of about 2-15% obtainable with a conventional pneumatic nebulizer, and (3) the introduction of a homogeneous medium into the atomizer or plasma. The hydride generation technique is the most widely used, accepted and typical gas-phase sample introduction method for atomic spectroscopy and provides us with an excellent procedure that overcomes many of the problems associated with commonly used pneumatic nebulization of liquid samples.

Hydride Generation Techniques in Atomic Spectroscopy 141

Initially, the primary technique for the quantitative determination of the hydride- forming elements was AAS. However, published applications for hydride determinations by atomic emission spectrometry (AES), atomic fluorescence spectrometry (AFS) and, recently, inductively coupled plasma mass spectrometry (ICP-MS) have appeared.

The elements that form volatile hydrides at ambient temperatures are arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium. The accepted procedure by which the hydride is generated is to reduce the analyte of interest to its volatile covalent hydride and subsequently sweep the generated hydride into an atom reservoir or other spectroscopic source where quantitative spectrometric measurements can be carried out. In contrast to solution nebulization, the volatile hydrides of these elements are more efficiently transported to the high-temperature source which, in turn, can more efficiently produce (i.e., atomize) and excite the free atoms and ions needed for atomic spectroscopy and mass spectrometry. In addition, the resulting separation of analytes from the elements of the sample matrix and from the sample solvent both reduces potential spectral interferences and enables preconcentration of the analyte. A review of hydride generation is available from Nakahara (1983a, 1990, 1991 ), Robbins and Caruso (1979), Verlinden et al. ( 1981 ), Godden and Thomerson (1980), D~dina (1988), and Campbell (1992).

In this chapter, a general description of hydride generation, its combination with AAS, plasma AES, AFS and ICP-MS, and recent developments and applications of the hydride generation technique will be provided in order to give a better understanding of its capabilities and to illustrate its application in a number of investigations published mainly over the last five years.

!!. HYDRIDE GENERATION

A. Covalent Hydrides

Elements of Group IVA, VA and VIA of the Periodic Table form covalent hydrogen compounds, called hydrides, which are of particular importance to analytical atomic spectroscopy for the determination of such elements as arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium. These hydrides are all volatile and thus easily generated in sufficient amounts to be of analytical use in AAS, AES, AFS and ICP-MS.

The hydride generation procedure can be divided into three steps. First, acidified sample solutions are treated with a reducing agent to generate the volatile covalent hydride of the analyte element. Second, the hydride is swept out of the generation vessel using a stream of inert gas (usually argon or nitrogen) into either an atom reservoir or a spectroscopic excitation source. Third, the hydride is decomposed to the gaseous metal atoms and spectroscopic measurements by AAS, AES, AFS or ICP-MS of the element are then carried out.


B. The Hydride Generation Reaction

A wide variety of reducing agents were investigated for use in hydride generation in the early stages, but the facile production of hydrides by reaction with sodium tetrahydroborate (sodium borohydride, NaBH4), introduced into this field by Braman et al. (1972), is now almost universally used for the generation of hydrides, as shown in the following equation (1):

NaBH4 + 3H20 + HC1 ---) H3BO3 + NaC1 + 8H ~ EH,, + H2 (excess) (1)

where E is the hydride-forming element of interest and m may or may not equal n. The solid reagent in pelletized form was initially favored but introduction of a solid into a reaction vessel gives undesirable, highly localized reagent concentrations. A freshly prepared solution of sodium tetrahydroborate is much more efficient and now preferred and also is the most obvious reagent for continuous-flow systems to be mentioned below. The concentration of this reducing agent must be optimized for the particular analyte element and for the equipment concerned. A variety of its concentrations are recommended, usually 0.1-10% (w/v) aqueous solution made alkaline with potassium or sodium hydroxide. After filtration through a 0.45-1am membrane filter this reducing solution is sufficiently stable for up to three weeks (Knechtel and Fraser, 1978).

For acidification, hydrochloric acid is most often used, although sulfuric and nitric acids are equally suitable in some cases. Optimum acidity ranges appear to be dependent upon the elements concerned and the types of hydride generators: 1-9 M for arsenic, bismuth and antimony, 1-3 M for germanium, 0.1-0.2 M for lead and tin, 2.5-5 M for selenium and 2.5-3.6 M for tellurium (Nakahara, 1983a). Hydrides of antimony, lead and tin were generated from non-aqueous media acidified with sulfuric acid (Azn~irez et al., 1987; Rabadan et al., 1990).

The efficiency of hydride generation is strongly dependent on the use of optimized chemical and physical parameters, but these are determined not only by the method of hydride generation, but also by the design of the apparatus including the heated quartz tube atomizer and the reaction chambers of large test tubes, glass vials, Erlenmeyer flasks, wash-bottles or pear-shaped flasks, and by the material of the reaction vessels with silanized and re-silanized glass. The radiotracer technique, therefore, has been used to find out the hydride generation efficiency (D~dina, 1986; Krivan, 1992). For example, D~dina (1986) found by use of a 75Se radiotracer that, under optimum conditions, selenium hydride is generated with an efficiency of 95% or better. Recently, Tesfalidet and Irgum (1989, 1991) proposed a unique, heterogeneous hydride generation in a packed membrane cell with an anion exchanger in the tetrahydroborate form for the determination of arsenic, selenium and antimony. More detailed information on the optimum conditions for individual analytes can be found in review articles (Nakahara, 1983a; Godden and Thomerson,


1980). Very recently, Lin et al. (1992) proposed a new electrochemical generation- flow injection system for the determination of arsenic, selenium and antimony.

!11. HYDRIDE TRANSPORT

Once generated, the hydrides formed have been manipulated in many ways. Generally the hydride is first flushed from the solution with argon, nitrogen or helium as a cartier gas. The hydride evolved is either conveyed directly into the atomization system as it is generated (direct-transfer mode), or some form of storage is used before transfer to the atomizer (collection mode). The former method uses a continuous-flow or flow-injection mode; in the latter, the hydride formed is introduced all at once as a plug into an atomizer.

A. Direct-Transfer Mode

After the introduction of sodium tetrahydroborate as a reducing agent, the direct-transfer mode became considerably popular because the reduction reactions shown in the equation 1 proceed much faster than those in the zinc reduction system. It is not necessary to collect the evolved hydrides, and most laboratory-made or commercially available hydride generators now allow direct introduction of the hydrides into an atomizer. Some hydrides that are generated very rapidly or that are unstable in the gas phase should be transported directly to an atomizer, whereas collection procedures have been proposed for the hydrides which are slowly evolved. Chapman and Dale (1979) have comprehensively compared the two methods, collection or direct transfer of the liberated hydrides, taking into consideration the differences in rates of hydride production of the elements concerned. They have concluded that the hydrides of arsenic, bismuth and selenium require the collection mode, while those of lead, antimony, tin and tellurium need the direct-transfer mode.

The direct-transfer method can be divided further into three modes: continuous- flow, flow-injection and batch. In the continuous-flow mode, both sample solution and tetrahydroborate solution are flowing continuously at a constant rate to the generator to establish a steady-state signal (in contrast to all the other modes which provide transient signals). Continuous-flow equipment, with peristaltic pumps for sample and reagent as in the auto-analyzer system, has been used for hydride generation both with (Yamamoto et al., 1985) and without (Sturman, 1985) air segmentation, and the hydride has been separated with conventional gas-liquid separators (Yamamoto et al., 1985) or membrane separation (Yamamoto et al., 1987). These systems have the advantage of intimate mixing of reagents with better pH control, and also they appear to be much more tolerant of elements which normally interfere in the hydride generation. However, they usually have a lower detection limit compared to batch systems. Such control over reaction conditions


using a peristaltic pump was applied to generate arsine selectively from various arsenic derivatives, allowing an estimation to be made for arsenic(III), arsenic(V), monomethyarsonic acid and dimethylarsinic acid in aqueous solution (Anderson et al., 1983a).

A direct-transfer mode that includes continuous sample flow has the advantage that it provides a constant output signal as soon as equilibrium is reached. However, use of the very successful and readily available flow injection-type equipment has provided methods in which discrete samples are introduced repeatedly with a resultant transient signal. Using this flow injection mode, Pettersson et al. (1986) attained a detection limit of 0.1 lag/1 for selenium. Pacey et al. (1986) combined a flow injection system, with intermittent sample introduction into a continuous reagent-flow, microporous membrane separator that had carrier gas on the outside of the membrane for hydride recovery, with a flame-in-tube type atomizer in their automated system for the determination of arsenic.

In the simplest operation of the batch mode, the generated hydride (together with excess hydrogen) is transported immediately to the atomizer normally in a carrier gas. In equipment of the "stopped flow" type (Welz and Melcher, 1981) the sweep of the hydride from the reaction vessel is delayed for a few seconds to allow the generation reaction to proceed to completion, but the inherent instability of the hydride must be taken into consideration when this technique is contemplated.

B. Collection Mode

Most of the older procedures included some form of collection of the hydrides. The collection may be done either in a closed vessel, where the hydride generated from the sample solution is collected under pressure together with hydrogen resulting from tetrahydroborate decomposition (pressure collection), or in a cold trap which is usually a U-tube, immersed in liquid nitrogen, through which hydrogen passes freely and is not collected (cold-trap collection). The marked advantage of a cold-trap collection, which currently is used more frequently than the pressure collection, is that the hydride can be collected from a virtually unlimited volume of sample. The trapping equipment of Piwonka et al. (1985) appears to be a very significant development, particularly for low analyte levels (in the pg range). Isotope tracer studies using 75Se have confirmed a 99% recovery of hydrogen selenide trapped in a straight quartz tube packed with silanized Chro- mosorb W which is retained by silanized quartz wool plugs. A modification of the collection mode is trapping of the gaseous hydrides in a heated graphite furnace atomizer with subsequent determination by AAS. Trapping (in-situ preconcentration) of the hydrides in the pre-heated graphite furnace at a low temperature (e.g., 600 ~ is followed by atomization at a high temperature (e.g., 2700 ~ (Sturgeon et al., 1986). One would expect the collection step to eliminate the possible influence of the hydride generation kinetics. Presently, however, direct-transfer methods unambiguously prevail over collection techniques.


IV. METHODS OF ATOMIZATION, EXCITATION AND IONIZATION

A variety of analytical atomic spectroscopic techniques have been coupled with the hydride generation method. In this section, methods of atomization, excitation and ionization will be described in four major categories of analytical atomic spectroscopy, i.e., AAS, plasma AES, AFS and ICP-MS.

A. Atomic Absorption Spectrometry (AAS)

The following atomizers are presently in use: flames, flame-in-tube atomizers, flame-heated quartz tube atomizers, electrically-heated quartz tube atomizers and graphite furnace atomizers.

Flames

The useful resonance lines for arsenic and selenium are below 200 nm in a region where spectral interferences from flame radical absorption is very damaging. Even radicals from the relatively transparent nitrogen (entrained air)- and argon (entrained air)-hydrogen flames give rise to considerable absorption in this area and many molecular species are present in the low-temperature flame. These diffuse flames are inferior to the other atomizers. The sensitivity is lower due to a marked dilution of the hydride with flame gases. The flame also has a high background absorption and its flicker noise deteriorates the detection limit. Presently, therefore, there is a trend toward use of other types of atomizers such as heated quartz tube atomizers to be described below.

Flame-in-tube Atomizers

The excess hydrogen evolved during hydride generation can normally be used to carry the generated hydrides to a T-shaped quartz tube. A small amount of oxygen or air is added to support combustion of the hydrogen and atomization of the hydrides. This type of quartz tube, internally heated with combustion flame, is called a flame-in-tube atomizer. A typical flame-in-tube atomizer (D~dina, 1982) is shown in Figure 1 and consists of two parts, an intake part and a T-tube, both made of quartz and connected by a standard joint. The horizontal bar of the T-tube is aligned in the optical path. The hydride transported from a generator by a flow of hydrogen enters the left side of the intake part. Oxygen is introduced through a capillary into the right side of the intake part. Its flow rate is much lower than that of hydrogen. Thus, a very small, almost invisible flame burns at the end of the capillary. The end is usually 2-10 mm in front of the T-tube junction, so that the flame burns in the inlet arm of the T-tube. A considerable range of hydrogen and oxygen flow rates has been used: 2-200 ml/s of hydrogen and 0.05-3.5 ml/s of


oxygen. The flame-in-tube atomizers are often used to investigate mechanisms of hydride atomization to be mentioned later.

Flame-heated Quartz Tube Atomizers

Thompson and Thomerson (1974) first reported the use of a flame-heated quartz tube atomizer for hydride atomization. They used a stoichiometric air-acetylene flame to heat an open-ended silica tube to which the liberated hydrides were directly introduced through a side arm of the tube. The excess hydrogen was prevented from igniting at the ends of the tube by a transverse flow of nitrogen. The advantages of this technique are that no collection vessel is required and the flame background absorption is virtually eliminated and thus better sensitivity is achieved than with flames and flame-in-tube atomizers. Very recently, Le et al. (1992) investigated the hydride generation efficiency of arsenic, antimony and tin in an air-acetylene flame-heated quartz tube atomizer by the radioactive tracer technique. Mukai and Ambe (1987) used an air-acetylene flame heated alumina tube for atomization of arsine, instead of a quartz tube.

Electrically-heated Quartz Tube Atomizers

As an alternative to flames, flame-in-tube atomizers and flame-heated quartz tube atomizers, an electrically-heated tube atomizer was first used for hydride atomization by Chu et al. (1972). Since then, the electrically-heated quartz tube atomizers

iI I I i I I

I

/ T - tube / /

H Z + ~ 0 2

hydride

/ I

intake part

Figure 1. Flame-in-tube atomizer. Reproduced with permission from D~dina (1982) and The American Chemical Society.


have become the most commonly used atomizers (Nakahara, 1983a, 1990; Godden and Thomerson, 1980; Verlinden et al., 1981). The sensitivity obtained in these atomizers is in the same range as are those of the flame-in-tube atomizers and flame-heated quartz tube atomizers, and considerably higher than those of flame atomizers or graphite furnace atomizers. The superior sensitivity of quartz tube atomizers is due to their large dimensions and to the low dilution of the hydride either by flame gases or as a result of thermal expansion. Both these factors increase the residence time of analyte atoms and thus the sensitivity. Another advantage of this type of atomizer is the possibility to control its temperature and thus obtain an optimum tube temperature for atomization of each analyte element.

For arsenic and selenium in particular, increased noise levels at shorter wavelengths may be observed if the hydrogen escaping from the end of the quartz tube ignites, and a tube with quartz windows (Welz and Melcher, 1981; Parisis and Heyndrickx, 1986) or a nitrogen-flow screen across the ends of the tube (Agterden- bos et al., 1985) is desirable at high hydrogen flow rates.

The nature of the surface of the quartz tube is extremely important. Occasional cleaning with hydrofluoric acid is desirable and mist carry-over from hydride generation must be avoided. A hydrofluoric acid-cleaned quartz tube was observed to give slowly increasing results for about thirty determinations but then to remain stable in atomization efficiency, provided that it was retained in a desiccator when not in use (Parisis and Heyndrickx, 1986). Very recently, D~dina and Welz (1993) investigated the effect of atomizer design, purge hydrogen flow rate and atomizer temperature on the sensitivity of arsenic in quartz tube atomizers.

Some mechanisms of hydride atomization in externally heated quartz tube atomizers will be discussed in detail below.

Graphite Furnace Atomizers

Some of the interferences which obviously arise from the formation of heat-stable molecular species in the quartz tube atomizer can be alleviated by the use of a higher temperature atomization system. The graphite tube of the conventional graphite furnace atomizer has a length which gives relatively low sensitivity when used as the atomizer for a hydride in a gas flow, but arsenic and selenium have been collected in a graphite tube at 600 ~ and then atomized at 2600 ~ with excellent sensitivity (detection limit: 30 pg As and 70 pg Se) (Sturgeon et al., 1986). The use of a graphite furnace for atomization of hydrides was reviewed by D~dina (1988) who used a continuous-flow system with steady-state signal output to study the mechanism of atomization of selenium hydride in a graphite furnace. Although the sensitivity of a flame-in-tube atomizer of similar tube dimensions to the graphite furnace is better than that attained with the low-temperature graphite furnace, the stability and reproducibility of the latter is superior and warrants further investigation for routine applications. Dittrich and Mandry (1986a) designed an argon- shielded graphite paper atomizer (with a tube length of 92 mm and an inner diameter


of 9 mm) which could be heated to 1850-2000 ~ This effectively eliminated the interferences of antimony in arsenic analysis, and of arsenic in antimony, selenium and bismuth determinations (Dittrich and Mandry, 1986b). Wang et al. (1986) have used a modified long-tube graphite furnace. In their system, the hydride (arsine) is swept with helium into an alumina tube (190 mm long) placed within the graphite furnace, and an excellent low detection limit (0.2 ng/ml) has also been attained at temperatures above 1200 ~ Very recently, Sinemus et al. (1992) and Shuttler et al. (1992) reported on the combination of a flow-injection system with in-situ trapping on a heated graphite furnace atomizer.

B. Atomic Emission Spectrometry (AES)

Inductively Coupled Plasmas (ICP)

At present, the use of an inductively coupled plasma (ICP) as an excitation source in AES has become widespread for routine single- and multi-element trace analysis because of a number of good analytical figures of merit and the recent availability of many types of commercially available instruments. An obvious development is to combine the advantages of hydride generation with those of ICP-AES which is now relatively common in analytical laboratories. Unfortunately, the plasma discharge is more sensitive to changes in operating conditions than are the flames and other types of atomizers used in conventional AAS, and initially the technique was restricted to instruments of relatively high power (Nakahara, 1983a, 1990, 1991). However, the problems associated with the introduction of large amounts of excess hydrogen and also a small volume of water vapor into an ICP now appear to have been overcome to the extent that many publications refer to the use of instruments in medium- to low-power range. Many hydride-forming elements are found in concentrations too low for determination by conventional ICP-AES, and one of the main advantages of the use of hydride generation for sample introduction is the increase in sensitivity (by the order of one or two orders of magnitude) gained over liquid sample nebulization techniques because of the high transport efficiency of the hydride and the possibility of preconcentration. Notable advantages of ICP- AES include the possibility of simultaneous determination of multiple elements, a linear calibration over an extended range of concentrations, and greatly reduced matrix effects (particularly, spectral interference).

Two general approaches, batch and continuous, have been used to form and introduce the volatile hydrides of the elements of interest in ICP-AES. Thompson et al. (1978a,b) first reported the use of continuous hydride generation for multielement ICP-AES and described the possibility of simultaneous determination of trace concentrations of arsenic, bismuth, antimony, selenium and tellurium using a simple phase (i.e., gas-liquid) separator shown in Figure 2. The main requirements (Thompson et al., 1978a, b) for continuous generation are efficient mixing of the acidified sample solution and the tetrahydroborate, a short reaction time, separation


Figure 2. Gas-liquid separator for continuous generation of gaseous hydrides. Re- produced with permission from Thompson et al. (1978a) and The Royal Society of Chemistry.

of the product gases from the liquids, uniform mixing of product gases with the argon carrier gas, and a small positive pressure to sweep the product gases into the ICP.

One problem inherent to hydride generation is that the reaction by-products such as hydrogen, carbon dioxide and water vapor can cause instability or even extinguish the plasma. Nakahara (1983b) designed a continuous-flow system, similar in concept to Thompson's, for the determination of the hydride-forming elements, as shown in Figure 3. This "mixing-coil, double-stripping method" employs a four- channel peristaltic pump and a premix manifold to control both sample and reagent flows, and two gas-liquid separators to eliminate as much of the excess hydrogen, carbon dioxide and water vapor as possible. A four-channel peristaltic pump enables the inclusion of other reagents such as potassium iodide and/or hydrogen peroxide.

A miniature hydride generation system that eliminated the formation of large bubbles and provided a waste gas outlet to reduce the amount of hydrogen reaching the plasma has been reported by Yokoi et al. (1990a,b). Hwang et al. (1990) used a simple, inexpensive Y-fitting as a reaction cell and a high-solids cross-flow nebulizer and spray chamber as a gas-liquid separator which they called an "in-situ nebulizer/hydride generator". Watling and Collier (1988) determined the amounts of arsenic, antimony, bismuth and selenium simultaneously, with detection limits

Gr] o

High voltage supply

1 Amplifier J,,

I Digital Chart

integrator recorder

Tuning and coupling

R F power generator

ICP

Plasma torch ~._

l Hydride

-- ---~ Gas -]controller

Ar

Water

Water

Peristaltic pump

Reaction coil

Water

~ NaBH4 solution Sample solution Auxiliary ( I ) Auxiliary (1I)

Buffer tank

""~ Liquid waste 1st gas-liquid separator

Liquid waste

2nd gas-liquid separator

Figure 3. Schematic diagram of continuous hydride generation/ICP-AES system. Reproduced with permission from Nakahara (1983b) and The Society for Applied Spectroscopy.


around 1 lag/1. In their hydride generator, which is linked directly to the plasma torch, they pump sample solution and sodium tetrahydroborate over a specially designed dual platinum screen, through which argon flows, to give nebulization together with rapid release of entrained gases. Microporous polytetrafluoroethylene (PTFE) membranes and tubing have been investigated as gas-liquid separators for arsine generation/ICP-AES (Wang and Barnes, 1988). Recently, Tao et al. (1990) examined the use of a hydrogen separation membrane module, consisting of about 500 hollow fibers (13 cm x 0.5 mm) of an aromatic polyimide, to selectively remove hydrogen and to allow for the use of low-power ICP (0.6-1.0 kW), with improved detection limits by a factor of three over the same hydride system without the membrane. More recently, Qin et al. (1991) have developed an integrated nebulizer-hydride generator system in which an upright spray chamber for pneumatic nebulization is modified for use in hydride generation. This system allows simultaneous determination of both volatile elemental hydrides and other elements.

In order to prevent the ICP from being extinguished, Fry et al. (1979) used batch generation with condensation in a liquid argon-cooled U-tube for the determination of arsenic. Eckhoff et al. (1982) used a U-tube trap filled with Teflon shavings in a liquid nitrogen bath, and subsequently carried out a gas-chromatographic separation of the hydrides of arsenic, germanium and antimony on a column of Chro- mosorb 102.

As with batch generation, the use of a flow injection analysis (FIA) system with hydride generation results in transient signals, even though reagents flow continuously. The main advantages of flow injection for hydride generation include the ability to use small-volume samples as well as rapid sample throughput and ease of automation. Liversage et al. (1984) injected equivalent volumes (170 ~tl) of sample solution and tetrahydroborate solution into a flowing stream of water in the ICP-AES determination of arsenic. Wang and Barnes (1988) proposed the use of a similar FIA procedure with both PTFE tubing and membrane gas-liquid separators in the determination of arsenic, lead, selenium and tin. Furthermore, Pyen and Browner (1988) reported on the comparison of FIA and continuous sample introduction for automatic simultaneous determination of arsenic, antimony and selenium by hydride generation/ICP-AES.

In addition, post-column hydride generation has been used with high-performance liquid chromatography (HPLC) for chemical speciation studies. Bushee et al. (1984) used continuous hydride generation with HPLC for arsenic analysis and separation. In their work, the arsenic anions from the HPLC column were converted to the corresponding hydrides by a simple device which used two T-fittings. The first fitting was fed by the HPLC effluent and by acid solution from one channel of a peristaltic pump. The outlet from the first fitting was connected to another T-fitting which was fed with tetrahydroborate solution from the second channel of the peristaltic pump. The arsine, the remaining column effluent and hydrogen were fed directly into a cross-flow nebulizer, thereby introducing all species eluting from the column into the ICP.


Direct Current Plasmas (DCP)

Presently, the direct current arc plasma (DCP) differs from the classical direct current arc only in that the plasma produced between the two electrodes is displaced by a stream of argon and a "transferred" arc or non-current carrying plasma is obtained. The DCP can also be transferred by means of a third (external) electrode (a three-electrode "plasma jet"). Miyazaki et al. (1977) first used a batch generation of arsine and stibine by metallic zinc reduction and subsequent liquid-nitrogen condensation for DCP-AES.

The first use of continuous-flow hydride generation with DCP-AES detection was by Panaro and Krull (1984). In their system, the anal yte hydride was introduced directly into the conventional nebulizer of the DCP-AES instrument. Subsequently they extended this method to the total tin determination and speciation for methylated organotins by HPLC/continuous on-line hydride generation/DCP-AES (Krull and Panaro, 1985). Very recently, Chen et al. (1992a) combined hydride generation with an FIA system in the determination of antimony by DCP-AES.

Microwave induced Plasmas (MIP)

It is well known that a microwave induced plasma (MIP) of relatively low power (in general, less than 200 W) in argon or helium possesses a number of advantages as an excitation source in AES for the determination of a significant number of elements with high sensitivity. However, one principal interfacing difficulty generally encountered with the hydride generation technique is the instability or incompatibility of atmospheric pressure MIPs with a copious amount of excess hydrogen produced during hydride generation. This has been circumvented by the use of a liquid nitrogen condensation and gas-chromatographic separation on a Chromosorb 102 column to separate the analyte hydrides from the hydrogen evolved during the course of the generation reaction (Mulligan et al., 1979). In conjunction with a unique batch generation method of injecting a micro-volume of sample solution onto a sodium tetrahydroborate pellet placed in a glass tube, Barnett (1987) reported the successful application of a tangential flow torch in MIP-AES. Recently, Matusiewicz et al. (1990) combined the in-situ preconcentration of arsine in a graphite furnace with MIP-AES with good results. More recently, Bulska et al. (1993) compared hydride cold-trapping and hot-trapping in a graphite tube for the simultaneous determination of arsenic, antimony and selenium by MIP-AES.

Capacitively Coupled Microwave Plasmas (CCMP)

Although, in comparison with ICE DCP and MIP, a capacitively coupled microwave plasma (CCMP) has some good spectroscopic properties as an atomic spectral excitation medium, it has received less overall attention. Thus, only a few papers


(Nakashima, 1976; Atsuya and Akatsuka, 1981; Uchida et al., 1990) concerning CCMP-AES coupled with hydride generation have been published to date.

Nakashima (197 6) produced the hydrides of arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium by reaction with sodium tetrahydroborate, swept them into the CCMP and obtained their detection limits of 0.01-2 ktg in a 1.0-ml sample. Atsuya and Akatsuka (1981) described the combination of the CCMP-AES with a batch generation method for the determination of arsenic. Recently Uchida et al. (1990) evaluated helium CCMP for the determination of inorganic tin followed by hydride generation and collection in a cold trap, and also for the speciation and determination of butyltin compounds by interfacing the helium CCMP with a gas chromatography system.

Alternating Current Plasma (ACP)

A helium alternating current plasma (ACP) was developed and successfully interfaced with HPLC (Col6n and Barry, 1990). Subsequently, Col6n and Barry (1991) evaluated this ACP detector with a mixture of arsenic- and selenium- containing compounds after post-column hydride generation. Detection limits for the compounds under study ranged from 45--60 pg/s. Determination of arsenic and selenium in spiked fiver water samples has been used to demonstrate the applicability of the technique.

Flames

Andreae and Byrd (1984) have proposed the use of an air-hydrogen flame emission-type detector for the determination of trace amounts of inorganic tin and ethyltin compounds, the molecular band emission of tin monohydride being at 609.5 nm. In their work, the liberated tin hydrides are cryogenically trapped on a U-tube in liquid nitrogen and subsequently, upon warming, are separated.

In molecular emission cavity analysis (MECA), the analyte hydride is swept into an MECA cavity with a stream of carrier gas. Henden (1985) carried out a simultaneous reduction of arsenic, germanium, antimony and tin followed by gas-chromatographic separation on a column of 10% E-301 silicone gum rubber on Porapak Q, and measured the emissions at 490 nm in an oxygen-hydrogen flame with a cavity. Burguera and Burguera (1986) proposed a simple and rapid determination of arsenic in microliter volumes of sample solutions by FIA and hydride generation coupled with MECA. A1-Zamil and Townshend (1988) described the determination of tin by hydride generation, cold trapping and MECA in a nitrogen- hydrogen flame and detection of the tin monoxide emission band which peaks at 408 nm.


C. Atomic Fluorescence Spectrometry (AFS)

In AFS, the method of determining arsenic, antimony, selenium and tellurium by tetrahydroborate reduction with subsequent AFS detection of their evolved hydrides was initially studied by Thompson (1975). In his work, the analyte hydrides were passed directly into an argon-hydrogen flame maintained on a Pyrex tube. The atomic fluorescence was excited by use of modulated microwave-excited electrodeless discharge lamps (EDLs) and detected by a dispersive system at 193.7, 231.1,196.0 and 214.3 nm for arsenic, antimony, selenium and tellurium, respectively. The detection limits for these elements in a 15-ml sample solution (i.e., batch hydride generation) range from 0.06 to 0.1 ktg/1. Ebdon and Wilkinson (1984) described a similar hydride generation-dispersive AFS system for arsenic and selenium using an air-hydrogen flame which burnt on a glass "Y burner." Brown et al. (1985) proposed the use of an electrically-heated silica tube atom cell as an alternative to flame atomization for the selenium determination by hydride generation-AFS technique. Thus, most of the elements that are advantageously determined by generation of their hydrides are detected by AFS in the ultraviolet spectral region below 250 nm. This is a particularly useful spectral range for AFS measurements because there is very little energy emitted in the far ultraviolet region by the heating process which is necessary to convert the analyte hydride to an atom vapor. Furthermore, the spectrum is no longer complex due to matrix species because of resultant separation of the analyte from the matrix by the hydride generation reaction. Therefore, a non-dispersive measuring system is very feasible as will be mentioned below.

The present author has published a series of papers using hydride generation non-dispersive systems for the determination of arsenic, bismuth, lead, antimony, selenium, tin and tellurium using an argon (entrained air)-hydrogen flame and microwave-excited EDLs (Nakahara, 1983a, 1990). A similar hydride generation non-dispersive AFS system was used for the determination of lead (D'Ulivo and Papoff, 1985) and dialkyl- and trialkyllead (D'Ulivo et al., 1986). D'Ulivo et al. (1985) have described a unique system for multi-element non-dispersive AFS, based on exposure of the atomic vapor (produced in an argon-hydrogen flame) to the radiation from four radio frequency-excited EDLs (RF-EDLs) simultaneously, each being modulated at a distinct frequency in the kHz range, and one photomultiplier having its output monitored continuously by four lock-in amplifiers, each tuned to the frequency of the relevant RF-EDLs.

D. Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Although not strictly within the category of analytical atomic spectroscopy, the use of hydride generation with inductively coupled plasma mass spectrometry (ICP-MS) is worth mentioning in this chapter. Date and Gray (1983) first proposed the use of hydride generation with ICP-MS for the detection of arsenic; however,


few details were given in their paper. Powell et al. (1986) described their hydride generation/ICP-MS system and optimized several parameters including forward power, ion lens settings, sample gas flow rate, and concentrations of sodium tetrahydroborate and acid. Wang et al. (1988) and Janghorbani and Ting (1989) employed hydride generation/ICP-MS for isotopic analyses for lead and selenium, respectively. The combination of FIA-hydride generation with ICP-MS has been reported (Vrllkopf et al., 1990; Dean et al., 1990). Heitkemper and Caruso (1990) reported the simultaneous determination of volatile hydride-forming elements by continuous generation with ICP-MS. A silicone-rubber tubular membrane gas- liquid separator was used by Branch et al. (1991) to eliminate the interference of chloride (as ArC1 § with the determination of arsenic by hydride generation/ICP- MS.

Batch generation has been used by Jin et al. (1991) with ICP-MS detection for the determination of germanium species. Hydrides of inorganic germanium and of three methylated germanium species were passed through water and carbon dioxide traps prior to condensation in a liquid nitrogen-cooled U-tube packed with quartz beads. The hydrides were then volatilized and separated according to their boiling points. The method was used to detect germanium species in natural and waste waters with absolute detection limits of 0.1 pg for germanium and precisions at the 10 pg level ranging from 1.5-5.2% relative standard deviations (RSDs).

V. ANALYTICAL FIGURES OF MERIT

The analytical performance of hydride generation-atomic spectroscopy is characterized by figures of merit, such as detection limit, linear dynamic range, and precision and accuracy of measurements.

Detection limits for hydride generation systems as well as those for conventional solution nebulization are summarized in Table 1. Detection limits for arsenic, bismuth, germanium, lead, antimony, selenium, tin and tellurium (listed in Table 1) enable comparisons of various atomic spectrometric methods including flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), ICP-AES, microwave induced plasma atomic emission spectrometry (MIP-AES), AFS and ICP-MS. Perhaps the most important comparison is that between hydride generation/AAS, hydride generation/ICP-AES and the recent hydride generation/ICP-MS. Relative to conventional solution nebulization, detection limits by hydride generation methods are better by a factor of up to 1000 for certain elements. In general, because of the wide range of hydride generators and atomizers and the resulting differences in optimized experimental conditions, it is very difficult to accurately compare published data with regard to analytical performance of the hydride generation-atomic spectroscopy. Moreover, there is much confusion over the definition of the term "detection limit", so users of such

Table 1. Detection Limits (ng/ml) for Volatile Hydride-Forming Elements Reported for Several Atomic and Mass Spectrometric Techniques a

FAAS I CP-A ES MIP-AE~ A Fff ~ ICP-MS c

Solution Solution Element nebulization Hydride GFAAS nebulization Hydride

Solution Solution Hydride nebulization Hydride c nebulization Hydride

. . .a

~ r l As 100 0.02 0.2 20 0.02

Bi 20 0.02 0.1 20 0.3

Ge 200 3.8 b 0.2 10 0.2

Pb 10 0.6 b 0.05 20 1.0

Sb 30 0.1 0.2 60 0.08

Se 70 0.02 0.2 60 0.03

Sn 100 0.5 b 0.2 40 0.05

Te 20 0.02 0.1 50 0.7

0.35 100 0.1 0.4 0.0015

m 5 0.005 0.04 0.0007

0.15 100 - - - -

0.5 50 0.1 0.02 0.0003

1.25 40 0.06 1.0 0.03

2.0 50 1.2 ~

5 0.08 0.03 0.001

Notes: aReported by Perkin-Elmer Corporation, January 1991, unless otherwise stated.

bFrom Nakahara, 1983a, 1990.

CFrom V'ollkopf et al., 1990.


data as shown in Table 1 and in the literature should always check the definition applied in the original papers.

Precisions reported as percent relative standard deviations (RSDs) usually range from 2% to slightly higher than 10%. Thus, in general, the hydride-forming elements can be detected at concentrations below 1 ng/ml, and concentrations that are 10 or more times the detection limit can be measured with precisions less than 5% RSD.

Linear dynamic ranges for the hydride generation-atomic spectroscopy vary from two to five orders of magnitude, depending on the particular method used. Since the figures of merit for continuous generation are comparable to those for batch generation, and the operating procedure is simpler, continuous generation is the present "method of choice" for sample introduction of the hydride-forming elements.

VI. OPTIMIZATION OF PARAMETERS FOR HYDRIDE GENERATION

As already mentioned, there are generally two basic methods of hydride generation: (1) the hydride evolved from a sample solutions is directly swept into an atomizer (direct-transfer mode), and (2) the hydride is collected until the evolution is completed (collection mode) either in a container under pressure or in a cold (in-situ) trap. The trapped hydride is either swept into an atomizer in an open system with simultaneous evaporation in a heating bath, or it is first evaporated into a closed volume in a warming bath. Method (1) is performed either in a continuous-flow system or by batchwise reduction of the hydride-forming elements. D~dina (1986) gave full details of the optimization of hydride generation methods for AAS both in order to develop an approximate mathematical description of the hydride generation process which would make possible an efficient optimization of the experimental arrangement, and to test the theory by comparing predicted data with experimental results obtained with selenium hydride generation.

The simplex multivariate optimization, as opposed to a univariate method limited to a small number of parameters, enables a large number of interrelated, continuously variable parameters to be optimized with relative ease and speed in hydride generation systems coupled with AAS (Randall et al., 1986), ICP-AES (Tioh et al., 1986; Watling and Collier, 1988; Pyen et al., 1986; Parker et al., 1985), and DCP-AES (Ebdon and Sparkes, 1987).

VII. INTERFERENCES

In the following section an evaluation of interferences, including their elimination or minimization, will be described, rather than a complete summary of all interfer-


ences reported so far in the literature. Generally, two types of interferences, spectral and chemical, are associated with the use of hydride generation.

It should be noted here that there are wide divergences of opinion in the literature regarding the presence and severity of these interference effects. These differences are undoubtedly due to variation in the mixing dynamics of the tetrahydroborate with the acidified sample solution, and the speed of phase (i.e., gas and liquid) separation after the reaction, variations which are caused by the differences in design of the various hydride generation and/or detection systems.

A. Spectral Interferences

Spectral interferences arise from overlapping spectral lines or molecular bands. In the hydride generation techniques, however, the analyte element passes into an atomizer as a gaseous hydride after separation from the interferents, while concomi- tants normally remain in the reaction vessel. Consequently, because of the relatively small number of components in the gas phase in the atomizer, spectral interferences can be virtually excluded, and background correction is usually considered unnecessary for hydride generation-atomic spectroscopy. Nevertheless, in a significant number of published works, background correction is employed, most often in connection with flame-in-tube or graphite furnace atomization.

B. Chemical Interferences

Chemical interferences may principally occur either in the liquid phase during hydride formation and its transfer from solution (liquid-phase interferences) or they can affect the analyte in the gaseous phase (gas-phase interferences).

There are, in general, numerous interferences possible in the liquid phase, arising either from a slower formation of the hydride or from an inhibition of the hydride formation, partially or completely. A reduced rate of formation of the hydride may be the result of a competitive reaction when an accompanying material uses up most of the reducing agent (e.g., sodium tetrahydroborate) and only a small portion is left for the analyte (Agterdenbos and Bax, 1986a). Additionally, a different valence state or a different chemical environment can reduce the rate of hydride formation. Quite often, a competition for reduction between the analyte and the interferent(s) results not only in a slower formation of the hydride but also in a coprecipitation or in the formation of an insoluble compound of the analyte element (Welz and Melcher, 1984). In higher hydrochloric acid concentration and/or with lower sodium tetrahydroborate concentrations the interferent (e.g., nickel) is reduced to the metal, which is the interfering species, to a lesser extent, resulting in extension of the interference-free determination by more than three orders of magnitude (Welz and Schubert-Jacobs, 1986a). The mechanism of this phenomenon and of most other interferences is due to a preferential reduction of the interfering ion to the metal. It is probable that the finely dispersed, precipitated metal then adsorbs and


decomposes the gaseous hydride (nickel and other Group VIII elements are effective hydrogenation catalysts and can adsorb hydrogen in large amounts). Insoluble nickel arsenide or similar compounds may then be formed in a secondary reaction (Welz and Melcher, 1984). This explanation is supported experimentally. The precipitation of the interfering metal ion is certainly less pronounced at lower tetrahydroborate concentrations, and the precipitate is more soluble in a concentrated acid. Further, iron(III) has been found to have a releasing effect on the interference from nickel or copper in the determination of arsenic and selenium (Bye, 1986, 1987; Fairhurst et al., 1987). The reason for the favorable effect of iron(III) on the nickel or copper interference is probably the preferential reduction of iron(III) to iron(II) by tetrahydroborate ion before reduction of the nickel or copper ions.

