JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JUNE 1993, VOL. 8 659

Performance of a Modular Thermospray Interface for Signal Enhancement in Flame Atomic Absorption Spectrometry Coupled On-line to Flow Injection or Liquid Chromatography

Erik H. Larsen* and Jean-Simon Blaist McGill University, Macdonald Campus, Department of Food Science and Agricultural Chemistry, 27 7 7 7 Lakeshore Road, Ste-Anne de Bellevue, Quebec, Canada H9X 3V9

A modular thermospray interface for improving sample transport efficiency in conventional flame atomic absorption spectrometry (AAS) burners has been developed and optimized for flow injection (FI)-AAS and high- performance liquid chromatography (HPLC)-AAS applications. The installation of this interface required no modification of the conventional flame AAS burner. The thermospray unit consisted of two separate thermoelectric elements which heated a 50 pm silica capillary carrying the mobile phase and the nebulizer gas (air), respectively. The capillary and the nebulizer gas were guided through their respective heating elements using inexpensive quartz tubes-Swagelok fittings assemblies. The capillary tube was inserted into the sample aspiration needle of the heated nebulizer nozzle and positioned in the mixing chamber where the overheated mobile phase expanded as a fine thermospray. Under optimum conditions, the thermospray unit completely nebulized 1 ml min I-' of the analyte solution. The unit was chemically compatible with water as the flow injection (FI) carrier and 0.1 mol I-' ammonium carbonate, 0.1 mol I-' pyridinium ion or 10-100 mmol-1 TRIS-HCI [TRIS = tris(hydroxymethyl)methylamine] buffer as HPLC mobile phases. In the FI-thermospray-AAS mode the signal-to-noise ratios were improved 6-8 times over conventional FI sample introduction, providing signal enhancements that were comparable to figures obtained with more complex and costly systems. An additional contribution to signal enhancement by the heated nebulizer gas (air) was attributed to desolvation of the thermospray aerosol in the mixing chamber. The absolute limits of detection were 1.9 ng of cadmium, 8.5 ng of copper and 27 ng of lead. The interface was also applied for the speciation of cadmium-metallothionein isoforms I and II in the HPLC-AAS mode. Keywords: Flow injection; high-performance liquid chromatography; flame atomic absorption spectrometry; thermospray interface; speciation

By combining the separation power of high-performance liquid chromatography (HPLC) with element specific atomic spectrometric detection, information on molecular forms of a metal or metalloid (chemical species) can be obtained. The interfacing of liquid chromatographic equip- ment with atomic spectrometric detectors for chemical speciation has therefore developed into an important field of research in analytical chemistry.' In atomic spectrometry, the analyte solution is normally introduced by continuous aspiration. However, when only small sample volumes are available and no speciation information is needed, sample introduction can be carried out by flow injection (FI).273 Ifan HPLC pump is used to deliver the mobile phase, the addition of a chromatographic column converts the FI system into a dedicated metal speciation instrument. Thus, the FI and the HPLC modes possess similar characteristics in the way the sample is introduced into the detector.

Atomic absorption spectrometry (AAS) has proved its potential for FI or HPLC detection with the introduction of devices such as a thermospray microatomizeI.4 or a thermo- chemical hydride generator-diffusion flame a t o m i ~ e r . ~ ~ ~ These devices improved the efficiency of the nebulization and atomization processes, and reduced spectral interfer- ences. Post-column derivatization systems have been de- signed to convert the analytes as volatile hydrides or ethylates with subsequent detection in quartz atomizer^.^.^ However, these approaches were applicable only to rela- tively few heavy metals and metalloids. More versatile HPLC-AAS combinations included off-line sampling of analyte followed by discrete detection with electrothermal AAS9 or sequential sampling-drying flame AAS.I0

More conventionally, the transfer of the analyte solution

*Visiting scientist from The National Food Agency of Denmark,

tTo whom correspondence should be addressed. Morkhoj Bygade 19, DK-2860 Soborg, Denmark.

from the FI or the HPLC system to the atomic spectrome- tric detector takes place through a standard pneumatic nebulizer-mixing chamber assembly which inherently has a low efficiency. For example, the nebulization efficiency of inductively coupled plasma (ICP) techniques is only 1 -2Y0 . l~ Improved efficiency therefore appears attractive since it leads to higher analytical sensitivity. However, at the same time, it causes an increased solvent load on the argon plasma, which might be extinguished owing to quenching of the ionization process. In comparison, the air-acetylene flame of the AAS is relatively insensitive to an increased solvent load. The nebulization efficiency of standard pneumatic nebulizers used for AAS is 5- 10%. By running the nebulizer under 'starved conditions' a much higher nebulization efficiency of 32% has been achieved for waterI2 with an uptake rate of 1 ml min-I.

