trimer causes hydroxyl groups to replace the bridging acetate groups. This hydroxyl group substition causes some of the bridging acetate groups to form bidentate acetate groups. Hydroxyl substitution also converts the cyclic structure to a linear trimer with two bridging ace- tate groups, four bridging hydroxyl groups, and a biden- tate acetate group on each end. A neutral complex forms when a third end-site is occupied by a unidentate hy- droxyl group. Additional hydroxyl addition causes pre- cipitation of the complex. In all of these solutions, de- pending on the pH, there is an equilibrium between trimeric cyclic and trimeric linear chromium acetate species with different degrees of hydroxyl substitution.

ACKNOWLEDGMENTS

The author expresses his appreciation to the management of Mar- athon Oil Company and USX for support of this work. He also thanks J. Ivan Legg of Auburn University for helpful discussions; Joel Gray,

Coors Analytical Laboratory, Golden, CO, for obtaining the 2H NMR spectra; and J. Carter Cook, VG Instruments, Savannah, GA, for the FAB mass spectrometer data.

1. R. D. Sydansk and P. A. Argabright, U.S. Patent 4, 683, 949 (Aug. 4, 1987).

2. B. N. Figgis and G. B. Robertson, Nature (London), 694 (1965). 3. M. K. Johnson, D. B. Powell, and R. D. Cannon, Spectrochimica

Acta 67A, 995 (1981). 4. A. Earnshaw, B. N. Figgis, and J. Lewis, J. Chem. Soc. (A), 1656

(1966). 5. L. Dubicki and R. L. Martin, Aust. J. Chem. 22, 701 (1969). 6. H. Erdman, Das Leder 14, 249 (1963). 7. J. E. Tackett, Appl. Spectrosc. 43, 483 (1989). 8. C. A. Green, H. Place, R. D. Willett, and J. I. Legg, Inorg. Chem.

25, 4672 (1986). 9. J. I. Legg, unpublished results.

10. M. J. Udy, Chromium (Reinhold, New York, 1956), p. 288. 11. F. A. Long, J. Am. Chem. Soc. 61, 570 (1939). 12. M. T. Beck and I. Seres, Chem. Analyst 50, 48 (1961).

Characterization of a High-Efficiency Helium Microwave-Induced Plasma as an Atomization Source for Atomic Spectrometric Analysis

L A R R Y D. P E R K I N S and G A R Y L. L O N G * Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061

Characterization studies of a He high-efficiency microwave-induced plas- ma, He-HEMIP, utilizing direct sample introduction with pneumatic nebulization for atomic emission and atomic fluorescence spectrometry are presented. These studies include diagnostic measurements and an- alytical characterization of the 150-W He-HEMIP. Diagnostic mea- surements include excitation temperatures with the use of aqueous and organic nebulized thermometric species, electron number densities, and ionization temperatures for the plasma. The effect of sample uptake rate on the emission intensity is investigated. Ionization interferences are minimal, and phosphate interferences were found not to occur. In ad- dition, the He-HEMIP is characterized as an atom source for metals and nonmetals with the use of atomic emission spectrometry and atomic fluorescence spectrometry. With AES, detection limits for metals and nonmetals are in the sub-ppm range. With AFS, detection limits for metals were determined to be in the low to sub-ppb range and were found to be not statistically different from those reported for HCL-ICP-AFS. Linear ranges for AES and AFS ranged from four up to five and one- half orders of concentrative magnitude.

Index Headings: Atomic emission spectrometry; Atomic fluorescence spectrometry; Microwave-induced plasma; Helium plasma; Solution nebulization; Metal determinations; Nonmetal determinations.