In their study of the interferences in AAS and ICP-AES, Hershey and Keliher (1986) observed that 19 from a total of 50 elements produced an interference effect greater than 10%. Cobalt(II), nickel(II), palladium(II) and platinum(IV) reduced the signal severely. It appears that transition-metal ions are the strongest interferents in the determination of the hydride-forming elements by this technique.

Dittrich and Mandry (1986a, b) carried out studies of matrix interferences in the presence of hydride-forming matrices. By thermodynamic calculations, they suggested the formation of diatomic molecules between the analyte and matrix as a major source of matrix interferences in the gas phase within the graphite furnace atomizer. Consequently they recommended the use of a furnace temperature greater than 2000 ~ (Dittrich and Mandry, 1986b). A comprehensive review of interferences is available from D~dina (1988). Barth et al. (1992) reported on cross-interferences of hydride-forming elements.

It would appear that the mechanism of interferences will remain the subject of considerable discussion and interest in this field.

C. Possible Methods for Overcoming Interferences

Potential approaches to minimize or to eliminate these interference effects have been suggested by a number of workers. Possible methods of overcoming interferences are summarized as follows (Nakahara, 1983, 1990).

1. Use of the standard additions method. 2. Increasing the acidity of reaction solution and/or the concentration of a

reducing agent. 3. Adding masking agents including ethylenediaminetetraacetic acid, potas-

sium iodide, thiourea, ascorbic acid, malic acid, 1,10-phenanthroline, thiosemicarbazide and L-cystine.

4. Adding interference-releasing elements such as iron, copper, tellurium and mercury.


5. Use of separation techniques which are also useful as a pre-concentration step, thereby improving detection limits (sensitivities).

(i) Solvent extraction. (ii) Coprecipitation with lanthanum hydroxide, iron hydroxide, magne-

sium hydroxide, hydrated manganese dioxide, aluminum hydroxide and zirconium hydroxide.

(iii) Adsorbing colloid flotation of iron oxide, zirconium hydroxide and indium hydroxide.

(iv) Application of chelating resins such as Chelex-100, poly(dithiocarbamate) and poly(acrylamidoxime), and of a surface-modified silica gel.

VIII. ATOMIZATION MECHANISMS

When hydride generation with AAS was first developed, the atomization mechanism of the hydrides was not fully considered. It was thought that the hydrides were atomized by thermal decomposition. According to Welz and Schubert-Jacobs (1986b), atomization of gaseous, covalent hydrides in a heated quartz tube atomizer is caused by free hydrogen radicals rather than a thermal decomposition. The mechanism of radical formation is not yet fully understood but traces of oxygen appear to play an important role in the generation of free radicals according to the following equations:

H + 02 ~ OH + O (2)

O + H2 ~ OH + H (3)

OH + H2 ~ H20 + H (4)

Consequently, only OH and H radicals are formed in the presence of excess hydrogen. Owing to the very fast reaction (Eq. 4), a balanced state between them is readily established in which, under given conditions, OH radicals are outnum- bered by H radicals at least by a few orders of magnitude and therefore can be neglected. Thus, one may assume that only H radicals are formed in quantities corresponding to the total amount of oxygen, i.e., two radicals for each oxygen molecule.

The actual mechanism of hydride atomization proceeds most probably via interaction of hydride species with H radicals. In the case of arsenic hydride, for example, three consecutive reactions with H radicals are proposed as the actual mechanism of atomization (Welz and Schubert-Jacobs, 1986b; D~dina and Welz, 1992):

AsH3 + H ---) AsH2 + H2 (5)

followed by


AsH2 + H ---> AsH + H2 (6)

and

AsH + H ---) As + H2 (7)

and other hydride-forming elements may react accordingly. From this atomization mechanism, it can be concluded that two types of interferences are possible in a quartz tube atomizer: those which reduce the number of H radicals and thereby the atomization efficiency, and those which affect the decay of analyte atoms (Welz and Schubert-Jacobs, 1986b).

The coexistence of arsine, atomic arsenic and polymers of arsenic was studied by thermodynamic data and experimentally (Bax et al., 1986a). As a result, the most obvious formula for the decomposition reaction is

2ASH3 ~ 2As + 3H2 (8)

A description of the decomposition reaction with H radicals as reactants does not agree with the amount of radicals present in or near the cuvette (Agterdenbos and Bax, 1986b). No argument has been found against a decomposition reaction formulated as

4ASH3 + 302 ~ 4As + 6H20 (9)

which is catalyzed by H or OH radicals (Bax et al., 1986). Similarly, for the decomposition of selenium hydride, not only the following reaction (Eq. 10)

Sell2 ---) Se + H2 (10)

is possible (Agterdenbos et al., 1985), but also a reaction of the type

2Sell2 + 02 ~ 2Se + 2H20 (11)

has been suggested (Agterdenbos et al., 1986). The positive effect of oxygen and hydrogen on the atomization of hydride-forming elements in an electrically-heated quartz tube atomizer has been indicated in the determination of arsenic, selenium, bismuth, antimony and tin (Parisis and Heyndrickx, 1986), tin (Donard et al., 1986a), arsenic (Narsito and Agterdenbos, 1987) and selenium (Bax et al., 1986b; D~dina, 1992). The role of both gases may be to form free radicals, which probably participate in reactions such as are shown in equations 2-11. Recently, Welz et al. (1990) have used molecular beam sampling mass spectrometry to investigate the mechanisms which control atomization of arsenic in a heated quartz atomizer.

The mechanism of hydride atomization in graphite furnace atomizers is not usually addressed. Shaikh and Tallman (1978) and Wang et al. (1986) assumed that arsine is decomposed in the furnace and that arsenic is deposited on the surface and then volatilized and atomized. Dittrich et al. (1986) found that sensitivity for arsenic in their graphite furnace (at a temperature around 1800 ~ and in an externally


heated quartz atomizer dropped in the absence of hydrogen to 70% and 7%, respectively. They concluded from this that a thermal atomization mechanism plays the main role in graphite furnaces. However, at equilibrium the opposite effect should be expected: hydrogen should reduce atomization efficiency by forming arsenic-hydrogen molecules. It seems to suggest that even a temperature of 1800 ~ is not sufficient to ensure the equilibrium and a mechanism other than simple hydride decomposition at least participates. Dittrich and Mandry (1986b) observed that sensitivity for arsenic was independent of temperature in the range 1800-2200 ~ This is additional, however indirect, evidence for non-equilibrium conditions in the graphite furnace at these temperatures since, at equilibrium, sensitivity should decrease considerably with increasing temperature in this range due to a reduction of residence time at higher temperatures.

The process of determination of the elements which form volatile hydrides by AAS can be considered as consisting of two completely independent steps: hydride generation, followed by atomization. The theoretical peak shapes (absorbance vs. time profiles) have been obtained by developing a kinetic model and compared with

H3AsO3 H3AsO 4 AsH 3 Arsenous Acid Arsenic Acid Arsine

"Arsenite . . . . Arsenate"

/ O H CH3As~.g H

Methylarsonic Acid

(CH3)2As~g H

Dimethylarsinic Acid

(CH3)3As-O

Trimethylarsine Oxide

CH3AsH2 (CH 3)2 AsH (CH 3)3As Methylarsine Dimethylarsine Trimethylarsine

+ /O (CH3)3AsCH2C,,x O

Carboxymethyl (trimethyl) arsonium zwitterion

"arsenobetaine"

[(CH3)3AsCH2CH2OH]+X"

2-Hydroxyethyl (trimethyl) arsonium salt

"arsenocholine"

CH3 I

O-~--As CH,, 0 I - i " Y " ~ O-CH2-CH-CH2-~

OH OH Dimethyl (ribosyl) arsine Oxides

X, ~: OH, OH OH, SO3H OH, OSO3H NH2, SO3H OH, OPO3CH2CH(OH)CH2OH

Figure 4. Chemical forms and names of inorganic and organic compounds of arsenic and their corresponding hydrides.


experimental results (Van Wagen et al., 1987; D~dina, 1986). The kinetic factors that control the peak shapes can be employed to optimize any configuration of a hydride generator used in AAS (Van Wagen et al., 1987). A more detailed mathematical description of atomization as well as hydride generation is available from D~dina (1988).

In ICP-AES, a mathematical model has been proposed for the hydride transfer between the hydride generator and ICP (Wang and Barnes, 1986) and for the solution pH dependence of hydride generation (Wang and Barnes, 1987). Their detailed mathematical calculations have been experimentally verified for arsenic, lead, selenium and tellurium hydrides in ICP-AES.

IX. CHEMICAL SPECIATION

Speciation of an element is the determination of the individual physico-chemical forms of the element which together makes up its total concentration in a sample. Knowledge of the speciation in some types of samples is important because toxicity, bioavailability, bioaccumulation and transport of a particular element depend critically on the chemical form. The hydride generation procedure coupled with atomic spectroscopy can afford several methods for inorganic and/or organic speciation of some hydride-forming elements. These arsenic compounds and their corresponding hydrides, for example, are identified by name and structure in Figure 4.

A. Inorganic Speciation

It is well known that arsenic, antimony, selenium and tellurium commonly exist in water samples in two oxidation states: arsenic(III) and (V), antimony(III) and (V), selenium(IV) and (VI), and tellurium(IV) and (VI). Many workers have mentioned the effect of the valence of the elements to be determined in the sample on the rate (or efficiency) of hydride generation. Potassium iodide has frequently been used as a pre-reductant to determine the total concentration of arsenic and antimony (Ebdon and Wilkinson, 1987; Donaldson and Leaver, 1988; Tanaka et al., 1986b; Pacey et al., 1986). Antimony(V) was reduced to the trivalent state by digesting the sample solution in a hydrochloric acid containing potassium bromide (Fukuda et al., 1987). For pre-reduction of selenium(VI) to selenium(W), the addition of potassium bromide (Tanaka et al., 1986b) and tin(II) chloride (Subrama- nian and Mrranger, 1984), and the procedure of boiling the sample solution in 4-5 M hydrochloric acid (Piwonka et al., 1985; Apte and Howard, 1986) have been carried out to determine the total selenium content. The pre-reduction of tellurium(VI) to tellurium(IV) can be performed in a similar manner by boiling the sample solution for a period of 10-20 min after adjusting to a hydrochloric acid concentration of 2-6 M (Andreae, 1984). Furthermore, Boampong et al. (1987) have demonstrated that both arsenic(III) and arsenic(V) can be determined as total


arsenic at pH less than 1.0, without the need for any pre-reduction step, because identical results can be produced from both oxidation states.

Selective (or differential) determination of selenium(IV) and selenium(VI) has been made by measuring the total selenium after conversion of selenium(VI) to selenium(IV) in 4 M hydrochloric acid on a boiling water bath and separately by hydride generation of selenium(IV) alone in 4 M hydrochloric acid (Apte and Howard, 1986). Selective determination of tellurium(IV) and tellurium(VI) can be carried out in the same way (Andreae, 1984). Arsenic(III) and arsenic(V) (Pacey et al., 1986), and antimony(III) and antimony(V) (Tanaka et al., 1986b) can be differentially determined by measuring their total amounts after conversion of their higher oxidation states to the lower, with potassium iodide added, and separately by hydride generation of arsenic(III) or antimony(III) alone without any pre-reduction step. Differential determination of tellurium(IV/VI) and selenium(IV/VI) has been performed by using select pH complexation by a poly(dithiocarbamate) resin (Fodor and Barnes, 1983).

B. Organic Speciation

In addition to separation and quantitation of some hydride-forming elements such as arsenic, antimony, selenium and tellurium in their different oxidation states, a number of investigations have been performed on the determination of various organic forms of the elements of interest in environmental samples and biological materials.

For organic speciation, the performance of a hydride generation system consisting of a hydride generator and a detector (an atomic absorption spectrometer or a plasma emission spectrometer), with or without a gas chromatograph, is influenced by the rates of formation of the hydrides in the hydride generator. To become independent of kinetic limitations, a cold trap cooled with liquid nitrogen is frequently placed just after the hydride generator. The hydrides are flushed with a stream of carder gas (e.g., helium) from the generator into the cold trap where they condense. When the reduction is complete, the cold trap is warmed and the hydrides are volatilized to be separated and detected. Additional traps may be used to remove from the gas stream water and carbon dioxide that may interfere with the detection. These hydride generation systems with element-specific detectors have excellent detection limits and produce precise and accurate results.

Hydride generation systems with arsenic-specific detectors have found wide application for the identification and quantification of inorganic and organic arsenic compounds that are reducible to volatile arsines. Examples of arsenic compounds convertible to arsines by sodium tetrahydroborate are: arsenite and arsenate (reduced to AsH3), monomethylarsonic acid (reduced to CHaAsH2), dimethylarsinic acid (reduced to (CHa)2AsH), and trimethylarsine oxide (reduced to (CH3)As), as shown in Figure 4. Arsenite is the only arsenic compound reduced at a pH of 5. All other reducible arsenic compounds including arsenite are converted to the respec-


tive arsines in an aqueous solution of approximately pH 1. This difference in reducibility allows arsenite to be determined separately and removed from the solution. The various arsines formed at pH 1 are collected in the cold trap and then volatilized in the order of increasing boiling point into the detector. Better separation of the arsines can be achieved with a gas chromatograph.

Marine organisms are known to contain, in addition to inorganic arsenic compounds and simple methylated arsenic compounds, the more complex arsenic derivatives: arsenobetaine and ribosyldimethylarsine oxides (Edmonds and Francesconi, 1988). These two complex derivatives are converted upon treatment with 2 M aqueous sodium hydroxide for three hours at 95 ~ to reducible arsenic compounds, arsenobetaine to trimethylarsine oxide (eq. 12) and ribosyldimethylarsine oxides probably to dimethylarsinic acid (eq. 13):

NaOH ( C H 3 ) 3 A s + C H 2 C O O - ---) (CH3)3AsO

CH 3 i

O = A s - - C H 2

[ OR

! i OH OH

NaOH

(CH3)2AsO2H

(12)

(13)

An aliquot of a sample that has not been digested with base when analyzed with this hydride generation system will provide information about the presence of inorganic arsenic compounds and simple methylated arsenic compounds. Analysis of another aliquot that has been treated with sodium hydroxide will produce a more intense signal for trimethylarsine if arsenobetaine was present and for dimethylarsine if a ribosylarsine oxide was present. The differences between the dimethylarsine signals are related to the ribosylarsine oxide concentration in the sample, and the differences between the trimethylarsine signals to the concentration of arsenobetaine (Kaise et al., 1988). Atallah and Kalman (1991) proposed on-line photo-oxidation conversion of organo-arsenicals into arsenate for speciation.

With such a cold-trapping procedure, organic speciation has been carried out for arsenic (Tanaka et al., 1986a; Mukai and Ambe, 1987; Van Cleuvenbergen et al., 1988), germanium (Hambrick et al., 1984), lead (Donard et al., 1986b) and tin (Balls, 1987; Andreae and Byrd, 1984; Donard et al., 1986a,b; Randall et al., 1986; Valkirs et al., 1987; Han and Weber, 1988) by AAS and for tin by flame AES (Andreae and Byrd, 1984). The liquid nitrogen traps for freezing out the hydrides consist of a silanized glass wool column (Balls, 1987) and a U-shaped Pyrex tube filled with OV-1 (Valkirs et al., 1987), OV-3 (Tanaka et al., 1986a; Andreae and Byrd, 1984), PEG-20M (Mukai and Ambe, 1987), poly-m-phenylether (Van Cleu- venbergen et al., 1988), silicone oil (Hambrick et al., 1984) or SP-2100 (Donard et


al., 1986a,b; Randall et al., 1986; Han and Weber, 1988) on Chromosorb W (Tanaka et al., 1986a), Chromosorb 101 (Mukai and Ambe, 1987), Chromsorb WHP (Valkirs et al., 1987), Chromosorb WAW-DMCS (Van Cleuvenbergen et al., 1988; Ham- brick et al., 1984; Andreae and Byrd, 1984) or Chromosorb GAW-DMCS (Donard et al., 1986a,b; Randall et al., 1986; Han and Weber, 1988).

As an alternative to the preceding cold-trapping procedures, selective generation of arsines from arsenic(III), arsenic(V), monomethylarsonic acid (MMA) and dimethyarsinic acid (DMA) in different reaction media has been developed for arsenic speciation (Anderson et al., 1986a, b; Tsalev et al., 1987). For example, the reaction media studied have been shown to allow the rapid determination of As(III) alone, DMA alone, As(III) + As(V) and "total" arsenic, i.e., As(III) + As(V) + MMA + DMA (Anderson et al., 1986a, b). A similar procedure for discriminating between R3Pb +, R2Pb 2+ (R = methyl or ethyl) and Pb 2+ compounds has been proposed for lead speciation by non-dispersive AFS (D'Ulivo et al., 1986).

The separation of reducible compounds can be accomplished by column liquid chromatography, ion chromatography, or high performance liquid chromatography before the reduction step. The column effluent is then passed into the hydride generator, and the arsines flushed into the detector (Ebdon et al., 1988). A trap for the collection of the arsines and a gas chromatograph are not needed in this continuous-flow system. Peristaltic pumps deliver and dose the reducing agent, most commonly an aqueous solution of sodium tetrahydroborate made basic by addition of sodium hydroxide. Mixing coils assure complete mixing of the analyte with the reducing agent and quick reduction, and gas-liquid separators send the gaseous arsines to the arsenic-specific detector and the liquids to waste. If necessary, the analytes can be concentrated on a suitable column before separation by liquid chromatography and reduction and detection in a hydride generation system (Ebdon et al., 1988). Arsenite and arsenate were determined in water samples after separation on a C18 column by high pressure liquid chromatography, reduction to arsine and detection with ICP-AES (Bushee et al., 1984). Methyltin compounds were identified and quantified with DCP-AES as the tin-specific detector (Krull and Panaro, 1985). Similarly, high performance liquid chromatography (HPLC) has been used for separation of reducible arsenic compounds (i.e., As(III), As(V), MMA and DMA) followed by continuous-flow hydride generation-AAS (Tye et al., 1985; Chana and Smith, 1987; Haswell et al., 1985).

Subsequently, Clark et al. (1987) and Clark and Craig (1988) developed a novel "on-column" hydride generation method for the production of volatile hydrides of arsenic, antimony and tin for the gas chromatographic analysis of dilute solutions. In the case of the three butyltin species (Bu3Sn +, BuzSn 2+ and BuSn 3+, nominally) and Pr3SnC1 as intemal standard, for example, a 5-1al volume was injected for hydride generation into the gas chromatograph (column, 10% SP-2100 previously doped with 0.01 g of solid sodium tetrahydroborate, temperature programmed from 80 to 200 ~ at 24 ~ with an AAS detection system.


X. APPLICATIONS

A number of criteria could be used in judging the potential of the hydride generation technique coupled with analytical atomic spectrometry as an analytical method, but perhaps one of the most fundamental considerations is whether or not the technique is suitable for analysis of a wide variety of practical samples.

A. Sample Digestions

In particular, some conditions for sample digestion as well as hydride generation are important to specific elements to consistently attain good precision and accuracy in the practical application works. Many decomposition procedures have been proposed for practical samples. For example, 10 digestion methods were investigated for the determination of arsenic in soils by hydride generation-AAS (Van Der Veen et al., 1985). These methods included dry ashing/digestion, several acid-leaching procedures, and digestions in a pressure decomposition vessel or a Kjeldal apparatus. As a result, a nitric/sulfuric acid digestion was found to be the most suitable for soil samples. In another paper (Pettersson et al., 1986), four digestion methods were compared for the determination of selenium in NIST SRM bovine liver: nitric acid-bomb digestion, nitric-perchloric-sulfuric acid digestion, nitric acid-magnesium nitrate digestion and nitric-perchloric acid digestion. All procedures gave concordant results, provided that the standard additions method was employed. Similarly, another four methods of dissolution were evaluated for marine sediment (de Oliveira et al., 1983).

1. Initial open vessel digestion at room temperature with concentrated nitric acid, followed by digestion with a mixture of nitric, perchloric and sulfuric acids on a hot plate.

2. Fusion with sodium hydroxide using porcelain or nickel crucibles. 3. Acid digestion with a mixture of nitric, perchloric and hydrofluoric acids in

sealed Teflon vessels. 4. Fusion with potassium hydroxide.

For arsenic, all values obtained by all four digestion procedures agreed with the certified values. For antimony, only the potassium hydroxide fusion procedure yielded accurate results. Two dissolution procedures provided consistent results for selenium" sodium hydroxide fusion and acid digestion in sealed Teflon vessels. Many digestion methods have been compared for a variety of practical samples (Sturgeon et al., 1986; Crock, 1986; Welz and Melcher, 1985; Hansson et al., 1987; Bunker and Delves, 1987). Recently, Tsalev et al. (1992) proposed on-line microwave sample pre-treatment for FIA-hydride generation for arsenic, bismuth, lead and tin.


B. Practical Applications

Table 2 summar izes applicat ions o f the hydr ide generat ion techniques coupled

with AAS, AES, AFS and ICP-MS publ ished over the last five years. It provides a

comprehens ive list of sample types and analytical a tomic (or mass) spect rometr ic

techniques for arsenic, bismuth, germanium, lead, antimony, selenium, tin and

tel lurium. The applications are intended to be representat ive, rather than inclusive.

Table 2. Recent Applications of Hydride Generation Techniques A. Blood, Serum, Plasma, Urine, Other Clinical Samples

AAS: As in urine [speciation] (Miirer et al., 1992a,b) and urine [HPLC separation] (Hakala and Pyy, 1992); Bi in plasma, erythrocytes and urine (Froomes et al., 1988), human blood and urine (Wan and Froomes, 1991); Se in human blood components (Hansson et al., 1989a), blood plasma and serum [FIA] (McLaughlin et al., 1990), blood serum (Mayer et al., 1992), and human body fluids (Negretti et al., 1990); Te in urine (Kobayashi and Imaizumi, 1991).

AFS: Se in urine and blood serum [non-dispersive] (D'Ulivo et al., 1993). B. Biological Tissues

AAS: As in clam [photo-oxidation of organic As by UV] (Cullen and Dodd, 1988), NIST biological S RMs (Arenas et al., 1988 ) and bovine liver (Mayer et al., 1992); Pb in biological samples [in-situ pre-concentration] (Aroza et al., 1989) and biological materials [aqueous slurry technique] (Madrid et al., 1989); Se in biological samples (Tamari et al., 1992); Sn in biological samples (Tsuda et al., 1988).

ICP-AES: As in biological samples (Tracy et al., 1991); Se in biological and environmental SRMs (Tracy et al., 1990); Sn in biological samples (Yokoi et al., 1990b).

ICP-MS: As in NIST SRM oyster tissue and freezed-dried urine (Heitkemper and Caruso, 1990); Se in biological samples [isotopic determination] (Janghorbani and Ting, 1989; Ting et al., 1989) and biological reference materials (Buckley et al., 1992).

C. Environmental Samples AAS: As in soil extract, river water and drainage water (Glaubig and Goldberg, 1988),

freshwater algae [speciation] (Maeda et al., 1987), solid environmental samples [cold acid slurry digestion] (Haswell et al., 1988), marine algae arsenosugars [speciation] (Howard and Comber, 1989), marine atmosphere [speciation] (Nakamura et al., 1990) and marine green algae (Takimura et al., 1990); Pb in coal fly ash, tea leaves, sediment and pine needles (Zhang et al., 1989b), airborne particulate (Nerin et al., 1989) and river sediment [atomization under low pressure] (Zhang et al., 1992); Se in sediments (Itoh et al., 1988; Velinsky and Cutter, 1990), soil and coal fly ash (Nham and Brodie, 1989) and fly ash, flue gas cooler condensate sludge (Ericzon et al., 1989); Sb in marine sediments [slurry hydride generation] (de la Calle Guntifias et al., 1991a); Sn in estuarine sediments [speciation] (Cooney et al., 1988), eelgrass [speciation] (Francois and Weber, 1988), marine sediment (Astruc et al., 1989), sediments and biological samples (Desauziers et al., 1989), seaweed and sediments [speciation] (Harriott et al., 1991b) and fiver sediment [cold trap-GC separation] (Cai et al., 1992); As and Se in environmental and agricultural samples (Hershey et al., 1988); As, Sb



ICP-AES:

AFS: ICP-MS:

D. Food Stuffs AAS:

and Se in oily wastes (Campbell and Kanert, 1992); As, Sb, Se and Sn in sediment standard materials [atomization under low pressure] (Baogui et al., 1989). As in sediment (Aizpun Fernandez et al., 1992) and environmental samples (Hwang et al., 1990); Bi in solid wastes (Sarzanini et al., 1991 ). Se in coal fly ash, sea water and fish [non-dispersive] (D'Ulivo et al., 1990). As in edible red alga [HPLC separation-speciation] (Shibata et al., 1990).

As in beer (Cervera et al., 1989a), tomato products (Cervera et al., 1989b), tuna, orchard leaves and other foods (Holak and Specchio, 1991), sea foods and human urine (Momplaisir et al., 1991), sea food products (Ybafiez et al., 1992) and fish (Navarro et al., 1992a); Pb in fresh mussel, tomato and hay powder (Madrid et al., 1988), food sample [slurry procedure] (Madrid et al., 1990a), wines (Sanz et al., 1989; Cacho et al., 1992), and fish, water and wine (Madrid et al., 1990b); Se in fish reference materials (Hansson et al., 1989b) and wines (Eschnauer et al., 1988); Sn in oyster samples (Rapsomanikis and Harrison, 1988) and food digests [on-line ion-exchange separation] (Fang et al., 1992); As and Se in whole fish (Brumbaugh and Walther, 1989) and foods (Alvarez and Capar, 1991); As, Sb, Bi, Pb, Sn, Se and Te in wines and beverages (Baluja-Santos and Gonzalez- Portal, 1992).

E. Geological Samples AAS: As in soil (Huang, J. et al., 1988); Bi in geological reference materials (Chan et

al., 1990); Ge in iron meteorites (Guo and Brooks, 1990); Pb in soils and ferromangano brass (Li et al., 1990); Sb in ores and related materials (Donaldson, 1990); As and Se in soil etc. (Chan and Sadana, 1992); Sb and Bi in soils (Asami et al., 1992); As, Sb, Bi and Se (Kuldvere, 1989).

ICP-AES: As, Sb and Bi in geological samples (Zhang et al., 1988). DCP-AES: Bi in geological reference samples (Per~'n~d et al., 1989); Sb in geological

samples (Per~n~iki and Lajunen, 1988). MIP-AES: Se in soil (Ng et al., 1989).

AFS: As, Sb and Bi in geological samples [non-dispersive] (Xiaowei et al., 1989; Guo et al., 1992).

ICP-MS: Bi in standard rocks (Akagi et al., 1990). F. Metallurgical Samples

AAS: As in Ni-base alloy [on-line matrix removal by strong cation-exchange micro- column] (Riby et al., 1989). Ni alloy (Hanna et al., 1992) and Ni-base alloy [FIA matrix isolation] (Tyson et al., 1992); Bi in steels and Ni-base alloys (Bettinelli et al., 1992); Sb in Cu-based alloys (Harriott et al., 1991a); Se in Cu metals [matrix isolation with cation-exchange resin] (Offley et al., 1991) and Cu and Ni material (Wickstrom et al., 1991); Sn in steel samples (Zhang et al., 1992; Mclntosh et al., 1992) and low-alloy steels (Welz et al., 1992); Se and Te in Pb alloys (Fox, 1990); As, Bi, Sb, Se, Sn and Te in pure In metal (Sentimenti et al., 1990).

ICP-AES: As in white metal, cast iron, cupro-nickel (Menendez Garcia et al., 1989); As and Sb in Ni-Fe base alloy (Huang, B. et al., 1988).

DCP-AES: As in iron, steel and water (Chen et al., 1992a); Sb in Fe and Cu samples (Chen et al., 1992b); Sn in Cu, Fe and low-alloy steel (Brindle and Le, 1988). Pb in common lead and various galena samples [isotope dilution] (Wang et al., 1988). ICP-MS:

G. Plant Matter AAS:

ICP-AES:

As in vegetable samples (Navarro et al., 1992b); As, Se and Sb in mangrove leaves, polyester film and instant Chinese medicines (Lin et al., 1992). Ge in plants and animals (Hara et al., 1990).

1 70 TAKETOSHI NAKAHARA

Table Z (continued)

It. Water AAS:

ICP-AES:

DCP-AES:

MIP-AES: AFS:

ICP-MS:

I. Miscellaneous AAS:

ICP-AES:

As in estuarine pore-waters [speciation] (Ebdon et al., 1987), river water [speciation] (Sturgeon et al., 1989), estuarine and coastal waters [speciation] (Com- ber and Howard, 1989), fresh water and seawater [selective pre-concentration of As(Ill) by complexation-type stationary phase] (van Elteren et al., 1990), estuarine waters [speciation] (de Bettencourt and Andreae, 1991), NIST SRM water (van Elteren et al., 1991), fishery water samples [As(Ill, V) speciation] (Burguera et al., 1992), river water (Ueda and Kagaya, 1992), water samples [in-situ trapping on a deposit of reduced Pd] (An et al., 1992), river waters (Driehaus and Jekel, 1992), estuary water and interstitial waters [UV photolysis of arsenobetaine and arsenocholine] (Howard and Comber, 1992) and sea-water [speciation] (Michel et al., 1992); Bi in river water (Haruta et al., 1989); Sb in sea water [FIA system] (de la Calle Guntifias et al., 1991b) and water (sea and surface) and sediments (soils) (de la Calle Guntifias et al., 1992); Se in sea water [speciation] (Itoh et al., 1989), sediment-water extracts [speciation] (Masscheleyn et al., 1991), hot-spring water (Yamaya et al., 1992) and water samples (Ornemark et al., 1992); Sn in seawater [speciation] (Chamsaz and Winefordner, 1987), artificial sea water [speciation] (Chamsaz et al., 1988), seawater [butyltin speciation] (Stallard et al., 1989), water and sediments [cold trap speciation] (Schebek et al., 1991), sea-water [solvent extraction for butyltin] (Ni et al., 1991) and natural waters [in-line photolysis of butyltin compounds] (Ebdon et al., 1991); As and Se in river water (Narasaki, 1988); As, Sb and Se in water samples and urine [in-situ pre-concentration in graphite furnace coated with Pd] (Zhang et al., 1989a); As, Bi, Sb, Se, Sn and Te in water samples, urine and body fluids [optimization of parameters] (Welz and Schubert-Jacobs, 1991). As in water samples [on-line anion-exchange pre-concentration] (Schramel and Xu, 1992), aquatic media [speciation] (Rauret et al., 1991) and water [microporous poly(tetrafluoroethylene) tubing gas-liquid separator] (Barnes and Wang, 1988); Pb in sea-water [Chelex-100 pre-concentration] (Reimer and Miyazaki, 1992); As, Se and Sb in water (Pretorius et al., 1992). As in water (Brindle et al., 1992); Ge in natural waters [pre-concentration by coprecipitation] (Brindle et al., 1991). As in nearshore marine reference water (Matusiewicz et al., 1990). Se in sea water, chlorine-treated water samples, fish muscle and fish liver [non-dispersive] (D'Ulivo, 1989) and natural waters (sea, river and tap) [nondispersive] (D'Ulivo, 1988); As and Se in certified reference water samples [non-dispersive] (Corns et al., 1993). As in water reference material (Branch et al., 1991); Ge in natural waters and wastewaters [speciation] (Jin et al., 1991); As and Sb in water and sea-water [FIA system] (Stroh and VOllkopf, 1993); As, Se and Sb in reference waters and a natural water sample [simultaneous determination] (Haraldsson et al., 1992); As, Bi and Se in NIST water standard (Vijayalakshmi et al., 1992).

As in glass [separation by solvent extraction] (Rohr and Meckel, 1992) and HF (Bombach and Weinhold, 1989); Sb in PVC (Sanz et al., 1988); As, Bi and Sb in InP (MileUa et al., 1993). Ge in GaAs and polyethylene terephthalate [FIA system] (Nakata et al., 1988).

Hydride Generation Techniques in Atomic Spectroscopy 1 71

The references cited may contain additional determinations, or trials, for a particular sample type. It can be noted from Table 2 that recent applications are mainly for waters, biological samples, metals and metallurgical products, geochemical samples and foodstuffs. The variety and number of samples indicate that future studies involving hydride generation would be readily applied to the analysis of more complex samples.

XI. C O N C L U S I O N S

The hydride generation technique is widely used in the determination of low levels of those elements whose salts (or ions) readily form volatile hydrides with sodium tetrahydroborate. However, those using this technique must be aware of the conditions which have been reported to give rise to interferences in the generation reaction and take steps to minimize or eliminate these. Such an interference effect is perhaps the most important and widespread difficulty. Therefore, a further area in which much more useful work may yet be done is in the elucidation of the mechanism of interferences and in their reduction or avoidance. On the other hand, the hydride generation technique has the advantage that it affords a pre-concentration step and the analyte element is conveyed to the atomizer or excitation source with a much higher efficiency than is ever attained in a conventional solution nebulization method. The hydride generation procedure also allows the speciation of many hydride-forming elements to be determined.

In AAS, although atomization in a heated quartz tube atomizer is subject to some interferences from other hydride-forming elements, it affords a large increase in sensitivity over flame atomization and is valuable for very low level determinations. The heated quartz tube atomizer is particularly useful for the determination of arsenic and selenium because absorption wavelengths for these two elements are below 200 nm in an area which is subject to intense interference from flame backgrounds.

The hydride generation technique also affords improved detection limits for hydride-forming elements when used with plasma AES. For multi-element determinations some compromise of optimum reaction conditions may be necessary and interferences that occur during hydride generation must be considered, although atomization interferences should be negligible when compared with those of AAS. It is important to adopt an experimental technique which eliminates or at least reduces the adverse effect on plasma stability of the large volume of excess hydrogen in the hydride forming reaction.

Atomic spectrometric methods on their own do not yield information about the nature and distribution of the chemical species in which trace elements can exist. Additionally, the methods used to enable samples to be measured frequently destroy the matrix and thus also the original speciation of the analyte of interest. In order to obtain data relating to the speciation, the hydride generation technique coupled


with atomic spectrometry is one of the most suitable methods. In particular, the coupling of various HPLC techniques with ICP-MS may become the most promising for chemical speciation.

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THE EXCIMER LASER IN ATOMIC SPECTROSCOPY

Terry L. Thiem, Yong-lll Lee, and Joseph Sneddon

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

II. Excimer Laser Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

III. Excimer Laser Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

IV. Excimer Laser Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

A. Sample Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

B. Laser-Enhanced Ionization Spectroscopy in Flames and Furnaces . . . . 187

C. Laser-Excited Atomic Fluorescence Spectrometry . . . . . . . . . . . . . 188

D. Photoionization of Small Molecules . . . . . . . . . . . . . . . . . . . . 189

E. Fluorescence Detection in Liquid Chromatography . . . . . . . . . . . . 189

E Excimer-Laser-Induced Emission Spectroscopy . . . . . . . . . . . . . . 190

V. Excimer Laser Industrial Applications . . . . . . . . . . . . . . . . . . . . . . 208

A. Thin Film Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

B. Ceramic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

C. Thin Film Deposit ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

D. Excimer Laser Processing of Embedded Fibers . . . . . . . . . . . . . . 210

VI. Conclusion: Future Developments in Excimer Laser Applications . . . . . . . 211

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211


179

180 TERRY L. THIEM, YONG-ILL LEE, and JOSEPH SNEDDON

ABSTRACT

The use and various applications of the excimer laser in atomic spectroscopy are described. A description, brief theory, and properties of the excimer laser are presented. Its main applications in atomic spectroscopy are in sample introduction of solids to atomizers/excitation sources, to produce a plasma for atomic emission spectrometry, and as a pumping source for other lasers, most notably the dye laser, for subsequent application in laser-enhanced ionization spectrometry and laser- excited atomic fluorescence spectrometry. Other applications have included fluorescence detection in liquid chromatography and photoionization of small molecules. Finally a discussion of the application of the excimer laser in industry is presented for completeness, including thin film preparation and deposition, its use in ceramic materials, and processing of embedded fibers.

!. I N T R O D U C T I O N

Excimer lasers are being found to have an increasing number of applications in laser spectroscopy as powerful sources of ultraviolet (UV) excitation. Since the construction of the first laser, a ruby laser, by Maiman (1960), a multitude of different media have been employed to produce laser radiation including many solid state (such as ruby, Nd: YAG, Nd: glass, etc.), liquid dyes, and gas-phase species (such as CO2, N2, metal vapor, He-Ne, and excimers). Solid state lasers, predominately Nd:YAG lasers, have been used extensively in chemical applications in atomic spectroscopy. However, excimer lasers are being increasingly used owing to their high-energy coupling with refractory metal surfaces. These include sample introduction, laser-induced breakdown emission spectroscopy (LIBS), and for pumping dye lasers.

Excimer lasers were originally a class of short wavelength lasers based on radiation given off from excited dimers. Excimer emission was discovered by Hopfield (1930a, b) who found He2 emission in the extreme ultraviolet spectrum. Work soon followed using Hg2, Cd2, and other metal vapor excimer emissions that were discovered in close relationship to their respective atomic lines. Other rare gas excimer emissions were discovered by Tanaka and Zielikoff (1954a,b) and Tanaka (1955) who studied the emission spectra of Xe2, Kr2, and Ar2. The first demonstration of an excimer laser system was made by Basov et al. (1971), who demonstrated stimulated emission of Xe2 near 170 nm. Rare gas excimer lasers based on Kr2 at 146 nm and Ar2 at 126 nm were later demonstrated by Hoff et al. (1973) and Hughes et al. (1974). The addition of oxygen-bearing impurities to rare gases at high pressure produced lasing by XeO (540 nm), KrO and ArO (558 nm) (Powell et al., 1974; Hughes et al., 1976).