In order to increase the mass transfer of analyte in the AAS nebulizer, it is essential to shift the droplet-size distribution of the nebulized sample to lower va1ues.l' This has been achieved using the thermospray effect. The analyte solution is pumped through a heated capillary tube and, at the outlet, a high velocity aerosol and vapour jet is formed. This mode of aerosol formation was first applied to liquid chromatography-mass spectrometry (LC-MS) interfac- ing.I3

In the first application of the thermospray effect to AAS, the liquid analyte solution was pumped through a steel capillary maintained at a high temperature by a heating block. The resulting thermospray emerged inside the mixing chamber directly below the burner head. Total conversion of liquid to aerosol and 4-10 fold increases in analytical signal in the FI mode have been rep01-ted.I~ The system was also used in the HPLC mode for speciation of magnesium-chlorophyll complexes.IS This construction re- quired the removal of the original back-fire safety valve of the AAS mixing chamber to leave room for the heating

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660 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JUNE 1993, VOL. 8

block. This potentially hazardous modification required a ready access underneath the mixing chamber, which is not possible with most contemporary AAS instruments.

Recently, a hydraulic nebulization device has been developed for AAS applications.I6 Via an HPLC pump and an injector valve, the sample was forced through a 10-20 pm aperture nozzle, which replaced the conventional pneumatic nebulizer. The sudden pressure drop (4 x 10' Pa), combined with the use of an impact bead for further disintegration of the droplets, caused 50-60% of the fluid to be converted into a fine aerosol. The nebulizer was used for total metal determinations in the FI-AAS mode and for iron speciation studies in the HPLC-AAS mode. In addition to the onerous cost of the modifications to the burner assembly, the high back-pressures created in hydraulic nebulization might decrease the useful lifetime of the pumphalve seals and cause damage to HPLC columns with glass or polymer walls. In the FI mode, signal enhancement factors of 7.5 (peak height) or 10.1 (integrated absorbance) were reported for copper, with an apparent limit of detection (LOD) of between 3 and 5 ng.

An alternate approach to thermospray and hydraulic nebulization made use of a heated glass mixing chamber in conjunction with a standard nebu1i~er.I~ The increased analyte transport efficiency was attributed to desolvation of the aerosol droplets in the hot mixing chamber. Three to five times better LODs were reported for manganese. Compatibility with ICP atomic emission spectrometry was achieved by coupling the heated glass mixing chamber with a vacuum separator,'* which selectively removed the vaporized solvent.

In summary, the various methods for improved analyte mass transfer in conventional pneumatic nebulizers involve extensive and specialized modifications of the assembly. The purpose of this paper is to describe a simple and inexpensive thermospray interface that is readily connected to a standard flame AAS instrument without modification of the original nebulizer-mixing chamber-burner assembly. Specifications, figures of merit and quantitative results are reported for this accessory used with FI sample introduc- tion. Three metals of ecotoxicological and/or biological significance (cadmium, lead and copper) were selected as test analytes. Cadmium-metallothionein isoforms I and I1 were also determined using the thermospray accessory as an HPLC-AAS interface.

Table 1 Instrumentation and instrumental settings

HPLC system Pye Unicam Model LC-XPD HPLC Pump

Flow rate/ml min-I 1 Injection valve Rheodyne Model 7125 Sample loop volume/pI 97 (calibrated by mass)

Atomic absorption spectrometry Perkin-Elmer Model 3 100 Spectral bandpasshm 0.7 Resonance wavelengthlnm Cd 228.8; Cu 324.7;

Background correction No Signal integration time/s 0.5

Pb 283.3

Expansion factor x 10

Acetylene flow ratell min-' 2 Single-slot burner

Air flow rate/l rnin-' 15 Pneumatic suction rate/ml min-I 5 (water)

Data acquisition

Analogue input/V AID convector/Hz

JCL 6000 software (Jones Chromatograph, Littleton, Co, USA)

0- 1 10

Materials and Methods Instrumentation A double-beam atomic absorption spectrometer (Perkin- Elmer Model 3 100) was used for measuring the absorbance from sample solutions aspirated at 5 ml min-I. In the FI mode the samples were injected via a loop injector into the carrier stream, which was pumped by a dual piston HPLC pump. During conventional sample aspiration, or in the FI- AAS mode (without a thermospray system), pneumatic nebulization was active and the mixing chamber was equipped with the standard impact beadlflow-spoiler acces- sories. The connection from the injector to the AAS nebulizer was established by a short length of 0.5 mm id . Teflon tubing. In the thermospray nebulization mode, the beadlspoiler accessories became obsolete and were re- moved. The 0-1 V analogue signal output of the AAS instrument was fed into an analogue-to-digital (A/D) converter and processed by chromatographic software installed in a personal computer. Further details on instru- mentation and settings are given in Table 1.

Reagents and Standards All chemicals used were of ACS reagent grade or better. Working solutions in the range 0.044-1.0 pg ml-I of copper, 0.01 5-1.0 pg ml-I of cadmium and 0.11-10 pg ml-l of lead were prepared from 1 OOOpg ml-1 analytical- reagent grade stock solutions in dilute nitric acid (BDH, Montreal, Quebec, Canada). Doubly distilled water was used as the carrier stream throughout all the FI sample introduction experiments. Test mobile phases for HPLC consisted of aqueous solutions of 0.1 mol 1-I pyridinium ion in water adjusted to pH 2.8 with formic acid, 0.1 mol 1-1 sodium hydrogen carbonate and 0.1 mol I-' ammonium hydrogen carbonate, adjusted to pH 10.3 with sodium hydroxide and concentrated ammonia solution, respectively (BDH). A stock solution of partially purified horse kidney metallothionein I and I1 isoforms (Sigma, St. Louis, MO, USA) was prepared in 10 mmol 1-l TRIS-HCl [TRIS= tris(hydroxymethyl)methylamine] buffer, pH 7.2 1, preserved with 0.02% m/v sodium azide, to a total protein concentration of 50 pg ml-I.