INTRODUCTION

The microwave-induced plasma, MIP, has received considerable attention as an excitation source for atomic

Received 4 October 1988. * Author to whom correspondence should be sent.

emission spectrometry since the introduction of the Beenakker cavity in 1976.1 The attractiveness of this cavity lies in its ability to operate at low power and atmospheric pressure, as well as to support plasmas of various gases. These gases include argon, helium, nitro- gen, and air. Although all of these support gases have found their analytical utility, helium seems to be of par- ticular importance because of its high ionization poten- tial (24.6 eV). In addition, helium offers a simpler overall background spectrum, high metastable energy, and a greater optical transparency at shorter wavelengths, as well as the capability of exciting nonmetals and halogens. Primarily, gas-phase sample introduction into the heli- um microwave plasma is the common practice. Over the past ten years, few reports have appeared on the use of direct nebulization of aqueous samples for the deter- mination of metals and nonmetals in the helium micro- wave-induced plasma. T M This exiguity of studies with direct nebulization into the helium MIP focuses on the fact that the original Beenakker cavity does not afford for sufficient desolvation and excitation of the liquid samples introduced. However, direct nebulization of metal and nonmetal solutions into the helium plasma has not proven unattainable; using the Caruso type "two-stub internally tuned" cavity, Carnahan and co-workers in- troduced aqueous chlorine, bromine, and iodine samples using helium as the support gas via direct nebulization. Power levels ranged from 500 W to several kilowatts and

Volume 43, Number 3, 1989 0003-7028/89/4303-049952.00/0 APPLIED SPECTROSCOPY 499 © 1989 Society for Applied Spectroscopy

TABLE I. Instrumentation.

Component Model /s ize Manufacturer

Microwave cavity High-efficiency Laboratory built TM01o

Generator HI-2450 Holiday Industries Edinia, MN

Discharge tube Tangential Laboratory built

Coaxial cable RG 214 Times Fiber Comm. Wallingford, CT

Monochromator 01-512 PTI 0.25 m Princeton, NJ

PMT R955 Hamamatsdu Corp. Bridgewater, NJ

Nebulizer Concentric J.C. Meinhard TR-50-C2 Santa Anna, CA

Spray chamber Scott Laboratory built

Chopper Model 125A EG & G Princeton, NJ

Lock-in Model 5101 EG & G Princeton, NJ

Lens [/3, Suprasil Oriel Corp. Stratford, CT

Computer Apple IIe Apple Computer, Inc. Cupertino, CA

Flow controllers MM3 Air Products Allentown, PA

plasma gas flow rates up to 50 L/min. 3,4,5 Detect ion limits employing UV-visible lines were obta ined at and below one par t per million.

This use of large power levels and gas flow rates clearly diminishes the low power advantages of the MIP (i.e., simplicity, versatility, and low cost). Fur thermore , the use of high power levels requires cooling (due to excessive heat ing of the cavity) and the util ization of special dis- charge tubes (changes up to one every 0.7 h) at the ex- pense of increased background emission, microwave leakage, and cost.

This repor t focuses on the use of a helium high-effi- ciency MIP, H e - H E M I P , for the de terminat ion of metals and nonmetals using direct nebulization. The H E M I P was developed by Matus e t al. as a modification of the original Beenakker cavity tha t precludes the use of ex- ternal matching devices. 6 Instead, a capacitive an tenna probe is used for proper coupling. The capacitive of the probe is adjusted by varying the surface area at the end of the probe. Coupling is fur ther adjusted by sliding the probe along the radial face of the cavity to match the capacitive and inductive coupling of power to match the impedance of the microwave generator. The power trans- fer efficiency was de te rmined to be greater than 90 %.

Using this cavity, Long and Perkins were able to sus- tain a centered and stable 1 L /min Ar plasma at 36 W while nebulizing 1 mL/min of water wi thout the use of desolvation apparatus. 7 In another study, Perkins and Long evaluated the use of this cavity as an atomizat ion cell for atomic fluorescence spectrometry, using hollow cathode lamps and the Xe-arc as excitat ion sources, s Fur thermore , the H E M I P tends to be more amenable to the in t roduct ion of direct nebulizat ion of aqueous and organic solutions, when compared to Beenakker cavities in t roduced to date. 7 In contradict ion to tha t observed by

TABLE II. Operational parameters.