A major development in the excimer laser field was the discovery of rare gas monohalide emissions. In studies of product yield from argon metastable reactions, Golde and Thrush (1.974) identified an ArC1 continuum band at 170 nm. Velazco

The Excimer Laser in Atomic Spectroscopy 181

Table 1. Rare Gas Excimer Laser Wavelengths

Species Wavelength (nm) Reference

Ar2 126 Hughes et al. (1974) Kr2 146 Hoff et al. (1973) Xe2 172 Kohler et al. (1972) ArC1 175 Waynant (1977) ArF 193 Hoffman et al. (1976) KrC1 222 Murray and Powell (1976) KrF 248 Ewing and Brau (1975), Bhaumik et al. (1976) XeBr 282 Searles and Hart (1975) XeC1 308 Ewing and Brau (1975) XeF 351,353 Brau and Ewing (1975) XeO 540 Powell et al. (1974) ArO 558 Powell et al. (1974) KrO 558 Powell et al. (1974)

and Setser (1975) observed ultraviolet emissions from XeF, XeC1, XeBr, and XeI. Table 1 displays the respective wavelengths and a listing of rare gas excimer laser emissions.

I!. EXC|MER LASER THEORY

It is beyond the scope of this chapter to describe in detail excimer laser theory, which can be found elsewhere (Ewing, 1986). Excimer lasers operate on broadband transitions and therefore have small stimulated emission characteristics. Radiation from the rare gas excimers (RGE) occurs as the result of transitions from states in the bound, excited lY:+u and 31~+u electronic levels to the 1E+g repulsive ground state. Excimer lasers are most commonly pumped by electrical discharges (Hutchinson, 1980; Brau and Ewing, 1975; Burnham et al., 1976a, b; Wang et al., 1976) or by electron beams (Hutchinson, 1980; Brau and Ewing, 1975). The spatial characteristics of the radiation produced from the excimer laser depend on the type of pumping, usually electrical discharge, and on the configuration of the laser cavity. Electrical discharge excitation with UV pre-ionization typically produces rectangular beams. The spectral distribution of the radiation depends on the type of excimer laser in question as well as the configuration of the laser itself. The fluorescence from the laser with bound lower states typically contains a line structure caused by the transitions from the lowest vibrational level of the lowest excited electronic state to various vibrational levels of the ground state.

In rare gas-halogen mixtures, electron attachment followed by ion recombination is very important. For example, an argon fluoride excimer laser contains a mixture of Ar, F2, and helium buffer gas.


When the electrical discharge occurs, the reaction

e + F2 = F + F (1)

occurs in about 10 ns. This discharge also ionizes the Ar and results in the reaction:

Ar + + F- + He = ArF* + He (2)

In the three-body reaction, the helium simply acts as a buffer. Because the argon fluoride produced is electronically excited and has a short lifetime (about 2.5 ns), it rapidly decays by photon emission to the lower energy level. Since this is an unbound state, in which the force between the atoms is always repulsive, the diatomic molecule immediately dissociates. Thus this state never attains a large population and the needed population inversion exists between it and the higher energy bound state. The decay transition can thus be stimulated to produce laser emission with high efficiency, typically around 20%. A typical mix contained in the sealed cavity is 2% of the rare gas (Ar, Kr, or Xe), 0.2% halide gas (F2, C12, or Br2), and 97.8% helium, resulting in an overall pressure of 2.5-3.0 atmospheres (Andrews, 1986).

iil. EXCIMER LASER PROPERTIES

Excimer lasers present certain distinct advantages for material-processing applications in comparison to the other types of lasers. Most excimer lasers emit in the ultraviolet region. At these short wavelengths the reflectivity of most metals and ceramics is lower than at longer wavelengths and the absorption higher. For example, the percent energy absorbed by copper is only 1% at 10.6 lam (10,600 nm) whereas at 0.25 lam (250 nm) it is 70%. The same holds true for nickel and silver with values of 5% and 1%, respectively, at 10.6 lam and 58% and 77%, respectively, at 0.25 ~tm. In addition, ultraviolet photons are more energetic (4-6.3 eV) than those of longer wavelengths and therefore interact nonthermally with molecular materials and, in general, may induce photochemical reactions (Hon- tzopoulos and Damigos, 1991).

Woodroffe et al. (1980) measured the thermal and impulse coupling of a pulsed KrF excimer laser (248 nm) to an aluminum surface. They calculated the amount of heat deposited on the aluminum target following irradiation to be about 40% of the incident pulse energy for long pulses (0.5 s) at moderate flux levels (106-108 W/cm 2) and a spot diameter of > 0.5 cm.

The properties of several commercially available lasers from Lamda Physik Inc. (Acton, Massachusetts) are shown in Table 2. It is possible to obtain a pulse energy of as much as 1000 mJ (Lextra 300 in a KrF medium, see Table 2) at relatively high repetition rates. Current excimer lasers have overcome many of the problems of early designs and are more reliable, rugged, require less maintainence and are more straightforward to operate than early designs.

Table 2. Properties of Selected Commercially Available Excimer Lasers from Lamda Physik Inc. LASER Lextra 50 Lextra 100 Lextra 200 Lextra 300

Laser Medium ArF KrF XeC1 XeF

Wavelength (nm) 193 258 308 351

Max. Pulse Energy (mJ) 200 300 200 100

Max. Repetition Rate (Hz) 30

Max. Average Power (Watts) 3.5 9 5 3

Pulselength (ns) FWHM 17 23 17 20

Pulse to Pulse Stability (+) typ. 1 ~ 3 1.8 1.8 1.8

Gas Lifetime (106 pulses) 0.4 2 20 4

Beam Dimensions (mm 2, v x h) 5-12 x 23

Beam Divergence (mrad, v x h) 1 x 3

Time Jitter, typ. + (ns) 2

ArF KrF XeCI XeF ArF KrF XeC1 XeF

193 248 308 351 193 248 398 351

200 300 200 100 400 600 400 320

50 30

6 15 8 5 10 16 11 8

17 23 17 20 23 34 28 30

3 1.8 1.8 1.8 3 1.8 1.8 1.8

0.4 2 20 4 0.4 1 10 2

ArF KrF

193 248

800 1000

8

20

3

0.25

10

25

1.8

0.5

XeC1 XeF

308 351

800 650

10

8 6

30 25

1.8 1.8

1.5 0.5

5-12 x 23 5-12 x 23 10-15 x 30

l x 3 l x 3 l x 3

2 2 2


IV. EXCIMER LASER APPLICATIONS

A. Sample Introduction

Most conventional methods of spectrochemical analysis utilize a single high- temperature source both to vaporize and to atomize the sample. Since these are separate processes, the ideal spectrochemical technique would be able to separate and optimize the individual processes. Several different combinations of techniques, primarily using atomic emission spectrometry, have been employed to separate the processes, including: laser vaporization-inductively coupled plasma (Thompson et al., 1981; Carr and Horlick, 1982; Ishizuka and Uwamino, 1983), laser vaporization-direct current plasma (Mitchell et al., 1986) laser vaporization- spark excitation (Van Deijcke et al., 1979), laser vaporization-microwave-induced plasma (Ishizuka and Uwamino, 1980), laser vaporization-atomic absorption (Ishizuka et al., 1977; Wennrich and Dittrich, 1982; Wennrich et al., 1984), and laser ablation-inductively coupled plasma mass spectrometry (Denoyer et al., 1991). Laser ablation to introduce solid samples into various excitation sources is very attractive because of minimal sample preparation, efficient atomization of the solid sample surface regardless of the sample conductivity, and local analysis.

Most applications of the laser for sample introduction have utilized solid state lasers such as the Nd: YAG, using its primary (1064 nm) or secondary (532 nm) harmonic. The excimer laser, however, is starting to be increasingly used in this area owing to the efficient coupling of UV radiation to metal surfaces, as discussed in the excimer laser applications section. Chan and Russo (1991) studied laser-material interactions using an inductively coupled plasma (ICP) in order to optimize the conditions necessary for peak sample introduction performance. A KrF excimer laser was used with varying pulse energies of 200 to 550 mJ/pulse. The amount of material removed by laser sampling depended on the surface characteristics of the sample; a larger quantity of material was removed from an oxidized surface. The amount of material removed increased with the laser power density as well as the repetition rate of the laser.

In a similar study, Tremblay et al. (1987), ablated metals for sample introduction into an ICP using an XeC1 excimer laser for the analysis of nickel and chromium in stainless steel samples. They were able to determine both the temperature of the plasma produced by the ablation and the amount of sample that was being ablated. The estimation of 13,500 + 2000 K for the plasma temperature is in good agreement with the temperature determined by Lee and Sneddon (1992) for an ArF excimer- laser-ablated plasma. They determined that (3 + 0.7) x 1012 atoms of copper were removed as a result of each laser shot using a 99% copper rod and (6 + 1) x 1012 atoms of iron were removed after each laser shot using National Bureau of Standards no. 448 which contained 85.3% iron. In addition, they obtained analytical calibration curves for nickel at 232.003 nm and chromium at 267.716 nm that were linear over the range of 0.52% to 24.8% for nickel and 2.99% to 23.79% for


chromium. The detection limits (3(~) were 0.34% for nickel and 0.53% for chromium.

Hwang et al. (1991, 1992) studied the quantitative effects of laser radiation from an ArF excimer laser and developed a model which correlated a theoretical model for mass vaporized and increasing laser energy. The relationship was as follows:

m(t) = A I t + Bl2t 3/2 (3)

where A and B are constants determined by the thermal properties of the metals investigated (four in this series of experiments), m(t) is the mass ablated and I is the laser intensity.

Dye et al. (1991) studied Y203 pressed powder ablated pulses from an XeC1 laser operated at 308 nm, 150 mJ/pulse, 15-ns pulsewidth and 20 Hz. Their principal goal was to study the emission spectra produced in an oxygen atmosphere in the range of 10 -5 to4 x 10 -1 torr, at a peak fluence of 4 J/cm 2. A kinetic model was developed

to describe the results and the application to the production of laser-deposited high-temperature superconductor films was presented.

In another experiment, Pang et. al. (1991), utilized laser ablation for sample

introduction both into an ICP and for inductively coupled plasma-mass spectroscopy (ICP-MS). They used a high repetition rate (100-Hz) XeC1 excimer laser for sample ablation. The high repetition rate and relatively low laser power (50 mJ) was used in an attempt to improve the precision and sensitivity of the measurement. Their study included an experiment to determine the dependence of emission line intensity on the laser repetition rate. The result of this study can be seen in Table 3. As expected, more sample was ablated and transported to the ICP per unit time with the higher repetition rates. An absolute detection limit for titanium in the steel sample was calculated to be about 12 ktg/g at a signal to noise ratio (S/N) of 3 and a measurement time of 3 s.

There are several possible reasons for the improvement in precision of the measurement at a higher repetition rate. The moderate laser power and shorter wavelength may generate finer particles than do the higher power (100-500 mJ/pulse) Nd:YAG lasers commonly used. The formation of small particles increases the sample transfer efficiency and minimizes spiking cause by large

Table 3. Dependence of Emission Line Intensity on Laser Repetition Rates (3-s Integration Time)

Repetition Rate (Hz) Fe(l) 323.45 nm Ti(ll) 323.45 nm Cu(I) 324. 75 nm V(II) 326. 77 nm

10 3600 790 1300 60 50 20000 4500 7700 460

100 38000 9200 14000 860


Excimer Lascr

I ! ---

: lens ICP

t , II

__t._~.._ Sample Chamber

lens

spectrometer

, = t ,,,

Microcomputer

Ar gas IN

Figure 1. Experimental setup of the laser sampling ICP-AES system (reproduced, with permission, from Chan and Russo, 1991).

particles entering the ICP torch. In addition, signals from more vaporization events improve the statistics.

Sample was also introduced into an ICP for ICP-MS. Detection limits of about 4 parts per billion (ppb) for nickel and 3 ppb for chromium were determined using an S/N of 3. This values were comparable to or slightly better than those obtained from other laser ablation ICP-MS experiments using high-power, low repetition rate Nd: YAG laser systems.

In a subsequent study by Chan et al. (1992), the composition of the ablated vapor from the superconducting material Bi-Sr-Ca-Cu-O was monitored by introducing the vapors into an ICP. A picosecond, pulsed Nd: YAG (266 nm, 7 mJ) or a nanosecond, pulsed KrF excimer laser (248 nm, 30 mJ) beam was focused onto a sample at a 90 ~ incident angle. The vaporized species were transported through the central tube of the ICP torch with an argon gas carrier flow, and the elemental emission intensity from the ICP was monitored with a photodiode array spectrometer. A schematic of the experimental setup is shown in Figure 1.

The composition of the bulk sample was determined by dissolving a portion of the sample in 10% nitric acid and a small amount of hydrochloric acid. The resulting solution was then introduced into the ICP through liquid nebulization. The results of the comparative study between the different analyses are shown in Table 4.


Table 4. Comparison of Composition by Various Means Method Bi (%) Ca (%) Cu (%) Sr (%)

Nominal 28.6 23.8 31.8 15.8

Solution Nebulization 29.3 20.7 33.9 16.2

Excimer Laser 39.3 8.2 44.7 7.8

Nd:YAG Laser 38.9 10.5 40.8 9.8

The data in Table 4 for both excimer and Nd: YAG laser sampling show a

reduction of calcium and strontium and an increase in bismuth and copper in the

sampled vapor compared to the composition of the bulk material. This discrepancy indicates preferential vaporization of bismuth and copper which, if the surface temperature of the bulk sample due to irradiation is < 3000 ~ C (the boiling point of SrO), is expected, due to the higher vapor pressures of Bi203 and CuO compared to that of CaO and SrO.

B. Laser-Enhanced Ionization Spectroscopy in Flames and Furnaces

The need for the measurement of elements at extremely low concentrations is a major concern in many scientific fields and especially in the semiconductor industry. Graphite furnace atomic absorption spectroscopy (GFAAS) is the most widely used and standard technique for the determination of metals at low concentrations and has detection limits in the low picogram range for many elements. Although adequate for most types of analysis, in the semiconductor industry absolute detection limits between 0.1 fg and 0.1 pg are essential for dopant purity validation. One analytical method that has shown promise of achieving these low detection limits is laser-enhanced ionization (LEI) spectroscopy (Butcher et al., 1991).

LEI is a very sensitive laser-based spectroscopic technique which has been shown to be suitable for trace element analysis of solutions in flames and furnaces (Axner, 1990). In LEI, the atoms to be detected are excited from lower lying states, usually the ground state, to a higher lying, excited state by resonant light from one or two dye lasers. Ionization of these excited atoms occurs via collisions with other species, molecular or atomic, present in the flame or furnace, and hence an increased number of ions is produced. The ions formed are detected electronically as an increase in the current, which is measured when an electrical field is applied across the interaction region. The method has been shown to be very sensitive and detection limits in the picogram per milliliter (ppt) and nanogram per milliliter (ppb) range have been obtained for a number of elements. The high charge collection efficiency (close to 100%) is one of the reasons for the method's high sensitivity.


Many groups have used excimer lasers as pumping lasers for LEI research. Axner and Sjostrom (1992) used an XeC1 (308 nm) excimer-pumped dye laser system to study the one-step signal strength versus excitation transition. They proposed a general theory for the determination of the most sensitive one-step transitions for atoms in flames and validated a simplified expression for the one-step LEI signal strength as a function of the excitation wavelength. The derived equation predicted the optimum unsaturated one-step transitions in atoms to be states that are positioned approximately (1.5 + 4.5kT/Eion) below the ionization potential of the atom where Eion is the atom's ionization potential, k the Boltzmann constant, and T the temperature in degrees kelvin of the atomization source. The expression was verified experimentally by measurements in two simple atomic systems, lithium and sodium. Chekalin et al. (1991) used two tunable dye lasers pumped by an excimer laser (XeCI, 308 nm) with a pulse energy of 60 mJ, in conjunction with an electrothermal microsample evaporation from a heated graphite rod into a flame, for the laser-enhanced ionization analysis of high-purity germanium and alloys.

Butcher et al. (1991) employed an excimer-pumped (XeC1, 308 nm) dye laser in probe atomization for LEI in a graphite tube furnace. For elements with excitation transitions which promoted the atoms to energy levels within 7000 crn -1 of the ionization potential, such as thallium, indium, and lithium, detection limits in the 0.7-2 pg range were obtained. Lead, magnesium, and iron, which were excited to levels between 24,000 crn -1 and 31,000 cm -1 from the ionization limit, had detection limits between 10 and 60 pg. The linear dynamic range for each of the elements was between three and four orders of magnitude, and the precision for aqueous standards was between 12 and 16%.

Johnson et al. (1992) used an excimer laser (XeC1, 308 nm) to pump two dye lasers for the analysis of thorium isotopes by resonance ionization mass spectrometry. The autoionization level structure for thorium was about 2000 cm -1 above the ionization potential with lifetimes of a few picoseconds, and an ionization cross- section of about 10 -15 cm 2 was found. The authors felt that this information could be useful for future work on determining thorium in geological samples up to the 350,000-year range.

Smith et al. (1990a) studied the weak, forbidden transitions in laser-enhanced ionization spectrometry in an air-acetylene flame using an excimer-pumped dye laser. The authors used well-characterized laser pulses and a calibrated detection system to evaluate the absorption oscillator strength of the weak transitions. The authors used the example of the well-known intercombination line of magnesium at 457.110 nm for the measurement.

C. Laser-Excited Atomic Fluorescence Spectrometry

Laser-excited atomic fluorescence spectrometry (LEAFS) is an extremely sensitive method for detecting low levels of metals and has recently been reviewed by Butcher (1993). The technique involves creating ground state atoms in an atomizer


such as a flame or graphite furnace and using a laser to excite the atoms to a higher transition level and monitoring the subsequent re-emission or fluorescence. The most widely used systems have been with an excimer laser used to pump a dye laser.

Irwin et al. (1992) used an pulsed excimer laser (XeC1, 308 nm) to pump the dye laser to achieve graphite furnace LEAFS measurements for the determination of lead and cobalt in standard reference materials. Work by Liang et al. (1993a) determined tellurium and antimony in nickel alloys using LEAFS. Further work by Liang et al. (1993b) used a capacitive discharge LEAFS system to study thallium.

Simeonsson et al. (1990) measured thallium and indium by LEAFS with resonance line laser excitation. The sensitivity obtained was comparable to using a nitrogen-pumped dye laser with the advantage of this system being in the simplicity of design, ease of operation, no wavelength selection or wavelength drift, and good sensitivity.

Smith et al. (1990b) optimized a system for the laser-induced (excited) atomic fluorescence spectrometric determination of thallium in a graphite furnace. Careful attention to optics is required, particularly with regard to the rejection of spurious background due to fluorescence from surfaces in the optical path. A limit of detection of 0.1 femtogram for thallium was obtained.

D. Photoionization of Small Molecules

Allen and Cody (1985) described the multiphoton absorption of ArF excimer laser radiation (193 nm) by nitric oxide and water to produce positive ions, which were then measured with a simple parallel plate detector. For nitric oxide, the experiments indicate that the ion current followed a power law dependence, being proportional to the laser flux with a power of 1.75. This is consistent with a 2-photon absorption picture. For water, a power of 4 was observed, whereas a power of 2 was expected. The authors concluded that this anomalous behavior in the absorption of water by laser radiation underscores the need for more detailed experiments with water.

E. Fluorescence Detection in Liquid Chromatography

Van de Nesse et al. (1991) used an excimer-laser-pumped dye laser to study the two-photon-excited (TPE) fluorescence in conventional-size column liquid chromatography. Excitation was performed at three different wavelengths, 514 nm, 586 nm, and 650 nm, and emission light was collected at around 410 nm. The method was compared to conventional one-photon excitation under the same experimental conditions. Sufficient differences were obtained to suggest distinctly different selectivities. With this method, a detection limit for the dye 4,4'-diphenylstillbene of 1.0 nM was obtained.


F. Excimer-Laser-lnduced Emission Spectroscopy

Laser ablation, with the subsequent development of a high-temperature plasma, is receiving increased attention as an alternative to conventional plasma sources [i.e., inductively coupled plasma (ICP), direct current plasma (DCP), glow discharge, and so forth] for spectrochemical analysis of solid materials. When a high-power laser pulse is focused on a target, the irradiation at the focal point leads to rapid local heating, intense vaporization, and, in the case of laser powers greater than 109 W/cm 2, ionization of the vaporized species. The degree of interaction or plasma formation is dependent on many factors, including the characteristics of the laser and the material being vaporized.

Typical Experimental Setup

A typical experimental setup for space-resolved measurements is shown in Figure 2. In this setup, the pulsed excimer laser was used at a wavelength of 193 nm and a pulse energy of 100 mJ/pulse at a repetition rate of 1 Hz. The pulsed laser beam was focused onto the surface of a metal with the use of a focusing and collecting lens. One of the lenses was directly attached to a laboratory-constructed chamber. The focused spot size of the surface was typically 0.5 x 1.3 mm (0.65 mm2). The chamber allowed the atmosphere and pressure to be controlled and evaluated around the excimer-laser-ablated plasma. Once the plasma emission signal was generated, a lens collected and directed it to a monochromator and photodiode array system, and data collection and reduction was done by the computer. Space- resolved studies involved mounting the monochromator on a multiaxis stage to

GAUGE TO ~ ~ ,., VACUUM

, LI !

EXCIMER FOCUS DRYING / LASER LENS UNIT

Illill

C~vlPt.~R

U~NS ~"""]_M~CROMe-rER I I "

I I

INTERFACE

Figure 2. A typical experimental setup for space-resolved measurements of an ArF excimer laser-ablated plasma (reproduced, with permission, from Lee et al., 1992b).


Table 5. Spectroscopic Constants of Neutral Copper Lines Used in Boltzmann Plot Temperature Determination

Transition Wavelength Upper Energy Statistical Weights Probability [A] Uncertainties [~.(nm)] Level [Ek(cm-1)] [gk] [lOSs -1] [A( %)]

427.51 62403 8 0.345 25 465.11 62403 8 0.380 25 510.55 30784 4 0.020 25 515.32 49935 4 0.60 25 521.82 49942 6 0.75 25

allow the plasma emission to be monitored at different positions relative to the sample surface.

Excitation Temperatures of the Plasma

Temperature is one of the most important properties of a new excitation source and contributes to an understanding of dissociation, atomization, ionization, and excitation processes occurring in the plasma itself. Lee et al. (1992a) employed the Boltzmann plot method to estimate the excitation temperature distributions in an ArF-ablated plasma for both copper and lead. The relative atomic intensities of the five Cu(I) and Pb(I) lines used for this calculation are shown in the Tables 5 and 6.

Typical Boltzmann plots are shown in Figures 3 and 4. The lines were fitted to the data by least-squares regression.

The excitation temperatures at different positions were calculated from the slope of straight lines and are shown in Table 7. The Boltzmann plot method is limited in temperature determination because the transition probabilities are estimated with uncertainties up to + 50%.

The excitation temperature of the plasma obtained as a function of axial distance is shown in Figure 5. The temperatures were relatively high, ranging from 13,200

Table 6. Spectroscopic Constants of Neutral Lead Lines Used in Boltzmann Plot Temperature Determination

Transition Wavelength Upper Energy Statistical Weights Probability [A] Uncertainties [~,(nm)] Level [Ek(cm -1 )] [gk] [ 108s -1 ] [A (%)]

357.27 49440 3 0.99 50 363.96 35287 3 0.34 50 368.35 34960 1 1.5 50 373.99 48189 5 0.73 50 405.78 35287 3 0.89 50

15-

A

,r 14

,.~ 13

12 e ~

,_1

11 30000

�9 I ' " ' I " ' I '

40000 50000 60000 70000


ENERGY ( cm' l )

Figure 3. Typical Boltzmann plot [In(Ik/gA) versus energy] for Cu(I) spectral lines in the laser-ablated plasma (reproduced, with permission, from Lee et al., 1992a).

to 17,200 K for copper and 11,700 to 15,300 K for lead. The region of maximum temperature was at a position some distance from the surface, where the temperature was unexpectedly low.

In a subsequent study, Lee et al. (1992b) found that atmosphere played a large role in the excitation temperature achieved in the plasma. The estimated excitation temperatures in the center of the plasma of selected gases (air, argon, and helium) and various pressures is shown in Figure 6. The temperatures for the ablation of copper were again very high, ranging from 12,000 to 18,000 K depending on the atmosphere and the gas pressure. In general, it was discovered that temperature decreases with decreasing pressure. This may be due to less confinement of the plasma, allowing it to be distributed over a larger volume. At 760 torr in air and

1.o597e.,x RA2 - o. ,o I

,~, 8.0

7.0 , i I I �9 i �9 I �9

30000 35000 40000 45000 50000 ENERGY ( c m "1 )

Figure 4. Typical Boltzmann plot [In(IZ,/gA) versus energy] for Pb(I) spectral lines in the laser-ablated plasma (reproduced, with permission, from Lee et al., 1992a).

Table 7. Excitation Temperature with Increasing Distance from Copper and Lead Surface

Metal Axial Distance ( m m ) Temperature Correlation Coefficient

Copper

Lead

0.1 13,235 0.975 0.2 15,926 0.914

0.3 17,071 0.888 0.4 15,816 0.916

0.5 16,006 0.911

0.6 15,578 0.891 0.8 17,218 0.875 1.0 16,648 0.862 1.4 15,589 0.849 1.8 14,700 0.830 0.8 14,180 0.904 1.0 13,573 0.960

1.2 13,573 0.960 1.6 13,575 0.960 1.8 15,277 0.910

2.0 14,563 0.880

2.4 14,540 0.852 3.0 13,465 0.705

3.6 11..669 0.535

20000

18000 LLI I:Z: 16OO0 I-- ,r 14000 rr" LL! 12000

LU 10000 1--

8O00

. ~ . _ ~ ' a )

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4

A X I A L D I S T A N C E ( mm )

Figure 5. The distribution of the excitation temperature for laser-ablated plasma with copper and lead in the axial direction calculated at different positions from the surface in an atmosphere of air. (a) copper; (b) lead (reproduced, with permission, from Lee et al., 1992a).

193


17000

A 16000

tU 150O0 n"

I-- ,a: 14000

LU 13.

I, Li !'- 13000

12000

o Air

x Argon

0

�9 ' 1 ' i �9 I �9 i �9 i �9 i . . . . " . . . . . . . ! " " . . . . . . . . .

0 100 200 300 4 0 0 500 600 7 0 0 800

P R E S S U R E ( torr )

Figure 6. Estimated excitation temperature in the center of the plasma formed at a copper surface in three different atmospheres (reproduced, with permission, from Lee et al., 1992b).

argon, highly confined hot plasma does not emit a strong analytical signal but in fact emits an intense background continuum.

Space-Resolved Studies

Knowledge of the spatial distribution of emission from the laser-ablated plasma might afford some insight into the space-resolved techniques for the improvement of analyte detection in atomic emission spectroscopy. The distribution of the plasma is due to the shock wave propagation. The shock wave is created initially at the sample surface and propagates along the axial direction.

The appearance and spectral characteristics of the laser-ablated plasma are quite different, depending on the target species. Figure 7 shows the space-resolved spectra of the plasma formed with copper at different positions over the range 0.0-2.00 mm in front of the solid metal by use of a laser energy of 100 mJ/pulse. The spectrum displays the characteristic peaks of neutral copper (I) and several of copper (II). The copper in the plasma is at a high temperature and there may be considerable thermal ionization immediately above the surface because copper has high thermal conductivity (4.01 W/cm2K). Strong emission signals were observed with strong background continuum near the surface, which decreased farther away from the surface. The outer sphere of the plasma showed relatively low background continuum emission.

Lead, however, formed a plasma quite different from that of copper in both plasma size and spectral characteristics. Figure 8 illustrates the emission spectra

�9 S, ZJIA

WAVELENGTH (nm)

~.) c I , I t a b

,]] ]l Figure 7. Space-resolved spectra of laser-ablated plasma with copper in 760 torr air at A (a) 0.0 ram, (b) 0.1 ram, (c) 0.2 ram, (d) 0.3 ram, and B (e) 0.5 ram, (f) 0.8 ram, (g) 1.2 mm, (h) 1.6 mm, and (i) 2.00 mm from the surface. The characteristic peaks of neutral copper(i) are a-427.51 nm, e-465.11 nm, f-510.55 nm, g-521.82 nm, and 529.25 nm and copper(il) ion are 438.54 nm, b-450.6 nm, and c-459.69 nm (reproduced, with permission, from Lee et al., 1992b).

195

g ( X ) - A

!

r ~ 3oo- h e tz (c )

i c d , b

. . . . . . . . . . . , A (a

- 6 0

4nO 4~0 5O0

WAVELENGTH (nm) - , ,

B

f

lno- h (d )

] O 0 - ~

_ . . . . . . ,, , T

WAVELENGTH (nm)

Figure 8. Space-resolved spectra of laser-ablated plasma with lead in 760 torr air at A (a) 1.6 mm, (b) 2.0 ram, (c) 2.2 mm, (d) 2.4 mm, and B (e) 2.6 mm, (f) 3.0 mm, (g) 3.6 mm, and (h) 4.0 mm, from the surface. The characteristic peaks of neutral lead atoms are a-357.27 nm, b-363.96 nm, c-368.35 nm, d-373.99 nm, e-401.96 nm, f-405.57 nm, g-416.80 nm, and j-500.54 nm. No lead ions were found in the inner sphere (reproduced, with permission, from Lee et al., 1992b).

196


.m r

tl r Z0 b g

a h i

,, ]

I~11-

I l i t ) -

~0-

S

i

g h i

WAVELENGTH (nm)

Figure 9. Spectrum of laser-ablated plasma formed with lead in air at 1.6 mm (A) and 3.6 mm (B), from the surface (reproduced, with permission, from Lee et al., 1992b).

profile of the plasma formed with lead measured at different distances from the surface, ranging from 1.6 to 4.0 mm. In the inner sphere, atomic emission lines were predominately observed. No significant ion lines were present. A very low background continuum was observed within the inner sphere of the plasma, which extended to 1.8 mm from the surface and showed narrow atomic emission lines as shown in Figure 9.


320

>,, 280

240 O3 Z 200 i!1 I-- 160 Z " 120 J Ud 80 E 4O

0

(b)

' i ' ~ ' '1 " i " i

0.8 1.6 2.4 3.2 4.0 4.8

A X I A L D I S T A N C E ( mm )

Figure 10. Spatial distribution of the plasma emission intensity of Cu(i)at 465.11 nm and Pb(I) at 405.78 nm in air" (a) copper; (b)lead (reproduced, with permission, from Lee et al., 1992b).

In the region from 1.2 to 1.8 mm, an especially high signal to background ratio was found in air. This is the best region for spectrochemical analysis of this material. In the outer sphere of the plasma, which extended to 5.0 mm from the surface, neutral atomic emission lines decreased gradually in intensity, and the presence of ionic emission lines was detected. Lead may be vaporized and migrate noticeably from the surface without heat transfer because of its low thermal conductivity and boiling temperature as compared to the copper sample previously discussed.

The intensity of emission for both the copper and the lead was examined with respect to distance from the surface. The emission intensity profiles of copper at 465.11 nm and lead at 405.78 nm are shown in Figure 10. As can be seen, the highest intensity of emission for copper is at approximately 0.3 mm from the surface. Intensity decreases gradually and is almost completely gone by 2.0 mm. Lead, however, does not show emission until approximately 1.6 mm from the surface and peaks at approximately 2.4 mm. Lead's emission decreases gradually and is almost completely gone at 5.0 mm.

Atmospheric Effects

Lee et al. (1992b) have studied the atmospheric effects on the plasma formation, both its intensity and shape, for metal species using an ArF excimer laser. The form of the laser-ablated plasma for different atmospheric conditions on a target surface was observed photographically. The appearance of the plasma varies with the pressure and composition of ambient gas, owing to the ablated metal atoms diffusing away from the surface and colliding with the ambient gas. A sequence of single laser shot images that illustrate the dependence on pressure in selected ambient gases (air, argon, and helium) is shown in Figure 11. All photographs of

_

the laser-ablated plasma were taken from a direction at a right angle from the laser

Figure 11. Photographs of the single shot laser-ablated plasma emission formed with copper under various atmospheres [air (A), argon (B) and (C) helium] at different pressures [(a) 760 torr, (b) 400 tort, (c) 200 torr, (d) 100 torr (e) 50 torr, and (f) 10 torr]. Position of the metal surface is indicated by the white line (reproduced, with permission, from Lee et al., 1992b).

199


beam. As can be seen, the size of the plasma increases with a decrease in pressure in the ablation chamber, as is expected because of the increased diffusion and decrease in collisional deactivation.

In the case of the highest pressure used in this study (760 torr) and in the air and argon atmosphere, the shape of the plasma is nearly hemispherical. However, with a decrease in the pressure, the shape is gradually deformed, becoming flattened and extending along an axial direction from the surface, and changes to a jet-like shape. In the helium atmosphere, the jet-like shape is observed even at atmospheric pressure (760 torr). The results show that the shape and size of the plasma take on a significantly different character with decreasing pressure.

When the pressure is reduced below 50 torr in an air or argon atmosphere, the plasma consists of two distinct regions. One is near the target surface, called the "inner-sphere plasma," which emits a strong continuum signal. The other, called the "outer-sphere plasma," gives strong, sharp peaks of the ablated species with a minimal background signal. At pressures lower than 1 torr, the edge of the outer-sphere plasma disappears, whereas under a helium atmosphere, the two distinct plasma regions are observed at a pressure of 760 torr.

The relationship between the axial spread length of the plasma and the pressure of the surrounding gas is illustrated in Figure 12. The plasma is spread within about 2.2 mm at 760 torr, expands to 4.4 mm at 200 torr, and further to about 11 mm in axial distance from the target surface at 10 torr in an air atmosphere. The size of the plasma in an argon atmosphere is slightly smaller than that in air. The axial spread length of the plasma is considerably larger under a helium atmosphere, in comparison to those of argon and air. It is approximately 7.2 mm at 760 torr, 10.6

Figure 12. The relationship between the axial radius of the plasma formed with copper and the pressure of the surrounding gas (reproduced, with permission, from Lee et al., 1992b).


I-- z

0 r

200 COUNTS

A (a)

(b)

450 560 WAVELENGTH (nm)

(c)

o

500 COUNTS

450 560

(a)

J

WAVELENGTH (nm)

Figure 13. Emission spectra of the laser-ablated plasma in the center of the plasma under different pressures [760 torr (A), 50 torr (B)] and various atmospheres [(a) argon, (b) air, and (c) helium] (reproduced, with permission, from Lee et al., 1992b).

mm at 200 torr, and 17.4 mm at 50 torr. Under the atmosphere of air or argon, the size of the plasma, both inner-sphere plasma and outer-sphere plasma, was increased by reducing the pressure. The size of the inner-sphere plasma under a helium atmosphere was nearly the same in reduced pressure, but the outer-sphere plasma was spread further by reducing the pressure.

The atmosphe.ric condition influences the production of the ablated plasma as well as its excitation. As discussed by several authors (Iida, 1989, 1990; Talmi et al., 1981; Piepmeier and Osten, 1971), the surrounding atmosphere affects the emission spectra, the crater size, and the amount of sample vaporized with the laser beam.

Emission spectra were observed as a result of the plasma formed from the interaction of the forced laser beam and a copper target. The maximum emission


spectra within each copper plasma at various positions in different ambient gases (air, argon, and helium) at 760 torr and 50 torr are shown in Figure 13. The characteristic peaks of the neutral copper atom and the copper ion were observed in the plasma emission. The assignments of these peaks were shown in the previous section. The pressure of the surrounding atmosphere plays an important role in the development and emission of the plasma. Reducing the pressure from 760 to 10 torr leads to a 7-fold increase in air and an 11-fold increase in argon of the Cu(I) line intensity at 521.82 nm, but further pressure reduction results in a decrease in the line intensity. Despite the fact that the size and excitation temperature of the plasma change with the pressure, the primary reason for the pronounced maximum at 10 torr in argon and air seems to be an optimum in the atomization and excitation processes. The maximum emission intensity was obtained at 50 torr in helium atmosphere, a 1.5-fold intensity increase over that obtained in 760 torr, and then reduced at further low pressure. It was found that not only the diameter of the plasma but also the characteristics of the emission spectra vary with the gas pressure and composition.

Laser radiation absorption by the ambient gas in the plasma plume plays a substantial role in the target metal vaporization processes and the spectral emission characteristics. Ambient gas breakdown plasma emission was observed in air and argon atmospheres at relatively high pressure (>100 torr). Nitrogen ion (II) emission at 399.5 nm was detected at 760 torr and gradually decreased until it disappeared below 50 torr in an air atmosphere. An argon breakdown plasma in front of the metal vapor plasma can be seen clearly in the photographs of the plasma above 100 torr (Figure 11). An atmospheric plasma may occur in front of a plasma of sample material and propagate rapidly through the atmosphere along the direction incident laser beam.

Unlike the experimental results obtained by Iida (1989), a complex high-continuum background was observed at 760 torr of air because of the occurrence of chemical reactions, and dissociation of metal species, molecules, and radicals, as well as formation of stable molecules and ionization. Metal oxide formation with oxygen in air can give a broad molecular emission band. The background continuum was clearly reduced in the argon atmosphere, and especially under the helium atmosphere, in comparison to the air atmosphere.

The oxide formation of ablated metal species has also been reported by Beenen and Piepmeier (1984). They found that a significant quantity of metal oxide was formed in an atmosphere containing only trace amounts of oxygen. In fact, laser ablation of aluminum in an oxygen-rich atmosphere has been studied spectroscopi- cally to yield data on the reaction:

A1 + 1/2 02 = A10* = A10 + hv (4)

Figure 14 displays the spatial distribution of the emission intensity of the neutral copper atom at 521.82 nm at various pressures for different atmospheric gases (air, argon, and helium). The intensity was calculated as the difference between the total

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1500

1000

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0 0.0

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A X I A L D I S T A N C E ( r a m )

1500

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A X I A L D I S T A N C E ( m m )

1250 . . . . . . . . . . . . 50 t o r r

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A X I A L D I S T A N C E ( r a m )

Figure 14. Spatial distribution of the emission intensity of the plasma at various pressures for the surrounding gas of (A) argon, (B) air, and (C) helium (reproduced, with permission, from Lee et al., 1992b).