AES AFS

Forward power 150 W 150 W Reflected power <10 W <10 W Observation height a 2-3 mm (+0.5 mm) b 8 mm Nebulizer uptake 0.46 mL/min 0.46 mL/min Aux. He flow 0 mL/min 0 mL/min Total He flow 1 L/min 1 L/min Probe penetration 96 % 96 % Time constant 3 s 3 s HCL power -- 10-25 mA

a Radial height above the top of the cavity. h Axial distance from the center of the cavity.

Cull and Carnahan, ~ nonmeta l emission using direct in- t roduct ion into the H e - H E M I P for C1 determinat ions was observed at 90 W.

Metal species were de te rmined in the UV-visible re- gion with the use of atomic emission and atomic fluo- rescence spectrometry. Nonmeta l emission spectra in the UV-visible were also determined. No desolvation appa- ratus was employed.

Measurements of exci tat ion tempera tures , electron number densities, detect ion limits for AES and AFS, in tere lement effects, l inear ranges, plasma profiles, and sample uptake studies are discussed.

E X P E R I M E N T A L

Reagents . All chemicals used were analytical reagent grade. Water was distilled and deionized. Stock solutions of all aqueous metals and nonmetals were purchased as 1000 ppm (Buck Scientific, Inc.) or p repared following s tandard procedures. The source of Fe for organic solvent de terminat ions was prepared by dissolving ferrocene in xylenes to obtain a 1000-ppm solution of Fe. Volumetr ic dilutions of these solutions were made to obtain the de- sired concentrat ions. The plasma gas used was Airco an- alytical-grade helium.

Instrumentation. The experimental atomic emission and atomic fluorescence systems used in this work have been described in previous papers 7,s and are summarized in Table I. Modifications to the previous sytems include the use of a 0.25-m monochromator , 300-W generator, and high-efficiency red-sensit ive P M T . The op t imum oper- ational parameters for AES and AFS are listed in Table II. A 25-#m slit width providing a spectral bandpass of 1.5 A was used, unless otherwise stated.

All studies were conducted with the plasma sustained by the nebulizer gas alone (1 L/min). No auxiliary flow was employed. In operation, the H e - H E M I P is self-ig- niting. On those occasions where the plasma did not self- ignite, a small tungs ten wire, a t tached to a rubber po- liceman, was inserted within the cavity to inductively heat the wire to cause a seed of the He gas.

Studies of meta l atomic emission and atomic fluores- cence were conducted with the plasma in the radial po- sition (side-on), unless otherwise stated. An axial obser- vation position (end-on) was used for nonmeta l studies.

D a t a Presentat ion. The da ta presented in this repor t are shown as background-corrected values. For AES and AFS profiles, the background signal was subt rac ted by the computer f rom the analyte signals. Working curve and in tere lement effect plots were similarly t reated.

500 Volume 43, Number 3, 1989

TABLE IIl. Excitation temperatures by the slope method.

Thermometric species Temperature (K)

He 6100 He + H20 5800 Fe (aqueous) 5600 (axial)

5800 (radial) Fe (Ferrocene) 5100

Data sets for temperatures and electron number den- sity studies represent an average of at least five repeated experiments.

Limits of Detection. The limits of detection are all calculated in accordance with IUPAC guidelines. 9 For each limit of detection calculation, 20 background read- ings were taken. The analytical sensitivities were cal- culated from the working curves of the element, which spanned at least two orders of concentrative magnitude. The k value used for all calculations was 2, rather than 3. This lower k value was used only for the purpose of allowing comparison with the existing literature.

Electronic Excitation and Ionization Temperature. The electronic excitation temperature of the He-HEMIP was determined from the spectral emission intensities of He and Fe atom lines with the use of a 20-#m slit width and employment of the slope method. 11 All constants used were taken from Ref. 10. An iron solution of 1000 ppm was introduced into the plasma, and the relative inten- sities of the following iron atom lines were measured: 370.5, 372.2, 373.5, 374.8, 375.8, 382.0, 385.6, 389.6, and 390.0 nm. The helium emission atom lines employed in this study were 388.87, 447.48, 471.31, and 501.57 nm.

Ionization Temperature. The spatial ionization tem- perature, Tio,, was determined from the relative emission intensities of the cadmium atom (228.8 nm) and cad- mium ion (226.5 nm) lines, with the use of the same position and resolution as used for the measurements for the excitation temperature and with calculations based on the Saha-Eggert relationship22

Electron Number Density. The method used for the determination of the relative electron number densities was based on the measurement of the Stark broadening of the H~ Balmer line at 486.1 nm. 13 The spatially inte- grated emission intensities and half-widths of the hy- drogen beta line were measured for the He plasma with and without the nebulization of 0.46 mL/min of water. The number density values are thus an average for the volume of the plasma viewed. A 20-#m slit width was employed.

RESULTS AND DISCUSSION

Excitation Temperature. The excitation temperature was determined with the use of several thermometric species (He with and without water and aqueous and organic Fe) and are tabulated in Table III. The temper- atures varied from 5100 (organic Fe nebulization) to 6100 K (radiance of He lines). It is interesting to note that the excitation temperature with the nebulization of water alone into the plasma and the introduction of iron as the thermometric species resulted in values that were not statistically different. The random errors for the esti- mation of the excitation temperature are less than 90 K. The decrease in the excitation temperature for organic

15 n e - X 10

3.5

3

2.5

2

1.5

1

÷ + +

+ + + +

+ ~ /~

-3 -2 -1 0 1 2 3

Position (mm)

FIG. 1. Spatially integrated electron number density profile: (+) rep- resents data taken without the nebulization of water; (A) represents data with the nebulization of water.

nebulization into the plasma, although not experimen- tally confirmed, can be attributed to the absorption of energy by molecular species (C2, CN, CO, etc.) from the plasma. However, it should be noted that this effect of lowering of the excitation temperature by the nebuli- zation of organic liquids into the plasma is less than that found for a 1.75-kW ICP, where a reduction in the ex- citation by the nebulization of organics was found to be approximately 1500 K. 14

Excitation and Ionization Temperature. Spatially in- tegrated intensities of the cadmium emission atom (228.8 nm) and cadmium ion (226.5 nm) lines were measured for the determination of the ionization temperature. Re- sults of the ion/atom line ratios were tabulated and sub- stituted into the Saha-Eggert relationship, and Tip, was calculated according to an iterative process. The Tip . as determined by the Saha-Eggert equation was 6200 K.

Electron Number Density. Figure i depicts the ne- pro- files across the face of the cavity (with and without the nebulization of water at 0.46 mL/min) as determined by the half-width from the broadening of the H~ Balmer line at 486.1 nm. These number density determinations are, at best, an approximation due to the resolution of the monochromator employed; however, we feel that these relative values are useful in illustrating an ne- trend with the introduction of samples into the plasma. The electron number density profile is demonstrated to be essentially fiat, with increases at 0.5-1 mm without the nebulization of water. With the introduction of water by nebulization, the number density profile continues to increase above 0.5 mm, peaks at 1 mm, and basically levels off at 1.5- 2.5 mm. This increase in the number density with water nebulization may be the result of the ionization of hy- drogen and oxygen species in the plasma.

Although the plasma visually appeared to be sym- metric, the number density profile suggested that the plasma is asymmetrical. This asymmetry is attributed to errors in the flow pattern of the laboratory-constructed torch.