203


emission and the background. The emission intensity increases with decreasing pressure of gas. The maximum emission intensity was found at a point, near the edge of the inner-sphere plasma, some distance from the metal surface. With reduction in the pressure under an air or argon atmosphere, the position of maximum intensity moved away from the target surface, as can be seen in Figures 14a and 14b. However, in the case of helium, the position of maximum intensity changed little with a reduction in pressure, moving a little closer to the surface under the lower pressure (Figure 14c). Emission of the plasma varies with time as well as space in the plasma. The temporal behavior of the plasma emission of neutral copper atom [Cu(I)] at 324.75 nm at an axial distance of 0.4 mm from the surface at 760 torr in an air atmosphere in shown in Figure 15. The maximum intensity occurred at approximately 20s, and decayed gradually until about 250s.

The line to background (L/B) ratio is an important factor in spectrochemical analysis. Figure 16 shows the effect of the ambient atmosphere on the L/B ratio of the plasma emission near the center of the plasma. The background increased with an increase of the ambient gas pressure of air and argon. The background intensities were higher in air than in argon at higher pressures, but similar at lower pressures in air and argon. This background, however, was much less intense than the spectral line emission. With a reduction in the pressure, the L/B ratio was increased in air and argon atmospheres. The spectrum produced under helium at 760 torr, because of the lower excitation temperature by the relatively high thermal conductivity of helium gas, yields a more favorable line to background ratio (L/B = 22), although the line intensity is lower than those under low-pressure argon gas atmosphere. This

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000000 0

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Figure 15. Time-resolved emission intensity at 324.75 nm of neutral copper atom [Cu(I)] at 0.4 nm from the target surface at 760 torr of air (reproduced, with permission, from Lee et al., 1992b).


25

20

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O HEUUM

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PRESSURE (Torr)

Figure 16. Line to background (L/B) ratio of the plasma emission spectra formed with copper in different atmospheres (reproduced, with permission, from Lee et al., 1992b).

phenomenon is an important discovery in the analytical application of direct elemental determinations by laser emission spectroscopy.

The L/B ratios of the spectra were also found to be atmosphere and position dependent in the plasma. The region near the metal surface is energetic enough to be a luminous plasma, which emits an undesirable spectral continuum background. This continuum is preferentially excited near the surface, whereas the spectral lines extend much further. Because the continuum background is very low in the outer-sphere plasma, it is possible to improve the L/B ratio by using an appropriate region.

Although the greatest intensity of atomic lines is emitted from the center of the plasma, the greater intensity of continuum emission results in decreased L/B. High-line intensity is not the only criterion for analytical purposes. The spectra emitted by the laser-ablated plasma vary, depending significantly upon the observation position as well as the period of time after the impact of the laser pulse.

Elemental Analysis of Solid Samples

In recent years there has been considerable interest in the use of the laser-induced plasma for the direct analysis of samples by emission spectroscopy. There are several advantages to laser ablation-atomic emission spectroscopy (LA-AES), including the high spatial resolution provided by the focused laser pulse, the small (nanogram to gram) sampling size, the ability to atomize and excite the solid in one step without extensive sample preparation, and the capability to analyze noncon-


ducting samples and to analyze through a window or at a certain distance from the sample.

A study by Lee and Sneddon (1992) using an ArF excimer laser for the quantitative analysis of chromium in standard low-alloy steel samples demonstrated that controlled atmosphere and position in the plasma gave the best signal to background ratio for elemental analysis. A calibration curve was developed which related the chromium concentration in the steel matrix to the intensity ratio of Cr(I) 520.84 nm to Fe(I) 516.75 nm. A detection limit of 20 ~tg/g (approximately 0.002%) was estimated.

Franzke et al. (1992) employed an excimer laser to pump a dye laser for the analysis of minerals such as pyrite (FeS2) and zinc blend (ZnS). The experimental setup consisted of an XeC1 excimer laser that was used to pump a dye laser thus producing 8 mJ at wavelengths in the 370-390 nm range. The unfocused 2-mm diameter dye laser output produced a laser power density of 2.5 • 107 W/cm 2. The ablation depth was estimated from profilometer traces of a planar CuToZr30 alloy target. The average ablation depth was 1.6 ktm per pulse. This corresponds to 5 • 10 -6 cm 3 of material or about 41 tug/pulse.

When minerals such as FeS2, PbS, or ZnS were investigated, a single laser pulse was sufficient to record an emission spectrum with good sensitivity. All lines observed in the laser ablation of FeS2 were identified as emission from neutral iron atoms. Analysis of the spectrum obtained from zinc blend displays a large number of emission lines of iron, a common impurity in this mineral. Manganese lines were also detected and the iron and manganese contaminants were confirmed using secondary ion mass spectroscopy (SIMS).

Other minerals where atomic emission lines were observed include ZrO2, PbCrO4, K2S, Sb2S3, and Mo2S3. A large number of other inorganic solids were also te,,ted but resulted in no emission spectra being observed at the laser power employed (2.5 x 107 W/cm2). Negative test results were obtained for substances characterized by absorption lengths greater than 100 nm and heats of formation greater than 250 kJ/mol. This includes carbonates (CaCO3, BaCO3), halides, oxides, and reflecting metals such as copper. This behavior has been observed in studies using Nd:YAG laser systems (Thiem, 1993).

The high sensitivity of the method by Thiem (1993) generated interest into the possible application of this method for liquid samples. The basic procedure consisted of the deposition of a few drops of the solution of interest onto a target material. The solvent was then evaporated, vaporized by the laser, and emission lines were detected. Metals such as aluminum, copper, and sheet iron were investigated since they did not give an emission spectrum at the laser power used. Laser ablation of a dried 100-ppm PbC12 solution provided adequate signal for detection. Detection limits for this experiment are less than 10 ppm.

Chen and Mazumber (1990) studied the emission spectra during excimer laser (KrF) ablation of graphite. The emission spectrum was dominated by the C2 Swan bands (d3Pg-a3Pu) but the CN violet bands and weak C2 Deslandres-d'Azambuja


bands were also present. The CN emission indicates a secondary reaction with the residual air of which CN is a product. Also present in the spectra were the emission lines of C(I) at 477.1 nm and C(II) at 657.8--658.2 nm. Chen and Mazumber (1990) also determined vibrational temperatures for the vaporized species, ranging from 11,000-15,300 K.

Deshmukh et al. (1988) investigated the emission spectra from ArF (193 nm) laser ablation of the high-temperature superconductor Bi2CaSr2Cu209 and samples ofBi203, CaO, CuO, and Sr(OH)2 �9 8H20, in order to show that the emission could be used to characterize the bulk material. Photoablation-induced emission studies were carried out in a vacuum chamber (ca. 10 -3 torr). Dyer et al. (1988) studied the KrF laser ablation of Y-Ba-Cu-O solid using UV-visible spectroscopy and ion probes. Principal luminescent species, expansion velocities, and the extent of ionization in the ablation plume were determined, which provided information on the interaction of relevance to the ablation as a film deposition source.

Dyer (1989) studied the ionization and onset in the ablation plume from a KrF laser-irradiated copper target. Using a 30-ns pulse, the plasma initiated at around 108 W/cm2; electron temperatures of 7 eV and ion energies of several hundred volts were produced at 2.2 x 108 W/cm 2.

Weimer (1988) and Geyer and Weimer (1989) studied the plasma emission and characterization of the ablation process from the high-temperature superconductor YBa2Cu307 using an ArF excimer laser. It was concluded that the resulting plasma was similar in content to individual components of BaCO3, CuO, and Y203. Gupta et al. (1991) studied the evolution dynamics of the fragments from the KrF (248 nm, 25 ns) ablation of YBa2Cu307 using ultrafast photography. Results showed that fragment removal is initiated near the beginning of the laser pulse (10 ns), continues for a maximum of a few hundred nanoseconds, and has an expansion-front velocity that suggests a target temperature varying from 1500 to 4000 K or greater.

McKee (1988) measured broadband laser emission spectra provided by ArFm, KrE and XeC1 excimer lasers and compared the wavelengths found with reference data. Muenchausen et al. (1990) investigated broadband angular distributions observed for XeC1 laser ablation plumes used in the deposition of YBa2Cu3OT. These were measured as a function of laser fluence, beam shape, and oxygen pressure.

Saenger (1989) used time-resolved studies for the optical emission from the ablation plume of ArF excimer-laser-irradiated Cu, CuO, Bil.TSrl.3Ca2Cu3Ox and Y1Bal.7Cu2.7Oy targets. Data were collected from several ionic and neutral species, including Sr § Sr, Ca § Ca, and Y. Probable velocities of (3-10) x 105 cm/s for neutral species and (1-2) x 106 cm/s for ions were obtained.

Schaub et al. (1989) used a dual-pulse KrF excimer laser (248 nm) for the determination of ejected-material velocities resulting from an aluminum surface under vacuum. Velocities measured 200-400 ns when arrival of the incident pulses ranged from 450-1200 m/s. Sell et al. (1990) used a helium-neon laser beam deflection to study laser ablation of polymers and the superconductor YBaCu307.


Models of thermal deflection at low fluence allowed the measurements of the thermal diffusivity of the air.

Simpson and Williams (1991) investigated the ablation of ZnS using pulsed 193-nm radiation from an ArF excimer laser. The plasmas obtained did not exhibit overall electrical neutrality but had a net positive charge. The authors suggested that the ablation process was the result of a buildup of photogenerated holes at the surface sufficient to result in the disintegration of the lattice and rapid vaporization at low thermal energies.

V. EXCIMER LASER INDUSTRIAL APPLICATIONS

The 1980s saw an increased use in the laser for industrial applications, particularly for machining and material processing. A brief outline of the use of the excimer laser is described in the following section for completeness.

A. Thin Film Preparation

Laser ablation is becoming increasingly recognized as a successful method for the deposition of oxide superconductor thin films. A particularly important characteristic of laser ablation is that it transfers the stoichiometric composition of a bulk superconductor target to the thin film. Many research groups are currently engaged in producing superconducting thin films. Walmsley et al. (1992) have deposited Y-Ba-Cu-O films with (001) orientation onto MgO using laser ablation at 248- and 193-nm wavelengths. Von der Burg et al. (1992) used continuous-wave CO2 to heat the substrate of SrTiO3(100) and polycrystalline ZrO2 during the thin-layer deposition of Y-Ba-Cu-O by KrF laser ablation.

O'Brien et al. (1992) studied the effects of laser fluence and shot number on the deposition of YBa2Cu307 using a KrF excimer laser system. They plotted the deposited thickness of the resultant thin film as a function of fluence as measured directly by scanning electron microscopy.

B. Ceramic Materials

Ceramic materials are especially difficult to work with in industrial applications. Applications of lasers for ceramic surface modification are attractive because photon energy may be provided in a temporally and spatially controlled manner and in a contact-free and clean process (Hontzopoulos and Damigos, 1991).

In a study using excimer laser ablation as a surface modification method for ceramics, Hontzopoulos and Damigos (1991) were interested in improving the ceramic surface properties in such a way that they could be used instead of metals for high-temperature applications or, more generally, in environments inappropriate for metals. They employed a variety of different parameters for the excimer laser


treatment, including treatment in air, under vacuum (10 -4 torr), and in inert (helium) atmospheres. SEM studies of the ceramic sample surfaces prior to irradiation showed them to be rough and irregular. KrF laser treatment of these samples with power densities of 40-100 mW/cm 2 leads to surface smoothing in the three classes of ceramics tested [partially stabilized zirconia (PSZ), tetragonal polycrystalline zirconia (TPZ), and silicon nitride (Si3N4)]. The formation of a smooth, glassy layer in these ceramics will increase their corrosion and erosion resistance. Furthermore, the glassy surface will protect the bulk ceramic from water penetration and in this way the stability of the ceramic material will be improved.

C. Thin Film Deposition

Pulsed laser deposition methods are emerging as some of the leading techniques for producing thin films of varying composition. The technique has been applied to the fabrication of high-temperature superconducting and semiconductor films of complex composition, and is particularly useful because of the retention of stoichiometry in going from the laser target to the thin film (Singh et al., 1990a, b; Olander, 1990; Inam et al., 1987; Marine et al., 1989, 1992). In spite of the expanding application, the mechanism of the laser ablation process remains poorly understood. Wang et al. (1976) studied the velocity distribution of laser-ablated aluminum at different wavelengths (193, 248, and 351 nm). They employed three different methods for studying the ablated atoms' kinetic energy. First, they monitored the aluminum atoms directly by exciting the 42S-32p3/2 transition centered near 394.4 nm and measuring its fluorescence through a narrow bandpass filter at 396.2 nm. Time-of-flight measurements are obtained by measuring the intensity of the atomic fluorescence signal as a function of delay time, with the probe laser at a fixed distance from the aluminum target. Calculated velocity distributions were in the range of 4.5-6.5 x 105 cm/s. The second method employed for determining the aluminum atoms' velocity was Doppler spectroscopy. Based on the measured linewidths of 0.0145 nm, a velocity of 4.7 x 105 crn/s was calculated, in good agreement with the previous time-of-flight measurements. The third method used for characterizing the velocity of the aluminum atoms was to produce A10 by introducing trace quantities of oxygen into the reaction vessel. The AIO is detected by LIF of the B-X (1,0) transition near 465 nm. Based on these results, aluminum atom velocities of > 2 x 105 cm/s were calculated, again in good agreement with the two other methods.

Y-Ba-Cu-O (YBCO) belongs to a class of new superconducting materials with high transition temperatures (Bianconi et al., 1992). One of the most successful methods of producing YBCO is using pulsed UV-laser irradiation of a sintered target (Brousse et al., 1992). This method takes advantage of the essentially nonthermal nature of the ablation process--because of the short wavelength of the laser beam, the chemical bonds are directly broken and the atoms are ejected from the target surface towards the substrate. Thus, under proper conditions, a near-


stoichiometric deposition of multicomponent material is obtained. Blank et al. (1992) studied the deposition of XeC1 (308 nm) laser-deposited YBa2Cu307 films and the density of droplets coming from the target using various laser pulse energies, laser spot size, and target-to-substrate distance. The deposition was found to be linearly dependent on the laser energy and quadratically on the spot size of the target.

Unamuno and Fogarassy (1992) deduced thermal calculations for Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O thin films, deposited under pulsed excimer laser radiation, from the resolution of the one-dimensional heat flow equations.

Venkatesan et al. (1988) used Rutherford backscattering techniques to measure the angular distribution of the composition of a Y-Ba-Cu oxide film that was deposited by the firing of excimer laser (30 ns, 248 nm) pulses at the substrate. Habermeier (1992) describes some recent developments in high-temperature thin film preparation using pulsed laser deposition. He stated that UV excimer lasers are used as the high-energy source because of the high absorbance (and efficiency) of UV energy by the substrates.

Razavi et al. (1992) used a KrF excimer laser (248 nm) for the in situ preparation of Bi2Sr2CaCu208 deposited on MgO single crystals. The substrate temperature, oxygen pressure, and distance between target and substrate were investigated. The critical temperatures of these deposited films are 60-70 K for deposition temperatures between 770 and 810 ~

D. Excimer Laser Processing of Embedded Fibers

The ability of the UV radiation to be focused to a very small spot size is being used to laser etch nanostructures in embedded optical fibers (Sigrist and Tittel, 1991). Thus etching has the ability to form small chambers via a "drilling process" in polymetric fibers embedded in epoxy resin. A possible application of this technology is that it may be used on optical fiber biosensors for in vivo blood gas analysis (Scheggi, 1989). For the etching process, it was found that the ArF laser (193 nm) gave the best results. Application of radiation at longer wavelengths (XeC1 at 308 nm or KrF at 248 nm) resulted in browning and burning of the epoxy resin, as well as browning and partial damage to the fiber, apparently owing to too-large absorption of radiation in the epoxy resin.

Ferrites, used in the fabrication of magnetic devices, have also been etched using excimer laser ablation. In a study by Tam et al. (1991), a KrF excimer laser was used for ablation studies. Previous work on ferrites used a focused continuous laser beam to scan the sample surface, which was placed in a gaseous or liquid chemical. Using the pulsed excimer laser, efficient etching was achieved in a dry environment without the use of additional chemicals. The authors were able to show that micropatteming of a large area of a ferrite ceramic surface is possible and, in fact, quite efficient approximately 100 A/pulse being removed at an incident fluence of 1 J/cm 2. They found an ablation threshold of 0.3 J/cm 2, below which scanning by


electron microscopy of the surfaces revealed the presence of frozen droplets attempting to leave the "glassy skin" of the ablated ferrite.

VI. CONCLUSION: FUTURE DEVELOPMENTS IN EXCIMER LASER APPLICATIONS

The future looks extremely bright for increased use of excimer laser systems in the field of analytical chemistry. The high coupling efficiencies of UV radiation with metal surfaces could make it the laser of choice for sample introduction and for direct elemental analysis by emission spectroscopy. The excimer laser should continue have increased use in the manufacturing sector also, for the production of thin film semi- and superconductors.

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Simeonsson, J.B., Ayala, N.L., Vera, J.A., Smith, B.A., Winefordner, J.D. Spectrochim. Acta 1990, 45B, 1025.

Simpson, J., Williams, J.O.J. Appl. Phys. 1991, 70, 2001. Singh, R.K., Singh, A.K., Lee, C.B., Narayan, J. J. Appi. Phys. 1990, 67, 3448. Singh, R.K., Narayan, J. J. Appl. Phys. 1990, 41, 8843. Smith, B.W., Famsworth, P.B., Omenetto, N. Spectrochim. Acta 1990a, 45B, 1085. Smith, B.W., Farnsworth, P.B., Cavalli, P., Omenetto, N. Spectrochim. Acta 1990b, 45B, 1369. Talmi, Y., Sieper, H.P., Moenke-Blankenburg, L.Anal. Chim. Acta 1981, 127, 71. Tam, A.C., Leung, W.P., Krajnovich, D. J. Appl. Phys. 1991, 69, 2072. Tanaka, Y., Zeilikof, M. Phys. Rev. 1954a, 93, 933. Tanaka, Y., Zeilikof, M. J. Opt. Soc. Am. 1954b, 44, 254. Tanaka, Y. J. Opt. Soc. Am. 1955, 45, 710. Thiem, T.L., Ph.D. Dissertation, University of Massachusetts, Lowell, 1993. Thompson, J., Goulter, J.E., Sieper, E Analyst 1981, 106, 32. Tremblay, M.E., Smith, B.W., Leong, M.B., Winefordner, J.D. Spectrosc. Lett. 1987, 20, 311. Uebbing, J., Brust, J., Sdorra, W., Leis, E, Niemax, K.Appl. Spectrosc. 1991, 45, 1419. Unamuno, S. de, Fogarassy, E. Mat. Sci. Eng. 1992, B13, 29. Velazco, J.E., Setser, D.W.J. Phys. Chem. 1975, 62, 1990. Van Deijcke, W., Balke, J., Maessen, EJ.M.J. Spectrochim. Acta. 1979, 34B, 359. van de Nesse, R.J., Mank, A.J.G., Hoornweg, G.P., Gooijer, C., Brinkman, U.A.T., Velthorst, N.H.Anal.

Chem. 1991, 63, 2685. Venkatesan, T., Wu, X.D., lnan, A., Wachtman, J.B. AppL Phys. Lett. 1988, 52, 1193. von der Burg, E., Grill, W., Diegel, M., Stafast, H. Mat. Sci. Eng. 1992, B13, 25. Walmsley, D.G., Sakeek, H.F., Morrow, T., Rowan, C., Turner, R.J. Mat. Sci. Eng. 1992, B13, 15. Wang, C.P., Mirels, H., Sutton, D.G., Suchard, S.N. AppL Phys. Lett. 1976, 28, 326. Wang, C.P. Rev. Sci. Instr. 1976, 47, 92. Wang, H., Salzberg, A.P., Weiner, B.R. Appl. Phys. Lett. 1991, 59, 935. Waynant, R.W. Appl. Phys. Lett. 1977, 30, 234. Weimer, W.A. Appl. Phys. Lett. 1988, 52, 2171. Wennrich, R., Dittrich, K. Spectrochim. Acta 1982, 37B, 913. Wennrich, R., Dittrich, K., Bonitz, U. Spectrochim. Acta 1984, 39B, 657. Woodroffe, J.A., Hisa, J., Ballantyne, A. Appl. Phys. Lett. 1980, 36, 14.

RECENT DEVELOPMENTS IN ANALYTICAL MICROWAVE-INDUCED PLASMAS

Robbey C. Culp and Kin C. Ng

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

II. Microwave-Induced Plasma Sources and Systems . . . . . . . . . . . . . . . 217

A. Beenakker TM010 Resonator Cavity . . . . . . . . . . . . . . . . . . . . 218 B. Surfatron and Surfaguide . . . . . . . . . . . . . . . . . . . . . . . . . . 221 C. Other Promising Microwave Discharge Designs . . . . . . . . . . . . . . 222

III. Chromatographic Detection Systems Using MIPS . . . . . . . . . . . . . . . 224 A. MIP-AED For Gas Chromatography . . . . . . . . . . . . . . . . . . . . 225

B. Element-Specific Detection with HPLC . . . . . . . . . . . . . . . . . . 244

C. Microwave-Induced Plasma as a Detector For Supercritical Fluid Chromatography . . . . . . . . . . . . . . . . . . 248

D. MIP as an Ion Source for MS . . . . . . . . . . . . . . . . . . . . . . . . 252

IV. Nonchromatographic Sample Introduction Methods . . . . . . . . . . . . . . 260

A. Aerosol Sample Introduction . . . . . . . . . . . . . . . . . . . . . . . . 260

B. Nonmetal Determinations Using MIP-AES and

Aerosol Sample Introduction . . . . . . . . . . . . . . . . . . . . . . . . 262


215

216 ROBBEY C. CULP and KIN C. NG

C. Metal Determinations Using MIP-AES with Aerosol Sample Introduction . . . . . . . . . . . . . . . . . . . . . . . . 265

D. MIP-AAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 E. Electrothermal Vaporization . . . . . . . . . . . . . . . . . . . . . . . . 272

V. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

I. I N T R O D U C T I O N

During recent years atmospheric pressure microwave induced plasmas (MIPs) have received increased attention as excitation sources for a wide variety of applications. As with most plasma emission sources, advantages are low chemical reactivity, high temperature, and relatively few interference effects. Additionally, the MIP has a high power density per power input and is able to use a variety of plasma support gases, including helium. Helium permits efficient excitation to emission of the halogens and other nonmetals not conveniently accessible to the more recognized argon inductively coupled plasma (ICP).

ICP is the current plasma source of choice for a variety of elemental analyses. Typically an ICP uses 17 liters per minute of argon or nitrogen with power consumption of 0.75-1.5 kW. Although there are a number of exceptions, MIP power requirements are normally well below 200 watts, with support gas flows, being typically below 1 L/min. Initial purchase cost varies similarly being above $15,000 for an ICP radio frequency power supply versus about $5,000 for a basic microwave generator.

The MIP-ICP cost differential and the ability to efficiently excite nonmetals to emission has driven MIP research. Historically, MIPs have been used most successfully as atomic emission spectroscopy (AES) detectors for gas chromatography (GC) (Bache and Lisk, 1965, 1966a; Bulska et al., 1991; McCormack et al., 1965; McLean et al., 1973). Current applications have expanded to include use as atom reservoirs for atomic absorption (AAS) (Ng and co-workers, 1983, 1987; Ling, et al., 1988; Lu et al., 1988, 1991) and atomic fluorescence (AFS) (Perkins and Long, 1988) spectroscopies. Recent developments in interface designs permit use of MIPs as AES detectors for liquid chromatography (Carnahan, 1981; Zhang et al., 1985) or ion sources for mass spectrometry (MS) (Eberhardt et al., 1992; Douglas and French, 1981).

Several excellent comprehensive reviews have been published discussing MIP properties and early development. Matousek, Orr, and Shelby (1984) and Beenak- ker et al. (1980) have reviewed the early development of the MIP, its physical properties and use as an excitation source for AES. Zander and Hieftje (1981) overview the historical, practical, and fundamental theoretical aspects of the MIP for AES. Since most of the fundamentals of microwave-supported discharges are reported elsewhere we will not duplicate them here, with the exception of a

Recent Developments in Analytical Microwave-Induced Plasmas 217

historical introduction, where necessary, for continuity. Instead, we will focus on the recent improvements in MIP technology and applications.

In the review that follows we have focused on recent improvements or important innovations involving MIP detection technologies. Specific focus is placed on the advantages and problems encountered in development of MIP sources for high-performance liquid chromatography (HPLC) detection and for direct determinations using AES, AAS, AFS, and MS. This chapter will review three major topics: the first is microwave discharge devices, which will also include microwave generator developments and tuners; the second focuses on chromatography detector applications, using MIP-AES; in conclusion, we report on direct elemental determinations using AES, AAS, AFS and sample introduction technologies with liquid and dry aerosol sample introduction.

I!. MICROWAVE-INDUCED PLASMA SOURCES AND SYSTEMS

Figure 1 illustrates the components required for sustaining an atmospheric microwave discharge. Microwave frequency power is generated in a microwave generator and delivered to the discharge device. A tuner is used to adjust for impedance differences between the generator, transmission line, and microwave discharge. A microwave torch, or discharge tube, is typically used to contain the microwave discharge. These range from simple small-diameter tubes, made of dielectric materials, to more complicated designs for stabilizing the MIP while cooling the torch to prevent its failure. The relatively low power and sensitivity to upset of the microwave discharge has resulted in interface designs specific to the application.

[ Microwave Generator]

Coaxial Cable or Wave Guide

I s'ample Introduction System

Argon, Nitrogen or Helium

Supply

Impedance Matching I Tuner ,

Microwave t Microwave-Induced Discharge Plasma Discharge

Device Optional Makeup Gas

Figure 1. Components for sustaining a microwave-induced plasma.


In large part, development of MIP applications has been due to development of new microwave discharge devices. Various Beenakker cavity-based designs and the surfacewave launcher (Surfatron) (Hubert et al., 1979; Moisan and co-workers, 1976, 1977) are the two devices most often used for supporting microwave discharges for AES. However, notable recent exceptions include a number of new capacitively coupled microwave plasma sources (Patel et al., 1987; Uchida et al., 1990; Forbes et al., 1991; Jin et al., 1989) and new coaxial sources (Jansen et al., 1985).

Microwave generators used for production of MIPs have been limited, for the most part, to commercial units with an operating frequency of 2.45 GHz. As a result, cavity-based MIPs are similarly limited to this frequency. A limited investigation of coaxial sources and surface launcher MIP sources has been done at other frequencies (Chaker and Moisan, 1984; Jansen et al., 1985). Detailed investigations of the applied microwave frequency effect on analytical performance remain to be done.

Applied power levels are the subject of a number of the papers discussed in the following section: low applied powers (< 100 watts) are used for GC; power levels ranging to over a kilowatt (Cull and Carnahan, 1988; Okamoto and co-workers, 1990, 1991; Wu and Carnahan, 1992a,b) have been used with liquid aerosol sample introduction for AES.

A. Beenakker TMolo Resonator Cavity

In 1976 Beenakker introduced (Beenakker, 1976) the TM010 resonator (Figure 2) consisting essentially of a 92.5-mm cylindrical cavity. Microwave energy is transferred to the plasma by inductive coupling using a short-circuited loop placed normal to the magnetic field (Beenakker and Bowmans, 1978; Beenakker, 1976). The system allows efficient coupling of low-power (< 150 W) 2.450 GHz microwave energy to a plasma support gas at atmospheric pressure. The initial design permitted stable atmospheric pressure MIPs with dry argon or with helium and wet argon (Beenakker, 1977; Beenakker and Bowmans, 1978; Beenakker et al., 1978). This permitted metal analysis using nebulized liquid aerosols injected directly into the argon plasma for elemental analysis by AES.

Further development has focused on increasing either the applied power or the plasma energy density. High-power plasmas have greater plasma volumes (Cull and Carnahan, 1989) while higher plasma gas flow rates are used to cool the plasma torch. A second approach achieves higher plasma energy densities by increasing the generator-plasma coupling efficiency. In these systems, plasma gas flow is minimized while refinements in the antenna and tuner are used to increase the plasma coupling constant (Cull and Carnahan, 1989; Matus et al., 1983; Long and Perkins, 1987; Perkins and Long, 1989; Quimby and Sullivan, 1990a).

In all but a few cases (Cull and Carnahan, 1988, 1989; Quimby and Sullivan, 1990a; Okamoto, 1991), the TM010 resonator uses a transmission line to carry power

Recent Developments in Analytical Microwave-Induced Plasmas cowave eotojt Coaxial Cable

Inert Gas and Analyte ~ ~ . . . . ~ Tuning S c r e ~

D i s c h ~ Capillary

Coupling Loop

I Plasma v Viewing

219

Tuning Screw

Figure 2. Original Beenakker TMolo resonator cavity using tuning screws, which permits radial or axial viewing of the plasma.

from the microwave generator. To deliver power efficiently to the load requires the impedance of the cavity to match that of the generator and transmission line (50 f~). In the event of an impedance mismatch, power is reflected back toward the generator as a standing voltage wave along the transmission line (Matus et al., 1983). Beenakker (1976) initially used tuning screws (Figure 2) that could reduce the load inductance, reducing reflected power. These had very limited range and were not effective at higher power levels.

More recent tuner development focused on external devices to reduce or eliminate reflected power. Commercially available triple stub tuners allow tuning over the entire range of impedance differences (Raglan, 1948; Thomas, 1972; Schrenk et al., 1972). Heat dissipation problems at power levels above 150 watts lead to breakdown of the dielectric elements and a reduced lifetime for triple stub tuners. Haas and co-workers (Haas et al., 1983; Haas and Carnahan, 1983) used internal tuning to permit use of 100 to 600 watts of applied microwave power by attaching two stub tuners internally to a lengthened coupling loop. The internally tuned cavity sustains moderate power plasmas of helium, argon and nitrogen (Michlewicz et al., 1985). Michlewicz and Carnahan (1984, 1986a) used 480 watts of applied power with a similar tuner arrangement and a water-cooled cavity to produce low ppm detection limits for chlorine, bromine and iodine using liquid aerosol sample introduction.

All stub-tuning devices operate on the principle that a reflecting element can be introduced to the load end of the transmission line. This permits a reflective wave


180 ~ out of phase and equal in magnitude to the load reflection (Matousek et al., 1984). In hulling a reflected wave, power present in the wave is partially radiated as heat in the tuning network and coupling element (van Dalen et al., 1978). The resulting net loss of efficiency decreases the actual operating power in the plasma and complicates plasma power measurement.

Mohamed and coworkers (1989a, b) proposed a different approach to producing a moderate-power MIP. They advocate the use of a pulse-operated MIP. The specially constructed microwave generator imposes a high peak power (<400 W, 50 ms), short duration pulse on a low bias dc microwave plasma (50 W). The mean power is kept low and reduces problems caused by joule heating, while also eliminating the need for a tuner. This results in an increased torch life and reduced plasma gas flow rates. A synchronized, time-resolved measurement system is used to monitor the plasma emission during a power pulse. This generator permits reductions of microwave output fluctuations by using a dc current to heat the magnetron heater.

Very high applied powers using a Beenakker cavity have been achieved without use of stub tuners. A 2.79-kW MIP, with peak power of 7.59 kW, is reported by Cull and Carnahan (1988). The 2.45-GHz microwave power supply is modulated at 120 Hz and delivered to a water-cooled cavity with a wave guide. Tuning is accomplished with a slot tuner. The line emission intensity for He (I) was discontinuous with time, as the plasma reignited with each cycle. The 2.79-kW plasma is stable with helium, argon, or nitrogen as support gases and with liquid aerosol sample introduction. However, the discontinuous plasma results in a very short plasma torch life (ca. 0.7 hours). Subsequent work utilized a modified microwave generator permitting a continuous modulated plasma (1.57-1.98 kW) with better application as a spectroscopic source (Wu and Carnahan 1992a, b; Cull and Carnahan, 1989). The new configuration results in much longer torch life.

Except for the system proposed by Mohamed et al. (1989a), moderate-power MIPs used for nonmetal determinations require high cooling gas flow rates to protect the plasma torch. These flows can be as high as 17 L/min (Urh et al., 1985) or more (Wu and Carnahan 1992a, b). With the exception of air MIP (Okamoto, 1991), high helium or argon flow rates may detract from the cost advantage generally attributed to MIPs. While moderate power plasmas are associated with larger plasma volumes and higher plasma temperatures (Wu and Carnahan, 1992a), analyte plasma residence time may be reduced by high plasma gas flow rates.

Essentially increasing the cavity coupling coefficient has been used by a number of workers to achieve good results at relatively low applied power levels (< 150 W) and low support gas flow rates. A modified Beenakker cavity, proposed by van Dalen et al. (1978), utilized an adjustable quartz rod inserted into the side of the cavity for frequency tuning and a variable-length antenna rod, parallel to the plasma discharge tube, for impedance matching. The quartz rod increases the cavity capacitance, lowering the natural resonant frequency. The importance of this design revolves around achieving both frequency tuning and an impedance match. Matus


et al. (1983) subsequently reported development of a "critically" coupled resonant cavity, later called the highly efficient MIP (HEMIP). As with van Dalen's cavity (1978), the HEMIP utilizes a quartz rod inserted into the cavity for frequency tuning. The HEMIP is matched to the generator and transmission line by translation of a capacitively coupled antenna probe along a radial slot. Movement of the antenna probe allows interaction with the magnetic and electric fields until it senses a 50-ohm load, thus providing the proper impedance match. When a microwave generator and plasma are critically coupled there is maximum transfer of microwave energy (Matus et al., 1983). Under these conditions the reflected power is minimized, increasing generator life as well as plasma power density. This cavity is used by Long and Perkins for analysis of metals (Perkins and Long, 1989; Long and Perkins, 1987) and nonmetals (Perkins and Long, 1989) with good success. With power levels as low as 70 W, using a concentric glass nebulizer and a modified tangential torch they were able to sustain a toroidal plasma with liquid aerosols.

The enhanced Beenakker cavity (EBC), based on van Dalen's design (Forbes et al., 1991; van Dalen, 1978) replaces van Dalen's quartz coupling antenna probe with an adjustable 9-mm disc (Slatkavitz, 1984). A single capacitive plate tuning screw is mounted in the face for frequency tuning. The adjustable disc probe provides a wider coupling range than that of van Dalen's design (Forbes et al., 1991). The EBC is stable at operating powers as low as 42 watts with 0.270 L/min helium flow rate (Forbes et al., 1991), whereas the discharge is heavily loaded with aqueous analytical aerosols (Reszke, 1992).

B. Surfatron and Surfaguide

Limitations attributed to resonator cavity MIPs include stability problems with relatively small sample loading and a limit to the methods that may be used for sample introduction. Selection of frequency cannot be used as a convenient op- timizable parameter since this is specified by choice of cavity width. The dimensions of the Beenakker cavity-MIP discharge are very small by comparison to ICP or DCP. To overcome some of these problems two surface launcher MIP devices have been developed. Changes in plasma parameters have little effect on matching the impedance of a surfacewave device to the microwave generator and transmission cable, as in the case of resonator-based MIPs. The surfacewave MIP has the advantage of increasing the plasma length while permitting a large frequency bandwidth (Selby et al., 1987). They also operate over a very wide power and pressure range, using argon or helium for plasma support (Abdallah et al., 1982).

Two surfacewave devices have been developed. The coaxial source, called a Surfatron (Hubert et al., 1979; Moisan et al. 1974, 1975) (Figure 3), is generally used at lower applied powers. A similar device using a rectangular waveguide for transmission of microwave power is typically used at moderate applied powers (500 W) (Moisan et al., 1984). Relatively few publications have explored the use of these devices for AES applications. However, when used, they compete well with


N-TYPE CONNECTOR

COUPLER ADJ~T

Q TUNING ADJUSTMENTS

FUSED SILICA TUBE

Figure 3. The Surfatron used by Selby and Hieftje (1987). The microwave generator is connected via a coaxial cable. Three tuning adjustments are possible, which are: vertical position of the coupling plate, gap length and depth of the internal chamber. Redrawn with permission (Selby and Hieftje, 1987) Copyright 1987, Pergamon Press Publishing.

Beenakker cavity based designs (Abdallah et al., 1982; Chevrier et al., 1982; Hanai et al., 1981; Robin et al., 1982).

C. Other Promising Microwave Discharge Designs

During the last several years a number of new microwave discharge devices have been developed for use in AES. These include the tubular electrode torch (TET) (Patel and co-workers, 1987, 1988), the microwave plasma torch (MPT) (Jin et al., 199 la; Yu et al., 1992), the annular-shaped microwave-induced nitrogen plasma (AnMINP) (Okamoto et al., 1990; Okamoto, 1991), the stripline source (SLS) (Forbes et al., 1991; Argentine and Barnes, 1992; Barnes and Reszke, 1990), the reentrant cavity (Quimby and Sullivan, 1990a; Brown et al., 1986) and a new resonator design by Matusiewicz (1992).

The TET and MPT are capacitively coupled designs which permit efficient coupling of higher applied powers. While accommodating a wide range of powers (200--1000 W), previous coaxial designs permitted contamination of the plasma by the central electrode. Both new designs incorporate a hollow tubular electrode, eliminating many of the previous contamination problems and increasing analyte plasma interaction. While using a waveguide to deliver power from the microwave


Vertically Sliding Collar

To Microwave 121 Generator

Spacer

f

,i

Antenna

ml

Screw

U[

L2

_A, Plasma Gas

Carrier Gas + Anlytical Sample

Figure 4. The capacitively coupled plasma torch used by Jin et al. (1991 a) incorporates a hollow tubular central electrode, producing a flame-like plasma that is viewed radially. The plasma does not contact the tubular electrode, preventing contamination of the plasma. Redrawn with acknowledgment to the authors Oin, Zhu, Borer, and Hieftje) Copyright, 1991, Pergamon Press Publishing.

generator, the TET generates a flame-like plasma at the tip of a tubular electrode that is viewed radially. Samples are introduced directly to the base of the TET plasma, accounting for the significant improvement of detection limits versus rod electrode capacitively coupled systems. The construction of the MPT is very similar in principle, but is functionally quite different from the TET. A capacitively coupled


plasma is formed at the tip of two concentric tubes (see Figure 4). The plasma has no contact with the generating apparatus and permits introduction of samples to the plasma center, in a manner similar to that of ICP. Stable, low-power operation is achieved at 150 W with and without desolvation. MPT detection limits (Jin et al., 1989) using desolvation are comparable to the best results for Beenakker cavity MIP.