Effect of Sample Uptake Rate. The effect of the sample uptake rate on atomic spectrometric signals is a very important parameter in system optimization, due to changes in solution viscosities. The effect of varying the

A P P L I E D S P E C T R O S C O P Y 501

Relative Intensity 70

60

5O

40'

3O

20

10

0 .17

FIG. 2. t e n s i t y .

@ @

r i i .37 .60 .80 1.00

Sample Uptake Rate (mL/min) E f f e c t o f t h e s a m p l e u p t a k e r a t e on t h e Ca(II) e m i s s i o n in-

aqueous solution uptake rate on the Ca(II) emission sig- nal at 393.6 nm is shown in Fig. 2 for a 50-ppm solution. The sample uptake rate was controlled by a peristaltic pump (Cole Palmer, Chicago, IL).

There appears to be little variation in the emission signal at a solution uptake rate of 0.17 mL/min up to 0.60 mL/min. Above 0.160 mL/min, the signal rises by 17 % and levels off between 0.80 mL/min and 1.0 mL/ min. These data suggested that there is very little ad- vantage to be had in using a peristaltic pump, compared to allowing the natural uptake rate to occur (0.46 mL/ min), using the system employed.

Organic Nebulization. It was determined that the high- efficiency helium MIP can operate continuously (solu- tions with concentrations to 1000 ppm Ferrocene in xy- lenes; concentrations above this were not studied) during direct nebulization of organic samples. Carbon deposits on the torch wall were minimal and did not disturb the plasma.

The resistance of the plasma to support of organic nebulization at 0.46 mL/min without extinction by the organic aerosol indicated the He-HEMIP's robustness, as compared with that of other MIPs which have been reported to have been used for organic vapor sample introduction for chromatography--where samples had to be vented, cooled, or heated to prevent extinguishing of the plasma25

Profiles. Figure 3 depicts intensity profiles for AES with the use of the optimized conditions outlined in Ta-

Relative Intensity 800

700

600(

5OO

400

300

200

100

0 0

FIG. 3.

o

o

©

o o o

. . . . . . . . . . o 9

2 4 6 8 10 12 14 16 18 20

Position (mm) T h e rad ia l e m i s s i o n prof i le for a 50-ppm Ca so lu t ion . Zero

m i l l i m e t e r r e p r e s e n t s f lush w i t h t h e t o p o f t h e cav i ty .

Relative Intensity 90

8O

7O

6O

5O

40

30

20, -3

o

-2.5

o

0

r i i i I i i r <~ -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Position (mm) FIG. 4. T h e a x i a l e m i s s i o n prof i le o f a 50-ppm I so lu t ion . Z e r o rep- r e s e n t s the c e n t e r o f t h e cav i ty .

ble II and a 6-mm-i.d. discharge tube. For all AES studies for metal determinations, the optimum height was 2-3 mm above the top of the cavity. The optimum obser- vation height for AFS measurements shifted to 8 mm above the top of the cavity. This shift in the AFS max- imum intensity above that of the AES observation zone is consistent with earlier studies performed with the Ar- HEMIP. 7 In this higher region of the plasma there exists a decrease in plasma flicker and spectral background. In addition, the analyte species have experienced a longer residence time, resulting in a more complete molecular dissociation of the original species.

Nonmetal determinations, however, were performed in the axial mode (Fig. 4). Generally, in the axial mode measurements were made at +0.5 mm from the center of the cavity.

Linearity. The linearity ranges obtained for metal de- terminations for AES (radial mode) typically span from four to five orders of concentrative magnitude, while non- metal AES determinations (axial mode) showed slightly

T A B L E IV. M I P - A E S and H C L - M I P - A F S m e t a l de tec t ion l imi t s in ppb (k = 2).