The AnMINP is one of two high applied power MIPs recently reported (Okamoto et al., 1990; Okamoto, 1991; Wu and Carnahan, 1992a, b). While operating at a kilowatt of applied power, it sustains an annular-shaped MIP with nitrogen or air. Few previous results for annular-shaped plasmas have been reported (Michlewicz et al., 1985; Bollo-Kamara and Codding, 1981; Haas and Caruso, 1984; Leis and Broekaert, 1984). Detection limits for zirconium and calcium using desolvated aerosols is less than 30 ppb and 1 ppb, respectively, which is well within an order of magnitude of typical ICP results (Winge et al., 1979). The result for zirconium is far superior to the limited previous results for a 100 W MIP-AAS system (Ng and Garner, 1993). Despite the high applied power, cooling is not required for the discharge device. Nitrogen or air tangential gas is supplied to the ICP-like tangential torch at a 10 L/min rate.

The SLS uses a stripline waveguide. Energy is coupled by an antenna plate and capacitive tuning screw and then transmitted to the plasma (Barnes and Reszke, 1990; Argentine and Barnes, 1992). Although operating at comparatively low power (90 W), it provides high energy transfer efficiency resulting in a stable plasma over a wide operational range (Forbes et al., 1991). Other work on stripline sources has been reported by Reszke and Parosa (1992). The argon plasma length is highly dependent on applied power with a 15-cm plasma observed with 100 W of applied power. The helium plasma is much less affected and barely extends beyond the face of the cavity.

i11. C H R O M A T O G R A P H I C DETECTION SYSTEMS USING MIPS

The past ten years has seen rapid evolution of gas chromatography (GC) and associated data-handling systems. Separation of extremely complex mixtures has been made possible with the advent of high-efficiency capillary columns. Owing to the large number of eluting peaks, it may be easier, in some cases, to interpret the resulting chromatogram if the detector responds only to individual compounds or groups of compounds. A number of selective detectors have been developed. These are either selective to functional groups, elements, or properties. Element- selective detectors for GC and supercritical fluid chromatography (SFC) in common use are the thermionic detector (NPD), electron capture detector (ECD), photoionization detector (PID), flame photometric detector (FPD), electrolytic conductivity detector (ELCD) and flame ionization detector (FID). These detectors


Table 1. Selective Detectors for Gas Chromatography (Harris, 1991 ; Buffington and Wilson, 1987)

Detector Selective for Typical D.L. LDR

FID Materials that ionize in flame 2 pg/s > 107 ECD Gas-phase as electrophores as low as 5 fg/s as chlorine 104 PID Materials ionized by UV 2 pg/s as carbon l07 FPD S, P species < 1 pg/s as phosphorous, > 104

< 10 pg/s as sulfur > 103 NPD N, P, heteroatoms 100 fg/s 105 ELCD N, S, halogens 0.5 pg/s as chlorine 106

2 pg/s as sulfur, 104 4 pg/s as nitrogen 104

FPD Specific P, S 20 pg/s as sulfur 103 0.9 pg/s as phosphorous 104

MSD Tunable for any species 10 pg to l0 ng; depends on selective ion l04 monitoring vs. scans of mass distribution

individually possess varying selectivity to small groups of elements, limiting their application to compounds of specific composition (Table 1). Detectors used for HPLC have limited selectivity. Of the detectors used for chromatography, only the mass-selective detector (MSD) and the atomic emission detector (AED) are universal detectors, tunable to any species. Depending on the system, the MSD detects either mass spectra or monitors specific masses for quantitative analysis. The AED allows detection of all elements except helium, based upon their elemental emission, over a broad dynamic range. The potential for simultaneous multielement detection is limited only by appropriate line selection and ability to collect and process the spectral data.

A. MIP-AED For Gas Chromatography

Direct current plasmas (Lloyd et al., 1978; Uden et al., 1978), inductively coupled plasmas (Sommer and Ohis, 1979; Windsor and Denton, 1979), glow discharge (Sommer and Ohis, 1979; Windsor and Denton, 1979; Evans, 1968), and microwave-induced plasmas (MIP) (Bache and Lisk, 1965, 1966a,b; McCormack et al., 1965) all have a long history of use as excitation sources for GC-AEDs. Still others show potential, such as the hollow cathode discharge (Ng et al., 1991) and microwave plasma torch (Jin et al., 1991 a). While each plasma source has advantages, the MIP remains the most popular and is currently used in the only commercial AED (Hewlett-Packard, HP 5921A AED). The MIP-AED offers an advantage over conventional detectors in many applications. For example, the electron capture detector, used for detection of halogenated species, has a response that is dependent on the electronegative atom's environment and may not detect the monohalogen-


Chart Recorder

i [ High Voltage ] Power Supply

[ "r 1L_! Reagent lCu~nt to I ,

MIP -- /,,Helium

Monochromator 0 ~1 / I"n:'Zo

I Microwave ] Power Suoolv

Figure 5. Basic instrument setup for gas chromatography using atomic emission detection with an MIP atom reservoir.

ated species (Buffington and Wilson, 1987) at all. The MIP-AED response depends solely on elemental composition, provided selectivity over matrix or background element emission is sufficiently high.

Before discussion of GC-AED applications and developments, an introduction to the general instrumental setup is helpful. The basic system is illustrated in Figure 5. Samples injected into the GC column are separated and enter a heated interface. Heating is necessary to prevent analyte condensation prior to the MIP. Most systems use a vent for removal of the solvent peak prior to the plasma. Eluents, makeup gas, and reagent gases are introduced prior to the plasma. The plasma is sustained by a device permitting the coupling of microwave energy into the sample carried by a stream of helium, argon, nitrogen or other plasma support gas. Excitation within the plasma leads to atomic emission, which is focused on the entrance slit of a monochromator. Figure 5 indicates that a monochromator is used for monitoring elemental emission. However, simultaneous multielement detection is common using a multichannel polychromator or photodiode array polychromator. Most setups will utilize a computer for data collection and processing.

GC Historical References (1965-1985)

The GC-MIP-AED was first reported by McCormack et al. (1965) who used low-pressure helium and atmospheric argon MIPs for element specific detection of carbon, fluorine, chlorine, bromine, iodine and sulfur in GC eluents. Bache and Lisk reported GC-AED detection of iodine (Bache and Lisk, 1966a) and phosphorus 03ache and Lisk, 1965) using an atmospheric-pressure argon plasma. Later work by the same group using a low-pressure argon plasma was accompanied by


significant improvements in detection limits (Bache and Lisk, 1966b). Subsequent papers by this group (Bache and Lisk, 1967, 1968) and Moye (1967) evaluated the more robust low-pressure helium plasma for phosphorus, sulfur and the halogens. Development of an AED selective to oxygen was first attempted by McCormack et al. (1965). Significant improvements by later workers (McLean et al., 1973; van Dalen et al., 1977; Brenner, 1978; Quimby et al., 1980; Wasik and Schwarz, 1980; Estes et al., 1982a) involved reducing the oxygen background emission. Determi- nation of elemental ratios with GC-AED was first investigated by Dagnall et al. (1975). Later work was done using multichannel (Eckhoff et al., 1983; Olsen et al., 1984) and rapid scanning (Mulligan et al., 1983) spectrometers, as well as the first commercial GC-AED (Applied Chromatographic Systems MPD 850) (Brenner, 1978; Hagen et al., 1983; Slatkavitz and co-workers, 1985, 1986; Bennekessel and Klier, 1978) for determinations of empirical ratios. Reviews of empirical molecular formula determinations have been prepared by Carnahan et al. (1981 ), Mohamed and Caruso (1987), and Uden and co-workers (1986, 1989).

Bache and Lisk (1971) reported the first use of a low-pressure helium MIP for detection oforganomercury compounds. Subsequent work includes determinations of mercury as the diethyl, dimethyl (Bache and Lisk, 1971) and diphenyl (Quimby et al., 1978) derivatives. Reported detection oforganolead compounds has included tetraalkyllead compounds (Quimby et al., 1978; Reamer et al., 1978) in gasoline (Sommer and Ohis, 1979; Uden, 1981; Ohls and Sommer, 1981), in atmosphere (Wasik and Schwarz 1980), and as trialkyllead chloride in water (Estes et al., 1981a). Detection of magnesium as the volatile methylcyclopenta-dienyltricar- bonyl derivative has also been evaluated (Quimby et al., 1978). Separation and determination of volatile organic derivatives of refractory metals are reported by Estes et al. (1981 a). MIP-AES detection has been very successful for detection of hydride-forming elements (van Dalen et al., 1977; Talmi and Bostic, 1975; Robbins and Caruso, 1979a, b; Fricke et al., 1978) and metal chelates (Kawaguchi et al., 1973; Grossman et al., 1972; Serravello and Risby, 1974).

Two extensive reviews of these and other references of historical note have been published. One of the most comprehensive is by Risby and Talmi (1983) discussing virtually all GC,MIP-AED detector developments to 1983. Ebdon et al. (1986) review these earlier references in detail to 1986 and include discussion of other types of AES detectors.

Microwave Discharge Devices for GC-MIP-AED

Several microwave resonator cavities and microwave discharge devices have been used for microwave-induced plasma AES and applied to GC detection. The most important of these, based upon their appearance in the literature, are the one-fourth-wave Evenson cavity (Fehsenfeld et al., 1965), tapered cavity (Fehsen- feld et al., 1965), Beenakker cavity (Beenakker, 1976). Surfatron (Moisan et al., 1977; Selby and Hiefkje, 1987; Chevrier et al., 1982; Hanai et al., 1981), stripline

228 ROBBEY C. CULP and KIN C. NG source (Barnes and Reszke, 1990) and reentrant cavity (Brown et al., 1986; Quimby and Sullivan, 1990a).

Beenakker Cavity for Gas Chromatography

The Beenakker TM010 resonator cavity sustains either atmospheric or low- pressure plasmas with helium, nitrogen or argon over a wide range of operating powers while being viewed axially or radially. Axial viewing (Figure 2) reduces discharge tube cleaning and replacement because of carbon deposits or devitrification. Axial viewing also permits use of more rugged, opaque, discharge tube materials, such as aluminum oxide (Cammann et al., 1983) or boron nitride (Gavick et al., 1990). A number of important improvements to Beenakker's original TM010

5 7 8 6 4

l@ii?iiiiimljlii!iiii!iiM!!@i[[[ II

16 ' i ......... W ~ I ] ? I ~ j [

,/ / / / / ~,~ i / / t 15 14 13

12

Figure 6. Reentrant resonator cavity is characterized by an axial center pedestal and increased thickness of the coupling loop: (1) capillary column, (2) purge flow outlets, (3) reagent and make-up gas inlet, (4) stainless steel plate, (5, 6) cooling water inlet and outlet, (7) quartz water jacket, (8) pedestal, (9) sparker wire, (10, 12) window purge inlet and outlet, (11) window, (13) brass conductor, (14) coupling loop, (15) silica discharge tube, (16) heater block. Reproduced with acknowledgment to the authors (Quimby and Sullivan, 1990a). Copyright, 1990, The American Chemical Society.


design have been discussed in the previous section devoted to improvements in microwave generators, discharge devices and tuners for the MIP.

One of the most important recent advances regarding GC-AEDs has been development of the reentrant cavity (Figure 6) by Quimby and Sullivan (1990a). Modifications to the traditional Beenakker TM010 resonator include reduction of the cavity diameter, with addition of an axial center pedestal and an increase in the thickness of the coupling loop. The gap between the front plate and the pedestal dictates the plasma length (3.9 mm). Quimby et al. (1990a) report that optimizing the cavity shape, coupling loop, and coaxial line provides for a very stable plasma, requiring no tuning. Line intensity drift is as little as 1% over 8 hours.

Gas Chromatography Using the Coaxial Plasma

Carbon deposits, solvent sensitivity, analyte memory effects, plasma discharge tube interactions, and exchange interactions have all posed challenges to usage of MIPs confined to a discharge tube (Tanabe et al., 1981). Halogens, especially fluorine-containing compounds, deteriorate silica or alumina discharge tubes. Huf and Jansen (1983) generate an unconfined helium-MIP for GC-AED between electrodes to mitigate some of the problems associated with cavity-based microwave discharges. The system is not limited to specific frequencies, as in the case of resonator cavity-based designs. Coaxial MIPs can be generated over a frequency range, permitting investigation of the applied microwave frequency on analytical performance. Construction of the device (Figure 7) is accomplished by connecting the central electrode to the inner coaxial line and the outer iris is connected to the shielding of a coaxial cable that is connected to the microwave generator. Imped- ance matching is accomplished when the inner and outer conductors are short- circuited using an adjustable piston (1/4 -)~ short circuit). When properly matched, the device spontaneously ignites with 18 watts of applied power at 2.450 GHz and sustains a plasma at applied powers as low as 2 watts. Typical operation at 30 watts requires 2 L/min of support gas. Two-sigma detection limits for fluorine at the 685.6-nm line and chlorine atJthe 479.5-nm line are somewhat higher than those reported by other groups (Quimby and Sullivan, 1990a), at 95 pg/s vs. 60 pg/s and 105 pg/s vs. 39 pg/s, respectively. Fluorine and chlorine selectivity is lower at 510 vs. 30,000 and 630 vs. 25,000, respectively. The large selectivity difference is attributed to plasma defocusing as it changes shape during passage of analyte. Linear dynamic range is two to three orders of magnitude. An important advantage is the tolerance for high sample loading. Simple modification of the system allows application to analysis of solid samples and thin layer chromatography (Jansen et al., 1985).


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!

0 t 1

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0.16(

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Figure 7. Cross-section of the coaxial plasma source used by Jansen and co-workers (Huf and Jansen, 1983; Jansen et al., 1985): (1) Piston positioning rod; (2) piston; (3) stainless-steel capillary; (4) N-type connector; (5) replaceable nozzle; (6) glass cylin- der. (Redrawn with permission (Jansen et al., 1985), Copyright, 1985, Pergamon Press Publishing.)

Gas Chromatography Using the Surfatron

The Surfatron structure consists of a length of coaxial transmission line that terminates at one end with a short circuit and at the other with a capacitive gap (Abdallah et al., 1982) (see Figure 3). Increases in applied power lengthen the plasma discharge without inflating its diameter. The Surfatron generates a stable and reproducible plasma under a variety of conditions. The surfacewave-sustained MIP (Surfatron) is used by several groups as a source for GC-MIP-AED (Chevrier et al., 1982; Hanai et al., 1981).

MIP GC-AED Interface Developments

A suitable interface between the GC and the MIP is critical to successful chromatography using an AED. The simplest interface uses a heated (150-300 ~ transfer line between the GC and the MIP to 150-300 ~ (Quimby and Sullivan, 1990a; Cammann et al., 1983). This avoids signal broadening due to condensation of vapors prior to the plasma. At high concentrations, organic compounds introduced to the plasma form carbon deposits on the walls of the quartz discharge tube, if not extinguishing the plasma altogether. At lower applied powers (<100 watts),


elution of the solvent may reduce background signals, quench coeluting peaks and form carbon deposits. Carbon deposits absorb part of the desired elemental emission while increasing carbon background emission and may necessitate more frequent discharge tube cleaning or replacement. Strategies to prevent these effects are to initiate the plasma after the solvent peak has eluted (Bache and Lisk, 1965) or use of an interface that vents the solvent peak prior to its entry into the plasma (Quimby et al., 1978). Prevention or removal of deposits has also been accomplished by adding traces of oxygen (McLean et al., 1973; Olsen et al., 1984; Serravello and Risby, 1974), nitrogen (Talmi et al., 1975), air (McCormack et al., 1965), or hydrogen (Olsen et al., 1984) as scavenging gases. The utility of reagent gases has been largely accepted. Recent development in this regard focuses primarily on development of a venting interface.

Solvent Venting Interfaces for MIP GC-AED

Sample response has been shown to be a function of the time between plasma initiation and peak elution (Bostick and Talmi, 1977). The early systems that simply initiated the plasma after elution of the solvent peak were found to be unsatisfactory as a result. Systems permitting solvent venting with continuous plasma operation are reported by several groups. Quimby et al. (1978, 1979, 1980) used a dual detector system with an FID and MIP-AED. The heated transfer line to the MIP incorporates a high-temperature valve which vents the solvent peak during continuous plasma operation. GC effluents are vented until the FID indicates passage of the solvent peak. The three-way valve is then turned to allow eluents to pass through the plasma for detection. A similar system is also reported by Cerbus and Gluck (1983) and Haas and Caruso (1985).

The system described above provides good results for stable volatile derivatives of lead, manganese, and mercury derivatives. However, the heated stainless steel interface is not suitable for chemically active compounds, such as lead or mercury organohalides (Estes et al., 1981). An improved design by Estes and co-workers (1980, 198 l a, 1982) uses a chemically inert, low dead volume interface with fluid logic gas switching instead of in-line valves for on-line venting. Similarly to previous systems (Quimby et al., 1978), a gas splitter provides for simultaneous FID and MIP-AES detection. A purge system isolates the detector plasma until passage of the solvent peak. During purging, helium is introduced to a tee-fitting immediately behind the plasma, reversing flow of capillary GC eluents to a vent up-stream from the plasma. With a purge gas flow of about 300 mL/min, half the flow is used to support the plasma during venting. After the solvent peak has passed, solenoid-activated valves simultaneously return the system to normal flow, isolating the front tee and allowing introduction of 10-20 mL/min of helium at the back tee for plasma support.

Practical improvements over previous designs are achieved by Zhang et al. (1990b). The vent system is constructed as part of a new torch assembly. Increases


.•, Plasma .___ Plasma

i ! ! Fused Silica Discharge Tube _ _ Fused Silica Discharge Tube

~i11:11"~-.[ F!tdirectio,,al Switching Valve ,i~ti,~,':l v~.~t o, ,,ode\

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Figure 8. Solvent-venting interface design. (A) Vent-off mode (chromatographic effluent routed to plasma); (B) vent-on mode (chromatographic effluent routed to waste). Reprinted with permission from the authors (Zhang et al., 1990b). Copyright, 1990, Elsevier Science Publishers.

in dead volume are eliminated by eliminating the need for tees in the quartz GC capillary. Rapid switching of vent on to vent off does not affect the plasma base line, as in previous systems. Figure 8 depicts the torch design proposed. Potential increases in resolution result with simplified construction of the solvent venting system. The torch used is of concentric tube design. Make-up gas flows over the outside of the central tube while the quartz capillary is contained in the center tube. During purging, flow in the central tube is reversed to vent. With proper adjustment of the purge rate, it is possible to prevent all column effluents from entering the plasma without significantly affecting the helium makeup gas flow. Figure 9 illustrates the negligible effect of alternating vent on and vent off on the chromatogram base line.

The Hewlett-Packard atomic emission detector (HP 5921A AED) also uses an atmospheric purge system. Total helium make-up flow is reversed during venting so column effluents are moved away from the torch to vent. Similar to the system


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Figure 9. (A) Baseline studies during vent-on and vent-off operations; (B) solvent peak during split injection in the vent-off mode; (C) chromatogram during split injection in the vent-on mode. Reprinted with permission from the authors (Zhang et al., 1990b). Copyright, 1990, Elsevier Science Publishers.

reported by Zhang et al. (1990a) no connections are made to the column capillary, minimizing dead volume.

Plasma Torch Designs for Gas Chromatography

The first torch designs used with GC-MIP were simple quartz tubes with inside diameters of 0.5 to 3.0 mm. In most cases, they provide detection limits on the order of picograms per second (Bache and Lisk, 1965; Beenakker, 1977; Estes et al., 198 lb). In these systems the plasma diameter (2 mm) permits intimate contact of the plasma with the torch walls. Problems of memory effects, nonlinear response and devitrification of the tube walls lead to frequent replacement of the plasma tube. The inability to accept even microliter quantities of some organic solvents without plasma extinction has been an added complication (Goode et al., 1983a). An alternative proposed by Bollo-Kamara and Codding (1981) utilizes a tangential flow torch (TFT) to constrict the plasma, preventing plasma tube contact. Similar


to torches used for ICE the sample is introduced to the center of a spiral gas stream just prior to the plasma. The tangential flow aids in centering and stabilizing the plasma while isolating the tube walls from the discharge. Goode et al. (1983a) critically evaluated this torch design as part of a GC-MIP-AED system. The relatively high tangential gas flow rate (0.4-2.5 L/min) required for this torch and other TFT designs (Haas and Caruso, 1985; Goode et al., 1983a) is generally thought to reduce analyte residence time owing to the increased linear velocity of the support gas (Bolainez et al., 1992a). However, the linear dynamic range and acceptance of solvent loading, as well as ease of operation, has led to their widespread acceptance and use.

The TFT plasma is generated in a highly turbulent swirling zone. The combination of relatively high tangential gas flow rates and a turbulent plasma zone may contribute to higher detection limits than those of capillary discharge tubes. The development of the laminar flow torch (LFT) is reported by Bruce et al. (1985). It is designed to minimize turbulence and prolong residence time in the plasma zone. A rearward facing step or center insert separated from an outer, concentrically placed tube results in a stable recirculating region downstream from the rearward facing step (Figure 10).

Bruce and Caruso (1985) compared the LFT, TFT, and capillary (0.5-mm i.d.) torches for analysis of pyrethroids and dioxins. The LFT uses a 65 mL/min column gas flow for plasma support while using 80 watts of applied power. Determination of elemental ratios for these compounds proved challenging when using either the capillary or tangential torches (Bruce and Caruso, 1985). The LFT provided significantly better precision and detection (Table 2).

Concentric tube LFTs have been recently reported by two groups (Sobering et al., 1988; Fielden et al., 1989). Concentric designs do not use a rearward-facing step. Instead, laminar flow is induced between the plasma tube and a centrally

RADIAL ALIGNMENT CENTRAL ~ . ~ . S Y S T E M

I TUBE /

CENTRAL ~'~ FLOW ~-~ GASES

ANNULAR FLOW GASES

DISCHARGE TUBE

Figure 10. Laminar flow torch. Redrawn with permission from the authors (Bruce et al., 1985). Copyright, 1985, The Society for Applied Spectroscopy.


Table 2. Select Nonmetal Detection Limits (pg sec), Based on Torch Design

Torch C H F CI Br Ref

Tangential 76 m m 120 - - (Haas and Caruso, 1985) Tangential 70 ~ ~ - - 40000 (Good et al., 1983a) Laminar 8.0 5 3.6 40 62 (Bruce et al., 1985)

Pyrethroids 8 5 4 40 62 (Bruce and Caruso, 1985) Dioxins 29 16 ~ 100 ~ (Bruce and Caruso, 1985)

Laminar 24 6 - - 12 - - (Fielden et al., 1989) Tangential 310 . . . . (Bolainez et al., 1992) WCCT 280 . . . . (Bolainez et al., 1992) WCCT 3.9 4.5 60 58 112 (Sullivan and Quimby, 1987)

placed sample introduction tube. Laminar flow automatically centers the plasma and cools the discharge tube. Plasma support gas and sample are injected into the center.

The concentric tube LFT design by Sobering et al. (1988) is characterized by

introduction of the laminar gas to the center of a 1/4-inch NPT tee fitting. Swagelok adapters hold the 16-gauge center concentric sample introduction tube and the outer

plasma tube. Discharge tube position in the tee-fitting can be adjusted for optimal laminar flow. No other devices are used to direct or modify flow. Visualization of

the sample flow with liquid aerosols shows the sample confined to the center of the discharge tube over a wide range of laminar gas flows (ca. 0.1-2.0 L/min).

Fielden et al. (1989) report optimization of a small-diameter concentric tube laminar torch (Figure 11) for GC-AED. The torch consists of a brass base containing two concentrically placed silica tubes. Positioning of the central tube is accomplished by careful adjustment of the three positioning screws. Sheathing gas i~,,

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Figure 11. Concentric tube laminar flow torch used by Fielden et al. (1989). The concentric torch replaces the center insert or step with a center tube. (Redrawn with permission of Fielden et al., 1989, Copyright, 1989, The Society for Applied Spectros- copy.)


SAMPLE GAS

COOLING WATER

A C D

, , , ,

F G B

COOLING WATER

Figure 12. Schematic diagram ofthe water cooled capillary plasma torch. (A) Quartz capillary tube, (B) water jacket, (C) Swagelok body, (D) end cap, (E) threaded fitting, (F) O-rings, (G) Teflon ferrule. Redrawn with permission of Bolainez (I 992a), Copyright 1992, The American Chemical Society.

introduced to an annular space where it is diverted through an array of circumferentially drilled holes to the annulus between the inner and outer tubes. The recirculating zone is located in front of the inner concentric tube and is separated from the main gas stream. Although a plasma can be sustained with flow rates as low as 1.0 mL/min, best operation is reported using 30--40 mL/min of helium make-up gas, with applied powers of 60-70 watts and 2.5-3.0 L/min of laminar gas. Detection limits are comparable to those of Bruce et al. (1985) (Table 2).

Quimby and Sullivan (1990a) have recently reported the development of a water-cooled capillary plasma torch (WCCPT). The torch is available as part of the Hewlett-Packard AED (HP 5921A). A quartz cooling water jacket extends through the cavity, permitting cooling of the 1-mm i.d. discharge tube. Cooling of the discharge tube reduces erosion of the tube wall and tailing of elements such as sulfur and phosphorus. Detection limits with this system are some of the best reported and are the result of a number of innovations as well as of the torch design. Bolainez et al. (1992a) contrast sensitivity of the WCCPT torch with a tangential flow torch. The WCCPT (Figure 12) consists of a 1.3-mm i.d. by 1.5-mm o.d. discharge tube, which is surrounded by a 2-mm i.d. by 6-mm o.d. quartz tube, cooling water jacket. The very thin cooling water jacket efficiently cools the discharge tube capillary while also minimizing absorption of microwave energy by the cooling water. Total plasma gas flows are maintained at 30 mL/min. Variation in the 127 mL/min cooling water flow rate did not significantly affect signal intensity.

Empirical Formula Determination

The application of GC-AED to determination of empirical formula ratios is predicated on a linear relationship between the atomic emission intensity for each


atomic species and the number of analyte atoms per molecule and is independent of the eluted compound's structure or that of the reference compound, as depicted in Eq. 1, where E is the number of atoms of interest and C is the number of carbon atoms or the carbon atom response in the analyte molecule.

E E (atoms in ref.) E (response unknown) C (response ref.) (1) - - X X

C C (atoms in ref.) E (response ref.) C (response unknown)

Empirical formula determination discrepancies have been reported by a number of workers (Dagnall et al., 1975; Evans et al., 1987; Dingjan and de Jong, 1983a). Studies by Slatkavitz et al. (1984) and Uden et al. (1986) used experimentally derived empirical ratios to calculate the coefficients of 13 chlorinated hydrocarbon molecular formulas. Relative errors for chlorine coefficients range from 0 to 10%. Valente and Uden (1990) plotted elemental ratios versus peak height ratios for the list of compounds previously evaluated by Slatkavitz et al. (1984). These plots permit correlation of elemental emission intensity ratios with known elemental ratios. In the event of no structural correlation the plot is linear. These graphs yield linear responses for C:CI ratios, and nonlinear relations for C:H and H:CI. Graphs of relative carbon responses vs. number of carbon atoms and a similar plot for chlorine yielded linear responses. The hydrogen graph was not linear. Wylie and Oguchi (1990) found that compounds with high chlorine content have elevated hydrogen responses. Wylie hypothesizes that the interference at high chlorine levels is due to an interaction of hydrogen with the discharge tube wall. They report that hydrogen doping corrects nonlinearity at the expense of an elevated hydrogen background. Data for higher concentration solutions reveal nonlinear behavior for both carbon and chlorine as well. Yie-ru et al. (1990) investigated the effect of reference compound selection on empirical formula determinations for n-alkene, n-alkane, aromatic, brominated and chlorinated hydrocarbons. The choice of reference compound is found to be unimportant for determination of empirical formulas using solutions of alkenes, alkanes, or mixtures. However, choice of reference standard for mixtures of aromatic hydrocarbons, alkenes, and alkanes did affect accuracy of empirical formula determinations. Alkane standards yielded best results for the alkanes in the mixture while aromatic standards led to best accuracy for aromatic compounds and so on. Halogenated hydrocarbon ratios were most accurate when reference compounds possessed similar hydrocarbon skeletons and similar numbers of halogen atoms. The effect is magnified by large differences, such as when a monobromo species is used as a reference standard for a tetrabromo species.

A second potential error (Estes et al., 1981 b) in multielement determinations of organic compounds is due to the molecular emission of CN, CO and C2 § leading to a continuum shift at the carbon line. This shift has the effect of inflating the carbon to halogen ratio. On-line (Hagen et al., 1983) or off-line (Estes et al., 198 lb) background correction schemes are used to avert this problem. Haas and Caruso


(1985), using a 190-watt plasma, report improvements in accuracy with off-line background correction. Maximum error is less than 1% for a set of three chlorohy- drocarbons with C:C1 ratios from 1 to 5.33.

The new Hewlett-Packard GC-AED system (Model 5921A) has also been evaluated for empirical formula determinations (Sullivan and Quimby, 1989, 1990; Wylie, 1989) for a variety of compounds. The photodiode array spectrometer permits multielement simultaneous data collection. A software algorithm is used for calibrating and measuring element ratios from multielement integrator results. Results for fatty acid esters indicate the C:O and C:H ratios are within 3%. Substituted phenol and polychlorinated biphenol results had significantly elevated hydrogen responses for compounds with high chlorine content. Adding a small continuous flow of hydrogen to the plasma gas during analysis was helpful to prevent the variable hydrogen response with compounds possessing one to five chlorine atoms. However, this was only at the expense of a 40-times higher hydrogen background and a 10-fold reduction in the signal to noise ratio. The nonlinear hydrogen response, due to a high chlorine number, could not be corrected by increasing the hydrogen background.

Reflected Power Detector

An innovative reflected power detector has been developed by Bolainez and Boss (1991) based on the highly efficient TM010 Beenakker cavity (Bums, 1987). When a cavity's resonant frequency is equal to the frequency of the microwave source and the plasma has an impedance equal to 50 ohms resistivity, the system is defined as critically coupled. A critically coupled cavity permits a very low reflected power (0 002-0.01 mW) (Matus et al., 1983). Reflected power deviations from background occur as the plasma is perturbed by introduction of organic vapors, resulting in plasma conductivity changes. Bolainez and Boss (1991) observed that reflected power was proportional to the mass of analyte present in the plasma, but did not find a linear relationship. Instead, a quadratic curve was observed. Signals for most detectors are based on voltage, which leads to a linear response. The nonlinearity of the calibration curves are explained by realizing that power is a square function of voltage. Taking the square root of each data point before peak area integration permits a linear response. The effects of applied power (6-26 W) and support gas flow rate (1.0-2.3 L/min) are evaluated. Optimal conditions for the 6.0-mm tangential torch are 12 watts of applied power with a 1.5-1.8 L/min flow of argon. Calculated mass detection is 1 t.tg of n-pentane per second. The minimum detectable carbon and hydrogen limits are 950 ng/s and 190 ng/s, respectively.

Bolainez et al. (1992a) followed up the above work using a water-cooled capillary plasma torch (WCCPT) to introduce GC eluents to a 30-watt microwave plasma for reflected power detection. Support gas flow is maintained at 33 mL/min. The low plasma gas flow permits detection limits of 0.6 ng/s for carbon. Evaluation of the origin of source noise that has prevented further depression of detection limits


with the reflected power detector was recently reported (Bolainez et al., 1992b). The largest contributor of noise in the reflected power detector background signal is fluctuations in the microwave power supply output.

GC-AED Detection of Metals

Indium. Uden and Wang (1990) use an MIP-AED to study ligand redistribution reactions of fluoridated gallium, indium, and aluminum [3-diketonates. Element-selective detection of carbon, gallium, indium, and aluminum provides a pathway for elucidating the qualitative and quantitative reactions of complex formation and ligand exchange. The quartz tangential plasma torch is optimized for indium with a 2.8 L/min helium flow. As there are very few results using indium to compare with, the authors report an absolute detection limit of 8 ng. This is much poorer than that reported for other metals used in the study. The authors surmise that the high detection limit is due to the interaction of indium with the plasma tube wall. The high flow rate results in a smaller residence time and is blamed for the relatively high indium detection limit.

Mercury. Organomercury halides typically possess poor chromatographic characteristics. Even with the use of high-efficiency capillary columns, success has been mixed (Berman et al., 1989; Lee and Mowrer, 1989). The high polarity of these compounds leads to partial on-column decomposition and anion-exchange processes taking place in the fused capillary column (O'Reilly, 1982). Bulska et al. (1991) use a Grignard reagent to alkylate organomercury halides to methyl- and ethylmercury derivatives, eliminating the very polar halide bond. The organomercury derivatives exhibit much better chromatographic characteristics. The technique is demonstrated using IAEA "lyophilized fish tissue" reference material (MA-B-3/TM). Results exhibit mercury recoveries well within the standard deviation of the certified value.

Lead. Greenway and Barnett (1991) report optimization of an MIP for selective detection of tetramethyl- and tetraethyllead as Pb and dimethylmercury and methyl- mercury (II) iodide as Hg. Simplex and univariate search methods were investigated for optimization of plasma operating parameters, PMT voltage, and choice of atom line. The most critical parameter reported is plasma gas flow. Several different torch designs were evaluated, including a tangential torch and a recrystallized alumina torch. The alumina torch is reported to exhibit a high background and low sensitivity. The authors report the best signal to noise ratio using a simple fused silica quartz plasma tube. Optimal applied power was found to be 100 watts with a helium flow rate of 830 mL/min.

Tin. Lobinski et al. (1992) have introduced a comprehensive method for determination of mono-, di-, tri-, and some tetraalkylated organotin compounds in


water and sediments by GC-MIP. Ionic organotin compounds are extracted with diethyldithiocarbamates into pentane, followed by evaporation to dryness. Deriva- tization with pentylmagnesium bromide in n-octane to produce the pentylated alkyltin compound yields a solution of the organotin species appropriate for gas chromatography (Dirkx et al., 1989). Solvent vent time is an important parameter when using the HP-5921A atomic emission detector owing to the volatility of some of the tin derivatives. It was necessary, in some cases, to use a more volatile solvent, such as hexane, to prevent loss of very volatile organotin compounds with the solvent. Maximum response was obtained with a flow rate of 240 mL/min and fell off rapidly with increases or decreases in make-up gas flow. While 20% higher than the manufacturer's recommended flow rates, response is improved by a factor of two. Tin is prone to formation of refractory oxides in the presence of oxygen and will condense on the discharge tube wall immediately in front of the hottest part of the discharge, leading to tailing. The authors mitigate tin deposition by doping a small unspecified percentage of hydrogen gas into the makeup helium. Hydrogen doping improves detection of hydride-forming elements such as aluminum, boron, phosphorus, tin, lead, arsenic and germanium (Estes et al., 1981b). Absolute detection limits for organotin by this method and instrument are 0.05 pg and are the best reported to date for GC tin detection. The analytical figures of merit for some metals are summarized in Table 3.

GC-AED Detection of Nonmetals

Isotopic Ratios by GC-AED. Carbon- 13 and deuterium are commonly used as tracers to determine the fate of labeled compounds. Combustion chambers and radiochemical detectors limit chromatic resolution available with modern capillary columns. The atomic emission lines of deuterium (656.1 nm) and hydrogen (656.3 nm) are sufficiently separated to allow selective detection of each isotope with typical laboratory spectrometers (Estes et al., 198 lb; McLean et al., 1973; Quimby and Sullivan, 1990a; Sullivan and Quimby, 1990).

Gas chromatography-mass spectrometry (GC-MS) is the most common method for determining isotopic ratios of carbon (vanden Heuvel, 1986; Chance and Adamson, 1989). GC-MS detection of 14C is limited owing to interference from 13C, 2H, 180, and so forth, which contributes to the M + 2 background for most organic molecules (Markey and Adamson, 1982). As an alternative, Markey and Abramson (1982) used a low-pressure MIP interface to convert capillary GC organic effluents to CO and CO2, which are detected by mass spectrometry. Quimby et al. (1990) suggested that another, more specific approach to determining 12C:13C ratios is to monitor the intensity of the 241.94-nm 12CO+ and 241.36-nm 13CO+ molecular bands. An algorithm (Sullivan and Quimby, 1990) that takes diode array signals and combines them to produce background-corrected chromatograms allowed the authors to produce separate chromatograms of each isotope. Detection limits for 12C and 13C using this technique were measured to be 110 pg/s. The

Recent Developments in Analytical Microwave-Induced Plasmas 2 41

Table 3. Organometallic Detection Limits [as nanograms (ng) or nanograms per second (ng/sec)], Wavelength (~.), Selectivity versus Carbon (vs. C), and Linear Dynamic Range (LDR) (power of 10) Using Gas Chromatography with Atomic

Emission Spectroscopic Detection

Detection Limit Selectivity

Element ng ng/sec (vs. C) LDR (lOx) ~, (nm) Ref.

A11 0.019 0.0050 3900 2.7 396.2 (Estes et al., 1981b)

Co I 0.003 60000 3 345.3 (Quimby and Sullivan, 1990b)

Cu I 0.001 100000 3 324.7 (Quimby and Sullivan, 1990b) Cr II 0.019 0.0075 108000 3 267.7 (Estes et al., 1981b) Fe 0.00005 1000000 3 302.1 (Quimby and Sullivan, 1990b)

Hg I 0.0001 300000 253.7 (Quimby and Sullivan, 1990b) Hg I 0.00083 1.8 x 106 253.7 (Greenway and Barnett, 1989)

Hg I 0.0013 1.8 x 106 253.7 (Greenway and Barnett, 1989)

0.0008 (Estes et al., 1982b) 0.0013 (Estes et al., 1982b)

In I 8 600 325.6 (Uden and Wang, 1990) Mn II 0.0077 0.0016 111000 3 257.6 (Estes et al., 1981b) Mo II 0.025 0.0055 24200 2.7 281.6 (Estes et al., 1981b)

Ni II 0.0059 0.0026 6470 3 231.6 (Estes et al., 1981b) Nb II 0.335 0.069 32100 2 288.3 (Estes et al., 1981b) Os II 0.034 0.0063 182000 3 225.6 (Estes et al., 1981b)

Pb I 0.0013 37000 405.78 (Greenway and Barnett, 1989) Pb I 0.0011 37000 405.78 (Greenway and Barnett, 1989) Pb I 0.0007 0.00017 24600 3 283.3 (Estes et al., 1982b) U II 0.035 0.0078 134000 3 240.3 (Estes et al., 1981b)

Sb I 0.005 19000 3 217.6 (Quimby and Sullivan, 1990b) Se I 0.004 50000 3 96.1 (Quimby and Sullivan, 1990b) Se I 0.062 0.0053 10900 3 204.0 (Estes et al., 1981b) Sn I 0.001 37000 303.41 (Sullivan, 1991) Sn I 0.0005 303.419 (Lobinski et al., 1992) V II 0.004 36000 3 292.4 (Quimby and Sullivan, 1990b) V II 0.026 0.10 56900 2 268.8 (Estes et al. 1981b)

vacuum ultraviolet molecular band centered at 171.4 nm is reported by Quimby and Sullivan (1990b) to be about 100 times more intense than the 241.94-nm band. Further improvements were achieved by optimizing reagent and make-up gas flows. Best performance was achieved using low make-up gas flow rates (20 mL/min), higher oxygen reagent gas flow rates (1.5%) compared to atomic detection conditions, and addition of hydrogen (0.005 mL/min) to the oxygen reagent gas. This resulted in a detection limit for 13C at 171.0 nm and ~2C at 171.4 nm of 10 pg/s.


Oxygen. A detector selective to oxygen-containing compounds is attractive for identifying oxygenates which may influence the refining or stability of petroleum and synthetic fuels. Typical helium GC-MIP detectors have a high oxygen background due to the presence of oxygen contamination in the helium carrier gas, air leakage into the system, and back diffusion of air into the plasma. Devitrification of fused silica discharge tubes also contributes to a higher oxygen background. Slatkavitz et al. (1986) added hydrogen as a scavenger gas to the helium plasma to reduce oxygen emission while using a boron nitride discharge tube to reduce oxygen background. With axial viewing, the oxygen sensitivity and oxygen to carbon selectivity were reported to be 0.2 ng/s and 10 to 1, respectively.

Goode and Kimbrough (1988) propose using the more intense molecular OH band (308.9 nm) for detection of oxygenates. They systematically evaluated the causes of elevated oxygen background common to most systems. Evaluation of possible air leaks, column bleed, and torch devitrification implied that they are not the major contributors to a high oxygen background. Argon was the preferred plasma gas. Other variations evaluated are: hydrogen added to helium, hydrogen added to argon, argon added to helium. Purification with a dry ice-acetone trap reduces the oxygen background. Water vapor in the plasma gases is believed to be the most significant contributor. Bradley and Carnahan (1988) obtained further reductions in oxygen background by sheathing the discharge tube with purified nitrogen. Using these precautions they further depressed detection limits for oxygen and increased oxygen to carbon selectivity to 1000. Detection limits for complex mixtures of oxygenates were apparently higher owing to difficulty in resolving individual oxygen signals associated with oxygen-containing compounds.

Fluorine. Detection of fluorine compounds has been challenging owing to reaction of fluorine with the discharge tube wall (Haas and Caruso, 1985; Goode et al., 1983b) as well as spectral interferences in fluorine and chlorine emission due to molecular emission interferences. These were investigated by Koirtyohann (1983), who recommended limiting plasma gas nitrogen levels to 0.001% or less for maximum fluorine emission. Brill et al. (1988) used 10 times the nitrogen concentrations recommended by Koirtyohann (1983) with little effect on fluorine emission and reported that fluorine emission rises sharply with increases in applied power from 50 to 110 W. Fluorine detection limits are an order of magnitude superior to those found by Quimby and Sullivan (1990a) using the Hewlett-Packard AED with a similar linear dynamic range. Sullivan and Quimby (1992) advocate use of a mixed oxygen and hydrogen reagent gas to improve peak shape and selectivity. Sensitivity of the HP-AED system is a factor of 30 superior to that of the system used by Brill et al. (1988).

Halogenated Compounds. With the exception of fluorine and oxygen, the majority of recent nonmetal determinations have involved determination of halogens in conjunction with other nonmetal elements of interest. Most systems


reported have multichannel capability permitting convenient calculation of empirical formulas. The majority of developments (Sullivan, 1991; Wylie and Oguchi, 1990; Quimby and Sullivan, 1990a,b; Goode and Kimbrough, 1988; Kovacic and Ramus, 1992) in this regard involve use of the Hewlett-Packard AED (model #HP 5921A, Hewlett-Packard Company, Avondale, Pennsylvania). An evaluation of the Hewlett-Packard AED has been reported by Quimby and Sullivan (1990a). The system has a very broad dynamic range (> 3k) with selectivities generally greater than three to four orders of magnitude. A purged optical path permits observation of the more sensitive resonance lines in the vacuum ultraviolet region. A water- cooled torch reduces discharge tube ablation and the reentrant cavity eliminates the need for periodic tuning. Reagent gas recipes have been developed for optimal determination of sets of elements simultaneously, using the diode array spectrometer (Quimby and Sullivan, 1990b). Wylie and Oguchi (1990) report use of the Hewlett-Packard AED for pesticide quantification and determination of approximate empirical formulas of 20 different herbicides in two different mixtures. Empirical formula data obtained for up to seven elements (carbon, hydrogen, nitrogen, oxygen, sulfur, chlorine, and fluorine) in the two mixtures did not allow precise calculation of the correct formulas. Calculated values for hydrogen, nitrogen, and sulfur were generally low when using a single internal standard.

Variation of the elemental response factors (ERF) used by the software algorithm for determination of empirical formulas was measured for carbon, chlorine, fluorine, and nitrogen by Kovacic and Ramus (1992). Five groups were evaluated for the structural dependence of emission intensity and the relative standard deviation (RSD) of the resulting ERE The first group was composed of nine complex heterocyclic compounds, including a number of molecular ring substituents composed of carbon, hydrogen, nitrogen, chlorine, fluorine, and oxygen. A second group was composed of four pyridine compounds with chlorine and fluorine ring substituents. Five chlorinated heterocyclic compounds constituted the third group, and five chlorinated aromatic compounds were evaluated as the fourth group. The fifth group included three chlorinated aliphatic compounds. Chlorine measurements associated with the third group of chlorinated heterocyclic compounds indicated dependence of emission on structure. Results were negative in this regard for carbon, fluorine, nitrogen, and oxygen, as well as for chlorine in groups other than the third one. Percent RSD variation for these elements is 3, 6, 5, 6 and 5%, respectively.

Miscellaneous. George et al. (1989) reported low picogram detection limits using a GC-AED for determination of hydrogen, methane and carbon dioxide contaminant gas concentrations in argon and helium. Seeley and Uden (1991) obtained picogram detection limits (12 pg) and a broad dynamic range (4 orders of magnitude) using a commercial MIP-AED for determination of boron in motor oils.

A novel optical system was used by Stilkenbohmer and Cammann (1989) in lieu of a polychromator or photodiode array spectrometer for determination of hydrogen


concentrations and H:C ratios in organic compounds, with automatic background correction. Hydrogen detection limit results are somewhat higher than those using conventional optical systems at 0.5 ng/s. Determinations of C:H ratios for the compounds evaluated have RSDs of 4-7%. A similar setup has been used by Bradter et al. (1989) for determination of fluorinated inorganic compounds with low ng/s detection.

B. Element-Specific Detection with HPLC

One of the most challenging areas of MIP research has been the development of an MIP-AED for liquid chromatography (LC). With typical operation in the range of 50 to 200 watts, MIPs possess high energy densities with relatively low total energy. As a result, MIPs can be intolerant of liquid aerosol sample introduction (Zander and Hieftje, 1981). Common organic HPLC solvents, when introduced to the MIP, may interfere with plasma coupling efficiency causing plasma instability or extinction (Boorn and Browner, 1982). Sample dilution prior to its reaching the MIP-AED, occurring first in the mobile phase during the chromatographic process and second when diluted into the plasma support gas, is also a factor. Despite these problems the MIP offers one of the best potentials as an LC element-selective detector.

One of the first attempts to construct a sample introduction interface for LC to MIP for AED was done by Billiet et al. (1983) and is shown in Figure 13. The proportional interface allows use of a variety of mobile phase systems. Low to moderate concentrations of acetonitrile (ACN) and water are found to have minimal effect on plasma stability. However, decreases in sensitivity are seen for tetrahy- drofuran (THF)-water mixtures with 60% or more THE According to the authors,

Capillary LC Elllent

~ , He + effluent . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . ~ Heated ~ Vapor to plasma===--------------~ ~ ........................ 9 He + 0.7 % 02

Drain

Figure 13. The proportional interface used by Billiet et al. (1983). This interface is unique in permitting use of a variety of mobile phase systems for HPLC with atomic emission detection. Reproduced with acknowledgment to the authors (Billiet, van Dalen, and Schoenmakers), Copyright 1983, The American Chemical Society.

Recent Developments in Analytical Microwave-Induced Plasmas

1 CM

T O R C H 1"

245

--- ~ A

D R A I N DRAIN D R A I N GF-A GF-B GF-C

Figure 14. The microfrit nebulizer interface used by Ibrahim et al. (1985) as a HPLC interface for atomic emission spectroscopy detection. The microfrit nebulizer permits use of high percent organic solvent mobile phase systems. (Reprinted with permission of Ibrahim et al., 1985, Copyright 1985, Preston Publishers.)

an increase of the percent oxygen doped into the plasma gas to 4% allows operation to levels of 90% THE In pure THF or pure ACN the AED yields broad peaks with severe tailing, preventing their use for residence-time calculations. Effluents leaving an LC capillary form a film on the metal wire. The analyte is introduced to the low-power (70-W) MIP by evaporation into a stream of heated helium plasma support gas. The partial insertion of the waste tube into the cross junction helps reduce the interface dead volume and to efficiently remove excess effluent. A small concentration of oxygen (0.7%) is doped into the helium plasma support gas during normal operation to prevent carbon deposits in the 2-mm i.d. quartz discharge tube.

A microfrit nebulizer (Figure 14) has been suggested by Ibrahim et al. (1985) as a possible interface capillary HPLC and MIP. The small sample effluent and low mobile-phase flow (100 ktL/min) rate are an advantage for MIP sample introduction. Fifty percent methanol solutions of tetramethyl- and tetraethyllead were separated and deposited on the glass frit nebulizer. Introduction to 430-watt MIP, sustained with a minimal argon plasma gas flow of less than 1L/min resulted in detection limits of 6.8 ng and 40.3 ng of lead for tetramethyl- and tetraethyllead, respectively. Unfortunately, good chromatographic separation was not achieved, due to column damage from previous work. Ibrahim et al. (1985) report that nebulizer proximity to the plasma had a profound effect on sharpness of the peaks and on detection limits.


Table 4. Detection Limits for L C-MIP

Species Wavelength (nm) Mass Detection Limit (lag) Detection Limit (ng/sec)

CIO- 479.45 2.1 94

C103 479.45 2.5 66

BrO2 478.55 2.9 110

BrO3 478.55 2.5 120

I- 206.24 6.0 94

103 206.24 1.5 66

Note: Reprinted with permission from the authors (Michlewicz and Carnahan). Copyright 1987, Marcel Dekker, Inc.

Michlewicz and Carnahan (1987) used conventional ion-exchange chromatography to separate a system of halides and oxohalogen salts. Chromatographic effluents are introduced to a 500-watt helium MIP via an ultrasonic nebulizer for AES detection. The high-power system accepts higher sample loadings. Optimal plasma support gas flow for the system is reportedly 21 L/min. The 2.5% organic mobile phase does not appear to have influenced the operation. Detection limits for the setup are listed in Table 4. Michlewicz and Carnahan (1987) attribute the unusually intense spectral background to the lack of significantly lower detection limits. Although not explicitly stated, the high support gas flow rate might also be partially responsible.

Jansen et al. (1985) evaluated a coaxial plasma source for AED. The low power (75-watt) microwave discharge does not require a plasma discharge tube, eliminating the problems of carbon deposition and memory effects. While good results are reported for detection of gas chromatographic effluents, results for LC were not analytically useful.

A moving wheel interface has been developed by Zhang et al. (1985) for removal of volatile mobile phase solvents prior to formation of the plasma. Effluents from a HPLC are introduced to a stainless steel wheel via a conventional nebulizer. The interface is configured to allow a stream of heated nitrogen to vaporize the lower boiling point mobile phase solvent, leaving the analyte on the stainless steel wheel. Subsequently, the residue is vaporized into 17.1 L/min of helium plasma support gas flow by passage within close proximity (1.5 mm) to the 500-watt MIP.

A subsequent, as yet unpublished work, also by Zhang et al. (1990a), offers a significant solvent venting interface improvement (Figure 15). The authors use a moving belt interface formerly used for mass spectrometry. This allows optimization of plasma support gas flow rates for increased analyte residence time. Samples are chromatographically separated using a conventional C18 reverse phase column with a water-methanol mobile phase. Chromatographic effluents are deposited on a moving belt. The polyimide belt allows even distribution of effluents in a 0.2-mm thick film on its surface. As the belt moves toward the vaporization chamber it is first subjected to heating and then to two separate vacuum chambers designed to

Recent Developments in Analytical Microwave-Induced Plasmas Hc

TMolo ~r Cavity

t

247

Inl'rarcd mm Locks ~ Rellccto'r LC

Loaded #2 #1 idicr

O Whcci

Pump Pump Whccls Plasma Tubc

Figure 15. Moving belt interface used by Zhang et al. (1990b). This interface permits desolvation followed by vaporization of the analyte into very small flows of plasma support gas. Redrawn with permission of (Zhang et al., 1990b).

remove the mobile phase solvent. In the last step, the analyte is flash vaporized into the helium support gas. Sample loading with this system is of less importance. Only the analyte enters the 80-watt plasma. Samples analyzed in this study were 4-chlo- robenzophenone (b.p. 323 ~ 9-chlorofluorene (b.p ca. 150 ~ and p-chloro- biphenyl (b.p. 291 ~ The very low plasma support gas flow rate (20 mL/min) reduced diffusion and promoted longer residence time in the plasma. This is further helped by the small (2 mm) diameter of the plasma discharge tube. The evaporation step would appear to limit analyte selection to those with relatively high boiling points.

Galante and co-workers (Galante and Hieftje, 1987; Galante et al., 1988b), as well as Downey and Hieftje (1983), have utilized the MIP in conjunction with replacement ion chromatography (RIC) for element selective detection. RIC ex- changes less analytically sensitive ions with a lithium replacement ion. The system works similarly to suppressed ion chromatography (Smith and Chang, 1983; Pohl and Johnson, 1980; Fritz et al., 1982), but uses a third column to replace the eluting ion or its co-ions with another ion. The lithium replacement ion is then introduced to a 210-W nitrogen-MIE Liquid aerosols were generated with a conventional pneumatic concentric glass nebulizer and spraychamber using 2.3 L/min of nitrogen for nebulization and plasma support. The aerosol was subsequently desolvated and introduced into the tangential inlet of a tangential flow torch. Make-up nitrogen (0.4 L/min) is supplied to the torch's axial inlet.

The MIP-AED works best when solvent loading is minimized and sample residence time is increased (Zhang et al., 1985). Aside from the promising work by Ibrahim et al. (1985), the use of microbore HPLC in conjunction with the MIP to reduce solvent loading remains to be reported in detail, as are recent promising nebulizer designs, such as the rotating disk nebulizer (Rademeyer et al., 1991) or thermospray nebulization (Vermeiren et al., 1991). The use of a proportional interface, such as that by Billiet et al. (1983), with a heated plasma gas, would


appear to offer a number of further possibilities. The use of flow injection analysis as a low dead volume interface for inductively coupled plasma (ICP) (Lawrence et al., 1984) has been explored; a similar study with the MIP remains to be done.

C. Microwave-Induced Plasma as a Detector For Supercritical Fluid Chromatography

A supercritical fluid is produced by compressing and heating a gas at high pressures and temperatures above its critical point, and it has properties intermediate to ordinary gases and liquids (Palmier, 1988). A supercritical fluid has the mobility of a gas and the solvation power of a solvent. This physical state can be altered through pressure and temperature variations. GC is known to provide efficient separations with sensitive detectors available, but is limited to volatile and thermally stable compounds. HPLC is used for high molecular weight compounds, but its analysis time is longer and analyte detection can be inhibited by solvent interferences. Supercritical fluid chromatography (SFC) can be used for compounds traditionally separable only by HPLC, and provides fast separation and sensitive detection by GC. Capillary SFC can separate a complex sample more efficiently than packed column HPLC, owing to the enhanced number of theoretical plates (Fjeldsted, 1984; Novotny et al., 1981; Palmier, 1989). The detectors used in GC may also be used for SFC. MIP-AED is an established detector for GC. Evaluation of MIP-AED for SFC is a logical step.

SFC-MIP Interface

Carbon dioxide and nitrous oxide are the twomost common supercritical fluids. When these molecular gases are introduced into an MIP, the analyte emission and the plasma stability are reduced, whereas the background levels are elevated. Therefore, the detection limits obtained in a supercritical-fluid-containing plasma are increased when compared to those that do not contain the fluid. A typical SFC-MIP system is shown in Figure 16 (Zhang et al., 1991). The chromatographic column is connected to the MIP in a way similar to conventional GC-MIP. The SFC is operated at elevated pressure while the gas plasma is operated at atmospheric pressure. The supercritical fluid in the gas phase enters the plasma. It is therefore essential that the inlet orifice into the plasma for the supercritical fluid (gas) is restricted, allowing a reasonable residence time for the separated compounds in the plasma. Furthermore, when the SFC mobile phase decompresses rapidly, the effluent may "freeze" onto the tip of the orifice (ca. 150 mm) (Luffer et al., 1988). This problem is eliminated by heating the plasma support gas. Figure 17 (Zhang et al., 1991) shows a typical SFC-MIP interface. Pressure programming is used in SFC. In SFC-MIP pressure change creates a base-line shift. By the use of a larger restrictor orifice, this problem is reduced. In the presence of CO2 the plasma torch deteriorates faster (Luffer et al., 1988).

Figure 16. Schematic diagram of a SFC-MIP system. (Reprinted with permission of Zhang et al., 1991, Copyright 1991, The American Chemical Society.)

Figure 1Z Schematic diagram of the SFC-MIP interface. (Reprinted with the permission of Zhang et al., 1991. Copyright 1991, The American Chemical Society.)

249


The SFC-MIP Performance

The MIPs most evaluated for SFC are the Surfatron (Galante et al., 1988b; Luffer et al., 1988), the TM010 Beenakker cavity (Motley and Long, 1990; Webster and Carnahan, 1990, 1992), and the MPT (Jin et al., 1990). The surfacewave-produced plasma has an annular shape due to the radial electric field. This annular MIP is believed to be more tolerant to materials injected. The Beenakker MIP, however, has the highest electric field in the center, making sample introduction tedious. The MPT, on the other hand, has a torch design similar to that of the ICP torch, resulting in more tolerance to the form of sample injection. An MIP can accept more SFC gas amounts when (a) the plasma-applied power is increased, or when (b) the plasma support gas flow rate is increased. Table 5 shows comparisons of helium MIP tolerance to CO2 SFC gas. The most effective plasma in this respect is the Surfatron, followed by the highly efficient TM010 plasma. The maximum CO2 allowable in a plasma indicates the type of SFC operation suitable. For example, a low CO2 flow rate is usable for capillary column chromatography, while a higher flow rate is used for packed column chromatography.

Motley and Long (1990) compared helium and argon MIPs for SFC. They found that when CO2 is introduced, the argon MIP experienced a drop of temperature by 30% and a reduction of electron number density by a factor of ten. The helium MIP, however, experiences only a 4% drop in temperature and no reduction of electron number density when CO2 is introduced. These findings indicate that a helium plasma is a more effective SFC detector than an argon plasma. Virtually all the SFC-MIP systems studied are helium plasmas (Olson and Caruso, 1992; Jin et al., 1990; Motley and Long, 1990; Webster and Carnahan, 1990, 1992; Galante et al., 1988b; Luffer et al.. 1988; Zhang et al., 1991), and applications performed are for nonmetals.

Table 5. Helium Plasma Tolerance to Carbon Dioxide

MPT* Surfatron TM010

(Jin et al., (Galante et Motley and (Webster and (Webster and 1990) aL, 1988b) Long, 1990) Carnahan, 1990) Carnahan, 1992)

Power level (W) 450 150 150 40

Plasma gas total 5.3 0.12 1 0.125 flow rate (L/rain)

Max. CO2 flow 46 18 120 0.51 rate permitted (mL/min)

CO2/He % (v/v) 0.87 15 1212 0.41

500

22.25

20

0.90

Notes: *microwave plasma torch. Motley and Long (1990) used a HE (highly efficient) MIP.


Table 6. Interference upon Major Nonmetal Emission Lines in the UV-VIS and NIR for a N20- and CO2-doped He-MIP

Possible Interference from SFC Mobile Phase Wavelength of

Nonmetal Emission (nm) N20 Doping C02 Doping

Carbon 247.86 NO band at 247.9 nm Mobile phase interference

940.57 No interference observed Mobile phase interference

Hydrogen 486.13 N2 band at 483.T nm No interference observed

656.27, 656.28 N2 band at 654.5 nm CO band at 651.4 nm

Oxygen 777.19 Mobile phase interference Mobile phase interference

Nitrogen 746.88 Mobile phase interference No interference observed

868.03 Mobile phase interference No interference observed

Sulfur 545.39 No interference observed C2 band at 543.5 nm

921.29 No interference observed No interference observed

Phosphorus 213.55, 213.62 No interference observed CO band at 211.3 nm


Chlorine 479.45 N2 band at 472.4 nm C2 band at 473.7 nm


Bromine 470.49 N2 band at 465.0 nm C2 band at 469.8 nm


Iodine 206.24 NO band at 206.2 nm CO band at 206.0 nm


Fluorine 685.60 No interference observed No interference observed

739.87 N2 band at 738.7 nm No interference observed

Note." Reprinted with permission from the authors (Webster and Carnahan). Copyright 1990. The Society for Applied Spectroscopy.

The background emission spectrum from an SFC-gas-containing plasma is considerably more complex than that without the SFC gas, precluding use of some regions ordinarily employed in conventional GC-MIP. The near-IR region is cleaner and is suggested for monitoring (Galante et al., 1988b; Webster and Carnahan, 1990). Webster and Carnahan (1990) have summarized spectral interference upon major nonmetal lines in an N20- and CO2-doped helium MIP (Table 6). The reduced signal-to-background emission ratio has elevated the detection limits. For example, Jin et al. (1990) obtained a 55 pg/s detection limit for chlorine using the conventional helium GC-MPT, but obtained a 300 pg/s detection limit for chlorine using the helium SFC-MPT. In general, the elemental detection limit is at the pg/s level for a SFC-MIP system. To avoid the complex emission spectrum of SFC-MIP, Olson and Caruso (1992) employed MS detection. They obtained elemental detection limits at the low to subpicogram levels for halogenated compounds.


D. MIP as an Ion Source for MS

A plasma is an ionized gas medium. A significant portion of analytes introduced into the plasma also are ionized. It is possible to extract the elemental ions from a plasma to achieve detection with high sensitivity. Indeed plasmas have been used as ion sources for mass spectrometry (MS). These include the glow discharge MS (Harrison et al., 1990), the ICP-MS (Houk, 1986), and the MIP-MS that is to be reviewed here. The elements are generally detected as singly charged monoatomic positive ions. The spectrum generated is simple, with isotope ratio information provided. The very fast spectral scanning of mass spectrometry supplies the spectrum with a large range of mass (mass-to-charge) information; therefore the technique may be classified as a simultaneous, multielement detection technique. The detection limits of ICP-MS are in the sub-parts-per-billion level (Houk, 1986).

Plasmas used as ion sources for MS are operated either at atmospheric pressure or at reduced pressure. The successful, commercial glow discharge MS has the discharge operated at reduced pressure. The powerful, commercial ICP-MS, however, has the plasma operated at atmospheric pressure (Houk, 1986). The glow discharge operation is generally more suitable for solid samples, whereas the ICP is more commonly used for liquid samples. The mass spectrometer is normally operated at 10 -6 torr, making the interfacing with atmospheric plasma more tedious. This interface difficulty is solved by incorporating additional pumping stages that reduce atmospheric pressure to an approximate pressure level. Typically, differential pumping reduces pressure to about 10-4-10 -5 torr prior to a final pumping, where pressure is reduced to 10 -6 torr.

Microwave-induced plasmas have been used as ion sources for MS. All the advantages (power level, gas consumption rate, noise shielding) of the MIP compared to ICP in optical spectroscopy are also applied here, while for the MS interfacing the MIP dissipates less heat and uses lower gas flow, reducing sampling cone deterioration and differential pumping requirement, respectively. Use of a larger sampling orifice compared with using an ICP also reduces its clogging (Douglas and French, 1981). More importantly, different gas MIPs, such as those with nitrogen, helium, or argon, can be used to optimize for the spectral mass of interest and to avoid or eliminate spectral interferences from the background ions.

The use of an MIP as an ion source was first reported by Douglas and French (1981). This early work demonstrated the inherent potential of the MIP-MS. Therefore, considerable MIP-MS research was generated in Hiefte's laboratory at Indiana University in the United States, and intensive MIP-MS studies were carried out by Caruso's group at the University of Cincinnati in the same country. The MIP-MS has been shown to give superior analytical performance for many elements, when compared to the established ICP-MS. The MIP used is essentially sustained in a Beenakker cavity. Their performance will be discussed in the sections following.


Elec t ron Io1~ M u l t i p l i e r Q u a d r u p o l e Lenses S k i m m e r

~ S a m p l e r 1 ' " i f I v l i c r o w a v e C a v i t y

i . , j

i , I I, , I I I I n j e c t o r | C h a m b e r

Smal l La rge Rotary T a n g e n t i a l Diffusion D i f l u s i ~ F low P u m p P u m p Z

X [J y Figure 18. Diagram of a moderate-power nitrogen microwave-induced plasma mass spectrometer system. Reprinted with permission of the authors (Shen et al., 1990a). Copyright, Society for Appl led Spectroscopy, 1990.

The MIP-MS Interface

Most of the mass spectrometers used in MIP-MS research are of the quadrupole mass spectrometer type, although the magnetic sector mass spectrometer has also been used (Eberhadt et al., 1992). The majority of the work employed a commercial ICP-MS system, replacing the ICP with the MIP. Houk (1986) has reviewed in detail the construction, operation, and analytical performance of ICP-MS. A typical MIP-MS system is shown in Figure 18 (Shen et al., 1990a). The MIP torch most suitable for the MS is the tangential design similar to that for the ICP, resulting in a high ion concentration in the center for extraction. When the MIP torch is sealed to the sampling cone and operated with a restricted plasma gas flow, a low-pressure MIP (Story et al., 1990; Olson et al., 1990) is produced due to the mass spectrometer pressure pumping.

The MIP as a Soft Ionization Source

Low-power (< 50-W) MIPs have been used for fragmentation and ionization of organic compounds. The MIP is operated at low pressure in the level between 10 -1 and 10 -2 mbar so that the kinetic temperature and fragmentation are reduced, increasing the intensity of the molecular ion peak. The low-pressure plasma is


sealed from the atmosphere, minimizing formation of unwanted ion species. Poussel et al. (1988) investigated a Surfatron MIP using argon, krypton, or xenon as the plasma gas (ca. 3 mL/min). The sample was injected externally to the plasma for minimizing fragmentation. They compared the intensity of molecular ion peaks to those obtained with 70-eV electron impact ionization, and found that all three different gas MIPs gave an overall higher intensity. Of the three gas plasmas, argon gave the most intense molecular peak. It is important to operate the MIP at a proper power level for obtaining the highest molecular ion peak intensity. For example, the molecular ion observed at 25 W disappeared at 45 W (Poussel et al., 1988).

Olson et al. (1990) used Beenakker cavity sustained, low-pressure MIPs for soft ionization. A helium plasma with a 26 mL/min gas flow rate and 20 W applied power was evaluated and compared to a conventional electron impact source. The sample was injected externally to the expanded plasma plume. They found similar fragmentation patterns from the two sources with the intensities being different. When the sample was injected through the plasma in a conventional manner, quantitative data could be obtained, whereas when the sample was injected externally, the large fragmentation gives structural information. They also explored the use of a nitrogen MIP for soft ionization.

The MIP as an Elemental Ionization Source

Such inert gases as argon, nitrogen, and helium have been evaluated for forming plasmas in MIP-MS. Different gas plasmas provide different detection sensitivities and different background ion spectra. A desirable background region is selectable by using a suitable gas. The various gas MIP-MS performances are reviewed as follows.

The Argon MIP-MS. The atmospheric (air-entrained) argon plasma with solution nebulization sample introduction produces ionic combination species of argon, hydrogen, oxygen, and nitrogen. The major background ions are 160+, 14N160+, and 4~ +. The NO + is produced through charge transfer between Ar § and NO. Because of the large population of NO + in the plasma, the degree of ionization (and therefore the sensitivity) for elements having an ionization potential less than that of NO (9.25 eV) is reduced. It is possible to decrease the amount of NO by using a bonnet to enclose the plasma from air, or by employing a low-pressure plasma. The elemental oxide ion formation also lowers sensitivity. A solvent-free sample introduction method, such as desolvation condensation or electrothermal vaporization, can minimize this undesirable effect. Satzger et al. (1988) obtained 7-70 ng/mL detection limits with solution nebulization into a 225-W MIP, whereas Douglas and French (1981) found 0.1 to 8 ng/mL detection limits with ultrasonic nebulization- desolvation condensation into a 200-W MIP. The MIP torch used in the Caruso group was a simple alumina tube leading to a wide plasma-ion-monitoring region; it was noted that use of a tangential torch could improve the detection level.


The Nitrogen MIP-MS. The nitrogen MIP has a long plume tail, facilitating the orientation of the hot plasma region to the skimmer orifice. The background ion species result from combinations of nitrogen, hydrogen, and oxygen. In the work of Wilson et al. (1987), who employed a 220-W plasma and glass frit nebulizer with condensation-desolvation, the highest ion signal is that of 14N160+ and the next most concentrated ion is N2H § at m/z 29. No background ions were observed at m/z >60. The high concentration of NO + in this plasma has limited the sensitivity of elements having ionization potentials of >9.25 eV. The authors obtained detection limits from 3 to 22 ng/mL for five elements. This level of detection is 1 to 2 orders of magnitude poorer than that of the argon ICP-MS. The oxide and hydroxide formations and matrix interferences are worse in the nitrogen MIP-MS than in the argon ICP-MS.

Shen et al. (1990a,b) took advantage of the non-argon nitrogen plasma to look at some elements which have isotope-mass-overlapping by argonized ionic species. For example, argon dimers at masses 74, 76, and 80 would interfere the detection of selenium when using an argon plasma-MS system. In the nitrogen MIP-MS, however, the selenium isotopes are detected with signal abundances closely matching those of the natural abundances (Figure 19) (Shen et al., 1990a). Similarly potassium, calcium, chromium, and arsenic can be detected more effectively in the nitrogen MIP-MS than in an argon plasma-MS system such as the ICP-MS. Detection of the 75As+ isotope is of particular interest since 4~ + can interfere when using ICP-MS. Chlorides are commonly present in real world samples or when using hydrochloric acid for sample digestion. On the other hand, background ions in the nitrogen plasma can interfere with detection of some masses. For example, N 4+ interferes with 56Fe§ By employing 310-350-W power levels and direct solution nebulization with a cooled (6 ~ spraychamber Shen et al. (1990a) obtained detection limits ten times superior to those of Wilson et al. (1987). The detection limits are about an order of magnitude inferior compared to those of the argon ICP-MS (Table 7) (Shen, 1990a) with the exception of those of potassium and calcium in the nitrogen MIP-MS, where they are 2000 and 20 times superior, respectively. Contrary to the finding of Wilson et al. (1987), Shen et al. (1990a) found that the elements that have ionization potentials of >9.25 eV (NO +) have similar detection sensitivities to those that have ionization potentials of <9.25 eV. The background spectrum showed that the two major peaks are N § and NO § (Shen, 1990a). The high sensitivity obtained for the elements that have ionization potentials of > 9.25 eV can be explained by the ionization by N +, which has an ionization potential of 14.5 eV.

In the dry atmospheric-nitrogen MIP, the background ion peaks of laN16OH§ and 1602+ are small relative to that of the 14N+ peak, showing potential for 31p+ and 32S§ detection. Both sulfur and potassium have an ionization potential smaller than that of nitrogen and can be ionized by N § effectively. Indeed, Story et al. (1990) applied GC for sample introduction into a low-pressure nitrogen or helium MIP, further

1500 . . . .

Natura l E x p e r i m e n t a l 1400 -

A b u n d a n c e A b u n d a n c e 1300 - - . . . . . . . . . . . . . . . . . . . . . . . . . . .

_$_e.__(7.4) i ............... 0 . 9 6 .. . . . . . . . . . . . . . . . . . . . . . 0_.97 ............... S o (76) 1 9 . 1 2 j 9 . 1 2

S e ( 7 7 ) l . . . . . . . . . . 7 : 5 0 ........... j ............ 8 ; 4 2 . . . . . . . . . . .

o se-- - ( -7-a)t . . . . . . . . . . 2 5 _ 6 i ........................... 2 a l a - i . . . . . . , . . . . . . . . . . . I ~ooo I-S-e-- CO (80 )1 . . . . . . . . . 4 9 - ~ 6 ............. ) . . . . . . . . . . . - 4 8 - , 0 2 ,

~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,;8-4-. .......... t ............ "-~7-._ . . . . . . . . . . . . . . . I 800..--t Se(78)

1200 "13 E 1100 - 0

L (1) o. o3 t - ::3 0 C.) v

co r - (1) r -

m

7 o o iit 600

400500 _t So(76)

300 - t /~!)t'l 200 $0(74) i' / " |

/ 0 I

73 75 I

7'7

/ . . . .

i

7 9

I .....

se(8o)

se(o2)

i 81

. . . . F- L I ' I]3 85

Mass Figure 19. Plot of 100-ppb Se isotope abundance determination spectrum in nitrogen MIP-MS. Reprinted with permission of the authors (Shen et al., 1990a). Copyright, Society for Applied Spectroscopy, 1990.


Table 7. Comparison of Detection Limits (ppb, 3(~) Elements Mass (amu) N2-MIP-MS Ar-ICP-MS

Li 7 0.35 0.06 Mg 24 0.43 0.1 K 39 0.48 1000 Ca 40 0.24 5*

V 51 0.31 0.03

Cr 52 0.44 0.02 Mn 55 0.6 0.04

Fe 56 6.8 0.2** Co 59 0.17 0.01

As 75 0.32 0.4 Se 80 1.4 1# Sr 88 0.39 0.02 Ba 138 0.3 0.02

Pb 208 6.35 0.02

Notes: *mass 44. **mass 57. #mass 78. Reprinted with permission from Shen et al., 1990a.

minimizing the formation ofNOH § and 02 +. They obtained subnanogram detection limits for sulfur and phosphorus.

The Helium MIP-MS. Helium plasma is a natural choice for ionization of nonmetals. The helium plasma also shifts the background ions to lower masses, avoiding spectral interferences at higher masses such as occur in the argon ICP-MS. Creed et al. (1989) used a 300-W atmospheric helium MIP-MS with solution nebulization and cooled (5.5 ~ spraychamber for aqueous solutions. They found the major background ion peaks in the presence of 1% HNO3 in solution to be 160+ > 14N+ > 4~ > 16OH+ > 14N2+ > 1602+ > 14N160+ > 4He2+> 4Hell+. The

detection limit for I + (0.04 ppb) is similar to that of the ICP-MS. The results for Br § and C1 § are 500 and 25 times better, respectively, than those from ICP-MS. Fluorine cannot be detected quantitatively due to the high background ion interferences at the isotope masses. The detection limits obtained for metals are at the sub-parts-per-billion level, and are similar to those of the ICP-MS.

By employing a similar system (Heitkemper et al., 1990), operated at 325-W plasma power with a-11 ~ to 2 ~ cooled spraychamber, 70% alcohol (methanol, ethanol, propanol) can be nebulized with a 1 mL/min solution uptake rate which does not extinguish the plasma. No sign of carbon buildup at the sampler or skimmer was noticed. Detection limits obtained in 20% methanol for arsenic, cadmium, tin, and lead were 0.4, 0.2, 0.2, and 0.2 ppb, respectively. A HPLC was


Table & Comparison of Absolute Detection Limits of Organotin Compounds

(as Sn) Obtained by Gas Chromatography Detector Definition Detection Limit (pg)

Mass Spectrometry - - 200

Electron capture 2c~ 31-85

Quartz furnace AAS 2cr 1000-1400

Quartz burner AAS 2a 100

Flame AAS 2~ 2000

FID - - 250-2000

HAFID - - 13-1800

FID quenching 2c~ 0.5

Flame photometry 2(~ 3

MIP-AES 2(~ 6.1

MIP-MS 2(~ 0.06-0.24

Note: Reprinted with permission from Suyani et al., 1989.

connected to the spraychamber for the separation of halogenated compounds. The detection was monitored at the major isotope mass of the halide positive ion. Dibromomethane, bromobenzene, ethyliodide, 1-iodobutane, and chlorobenzene were separated using a reverse phase column. The absolute detection limits are about 50 pg for bromine, 1 pg for iodine and 10 ng for chlorine.

In a dry 170-W atmospheric helium MIP, Brown and co-workers (Brown et al., 1988) found the highest background ion peak to be 4He+, followed by those of 160+ and 14N+. By doping helium with methyl halide gases, they estimated the detection limits for chlorine, bromine, and iodine to be 0.08, 0.05, and 0.14 pg/s, respectively. Mohamad et al. (1989) coupled capillary GC to the atmospheric helium MIP-MS via heated transfer line. They looked at the separation of halogenated compounds with the plasma operated at about 200 W. Detection limits for chlorine, bromine, and iodine were determined to be 9.2, 0.92, and 1.5 pg, respectively. These values are about an order of magnitude superior to those obtained with MIP-AES work from the same laboratory. Suyani et al. (1989) placed a tantalum tube inside the MIP torch to direct the capillary-GC-separated vapors into the 70-W atmospheric plasma, resulting in tin compound detection limits of 0.09 to 0.35 pg as Sn. They compared this GC-MIP-MS system to other conventional GC systems and showed this system to be the most sensitive for tin detection (Table 8).