HCL- HCL- MIP- M I P - MIP- MIP-

E l e m e n t ), (nm) A E S a A E S b A F S a A F S c

A g 328.1 7 120 10 40 A1 396.2 130 1400 80 700 Ba 553.5 - - 180 8 20 Li 570.8 - - 43 1.2 20 Ca(I) 422.7 5 40 1.7 20 Co 240.7 50 1800 19 100 Cr 357.9 70 8000 40 2000 F e 248.3 30 650 30 600 K 766.5 - - 24 1 20 Mg 285.2 6 630 1.3 20 Mn 279.5 11 6900 - - 500 N a 589.0 1 2 0.1 10 Sr 460.7 11 25 5 20 Zn 213.9 8 420 1.2 40

" T h i s work . b See Ref. 7. ': See Ref. 8.

502 Volume 43, Number 3, 1989

T A B L E V. M I P - A E S nonmetal limits of detection in ppm (k = 2).

E l e m e n t ~ (nm) LOD" L O D b

Br 478.5 2 60 C1 479.5 0.8 2 I 206.2 1 7 P 213.6 0.4 - - S 217.1 1.2 - -

a T h i s work. b See Ref. 4.

lower linear plots. However, linear ranges for AFS span up to five and one-half orders of concentrative magni- tude.

Limits of Detection. The limits of detection of metals and nonmetals with the use of the He-HEMIP are listed in Tables IV and V. These values were obtained for AES and AFS with the use of direct sample introduction via a Meinhard nebulizer and were calculated according to IUPAC standards. It should be noted that a k = 2 was used, instead of the prescribed value of 3 as recom- mended by IUPAC. This substitution was done in order that the experimental detection limits could be com- pared with existing limits of detection for MIP and ICP systems.

The superior atomization characteristics of the He- HEMIP are compared with those of the Ar-HEMIP in Table IV. The Ar data are from previous research done with this cavity. 7 In all cases, except for Na, the use of He resulted in a substantial decrease (factor of 8 to 100) in the detection limit value. This trend is also observed for AFS data, where the use of He for HCL-MIP-AFS resulted in a decrease in the detection limit from a factor of 4 up to 50. It should be noted that the current He- HEMIP limits of detection are not statistically different from those published for HCL-ICP-AFS. 16

Nonmetal detection limits are listed in Table II, where the results of the He-HEMIP values are compared to data from Michlewicz and Carnahan, who used a 21 L/min He plasma and an applied power of 500 W. 5 In addition, a MAK pneumatic nebulizer was employed. In the case of the He-HEMIP, the detection limits are significantly lower than those reported for the high-power MIP cavity. Also reported are detection limits for S and P. These detection limit values, as far as the authors know, are the first reported for helium MIP-AES with the use of

Relative Intensity 6

5.5

5

4.5

4

3.5

3 0

FIG. 5. so lu t ion .

J r ~ . _ _ 1 i

.01 .1 1 10 100 1000

Sodium Concentration (ppm) The effect of Na on the emission signal of a 10-ppm calcium

Relativelntensity 8

7.5

7>

6.5

6

5.5

5

4.5

4

X X X X X

i I i _ _ i

.01 .1 1 10 100 1000

Sodium Concentration (ppm) FIG. 6. T h e effect of N a on t h e f luo rescence s i gna l of a 1 0 - p p m c a l c i u m so lu t ion .

direct nebulization without desolvation and a Beenakker modified cavity.

Interelement Effects. The effect of an easily ionized element (EIE) on the Ca(I) atomic emission and fluo- rescence signal at 422.7 nm is shown in Figs. 5 and 6. With the use of the conditions outlined in Table II and a 10-ppm Ca solution, no discernible effect of the inter- ferent was observed to occur for AFS. However, a slight enhancement was observed for AES while a 1000-ppm solution of the interferent, Na, was introduced. The EIE effect in this He-HEMIP is significantly less than that observed with the Ar-HEMIP, where an increase in sig- nal of 300% was observed for AES and 100% for AFS with a 1000-ppm Na solution. 7,s

Importantly, as shown in Figs. 7 and 8, the classical phosphate interference was not observed for AES, nor for AFS. This suggests that the He-HEMIP is a robust atom cell that is able to efficiently cause complete dis- sociation of gaseous refractory molecules into free vapor- phase atoms.

CONCLUSIONS

In this work, the advantage of using He vs. Ar as the plasma gas for the high-efficiency MIP with direct aqueous sample introduction for metal and nonmetal determinations by AES and AFS has been demonstrated. Not only are detection limits superior to those of Ar- HEMIP, but the values obtained for He-HEMIP-AFS are now equivalent to those of HCL-ICP-AFS. The ro-

Relative Intensity 6r

5.5

5

4.5

4

3.5

3

FIG. 7.

J _ _ J

1 10 100 1000

Phosphate/Calcium Molar Ratio The effect of P 0 4 a - o n a Ca emission signal (Ca concentration

= 10 p p m ) .

APPLIED SPECTROSCOPY 50:3

Relative Intensity 8

7.5

7>

6.5

6

5.5

5

4.5

4

FIG. 8. centration = 10 ppm).

X X X

i _ _ 4 L ~

1 10 100 1000

Phosphate/Calcium Molar Ratio The effect of P O 4 3- o n a Ca 2+ fluorescence signal ( C a 2÷ c o n -

bustness of He-HEMIP over Ar-HEMIP has been shown by the larger excitation temperatures, the electron num- ber densities, and the freedom from interelement effects.

ACKNOWLEDGMENTS

This work was supported by the Department of the Interior's Mineral Institutes Program administered by the Bureau of Mines under allot- ment Grant #Gl174151. The authors would like to express their ap- preciation to Phillips Petroleum Co. for the donation of the microwave generator, to Dr. Charles Boss at North Carolina State University for

stimulating discussions on the He-HEMIP, and to Andrew Mollick for construction of the microwave torches.

1. C. I. M. Beenakker, Spectrochim. Acta 31B, 483 (1976). 2. L.D. Perkins and G. L. Long, Fourteenth Meeting of the Federation

of Analytical Chemistry and Spectroscopic Studies, Detroit (1987), Abstract 40.

3. K.G. Michlewicz and J. W. Carnahan, Anal. Chem. 57,1092 (1985). 4. K. G. Michlewicz and J. W. Carnahan, Anal. Chem. 58, 3122 (1986). 5. K. B. Cull and J. W. Carnahan, Appl. Spectrosc. 42, 1061 (1988). 6. L. G. Matus, C. B. Boss, and A. N. Riddle, Rev. Sci. Instrum. 54,

1667 (1983). 7. G. L. Long and L. D. Perkins, Appl. Spectrosc. 41, 980 (1987). 8. L. D. Perkins and G. L. Long, Appl. Spectrosc. 42, 1285 (1988). 9. "Nomenclature, Symbols, Units, and Their Usage in Spectrochem-

ical Analysis II," Spectrochim. Acta 33B, 242 (1978). 10. W. L. Wiese and G. A. Martin, Wavelengths and Transition Prob-

abilities for Atoms and Atomic Ions, NBS Monograph NSRDS 68 (National Bureau of Standards, Gaithersburg, Maryland, 1980), Part II, p. 359.

11. D.J. Kalnickicky, V. A. Fassel, and R. N. Kinseley, Appl. Spectrosc. 31, 137 (1977).

12. M. W. Blades, B. L. Caughlin, Z. H. Walker, and L. L. Burton, Prog. Analyt. Spectrosc. 10, 57 (1987).

13. H.R. Griem, Plasma Spectroscopy (McGraw Hill, New York, 1964). 14. M. W. Blades and B. L. Caughlin, Spectrochim. Acta 40B, 579

(1985). 15. J. C. Van Loon, Anal. Chem. 51, 1139A (1979). 16. D. R. Demers, D. A. Busch, and C. D. Allemand, Am. Lab. 42, No.

3, 167 (1982).