The same laboratory (Creed et al., 1990) also investigated low-pressure dry 60-W helium MIP-MS. The highest background ion peaks followed the sequence of 4He+ > 14N2+ > 28Si+ > 1602+. Nitrogen and oxygen may be present as impurity gases, or due to air leaking into the plasma. Silicon might originate from the torch


construction material. The presence of He + in the low-pressure plasma may lead to a different route of ionization. The authors found GC-separated compounds were detected with limits of 0.1, 3.5, and 25 pg as iodine, bromine, and chlorine, respectively. Thus, iodine is detected with an order of magnitude improvement over the atmospheric plasma. The high bromine and chlorine detection limits are results of spectral interferences. Interference to 79Br+ is possibly because of formation of a nickel oxide or nickel hydroxide polyatomic species (at m/z 79) due to plasma ablation of sampler material. The 35C1+ interference is caused by background species at m/z 35 and 37.

ETV-MIP.MS

Electrothermal vaporization (ETV) (Ng and Caruso, 1990) is becoming an important sample introduction method for ICP-MS (Beres and Ediger, 1991; Ediger, 1991; Gregoire et al., 1991; Caruso et al., 1991). ETV introduces a solvent-free sample, leading to a cleaner spectral background than is possible with solution nebulization. ETV can also char away some of the unwanted sample matrix, reducing spectral overlap. Furthermore, ETV provides an overall 10-fold improvement in detection limits when compared to solution nebulization ICP-MS (Park et al., 1987). Evans et al. (1991) evaluated a tantalum tip ETV-MIP-MS system and found that detection limits for three metals were at the subpicogram level. It is predictable that more ETV-MIP-MS research activities will be forthcom- ing, leading to better assessment of the technique.

Conclusion

The atmospheric MIP-MS is operated at about 350-W power and about 7 L/min total gas consumption rate whereas the power and consumption rate in ICP-MS are about 1.5 kW 17 L/min, respectively. The operation cost for MIP-MS is considerably less than that of the ICP-MS. The detection limits of the two atmospheric systems are similar. The linear dynamic range for ICP-MS is about 106 whereas for MIP-MS it is about 104. Matrix effects such as oxide formation are lower in the ICP-MS.

The ability for low-pressure plasma operation is an attractive feature of MIP-MS. When the plasma is isolated from the atmosphere the complexity of spectral background ions generated is reduced and the ionization ability of the plasma is enhanced. The low-pressure MIP-MS is particularly suitable for GC interfacing. The GC-MIP-MS gives a spectrally clean background and high ionization capability. The GC-MIP-MS is predicted to enjoy a success similar to the now-established GC-MIP-AES.


IV. NONCHROMATOGRAPHIC SAMPLE INTRODUCTION METHODS

MIPs are best known as excitation sources for AES. However, recent developments have included applications for AAS and AFS. Regardless of the method, the quality of the MIP sample introduction interface is very important for obtaining good results. The most popular and successful sampling method for MIP is vapor-phase sample introduction. These methods include gas chromatography eluates (discussed previously), hydrides, acid halides, or other chemical vapor, laser ablation and electrothermal vaporization, which have the advantage of high mass transfer efficiencies and elimination of solvent load. In many instances vapor phase sam- piing is inconvenient or impractical which has led to the use of aerosols. These are either introduced into the plasma directly as liquid aerosols or desolvated, allowing introduction as a dry aerosol. A third form of sample introduction is solid sampling in which case a solid powder is introduced to the plasma.

In the balance of this section we will present some of the improvements in sample introduction systems for AES, AFS and AAS.

A. Aerosol Sample Introduction

Until the development of the Beenakker cavity and Surfatron, MIPs were either extinguished or destabilized by liquid aerosols (Beenakker et al., 1980 ). Even with these devices, early attempts using liquid aerosol sample introduction led to nonlinear calibration curves, memory effects, and relatively high detection limits (Beenakker et al., 1980). Liquid aerosols introduced to the MIP must be desolvated, melted, and vaporized before atomization and excitation leading to atomic emission can occur. The limited total energy and small analyte plasma residence time places a high premium on the quality of the sample introduction interface.

Excitation of nonmetals requires use of helium as the plasma support gas, which has unique properties when used for production of pneumatic aerosols. The natural solution uptake rate for argon is ten times (Michlewicz et al., 1985) that of helium for identical nebulizers. As a result, a peristaltic pump is normally used for sample feed, rather than depending on natural aspiration. The second important parameter is droplet diameter. The MIP produced by the Beenakker cavity possesses a small plasma volume for low-power applications. As a result, the amount of time that droplets have contact with the plasma is very limited. Higher applied power MIPs possess greater plasma volumes (Urh et al., 1985) but generally at the expense of increased plasma gas flow rates (Beenakker et al., 1980; Michlewicz et al., 1985; Michlewicz and Carnahan, 1986a; Michlewicz and Carnahan, 1987; Wu, 1992a, b). Therefore, residence times are similarly limited. Nebulizers that produce small droplet diameters with small size distributions are preferred, since they are more likely to permit complete desolvation and vaporization.

Nebulizer s f ~ ~ ,- ..... J

To Waste

Recent Developments in Analytical Microwave-Induced Plasmas To Plasma

261

Figure 20. Example of a CGN and spray chamber for MIP liquid aerosol sample introduction.

Pneumatic nebulizers are perhaps the most convenient method of MIP sample introduction. The high quality aerosol required by the MIP is produced to a limited extent by concentric glass (CGN) (Perkins and Long, 1989; Haas and Jamerson, 1987; Brown et al., 1987; Long and Perkins, 1987) (Figure 20) and MAK cross flow nebulizers (Michlewicz and Carnahan, 1986a; Michlewicz et al., 1985; Ng and Shen, 1986; Ng and Garner, 1993) (Figure 21). Both CGN and MAK nebulizers have low mass transfer efficiencies (< 1%). The CGN is characterized by broad distributions of droplet sizes with large mean droplet diameters by comparison to glass frit (GFN) or ultrasonic (USN) nebulizers. The MAK cross flow nebulizer,

Figure 21. The MAK nebulizer. Reprinted with acknowledgment to Meddings, Anderson and Kaiser.


named for Meddings, Anderson, and Kaiser (Meddings et al., 1981), produces an aerosol of sufficient quality to allow determinations of nonmetals (Michlewicz and Carnahan, 1986a; Michlewicz et al., 1985) and is commonly used for metals detection as well (Ng and Shen, 1986; Ng and Garner, 1993). While operating with a high back pressure (200 psi), the MAK nebulizer produces an aerosol with small mean diameters relative to other pneumatic nebulizers (Cull and Carnahan, 1987). The volume of gas required for nebulization with either nebulizer contributes to the overall plasma gas flow rate, which may be a disadvantage for low-power applications.

Ideally, a nebulizer used for MIP sample introduction should produce aerosols of small diameter with minimal size distributions at a low helium flow rate, along with high mass transport efficiencies, permitting convenient control of sample loading without large increases in the total plasma gas flow rate. The GFN (Galante et al., 1988c) produces mean droplet diameters below llam and transport efficiencies as high as 90% (Layman and Lichte, 1982). High GFN transport efficiency requires low sample flow rates (Browner and Boom, 1984). The use of helium for nebulization results in much smaller sample transport efficiencies. Thus, despite the high transport efficiency, low net mass transfer rates are associated with GFNs. USNs have transfer efficiencies of up to 30% with slightly larger droplet size distribution, ranging from 1.5 to 2.5 gm (Browner and Boom, 1984).

Both the MAK and USN nebulizers allow higher net analyte mass transfer rates to the plasma than does the GFN (Michlewicz and Carnahan, 1986a). Desolvation is necessary to prevent overloading the plasma with solvent (Browner and Boom, 1984). Desolvation occurs when the aerosol solvent is vaporized in a heated tube following the nebulization and then condensed. The aerosol may be further dehy- drated by passing it over a concentrated sulfuric acid desiccation cell (Jin et al., 1989; Que et al., 1989), allowing only the dried aerosol to enter the plasma.

B. Nonmetal Determinations Using MIP-AES and Aerosol Sample Introduction

The high ionization potential of nonmetals places further constraints on their determination. Resonance transitions for the nonmetals are found in the vacuum ultraviolet region of the spectrum. Standard air path spectrometers cannot be used in this wavelength region due to absorption of these wavelengths by oxygen and nitrogen. Therefore, with few exceptions, the less sensitive nonresonance lines in the ultraviolet, visible and the near-infrared regions (200-1000 nm) are used. Gas phase halogen species typically have strong halogen emissions at their ionic transitions [C1 (II) (479.5 nm, 481.0 nm, and 481.9 nm), Br (II) (470.5 nm, 478.6 nm, and 481.7 nm), and I (II) (546.46 nm)]. These highly energetic transitions are quenched in the presence of water vapor (Galante et al., 1988a) and are only observed with Beenakker cavity helium MIP (Michlewicz and Carnahan, 1986a; Wu and Carnahan, 1992a, b; Perkins and Long, 1989). Low helium flow rates and


increased residence time, available using lower applied power (150 W), allow detection limits (Perkins and Long, 1989) competitive with those obtained using moderate- and high-power MIPs (Michlewicz and Camahan, 1986a). The most sensitive lines for the low-power Surfatron are in the near-IR region (Br at 827.2 nm, C1 at 912.1 nm, S at 921.3, and P at 979.7 nm) and have yielded promising results (Galante et al., 1988a). For all MIP sources the most sensitive lines for I and P are at 206.2 nm and 213.6 nm, respectively. Although suffering spectral overlap from NO molecular bands, they still have greater analytical sensitivity than the P and I lines in the near-IR at 905.8 nm and 975.1 nm, respectively (Galante et al., 1988a).

The approach to determinations involving liquid aerosols has been characterized by high and low applied power approaches. Low applied power has the advantage of generally lower support gas and cooling gas flows rates (< 1 L/min). Problems related to MIP torch design, overheating of the coaxial cable, and efficient tuning and matching of the power supply to the plasma are relieved by the use of low applied microwave powers (< 200 W). Galante et al. (1988a) uses a low-power helium Surfatron to obtain low ppm and subppm detection limits for C1, Br, I, C, P, and S using GFN and desolvated aerosols. The Surfatron is easily tuned and more resistant to detuning due to solvent and solute loading than is the TM010 cavity (Selby and Hieftje, 1987). The low applied power permits a stable Surfatron MIP with helium flows of 300-400 mL/min. Signal to noise ratios peak at 160 to 170W for C1, Br, I, C, P and S. Addition of easily ionized elements enhances nonmetal emission by as much as 30%, similar to effects recorded for metal determinations elsewhere (Selby et al., 1987; Brown et al., 1987). Perkins and Long (1989) report lower detection limits by utilization of the TM010 resonator at low applied power (150 W), using CGN with a Scott spraychamber and desolvation. As in the low applied power Surfatron, the helium flow rate is very low at 1 L/min.

Most nonmetal determinations have been obtained with use of moderate to high applied powers (0.45-1.2 kW). These are more tolerant to liquid aerosols and accept greater sample loading rates. Unlike the low-power Surfatron, emissions are observed at the visible ion lines for most of the halides. Although samples can be introduced without desolvation, the best results are obtained with this step (Michle- wicz and Camahan, 1986a). With a helium flow rate of 21 liters per minute, the water-cooled, internally tuned Beenakker cavity (Michlewicz et al., 1985) with a tangential flow torch permits detection limits generally similar to the low-power HEMIP and below those of the low-power Surfatron (Table 9). Using desolvated aerosols, this system's detection limits are improved by a factor of 5 to 20 with a USN rather than MAK pneumatic nebulization. The difference is attributed to the increased analyte mass reaching the plasma without an increase in the total mass of foreign material in the plasma (Michlewicz et al., 1986a). The most intense emission for Bris the 470.49-nm ion line, with lesser lines at 478.55 nm and 481.67 nm. Spectral interference by the molecular second order band head of the NO (237.02 nm) species appears at 470.5 nm, requiring use of the less intense Br line

Table 9. Nonmetal Detection Limits Obtained using 30 (t.tg/mL) with Solution Nebulization and He-MIP-AES

a b

(Galante et al., (Perkins and Wavelength 1988a) 170 W Long, 1989)

Element (nm) GFN DSL 150 W CGN

c d e (Gehlhausenand (Michlewicz

Carnahan, and Carnahan, (Michlewicz 1989) ca. 190 1986a) 480 W and Carnahan, W USN DSL MAK 1986a) 480

f (Michlewicz

and Carnahan, 1986a) 480 W

USN DSL

g

(Wu and Carnahan

1992a)

h ( Wu and

Carnahan 1992 b) 1600 W

USN DSL

F (I) 6 8 5 . 6 - - - - 15 - -

CI (II) 4 7 9 . 5 - - 1.2 m 3 6

CI (1) 912 .1 < 30 . . . .

B r (II) 4 7 8 . 6 - - 3 m 90 6

B r (I) 827 .2 < 3 0 . . . .

I (I) 2 0 6 ' 2 24 2 - - 11 17

9 0 5 . 8 12 . . . .

C ( 1 ) 193.1 18 . . . .

P (I) 2 1 3 . 6 0 .55 . . . .

2 5 3 . 6 . . . . .

9 7 9 . 7 51 . . . .

S (I) 5 4 5 . 4 . . . . .

921 .3 < 30 1.8 ~ ~

0 .6

5

1.2

w

2 .2

m

0 .4

1.4

9 .7

Notes: (b-h) use a Beenakker cavity and tangential flow torch except for (a) which uses a Surfatron.

(b and c) do not use aerosol desolvation (DSL).


at 478.55 nm. The signal to noise ratio at the I (I) 206.16-nm line is 14 times that of the I (II) ion at 546.46 nm. Use of the most intense C1 (II) line at 479.45 presented no spectral interference problems.

The very high ionization potential of fluorine has made its AES determination particularly challenging, with only one result reported (Gehlhausen and Carnahan, 1989) for liquid aerosol sample introduction. The F atomic resonance line appears at 95.5 nm, precluding its use for analytical determinations. While other atomic lines are observed, the 685.6-nm line is the most intense and permitted the best detection limits using a 500-W internally-tuned TM010 resonator cavity, operating with 17.5 L/min of plasma gas and 1.2-1.4 L/min of carrier gas through the USN.

The helium-kilowatt-plus MIP (He-kiP-MIP) (Cull and Carnahan, 1989; Wu, 1992a,b) offers still higher applied powers for nonmetal analysis. The two TM010 resonators investigated (Wu, 1992a) are similar except for cavity depth. With depths of 2 cm and 3 cm, C1 (I) detection limits are 2.2 and 3.3 ~g/mL, respectively. Noise is the same for both cavities; the decrease in detection limits is due to the increase in signal intensity using the 2-cm cavity depth. Similarly, free electron density and excitation temperature is 10 and 26% greater, respectively. Yet despite the high applied powers, the detection limits reported are higher than those for moderate- and low-power MIPs (Michlewicz and Carnahan, 1986a; Perkins and Long, 1989).

C. Metal Determinations Using MIP-AES with Aerosol Sample Introduction

Despite the success of the well-entrenched commercial techniques of ICP-AES, flame AAS, and graphite furnace AAS (GFAAS), interest in the MIP as a spectrochemical source remains high. Development of more efficient resonator cavities (Perkins and Long, 1989), higher applied power systems (Haas and Caruso, 1984; Okamoto, 1991), and introduction of desolvation systems (Jin et al., 1991; Luet et al., 1991) has permitted many results that approach or often ~xceed (Haas and Caruso, 1984; Brown et al., 1987; Haas and Jamerson, 1987) those typical of ICP-AES (Winge et al., 1979). MIPs already offer significantly lower operating costs compared to ICPs. The ability to sustain an MIP with nitrogen or air may contribute to further cost effectiveness (Urh et al., 1985; Okamoto, 1991). Recent results using helium HEMIP AES and AFS for metal determinations indicate the potential of using a universal excitation source (Perkins and Long, 1989).

Metal Determinations in a Simple Matrix

As was the case for nonmetals, the MIPpermits metal determinations over a wide range of applied powers (50 W to 1 kW). With several exceptions, generally better results for the transition metals are available using higher and moderate applied power, whereas other elements have the best detection limits with low-power techniques. The 150-W MPT with a desolvation-condensation system offers sig-


nificantly better detection limits than those using moderate-power MIPs for aluminum, barium, lead and calcium. Detection limits for these elements are 14, 6.8, 27 and 0.24 ppb (ng/mL), respectively, with a linear dynamic range of 4-5 orders of magnitude. Results similar to those achieved with moderate applied power techniques (Haas and Caruso, 1985; Brown et al., 1987) are obtained for copper at 1.7 ppb. The poor detection limit of aluminum is probably due to the elimination of a plasma confinement tube. Aluminum is a common interferent species in the cavity MIPs that use alumina plasma confinement tubes (Brown et al., 1987). A second system uses helium plasma gas with the highly efficient Beenakker cavity (HEMIP) for AES and AFS. The self-igniting, low-power (150-W) system using 1 L/min of helium, without desolvation, is most successful when used in conjunction with a hollow cathode lamp (HCL) for HCL-AFS. Samples are introduced to the plasma via a tangential flow torch through the tangential flow gas inlet, with no auxiliary flow. Detection limits for potassium, sodium, and lithium (at 570.8 nm) as well as for cobalt, strontium, and zinc are the best reported for MIP. These are 1, 0.1, 1.8, 28, 7, and 1.8 ppb, respectively. The same group evaluated AES versus AFS for determination of a number of metals (Table 10, columns f and g). Silver and iron yielded only slightly better or equivalent results, respectively, using AES rather than AFS. All others had better results using HCL-MIP-AFS. The authors reported linear ranges for AFS up to 5.5 orders of magnitude.

Best results using moderate applied powers and the internally tuned Beenakker cavity (Haas and Caruso, 1984) are obtained for the first and second row transition elements. The system utilized by Brown et al. (1987) operates at 510 W using a low argon flow rate (0.45 L/min). Aerosols produced by the CGN are introduced directly to the plasma without desolvation. Five-millimeter i.d. alumina (m.p. 2075 ~ and yittria-stabilized zirconium oxide (m.p. 2700 ~ plasma containment tubes were evaluated. The yittria-stabilized zirconium oxide provided much better service than did the alumina tube, which required frequent replacement owing to thermal shock. Detection limits for cadmium, chromium, copper, iron, manganese, and nickel are the best reported and are listed in Table 10. The linear dynamic range is greater than 4 orders of magnitude.

A similar system is used by Haas and co-workers (Haas and Caruso, 1984; Haas and Jamerson, 1987). The internally tuned Beenakker cavity system uses 510 W of applied power and 0.45 L/min of argon for nebulization and plasma support with a 5-mm i.d. alumina plasma confinement tube. The system yields the best detection limits reported for mercury and magnesium at 36 and 3.9 ppb, respectively, when solution nebulization without desolvation is used for sample introduction. The result for copper is statistically equivalent to that of the 150-W MPT (Jin et al., 1991a) and the moderate power system by Brown et al. (1987). Use of the same system with a slightly higher argon flow rate (0.55 L/min) permitted Haas and Jamerson (1987) to obtain the best reported result for lithium at 0.24 ppb.

Preliminary results by Okamoto (1991) using the annular-shaped 1 kW air or nitrogen MIP with 10 L/min of plasma gas is very encouraging. The 30 and 1 ppb

Table 10. Metal Detection Limits Obtained using 30 (ng/mL) Solution Nebulization and MIP Atomic Systems

(nm)

a b c

(Brown et (Haas and (Haas and aL, 1987) Caruso, Jamerson,

AES 510 W 1984) AES 1987) AES CGN 510 W CGN 450 W CGN

(Okamoto, 1991)AES

l k W

e f g h i l (Perkins (Perkins (Jin et al.,

(Jin et al., (Perkins and Long, and Long, (Ng and 1991a) AES 1989) AES and Long, 1989) AFS 1988) AFS Shen, 1986) 150 W USN USN DSL 1989) 150 W CGN 70 W CGN AES DSL

t ~

"-4

A g ( I )

A1 (1)

A u (1)

B a ( l )

C a ( I )

C d ( I I )

C d (1)

C o

C r ( I I )

C r ( I )

C r ( I )

C s ( I )

C u ( I )

3 2 8 . 1 . . . .

3 9 6 . 2 m 9 7 0 k w

2 4 2 . 8 2 . 2 ~ ~

5 5 3 . 5 . . . .

4 5 5 . 5 . . . .

3 9 3 . 4 ~ ~ ~ 1

4 2 2 . 7 . . . .

2 1 4 . 4 ~ 6 . 9 ~

2 2 8 . 8 1 .0 1.5 - -

2 4 0 . 7 . . . .

2 6 7 . 7 12 18 ~

3 5 7 . 9 . . . .

4 2 5 . 4 3 . 3 6 . 0 ~

4 2 9 . 0 . . . .

4 5 5 . 6 ~ m 1 7 0

4 5 9 . 3 ~ ~ 1 3 0 0

3 2 4 . 7 5 . 2 1.8 - -

3 2 7 . 4 1 .7 - - ~

1 . . . . .

m 11 15 6 0 m 3

4 3 . . . . .

1 9 0 1 2 0 1 0 0 0 - - 1 4

3 . 0 . . . . .

k 12 3 0 ~

. . . . . 6 . 8

7 2 . 6 3 0 ~ 0 . 2 4

0 . 1 2 . . . . .

. . . . . 18

7 0 2 8 1 5 0 0 ~

m 1 o o 6 0 3 0 0 - -

. . . . 6 2

. . . . . 9 . 2

2 . 0 . . . . .

. . . . . 1 .7

19 . . . . .


~, (nm)

a b c

(Brown et (Haas and (Haas and al., 1987) Caruso, Jamerson,


d e f

(Okamoto, (Jin et al., 1991) AES 1989) AES

1 kW USN DSL

(Perkins and Long,

1989)

g h i 1 (Perkins (Perkins (Jin et al.,

and Long, and Long, (Ng and 1991a) AES 1989) AFS 1988) AFS Shen, 1986) 150 W USN

150 W CGN 70 W CGN AES DSL

F e ( I )

F e ( I I )

F e ( I )

H g (11)

In ( I )

K ( I )

L i ( I )

M g II

M g ( I )

M n ( I )

N a ( I )

N i ( I )

P b ( I )

2 4 8 . 3 m ~

2 5 9 . 9 4 .1 6 . 2

3 7 2 . 0 3 . 6 12

1 9 4 . 2 ~ 141

4 5 1 . 1 m - -

4 0 4 . 4 ~ - - 55

7 6 6 . 5 ~ - - 2 . 0

4 6 0 . 3 ~ ~ 33

5 7 0 . 8 ~ ~

6 1 0 . 4 ~ - - 18

6 7 0 . 8 ~ ~ 0 . 2 4

2 7 9 . 6 ~ 3 . 9

2 8 5 . 2 - - 7 . 2

2 5 7 . 6 1 .4 2.1

2 7 9 . 5 ~ ~

4 0 3 . 1 3 . 9 5 . 9

3 3 0 . 2 ~ ~ 23

5 8 9 . 0 m ~ 1.3

5 8 9 . 6 m _ _ 1.8

2 3 1 . 6 3 0 4 5

3 4 1 . 5 6 . 7 10

m

m

m

J

w

m

D

m

m

m

4 0 4 0 9 0 0 m 38

. . . . 18

m 1 3 0 _ _ m

~ 1 . 8 3 0 ~

. . . . . 0 . 9 9

0 . 5 . . . . .

6 1 . 8 3 0 ~

6 . 5 . . . . .

1 6 ~ 1 0 0 0 ~ 8 . 0

. . . . 18

1 0.1 15 m

. . . . . 8 .7

9 . 5 . . . . .


(nm)

a b c

(Brown et (Haas and (Haas and al., 1987) Caruso, Jamerson,


d e f

(Okamoto, (Jin et al., (Perkins 1991) AES 1989) AES and Long,

I k W USN DSL 1989)

g h i l

(Perkins (Perkins (Jin et al., and Long, atut Long, (Ng and 1991a) AES 1989) AFS 1988) AFS Shen, 1986) 150 W USN

150 W CGN 70 W CGN AES DSL

2 6 1 . 4 - - 33

4 0 5 . 8 - - 54

R b (I) 4 2 0 . 2 - - ~ 77

4 2 1 . 6 - - ~ 290

Sr (I) 4 6 0 . 7 ~ ~

V (I) 4 3 7 . 9 ~ ~

Z n (II) 2 0 2 . 6 7 .4 11

Z n (I) 2 1 3 . 9 2.3 3 .4

Z r (I) 3 4 3 . 8 - - ~

360 .1 ~ - -

m

m

m

m

m

30

m

m

0 .5

w

- - - - 139 27

11 7 30 13 m

12 1.8 60 ~ 14

D - - 3 9 4 5

Notes: (a--c, g- j ) use a Beenakker cavity with Ar for plasma support.

(e and f) use a Beenakker cavity with He for plasma support.

(1) uses the MPT with Ar for plasma support.


detection limits for zirconium and calcium, respectively, are the best reported to date for these elements. For zirconium, this is the only result reported that approaches the typical 7-ppb detection limit (Winge et al., 1979) of argon ICP-AES. The high boiling point of zirconium and its oxides has traditionally made it a particularly challenging species for the MIP-AES.

Alkali Metals. Maximum emission using argon ICP for alkali metals occurs very low (5-7 mm) above the ICP load coil (Haas and Jamerson, 1987), owing to their very low ionization potentials. Because the torch position is generally set to monitor elements high in the plasma, sensitivity is generally poor. Haas and Jamerson (1987) use axial viewing of a 450-W Beenakker cavity MIP to eliminate this problem. As previously shown with ICP, axial viewing (Davies et al., 1985; Faires et al., 1985) reduces detection limits. Results for the internally tuned MIP sustained with 0.55 L/min of argon and liquid aerosol sample introduction are an order of magnitude better for lithium or sodium and more than two orders of magnitude better for potassium, rubidium, and cesium than results from ICP with the same spectrometer. Further improvement has since been reported with use of helium HEMIP-HCL-AFS for sodium and potassium.

Easily Ionized Elements (EIE)

The presence of easily ionized elements in analytical samples used with ETV or aerosol sample introduction (Ng and Shen, 1986; Haas and Caruso, 1984) has been used to further depress detection limits for a number of elements measured at their atomic lines, using argon MIP. Detection limits measured at the ionic lines are generally decreased (Haas and Caruso, 1984). An evaluation of EIE effect on emission intensity enhancement (IE) and absorbance enhancement (AE) based on operational parameters has been undertaken by Jin et al. (1991b). They find that EIE effect is not correlated with the EIE ionization potential, plasma gas flow rate, or, at least in the case of AE, with the EIE species or concentration. Choice of analytical line had a very significant effect on IE but little on AE. The effect of EIEs is also dependent on plasma confinement tube diameter, with larger diameter tubes magnifying the effect.

EIE effect on AES and AFS is increased when using argon for plasma support rather than helium (Perkins and Long, 1989). Use of 1000 ppm sodium solutions containing 10 ppm calcium resulted in minimal Ca (422.7-nm) signal enhancement in AES and no discernible effect in AFS. The same experiment using argon resulted in 100% and 300% signal enhancement for HEMIP-AFS and AES, respectively.

D. MIP-AAS

Several workers have experimented with ICP-AAS. The ICP has not been suitable for AAS for a number of reasons (Magyar, 1980, 1987). The ICP discharge

Recent Developments in Analytical Microwave-Induced Plasmas 2 71

favors ionic species over those of ground state atomic species, the high flow rate tends to dilute analyte species, and the absorption volume is small by comparison to flame methods. Lu et al. (1991) have used ultrasonic nebulization with a desolvation--condensation system to obtain 0.6 ppb detection of cadmium using a 50-W Surfatron-MIP. An absorbance tube is fitted over the Surfatron discharge. Variation of the absorbance tube diameter (1.2-5.0 mm) had little effect on absorbance, but did affect signal stability. The larger tubes had faster signal response and worse reproducibility. A change in applied power from 20 to 50 W steeply increases absorption as atomization efficiency increases. The absorbance power curve levels off at about 50 W and further increases lead to plasma instability.

The Beenakker cavity permits convenient axial viewing, which has prompted investigation in our laboratory of MIP-AAS. The length of the MIP discharge is a function of the cavity depth. This concept has been used to increase plasma residence time, permitting more complete atomization, and improved detection limits for MIP-AES have resulted (Haas and Caruso, 1984). We have used a specially constructed torch and a 2-cm deep cavity to increase path length. The deeper cavity permits a longer plasma with a length similar to that of GFAA. This results in both a longer absorption pathlength and a longer analyte plasma residence time. Results obtained using a low-power (100-W) Beenakker MIP-AAS and GFN with 0.2 L/min of argon flow for calcium, copper, manganese are 1.0, 1.0, and 0.3 ppm, respectively. The same system has also been evaluated using a M A K nebulizer in conjunction with a desolvation-condensation. The increased sample loading possible with the M A K nebulizer using aerosol desolvation-condensation overcomes the disadvantage of the increased plasma gas flow rate and is similar to AES comparisons of the two nebulizers (Cull and Carnahan, 1987). The detection limits listed in Table 11 for this second system are comparable with those of ICP-AAS.

Table 11. Comparison of 3c~ (ppm) for MIP-AAS and ICP-AAS using Hollow Cathode Lamps as the Line Source

Element (Ng et al., 1988)

MIP GFN

(Ng and Garner, (Lu et al. 1991) (Mignardi et al., (Mignardi et al., 1993) MIP Surfatron MIP 1990b) T- Torch- 1990a) ICP MAK DSL USN DSL ICP ICP USN CGN

Ag -- 0.1 - - - - 0.09 Ca 1.0 0.1 m 0.03 0.08 Cd - - 0.02 0.00064 - - Co - - 0.2 m 0.1 Cu 1.0 0.6 ~ -- 0.1 Cr ~ 0.9 ~ - - Mg ~ 0.04 ~ 0.008 0.007 Mn 0.3 0.1 ~ 0.1 Na ~ 0.02 - - - - N i ~ 0 . 2 ~ - -

P ~ 4 ~ ~


Thermal Vaporization

The MIP is an efficient excitation source for AES. However, the low total energy present in the discharge places increased emphasis on the sample introduction interface. Many of the steps involved in the transformation of an analytical sample to its atomic or ionic state in the atom reservoir consume energy. In the case of solution nebulization, the aerosol must be desolvated, the dry aerosol melted and volatilized, then dissociated to the atomic or ionic vapor. In this form, the atomic or ionic vapor may be further excited to the various states associated with its elemental emission. Solids present similar problems in that they must also be melted, volatilized, and dissociated by the plasma. Incompletion of these processes leads to memory effects and spectral interferences. Thermal vaporization (TV) permits better utilization of the limited potential energy of the plasma by using a device prior to the plasma for producing volatilized or atomized samples. The residence time of the atomized analytical sample in the plasma is increased when these steps are done prior to the plasma. Matrix interferences in the atom reservoir may also be reduced when samples are atomized prior to the plasma. Unlike most determinations using liquid aerosols, the MIP may be operated at loW applied powers (< 100 W) with capillary discharge tubes (Matousek et al., 1986), which have provided the best results when using GC vapor-phase sample introduction techniques.

E. Electrothermal Vaporization

Operation of ETVs (Figure 22) is quite similar, in many respects, to graphite furnace AAS (GFAAS). Microliter (> 5-1aL) amounts of liquid samples are deposited into a vaporization cell. A low current is applied to dry the sample, followed by a high current to vaporize the sample for introduction to the plasma. An optional "ash" stage can be used for solid samples or liquids with a complex matrix to simplify the sample composition prior to analyte vaporization (Broekaert and Leis, 1985). ETV cells are generally constructed of metallic (W, Ta, Pt) or graphite materials. Cell forms have included graphite tubes (Aziz et al., 1982), rods (Alder and da Cunha, 1980), cords (Beenakker et al., 1980), and cups (Fricke et al., 1975; Ng and Caruso, 1983), as well as tantalum strips (Fricke et al., 1975; Ng and Caruso, 1983), filaments (van Dalen et al., 1982), wires (Stahl et al., 1989; Timmins, 1987; Brooks and Timmins, 1985), and boats (Barnett and Kirkbright, 1986; Chiba et al., 1984; Tanabe, 1985). Detection limit results for these systems are presented in Table 12.

Determination of Nonmetals by MIP-ETV

Carnahan et al. investigated the use of ETV sample introduction with the moderate- (Wu and Carnahan, 1990) and high-power (Alvarado et al., 1992) helium


K

Figure 22. Electrothermal vaporization (ETV) device for sample introduction into an atom reservoir: A, sample and carrier gas inlet to the atom reservoir; B, sample injection port; C, ground glass stopper; D, glass dome; E, vaporization cell (carbon cup); F, support electrodes; G, carrier gas inlet; H, fixed support stainless steel blocks; i, water cooling system; J, Teflon base; K, aluminum base; L, power cable. Reproduced with permission of The Society for Applied Spectroscopy and the authors.

MIPs. The moderate-power (500-W) helium MIP is used for determination of cadmium, copper, bromine, and chlorine using a Varion Model 63 carbon cup atomizer without matrix modifiers. The Varion ETV is based on a previous design by Nixon et al. (1974) and incorporates a custom adapter which reduces the ETV chamber from 750 mL to 12.5 mL, permitting improved transport efficiency of the vaporized analyte. Detection limit results for cadmium and copper are slightly higher than those reported for ETV-ICP (Matusiewicz and Barnes, 1985). Bromine and chlorine detection using ETV is 150 times and 50 times better, respectively, than when using liquid aerosol sample introduction with an ultrasonic nebulizer (Michlewicz and Carnahan, 1986a). Detection of sulfur species using the same ETV device with the He-KiP-MIP (Alvarado et al., 1992) (1.6 kW) provides an order of


Table 12. MIP Detection Limits for Nonmetals [in nanograms (ng)], Wavelength (nm), Linear Dynamic R (LDR) (power of 10), Analyte, and Electrothermal

Vaporization System Type using Thermal Vaporization Sample Introduction Detection LDR ETV System

Element Lhle (nm) Limit (ng) (10 x) Analyte Type Ref

CI

Br

I

479.45 6.2 2-3 LiCI G.E (Matousek et al., 1986 )

479.5 0.18 m H4C1 G.E (Wu and Carnahan, 1990)

470.5 0.45 m NH4Br G.E (Wu and Carnahan, 1990)

206.24 1.0 2-3 KI G.F. (Matousek et al., 1986)

206.24 0.2 - - KI (Barnett and Kirkbright, 1986)

516.12 1 - - KI G.F. (Barnett and Kirkbright, 1986)

527.86, 13.0 2-3 MgSO4 G.E (Matousek et al., 1986) 527.89

527.86, 12.0 2-3 Thiourea G.F. (Matousek et al., 1986) 527.89

564.0 15 m ~ G.C. (Beenakker et al., 1980)

921.3 45 2-3 K2SO4 G.C. (Alvarado et al., 1992) or

NH4SO4

Notes: *Originally reported as 2 times S/N, normalized here to 3 times S/N for consistency. **Originally reported as ppm of sample solution.

magnitude improvement over using USN (Wu, 1990) with the same source. Sulfur peak areas were not significantly affected by a change of sulfur form (for example lead sulfate versus ammonium sulfate). However, group 1A EIE salts depressed the sulfur signal by an unknown mechanism within the plasma. Careful control of ETV temperature also permitted separation of a mixture of organic and inorganic sulfur-containing compounds, including ammonium sulfate, potassium sulfate, cystine, and rnethionine. A graphite furnace-ETV device is used by Matousek et al. (1986) for determination of chlorine, iodine, sulfur, and phosphorus. The low- power Beenakker cavity is coupled in close proximity to the ETV system. Greatest sensitivity was obtained with the highest applied power possible (100 W) that did not cause devitrification of the plasma torch capillary. Optimal helium flow rates are 1.7-2.1 L/min. In contrast to results obtained by Barnett and Kirkbright (1986) using a tantalum-ETV system, a molecular band in the vicinity of the I line at 206.24 nm is observed during the heating cycle using the graphite-ETV. The spectral interference is attributed to interference caused by the CO band head, made by reaction of oxygen impurities with the heated graphite. A second significant background signal is detected at ca. 500 nm and is attributable to the vibronic transitions of the C2 Swan system (Selby, 1984), which affects detection of sulfur at the unresolved 527.86/527.89 nm line. The 2t~ detection limits for iodine, chlorine, and sulfur are 1.0, 6.2 and 13.0 ng, respectively.


Determination of iodine in hydrochloric acid has been reported by Barnett and Kirkbright (1986) using a low-power helium MIP (150 W) and tantalum boat-ETV. Determinations are made using the method of internal standards. Line optimization was obtained with use of a device for continuous vapor-phase sample introduction of vaporized iodine from its crystalline form. Of the five analytical lines investigated (206.163 nm, 256.624 nm, 258.279 nm, 287.863 nm, and 516.120 nm) the I atomic line at 206.163 is the most sensitive by nearly an order of magnitude over the ionic line at 516.120 nm, which has been commonly used by other workers (McLean et al., 1973; Beenakker, 1977; Quimby et al., 1979; Dingjan and de Jong, 1983b). Optimal plasma gas flows are 200-300 mL/min. The 26 detection limits for iodine in concentrated liquid hydrochloric acid are 0.2 ng and 1 ng for the 206.163 nm and 516.120 nm I lines, respectively.

Direct Solids Sampling Using ETV

Broekaert and Leis (1985) developed a procedure for direct solids sampling. Milligram amounts of dry organic solids are introduced to a graphite furnace using a syringe developed by Grobenski et al. (1982). Relatively low power (50 W) is used with a plasma gas flow of 0.3 L/min and a carrier gas flow of 4.5 L/min. Decomposition of the organic matrix with venting of decomposition products is required. In the case of biological samples, thermal decomposition is obtained when the furnace temperature is gradually increased to 940 ~ Wetting the powdered samples permitted a thin layer of analyte to be formed on the inner wall of the graphite furnace. This resulted in higher analytical signals with better repeatability. Memory effects were minimal, permitting 20 samples to be run without rinsing the graphite sample cup. Detection limits for a bovine liver standard are reported in Table 13. Calibration is accomplished by standard addition of a suitable standard solution to the ca. 2 mg of powder sample to be analyzed. Analytical precision is improved when time-resolved and simultaneous measurement of background and line intensity is used.

Table 13. Detection Limits (3c~) for NBS 1577 Bovine Liver Standard Solids Determined by Graphite Furnace ETV-MIP (Broekaert and Leis, 1985)

Detection Limits in Solid Samples (mg/g)--50- W Ar-MIP

Dr), Solution Residue Direct Solids Element Line Accepted Conc. (mg/g) (2 mg/50 mL) Samplhlg (2 mg)

Cu 324.8 139 + 10 0.03 0.07 Fe 260.0 270 + 20 0.15 0.5 Mn 257.6 10.3 + 1.0 0.07 0.2


ETV Automation

While ETV sample introduction can be quite successful when using MIPs as a source, the technique has had limited precision due to errors introduced by hand pipetting and through plasma disturbances by entrained air. Air enters the ETV system when analytical samples are introduced to the ETV device. Precision for hand pipetting is typically 5-10% (Brooks and Timmins, 1985). Stahl et al. (1987) have fully automated a wire filament ETV to improve the overall analytical relative standard deviation (RSD) when using ETV sample introduction. The ETV part of the system utilizes a 1-mm tantalum loop and a specially modified graphite furnace power supply. A computer-controlled autosampler delivers highly reproducible droplets to the tantalum wire loop for vaporization to the 100-W argon MIP. The RSDs for the droplet mass precision is < 1% and is less than 5% for the overall determination of a low concentration of indium. The linear dynamic range is at least three orders of magnitude.

Direct Sample Vaporization

A second thermal vaporization device involves direct sample vaporization (DSV) into an analytical plasma source. DSV produces an aerosol from a microvolume (ca. 5 ktL) of solution deposited in a hollow cup on the end of a graphite rod or on a tungsten wire loop. The insertion device is then placed into the lower portion of the plasma. Direct inductive heating of the cup (Abdalla et al., 1991b; Salin and Horlick, 1979) or wire loop (Salin and Horlick, 1979) permits vaporization of the sample into the plasma with high efficiency. This system has been used successfully for ICP (Salin and Horlick, 1979; Salin and Sing, 1984) and recently with the capacitively coupled MIP torch (Abdalla et al., 1991 b) for AES. Careful positioning permits solvent evaporation followed by ashing and volatilization in to the plasma. The DSV system by Abdalla et al. (1991a) has also been useful for solids. Other TV systems in the literature include arc nebulization (Layman and Lichte, 1982; Deutsch and Hiefje, 1984; Zander et al., 1977; Zander and Hieftje, 1978) and spark nebulization (Helmer, 1984a, b) vaporization systems, as well as laser and spark ablation sample introduction.

V. CONCLUSION

Among the plasma sources available today for spectrochemical analysis, microwave-induced plasma (MIP) is the most universal. Although optical emission is the principal detection for analytical signal, the MIP has shown great promise for other types of detection such as atomic absorption spectroscopy, atomic fluorescence spectroscopy, mass spectroscopy, and laser-enhanced ionization spectroscopy. He- lium plasmawill continue to dominate the MIP research due to its capability for


nonmeta l determinat ion. GC-based MIP systems have shown the greatest success

and should cont inue to enjoy this status. It is predicted that great efforts will be

placed to deve lop H P L C and SFC-MIP systems because of the popular i ty o f these

chromatograph ic techniques.

ACKNOWLEDGMENTS

We are grateful to our many colleagues who generously responded to our request for reprints, pre-prints, and permission to reproduce; particularly to Professor J.A. Caruso of the University of Cincinnati and Professor J.W. Carnahan of Northern Illinois University.

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INDEX

AAS, 141,145-148 (see also "Hydride generation...")

flame-in-tube atomizers, 145-146 flames, 145 graphite furnace atomizers, 147-

148 graphite paper atomizer, 147-148

quartz tube atomizers, electrically- heated, 146-147

quartz tube atomizers, flame- heated, 146

ACP, 153 Aerosol sample introduction, 260-270

easily ionized elements (EIE), 270 absorbance enhancement (AE),

270 emission intensity enhancement

(IE), 270 and metal determinations using

MIP-AES, 265-270 alkali metals, 270 alumina tube, 266 detection limits, 267-269 HCL-AFS, 266 HCL-MIP-AFS, 266 helium HEMIP AES and AFS,

265, 266 HEMIP-HCL-AFS, 270

internally tuned Beenakker cavity, 266

MPT with desolvation- condensation system, 265- 266

in simple matrix, 265-270 and nonmetal determinations

using MIP-AES, 262-265 fluorine, 265 halogen species, 262 helium-kilowatt-plus MIP, 265 high applied powers, 263 liquid aerosols, 263 low applied power, 263 spectral interference, 263-265 Surfatron, 263 TM0~0 resonator, 263, 265

pneumatic nebulizers, 261-262 concentric glass, 261-262 desolvation, 262 dessication cell, 262 glass tilt, 261-262 MAK cross flow, 261-262 ultrasonic, 261-262

AES, 68, 141, 148-153 (see also "Hydride generation...")

alternating current plasma (ACP), 153

285

286 INDEX

capacitively coupled microwave plasmas (CMP), 152-153

direct current plasmas (DCP), 152 flames, 153

molecular emission cavity analysis (MECA), 153

inductively coupled plasmas (ICP) 148-151

flow injection analysis (FIA) system, 151

gas-chromatographic separation, 151

gas-liquid separator, 149-151 high performance liquid chro-

matography (HPLC), 151 schematic, 150 in situ nebulizer/hydride gener-

ator, 149 microwave induced plasmas

(MIP), 152 AFS, 14 l, 154, 155 (see also

"Hydride generation...") electrically-heated silica tube atom

cell, 154 electrodeless discharge lamps

(EDLs), 154 non-dispersive, 154

Alternating current plasma, 153 AnMINP, 222, 224 Argon:

in GC-AED detection of oxygen, 242

MIP-MS, 254 Arsenobetaine, 165 Atomic absorption spectrometry

(AAS), 141, 145-148 (see also "Hydride generation..." and "AAS")

Atomic emission spectrometry (AES), 68, 141, 148-153 (see also "Hydride generation..." and "AES')

Atomic and molecular fluorescence, laser-excited, in graphite furnace, 1-62 (see also "LEAFS'3

analytical results from LEAFS, 14-25

calibration curves, 23-25 detection limits, 14-24

background correction for, 25-47 blackbody emission, 25-26 conclusion, 46-47 excitation spectral scan, 27, 29-

32 laser scatter, 25-26 multichannel techniques, 27-32,

47 nonanalyte atomic fluorescence,

25 spectral scan, 27 wavelength, 27-34, 47 Zeeman, 27, 34-46, 47 (see also

"Zeeman...") conclusion--LEAFS, 52 instrumentation and spectroscopic

transitions for LEAFS, 3-14 detection systems, 10-12 graphite furnaces, 6-10 laser system, 3-6 spectroscopic transitions for,

12-14 introduction, 2 LEAFS, real sample analyses by,

47-51 impaction method, 50, 51 nickel-based alloy, 47, 50 MIST SRMs, 47, 50 slurry sampling, 47 solid sampling, 47

LEAFS reviews 1988-1992, 3 LEMOFS, 52-61 (see also

"LEMOFS'3 background correction, 58-60 conclusion, 60-61

Index 287

detection limits, 54, 57-58 instrumentation, 52-53 interferences with signal, 58 linear dynamic ranges, 57-58 molecules and optimization

procedures, choice of, 53-57 real sample analyses, 58-60

Beenakker cavity-based designs, 216, 218-221,228-229, 266 (see also" MIPs")

TM010 resonator, 218-221,228- 229, 263

coupling loop, 219 "critically" coupled resonant

cavity, 221,238 EBC, 221 HEMIP, 221 joule heating, 220 modified design, 220-221 moderate power MIP, produc-

ing, 220 reflected power detector, 238-

239 SFC-MIP performance, 250 stub-tuning devices, 219-220 torch life, 220 tuner, 218-219 tuning screws, 219

Blackbody emission, 10-11, 25-26

Calibration curves of LEAFS, 23-25 for ZETA LEAFS, 39-42

Capacitively coupled microwave plasma (CCMP), 91-92 (see also "CCMP")

Capacitively coupled radio frequency plasma discharge (CCP), 92

Capillary torches, 234 CCMP, 91-92

and atomic emission spectrometry (AES), 152-153

operating parameters for studies of, 107

CCP, 92 Ceramic materials, use of excimer

laser with, 208-209 Concentric glass nebulizer (CGN),

261-262 Concentric tube LFTs, 234-236 Copper vapor lasers, 4

DCP, 90-91 and atomic emission spectrometry

(AES), 152 commercial availability of, 98 operating parameters for study of,

107 Direct current plasma (DCP), 90-91

(see also "DCP") Doppler spectroscopy, 209 Double resonance fluorescence, 13-

14 Dye laser, 5-6

EBC, 221 (see also "Beenakker...") EDLs, 154 Electrodeless discharge lamps

(EDLs), 154 Electrodeposition, 92-93 Electrothermal vaporization as sam-

ple introduction into plasma sources for analytical emission spectrometry, 63-138, 272-276

abstract, 64 applications, 114-128

biological materials, 123-128 clinical materials, 123-128 environmental substances, 117-

121 metallurgical samples, 122 mineral substances, 117-121 technical products, 117-121 waters, 115

288 INDEX

commercial availability of systems, 98-108

conclusions, 129-130 detection limits, 108-114, 130 future studies, suggestions for,

131-132 instrumentation, summary of, 98-

107 introduction, 64-65

FAAS, 65 nebulization, 65

MIP-MS, 259 nomenclature, 66

and electrothermal atomizer, 66 GFAAS, 66

operating parameters for studies, 99-107

preconcentration techniques, 92- 95

electrodeposition, 92-93 HMDC, 95 HPLC, 95, 97 resin-ETV-ICP technique, 95 in situ, 94

sample introduction, 66-92 capacitively coupled microwave

plasma, 91-92 devices, three designs for, 67 direct current plasma (DCP),

90-91 inductively coupled plasma, 68-

78 (see also "ICP") inductively coupled plasma-

mass spectrometry 78-81 (see also "ICP-MS")

microwave-induced plasma, 81-89 (MIP), 81-89 (see also "MIP")

microwave-induced plasma- mass spectrometry (MIP- MS), 89-90

speciation, 96-98, 131 ETV-ICP-MS, 97-98, 108 HPLC-ETV-ICP instrument, 97

Emission spectrometry, plasma sources and, 63-138 (see also "Electrothermal vaporization...")

ETV, 63-138 (see also "Electrother- mal vaporization...")

ETV-CCMP, detection limits for, 113-114

ETV-DCP, detection limits for, 113 ETV-ICP, detection limits for, 110,

114 ETV-ICP-MS, 97-98, 108

detection limits for, 110-111, 114 ETV-MIP, detection limits for, 112,

114 ETV-MIP-MS, 259

detection limits for, 112 Evenson cavity, 227 Excimer lasers in atomic spectros-

copy, 4, 179-213 abstract, 180 applications, 184-208

analysis, elemental, of solid samples, 205-208

excimer-laser-induced emission spectroscopy, 190-208 (see also "Excimer-laser-induced emission...")

fluorescence detection in liquid chromatography, 189

GFAAS, 187 laser ablation-inductively

coupled plasma mass spectrometry, 184, 185

laser-enhanced ionization (LEI)spectroscopy in flames and furnaces, 187-188

laser vaporization-atomic absorption, 184

laser vaporization-direct current plasma, 184

laser vaporization-inductively coupled plasma, 184

Index 289

laser vaporization-microwave induced plasma, 184

laser vaporization-spark excitation, 184

LEAFS, 188-189 Nd:YAG, 184-187 photoionization of small mole-

cules, 189 as pumping lasers for LEI

research, 188 conclusions, 211 future developments, 211 industrial applications, 208-211

ceramic materials, 208-209 embedded fibers, processing of,

210-211 ferrites, 210-211 thin film deposition, 209-210 thin film preparation, 208

introduction, 180-181 from excited dimers, 180 history, 180 LIBS, 180 rare gas monohalide emissions,

discovery of, 180-181 properties, 182-183 theory, 181-182

rare gas excimers (RGE), 181 Excimer-laser-induced emission

spectroscopy, 190-208 atmospheric effects, 198-205

inner sphere plasma, 200 outer sphere plasma, 200

excitation temperatures of plasma, 191-194

Boltzmann plot method, 191, 192 experimental setup, typical, 190-191 space-resolved studies, 194-198

Excitation spectral scan, 27, 29-32

FAAS, 155 Ferrites, excimer laser ablation and,

210

FIA, 151 Flame atomic absorption spectrome-

try (FAAS), 155 Flame-heated quartz tube atomizers,

146 Flame-in-tube atomizers, 145-146 Flames:

and AAS, 145 and AES, 153

molecular emission cavity analysis (MECA), 153

Flow injection analysis (FIA) system, 151

Fluorescence, atomic and molecular, 1-62 (see also "Atomic and molecular fluorescence...")

Fluorescence detection in liquid chromatography by excimer laser, 189

Fluorine, GC-AED detection of, 242

Gas chromatography, MIP-AED for, 225-244 (see also "MIPs...")

Beenakker cavity for, 228-229 discharge tube materials, 228

coaxial plasma, 229-230 fluorine and chlorine selectivity,

229 fluorine-containing compounds,

229 helium-MIP, 229 impedance matching, 229

empirical formula determination, 236-238, 243

GC-AEDs, 225-227, 239-244 historical references (1965-1985),

226-227 instrumental setup, 226

simultaneous multielement detection, 226

metal detection, 239-240 indium, 239 lead, 239

290 INDEX

mercury, 239 tin, 239-240

microwave discharge devices for, 227-228

Beenakker cavity, 227 Evenson cavity, 227 reentrant cavity, 228, 229 stripline source, 227-228 Surfatron, 227, 230 tapered cavity, 227

MIP-AED, 225-244 advantage of, 225-226

MIP GC-AED interface developments, 230-231

background emission, 231 quartz discharge tube, 230 reagent gases, 231 venting interfaces, 231-233 (see

also "...venting interfaces") nonmetals detection, 240-244

carbon, 240 elemental response factors, 243 fluorine, 242 GC-MS detection of carbon,

240 halogenated compounds, 242-

243 isotopic ratios, 240 low-pressure MIP interface, 240 miscellaneous, 243-244 oxygen, 242

organometallic detection limits, 241

plasma torch designs for, 233-236 alumina torch, 239 capillary torches, 234 concentric tube LFTs, 234-236 laminar flow torch (LFT), 234 quartz tubes, 233 tangential flow torch (TFT),

233-234 water-cooled capillary plasma

torch (WCCPT), 236, 238

reflected power detector, 238-239 Surfatron, 230 venting interfaces for MIP GC-

AED, 231-233 dead volume, 232 FID and MIP-AES detection,

231 GC eluents, 231 make-up gas, 232 quartz GC capillary, 232

GC-AEDs, 225 (see also "Gas chromatography...")

metal detection, 239-240 nonmetal detection, 240-244

GFAAS, 1-62, 155, 187 Glass frit nebulizers (GFN), 261-262 Graphite furnace atomic absorption,

1-62, 155, 187 (see also "Atomic and molecular fluorescence...")

AAS, 23 and Zeeman background cor-

rection, 34-46 (see also "Zeeman ...")

continuous flow atomizer, 10 dispersive detection, 8-9 front surface illumination, 7-9

transverse Zeeman background correction, 43-46

graphite cups, 6 graphite tube atomizers, 6 lens, 9 nondispersive detection, 8-9, 12 open atomizers, 6 transverse illumination, 7, 9

Graphite furnace atomizers, 147-148 Graphite paper atomizer, 147-148

Halogenated compounds, GC-AED detection of, 242-43

Helium MIP-MS, 257-259 background ion peaks, 257, 258 low pressure plasma, 259

Index 291

HEMIP, 221 (see also "MIPs" and "Beenakker... ")

High-performance liquid chromatography (HPLC), 95, 97, 151

HMDC, 95 HPLC, 95, 97, 151

element-specific detection with, 244-248

interface, proportional, 244-245, 247-248

ion-exchange chromatography, 246

liquid chromatography, 244 microfrit nebulizer, 245 moving belt interface, 246-247 moving wheel interface, 246 organic mobile phase, 246

HPLC-ETV-ICP instrument, 97 Hydride generation techniques in

atomic spectroscopy, 139- 178

abstract, 140 applications, 167-171

practical, 168-171 sample digestions, 167

atomization, excitation and ionization, methods of, 145-155 (see also under particular head)

atomic absorption spectrometry (AAS), 145-148

atomic emission spectrometry (AES), 148-153

atomic fluorescence spectrometry (AFS), 154

inductively coupled plasma mass spectrometry (ICP- MS), 154-155

atomization mechanisms, 160-163 in graphite furnace atomizers,

161-162 molecular beam sampling mass

spectrometry, use of, 161 peak shapes, 162-163

chemical speciation, 163-166, 171- 172 (see also ("...speciation')

conclusions, 171-172 heated quartz tube atomizer,

171 covalent hydrides, 141, 160 figures of merit, analytical, 155-

157 detection limits, 155-156, 171

interferences, 157-160, 171 chelating resins, application of,

160 chemical, 158-159 colloid flotation, 160 coprecipitation, 160 interference-releasing elements,

159 masking agents to overcome, 159 methods for overcoming, 159-160 separation techniques, 160 solvent extraction, 160 spectral, 158 standard additions method of

overcoming, 159 transition-metal ions, 159

introduction, 140-141 AAS, AES and AFS, 141 elements forming volatile

hydrides, 141 gas-phase sample introduction,

140 ICP-MS, 141 pneumatic solution nebulization,

140 optimization of parameters for,

157 simple multivariate optimiza-

tion, 157 reaction, 142-143

hydrochloric acid, 142 radiotracer technique, 142 with sodium tetrahydroborate,

142

292 INDEX

speciation, chemical, 163-166, 171- 172

arsenobetaine, 165 cold-trapping procedures, 165-

166 column liquid chromatography,

166 dimethylarsinic acid, 165 gas-liquid separators, use of, 166 high pressure liquid chromato-

graphy, 166 inorganic, 163-164 ion chromatography, 166 organic, 164-166 pre-reduction, 163 ribosyldimethylarsine oxides, 165 trimethylarsine oxide, 165

transport, 143-144 batch mode, 143, 144, 148 cold trap collection, 144 collection mode, 144, 157 continuous flow mode, 143-144,

148 direct transfer mode, 143-144,

157 flow injection, 143, 144 in situ preconcentration, 144 pressure collection, 144 sodium tetrahydroborate as

reducing agent, 143 stopped flow type of equipment,

144

ICP, 68-78 (see also "Electrothermal vaporization...")

commercial availability of, 98, 108 graphite heating devices, 70-78 GRV, 71-72 mass spectrometry, 78-81 metal heating devices, 68-70 operating parameters for studies

of, 99-102 PTFE, 71

ICP-AES, 155 ICP-MS, 23-24, 78-81,141,154-155

commercial availability of, 98, 108 graphite heating devices, 78-80 isotopic analyses, 155 metal heating devices, 80-81 operating parameters for studies

of, 99-102 Indium, GC-AED detection of, 239 Inductively coupled plasma, 68-78

(see also "ICP") and atomic emission spectrometry

(AES), 148-153 flow injection analysis (FIA)

system, 151 gas-chromatographic separation,

151 gas-liquid separator, 149-151 high performance liquid chro-

matography (HPLC), 151 schematic, 150 in situ nebulizer/hydride genera-

tor, 149 Inductively coupled plasma-mass

spectrometry (ICP-MS), 23- 24, 78-81, 141, 154-155 (see also "ICP-MS" and "Hydride generation ")

Laminar flow torch (LFT), 234 Laser ablation, 184, 190-208 (see also

"Excimer-laser-induced emission...")

-atomic emission spectroscopy (LA-AES), 205-208

Laser-excited atomic and molecular fluorescence in graphite furnace, 1-62 (see also "Atomic and molecular fluorescence...")

Laser scatter, 25-26 LDRs, 23-25 Lead, GC-AED detection of, 239

Index 293

LEAFS, 1-62 analytical results from, 14-25

calibration curves, 23-25 detection limits, 14-24 graphite furnace AAS, 23, 34-46 ICP-MS, 23-24 optical saturation, 25

background correction for, 25-47 (see also "Atomic and molecular fluorescence...")

instrumentation and spectroscopic transitions for, 3-14

blackbody-noise, 10-11, 25-26 (see also "Atomic and molecular fluorescence...')

copper vapor lasers, 4 detection systems, 10-12 double resonance fluorescence,

13-14 dye laser, 5-6, 26 excimer lasers, 4 flicker noise, 11 frequency converter, 6 frequency doubting, 6 front surface illumination, 7-9,

14, 27 graphite furnaces, 6-10 (see also

"Graphite furnace...") grazing incidence dye laser, 5, 33 Hansch dye laser, 5 laser system, 3-6 linear dynamic ranges (LDRs),

23-25 Nd:YAG lasers, 4 nitrogen lasers, 4 nonresonance fluorescence, 13,

23 piezoelectric pusher, 5-6, 33 pump laser, 4-5 resonance fluorescence, 12 shot noise, 11 slitwidth, 10-12

solar blind photomultiplier tube, (PMT), 14

spectroscopic transitions for, 12- 14

stray-fight noise, 10-11 transverse illumination, 7, 9, 26

real sample analyses by, 47-51 (see also "Atomic and molecular...")

reviews 1988-1992, 3 and ZETA LEAFS, 36-43 (see also

"Zeeman...") detection limits, 37, 44-45

LEI spectroscopy, 187 excimer lasers as pumping lasers

for, 188 LEMOFS, 52-61 (see also "Atomic

and molecular fluorescence...")

background correction, 58-60 conclusion, 60-61 detection limits, 54, 57-58 interferences with signal, 58 instrumentation, 52-53 linear dynamic ranges, 57-58 molecules and optimization proce-

dures, choice of, 53-57 atomization optimization, 53,

56, 57 char optimization, 53, 55, 56, 57 chemical modifier, 53 -pulse energy, 57 saturation, 57 spectral scan, 57 vibrational transition, 57

real sample analyses, 58-60 LFT, 234 Linear dynamic ranges (LDRs), 23-

25 Linear-enhanced ionization (LEI)

spectroscopy, 187-188 excimer lasers as pumping lasers

for, 188

294 INDEX

MAK cross flow nebulizer, 261 MECA, 153 Mercury, GC-AED detection of, 239 Microwave-induced plasma (MIP),

81-89 (see also "MIP") and atomic emission spectrometry

(AES), 152, 155 Microwave plasma torch (MPT),

222-224 MIP, 81-89

chloride generation, 89 commercial availability of, 98 graphite heating devices, 85-88 metal heating devices, 81-85 MTES system, 84-85 nebulizer, indirect, 88 operating parameters for studies

of, 103-106 separative column atomizer

(SCA), 89 silver wool, mercury amalgamated

to, 88-89 MIP-AAS, 270-272

Beenakker cavity, 271 ICP, unsuitability of, 270-271 and ICP-AAS, comparison of, 271 MAK nebulizer, 271 thermal vaporization (TV), 272

MIP-AED, 225-244 (see also "Gas chromatography...")

MIP-AES, 155 MIP-ETV, determination of nonme-

tals by, 272-275 MIP-MS, 89-90, 252-259

commercial availability of, 98 as elemental ionization source,

254-259 argon MIP-MS, 254 helium MIP-MS, 257-259 nitrogen MIP-MS, 255-257 (see

also "Nitrogen MIP-MS") ICP-MS, 252 interface, 253

operating parameters for studies of, 103-106

as soft ionization source, 253-254 Surfatron MIP, 254 tangential design torch, 253

MIPs, analytical, recent developments in, 215-283

chromatographic detection systems using, 224-259

atomic emission detector (AED), 225

electrolytic conductivity detector (ELCD), 224

electron capture detector (ECD), 224

element-selective detectors, 224 ETV-MIP-MS, 259 flame ionization detector (FID),

224 flame photometric detector

(FPD), 224 gas chromatography (GC), 224,

225-244 HPLC detectors, 225, 244-248

(see also "HPLC...") as ion source for MS, 252-259

(see also "MIP-MS") mass-selective detector (MSD),

225 MIP-AED for gas chromato-

graphy, 225-244 (see also "Gas chromatography...")

MIP GC-AED interface developments, 230-231 (see also "Gas chromatography...")

photoionization detector (PID), 224

separation, 224 supercritical fluid chromato-

graphy, as detector for, 248- 251 (see also "Supercritical fluid...")

thermionic detector (NPD), 224

Index 295

conclusion, 276-277 introduction, 216-217

applications, 216 helium, 216 and ICP, 216 reviews, 216-217

nonchromatographic sample introduction methods, 260-276

for AAS and AFS, 260 aerosol, 260-270 (see also

"Aerosol...") direct sample vaporization, 276 direct solids sampling using

ETV, 275 droplet diameter, 260 electrothermal vaporization,

272-276 ETV, direct solids sampling and,

275 ETV automation, 276 helium as plasma support gas,

260 MAK cross flow nebulizer, 261 MIP-AAS, 270-272 (see also

"MIP-AAS'3 MIP-ETV, determination of

nonmetals by, 272-275 pneumatic nebulizers, 261-262

(see also "Pneumatic nebulizers")

vapor-phase, 260 sources and systems, 217-224

AnMINP, 222 Beenakker cavity-based designs,

218-221,228-229 Beenakker TM010 resonator,

218-221,228-229 (see also"Beenakker. . . 'O

capacitively coupled microwave plasma, 218

coaxial sources, 218 components, 217-218 cooling, 224

generator, 217-218 HEMIP, 221 (see also

"Beenakker... ") high applied power MIPs, 224 microwave plasma torch

(MPT), 222 reentrant cavity, 222, 228, 229 stripline source (SLS), 222, 224,

227-228 surfacewave MIP, 221-222 Surfaguide, 221-222 Surfatron, 218, 221-222, 227,

230 tubular electrode torch (TET),

222 tuner, 217

Molecular emission cavity analysis (MECA), 153

Molecular fluorescence, laser- excited, 1-62 (see also "Atomic and molecular fluorescence...")

LEMOFS, 52-61 (see also "LEMOFS")

MPT, 222-224 S FC-MIP performance, 250

Multichannel background correction, 27, 27-32, 47

beamsplitter, use of, 31-32

Nd:YAG lasers, 4 Nebulization, 65 (see also "Electroth-

ermal vaporization...") Nitrogen lasers, 4 Nitrogen MIP-MS, 254-257

background ion peaks, 255-257 selenium isotopes, 255

Nonresonance fluorescence, 13, 23

Optical fibers, excimer laser processing of, 210-211

Oxygen, GC-AED detection of, 242

296 INDEX

Photoionization of small molecules by excimer laser, 189

Photomultiplier tube, 14, 27-32 Plasma emission spectrometry, 63-

138 (see also "Electrother- mal vaporization...")

Plasma-Spec Spectrometer, 98 PMT, 14, 27-32 Pneumatic nebulization, 65, 140,

261-262 (see also "Electro- thermal vaporization...")

concentric glass nebulizer, 261-262 glass frit nebulizers (GFN), 261-262 MAK cross flow nebulizers, 261 ultrasonic (USN) nebulizers, 261-

262 Pulsed laser deposition methods,

209-210 Pump laser, 4-5

Quartz tube atomizers, flame-heated, 146

electrically-heated, 146-147

Reentrant cavity, 222, 228 Reflected power detector, 238-239 Resonance fluorescence, 12-13

SCA, 89 Secondary ion mass spectroscopy

(SIMS), 206 Separative column atomizer, 89 SFC, 248-251 (see also "Supercritical

fluid...") SIMS, 206 Solar blind multiplier tube, 14 Stripline source, 222, 224, 227-228 Supercritical fluid chromatography

(SFC), MIP as detector for, 248-251

S FC-MIP interface, 248-249 pressure programming, 248 schematic, 249

SFC-MIP performance, 250-251 helium plasma tolerance to

CO2, 250 MPT, 250 Surfatron, 250 TM0~0 Beenakker cavity, 250

Surfacewave MIP, 221-222 Surfaguide, 221-222 Surfatron, 221-222, 227, 230, 263

MIP, 254 S FC-MIP performance, 250

Tangential flow torch (TFT), 233- 234

TET, 222-224 Thin film preparation, use of exci-

mer laser for, 208 deposition, 209-210

Tin, GC-AED detection of, 239-240 Tubular electrode torch (TET), 222-

224

Ultrasonic nebulizers (USN), 261- 262

Venting interfaces for MIP GC- AED, 231-233, 244-248

dead volume, 232 design, 232 FID and MIP-AES detection, 231 GC eluents, 231 make-up gas, 232 quartz GC capillary, 232

Water-cooled capillary plasma torch (WCCPT), 236, 238

Wavelength background correction, 27-34, 47

WCCPT, 236, 238

Zeeman background correction, 27, 34-46, 47

conclusion, 46-47

Index 297

energy level diagrams, 34-35 etalons, intercavity, 44-45 illumination configurations, 34-36 intercavity etalons, 44-45 longitudinal, 34-43

summary, 42-43 polarizer, 35, 44 transverse, 34, 43-46

ZETA LEAFS, 36-43 applications of, 42 calibration curves for, 39-42, 45 inverse, 42 and LEAFS, comparison of, 37,

44-45 ZETA LEAFS, 36-43 (see also

"Zeeman...")

J A l

P R E S S

Advances in Atomic Spectroscopy

Edited by Joseph Sneddon, Department of Chemistry, McNeese State University, LA.

This series describes selected advances in the area of atomic spectroscopy. It is primarily intended for the reader who has a background in atomic spectroscopy; suitable to the novice and expert. Although a widely used and accepted method for metal and nonmetal analysis in a variety of complex samples, Ad- vances in Atomic Spectroscopy covers a wide range of materials. Each chapter will completely cover an area of atomic spectroscopy where rapid development has occurred.

Volume 1, 1992, 238 pp. $97.50 ISBN 1-55938-157-4

Volume 1 consists of chapters covering the development, instrumentation, and results of a wide range of materials, including: background correction lasers, inductively coupled-mass spectroscopy; plasmas, electrothermal vaporizers, sample introduction, and Fourier transform atomic spectrocopy.

CONTENTS: Preface, Joseph Sneddon. Analyte Excitation Mechanisms in the Inductively Coupled Plasma, Kuang-Pang Li and J.D. Winefordner. Laser-Induced Ionization Spectrom- etry, Robert B. Green and Michael D. Seltzer. Sample Intro- duction in Atomic Spectroscopy, Joseph Sneddon. Background Correction Techniques in Atomic Absorption Spectrometry, G. Dulude. Flow Injection Techniques for Atomic Spectrometry, Julian F. Tyson.

FACULTY/PROFESSIONAL discounts are available in the U.S. and Canada at a rate of 40% off the list price when prepaid by personal check or credit card and ordered directly from the publisher.

JAI PRESS INC. 55 Old Post Road # 2 - P.O. Box 1678 Greenwich, Connecticut 06836-1678 Tel: (203) 661- 7602 Fax: (203) 661-0792

Advances in Near-Infrared Measurements

Edited by Gabor Patonay, Department of Chemistry, Georgia State University

Research falling under the classification of near-infrared spectroscopy covers a wide spectrum of activities in industry and academia. During the last several years near-infrared (NIP,) spectroscopy has developed into a powerful quantitative tool. NIR analyses have become vital for several industries. In the last few years NIR spectroscopy has been rapidly moving into new fields and finding more and more applications. These advances are especially remarkable in the shortwave NIP, region. This series will cover topics from prominent experts working in this extremely diverse disci- pline of NIP, spectroscopy. All contributors have made significant impact in this field of NIR spectroscopy.

Volume 1, 1993, 144 pp. $97.50 ISBN 1-55938-173-6

The first volume is organized so that the reader will gain basic insight into how NIP, spectroscopy developed into an important general analytical tool in the last several years. This first volume includes a variety of topics covering areas from the lastest developments in classical NIR measurements to applications of the developing and more detailed aspects of NIP, measurement techniques. Particular emphasis will be placed on giving the reader a comprehensive description of important techniques. The aim is to provide a balanced report of the most important advances in the field for both the specialist and the non-specialist who wish to enter this field.

CONTENTS: Preface, Gabor Patonay. Remote Monitoring with Near Infrared Fiber Optics, Chris W. Brown, Steven M. Donahue and Su-Chin Lo, University of Rhode Island. Development of Raman Scattering Techniques Using Near- Infrared Lasers and Fiber Optics, S.M. Angel and M.L. Myrick, Lawrence Livermore National Laboratory. An Analysis of NIP, Data Transformation, Howard Mark, Bran and Luebbe Analyzing Technologies, New York. A Stationary Hadamard Transform Interferometer, J,D, Tate, Basil Curnutte, Jr., Joseph V. Paukstelis, Robert M. Hammaker, and William G. Fateley, Kansas State University. Pharmaceutical Applications of Near-Infrared Spectrometry, James K. Drennen and Robert A. Lodder, University of Kentucky. Near-Infrared Fluorescence: An Emerging New Method, Gabor Patonay, Georgia State University. Index.

.1 A 1

P R E, S S

.1 A l

P R E 5; S

Advances in Multidimensional Luminescence

Edited by Isiah M. Warner, Department of Chemistry, Emory University and Linda B. McGown, Department of Chemistry, Duke University

The multidimensional capabilities of fluorescence spectroscopy are discussed with particular emphasis on multicomponent analysis. Several of these capabilities are highlighted in the various chapters using specific examples from recent research.

Volume 1, 1990, 205 pp. $97.50 ISBN 1-55938-172-8

CONTENTS: Preface, Isiah M. Warner and Linda B. McGown. Introduction to Multidimensional Luminescence, Gabor Pa- tonay, Georgia State University and Isiah M. Warner, Emory University. Luminescence Techniques for the Characteriza- tion of Multicomponent Systems, A.H. Bates, S.R. Meech, He- riot-Watt University and I. Soutar, University of Lancaster. Biomedical Applications of Expert Systems in Analytical Spec- troscopy, Brian J. Clark, Anthony F. Fell, University of Brad- ford, and M. Howard Williams, Heriot-Watt University. Multidimensional Time-Correlated Single Photon Counting Fluorescence Lifetime Measurements, Gregory Nelson, Ten- nessee Eastman Company, Gabor Patonay, Georgia State University and Isiah M. Warner, Emory University. Multicom- ponent Determinations of Polycyclic Aromatic Hydrocarbons Using Synchronous Excitation Phase-Resolved Fluorometry, Linda B. McGown, Duke University and Kasem Nithipatikom, Texas Tech University. Fiber Optic Chemical Fluorosensors, Ming-Ren S. Fuh, 3M Company and Gary D. Christain, Uni- versity of Washington, Seattle. A New Fiber-Optic-Based Phase-Resolved Phosphorescence Spectrometer, Frank V. Bright, State University of New York at Buffalo, Curtis A. Mon- nig, University of North Carolina and Gary M. Hieftje, Indiana University. Organized Assemblies in Analytical Chemilumi- nescence Spectroscopy: An Overview, Willie L. Hinze, N. Srinivasan, Thuy Kim Smith, Wake Forest University, Skukuro Igarashi and Hitoshi Hosino, Tohoku University, Japan. Index.


Volume 2, 1993, 156 pp. $97.50 ISBN 1-55938-327-5

CONTENTS: Preface, Isiah M. Warner, Emory University. Preface, Isiah M. Warner, Emory University and Linda B. McGown, Duke University. Spectroscopic Studies in Orga- nized Media: An Overview, A. Munoz de la Pena, Uiniversity of Extremandura, T.T. Ndou, and I.M. Warner, Emory Univer- sity. Counterions and the Properties of Ionic Micelles, A.L. Un- derwood, Emory University and W.W. Anacker, Montana State University. Study of Solute-Micele Interactions by Laser Spectroscopic Techniques, M.J. Wirth, and S.-H. Chou, Uni- versity of Delaware. Fluoresence Probe Studies in Trihydroxy Bile Salts, Steven M. Meyerhoffer, Merck & Co. and Linda B. McGown, Duke University. Spectroscopic Studies in Cyclo- dextrin Solutions, Isian M. Warner and Jodi M. Schuette, Em- ory University. Dynamic Fluorescence Investigations in - Cyclodextrin Organized Media, Frank V. Bright, Gino C. Cate- na, Jingfan Huang, JoAnn Zagrobelny, and Jing Zhang, State University of New York at Buffalo. Interactions in Solid Matrix -Cyclodextrin Luminescence Analysis, Robert J. Hurtubise and Marsha D. Richmond, University of Wyoming. Crown Ethers Mediated Sensitive and Selective Determination of Lanthanide Ions, Chieu D. Tran, Marquette University. Index.

FACULTY/PROFESSIONAL discounts are available in the U.S. and Canada at a rate of 40% off the list price when prepaid by personal check or credit card and ordered directly from the publisher.


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Advances in Gas Phase Ion Chemistry

Edited by N/gel Adams, and Lucia M. Babcock, Department of Chemistry, The University of Georgia

Advances in Gas Phase Ion Chemistry is different from other ion chemistry series in that it focuses on reviews of the authors own work rather than give a general review of the research area. This allows for presentation of some current work in a timely fashion which marks the unique nature of this series. Emphasis is placed on gas phase ion chemistry in its broadest sense to include ion neutral, ion electron, and ion-ion reactions. These reaction processes span the various disci- plines of chemistry and include some of those in physics. Within this scope, both experimental and theoretical contributions are included which deal with a wide variety of areas ranging from fundamental interactions to applications in real media such as interstellar gas clouds and plasmas used in the etching of semiconductors. The authors are scientists who are leaders in their fields and the series will therefore provide an up-to-date analysis of topics of current importance. This series is suitable for researchers and graduate students working in ion chemistry and related fields and will be an invaluable reference for years to come. The contributions to the series em- body the wealth of molecular information that can be obtained by studying chemical reactions between ions, electrons and neutrals in the gas phase.

Volume 1, 1992, 329 pp. $97.50 ISBN 1-55938-331-3

CONTENTS: Preface, N/gel Adams and Lucia M. Babcock. Flow Tube Studies of Small Isomeric Ions, Murray J. McEwan. Anion Molecule Experiments: Reactive Intermediates and Mechanistic Organic Chemistry, Joseph J. Grabowski. Ther- mochemical Measurements by Guided Ion Beam Mass Spec- trometry, Peter W. Armentrout. Photoelectron Spectroscopy of Molecular Anions, Kent M. Ervin and W. Carl Lineberger. Ion Chemistry at Extremely Low Temperatures: A Free Jet Expansion Approach, Mark A. Smith and Michael Hawley. Theoretical Studies of Hypervalent Silicon Anions, Mark S. Gordon, Larry P. Davis and Larry W. Burggraf. Chemistry Ini- tiated by Atomic Silicon Ions, Diethard K. Bohme. Spectro- scopic Determination of the Products of Electron-Ion Recombination, N/gel G. Adams. Index.

advances in atomic spectroscopy, volume 2

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