Effect of Atomization Surface on the Quantitation of Vanadium by Electrothermal Atomization Atomic Absorption Spectrometry

PAUL PANTANO and J O S E P H SNEDDON* Department of Chemistry, California State Polytechnic University, Pomona, California 91768 (P.P.); and Department of Chemistry, University of Lowell, Lowell, Massachusetts 01854 (J.S.)

The effect of four different atomization surfaces (uncoated graphite, pyrolitically coated graphite, continuously in situ coated graphite, and molybdenum coated graphite) on the quantitation of vanadium by elec- trothermal atomization atomic absorption spectrometry was investigat- ed. For the four different surfaces, tube lifetimes ranged from a high of 410 firings (uncoated graphite) to a low of 230 firings (molybdenum coated graphite), mass changes in the range from 5.6 to 7.2% were found, and the peak shape from the uncoated and pyrolytically graphite surface approached a good degree of symmetry. The single addition of several matrix modifiers to an uncoated graphite surface affected the signal over a range of - 106.5% for ammonium molybdate to + 12.3% for magnesium nitrate. A combined matrix modifier improved the signal from all four surfaces in the range from 12.8 to 34.0%. Detection limits ranged from 2 #g/L for the pyrolytic surface to 19/~g/L for the molybdenum coated surface. Electron micrographs of the four surfaces at various stages of the tube lifetime showed the deterioration of the surface with increasing u s e .

Index Headings: Analysis for vanadium; Atomic emission spectroscopy; Instrumentation, electrothermal atomizer.

Received 25 October 1988. * Author to whom correspondence should be sent.

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

E l e c t r o t h e r m a l a t o m i z a t i o n a t o m i c a b s o r p t i o n spec - t r o m e t r y ( E T A A S ) is a w i d e l y u s e d a n a l y t i c a l t e c h n i q u e for t r a c e a n d u l t r a t r a c e m e t a l d e t e r m i n a t i o n in c o m p l e x s a m p l e s . T h e m a j o r a d v a n t a g e over o t h e r a t o m i c spec - t r o s c o p i c t e c h n i q u e s is in m i c r o v o l u m e a n d m i c r o g r a m s a m p l i n g , as wel l as low d e t e c t i o n l i m i t s ( sub ~g /L for m a n y m e t a l s ) . T h e m a j o r d i s a d v a n t a g e ove r o t h e r a t o m i c s p e c t r o s c o p i c t e c h n i q u e s is t h a t t h e t e c h n i q u e is p r o n e to i n t e r f e r e n c e s , p a r t i c u l a r l y in c o m p l e x s a m p l e s con- t a i n i n g a lka l i a n d a l k a l i n e e a r t h sa l t s . T h e s t u d y o f t h i s i n t e r f e r e n c e h a s r e c e i v e d c o n s i d e r a b l e a t t e n t i o n in t h e l i t e r a t u r e ; t h e i n t e r f e r e n c e has b e e n a t t r i b u t e d to t h e f o r m a t i o n o f m o l e c u l a r c h l o r i d e s on t h e a t o m i z e r sur - face, 1,2 c o v o l a t i l i z a t i o n of t h e m e t a l w i t h a m o r e vo l a t i l e m a t r i x , 3,4 a n d t r a p p i n g of t h e m a t r i x , w h i c h is t h e n ex- p e l l e d f r o m t h e a t o m i z e r b y t h e e x p a n d i n g gases on r a p i d h e a t i n g 2 ,s I r r e s p e c t i v e of t h e cause of t h e i n t e r f e r e n c e , i t is obv ious t h a t t h e a t o m i z a t i o n su r f ace p l a y s a m a j o r ro le in t h e i n t e r f e r e n c e . T h e a t o m i z a t i o n su r f ace wil l a l so

504 Volume 43, Number 3, 1989 0003-702S/89/4303-050452.00/0 APPLIED SPECTROSCOPY © 1989 Society for Applied Spectroscopy