Thermospray Assembly A schematic diagram of the thermospray unit is presented in Fig. 1. The key component was a 'liquid heater' (B), composed of a thermoelectric wire [Kanthal A-1, 4.53 Q m-', 0.7 mm diameter (Pyrodia, Montreal, Quebec, Canada)] wound tightly in 70 coils that covered 5 cm of an 1 1 cmx3.2 mm o.d.x2 mm i.d. quartz tube. The tube guided a 23 cm x 50 pm i.d. silica capillary, (F) (Chromato- graphic Specialties, Brockville, Ontario, Canada) which was connected to the steel HPLC tubing from the injection valve by a 1.6 to 0.8 mm reducing union and a zero dead volume Vespel capillary ferrule (Chromatographic Special- ties). The silica capillary was fastened and centred inside the quartz guide tube using two reducing Swagelok unions (3.2 to 1.6 mm) fitted with Vespel ferrules (E). The front end of the capillary was inserted directly into the sample aspiration needle of the Perkin-Elmer pneumatic nebulizer nozzle (A), which was maintained at an elevated tempera- ture using a 'gas heater' (C). The design of this heater was essentially similar to that of the liquid heater and consisted of a 14 cm x 6 mm 0.d. x 4 mm i.d. quartz tube which ended at a 90" angle. The system was heated over 8.5 cm by 140 coils of Kanthal A-1 thermoelectric wire. This unit was fixed to the air line with a clip and connected to the pneumatic nebulizer gas inlet with a reducing Swagelok

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Y

I

E

It:

2 180 3 CI

X 140

c

100 .- - 3 n 3 60 z

66 1

-

-

-

-

aaf lB- ,E -

B

C

I

F

Fig. 1 Modular thermospray interface: A, Perkin-Elmer AAS nebulizer unit; B, liquid heater; C, gas heater; D, 0.64-0.32 cm reducing union; E, 0.32-0.16 cm reducing union; and F, silica capillary (50 pm i.d. x 23 cm)

fitting (D) (6.4 to 3.2 mm), which was fitted with Vespel ferrules.

The heaters were embedded in refractive wool (Fiberfrax, The Carborundum Company, Niagara, NY, USA) for thermal and electrical insulation and protection against corrosion from the ambient air. The assemblies were then encased with cylindrical pieces of firebrick of approxi- mately 15 mm wall thickness. To eliminate short-circuits between coils, the resistive wire was surface-oxidized (red hot, 800-900 "C) for 15 min before embedding in refractive wool. The a.c. potentials applied to the heaters were controlled using variable transformers and monitored with a digital voltmeter.

Optimization of Operating Variables The effect of the voltages applied to the heating elements on the absorbance of lead (0.97 pg injected) was modelled using a composite 2* factorial design. Sequential triplicate values were obtained for each of the eight experimental data points and the three central data points of the orthogonal design. The simple, quadratic and interaction parameters of a three-dimensional model, predicting AAS response as a function of applied voltages, were obtained by multiple regression analysi~.~ At the optima voltages the effect of the depth of insertion of the capillary into the mixing chamber on the AAS response to lead was recorded over a range of 7 cm. Then, the AAS response to lead, copper and cadmium standards obtained under optimal interface settings was compared with responses observed with direct FI-AAS coupling (in the starved mode) and conventional sample aspiration.

Calibration was performed by triplicate analyses of at least six dilutions of standard solutions. Integrated absor- bance was correlated with mass of analyte injected by simple linear regression.

High-performance Liquid Chromatography of Metallothioneins Horse kidney metallothioneins 1 and I1 isoforms were separated isocratically (1 ml min-*) on a weak anion- exchange column (TSK-DEAE-SPW, 7.5 cm x 7.5 mm; Supelco, Bellefonte, PA, USA) using 10 mmol 1-I TRIS- HCl buffer adjusted to pH 7.21 as mobile phase and preserved with 0.02% sodium azide.

Results and Discussion Specifications of the Thermospray Unit In order to characterize the operation of the thermospray

loo 1 ( a )

0 2 4 6 8 1 0 1 2 1 4

~ 220

~

200 L

e 180 $ 160 y"

m 140

(0

UI

E 0

180 < I

140 2 F CL

100 f 4.4

L .- a 60

0 10 20 30 40 Voltage applied on air heaterN

Fig. 2(a) Effect of liquid heater voltage on temperature of carrier liquid (water) at capillary outlet and on back-pressure (without nebulizer gas flow); and (b) effect of gas heater voltage on nebulizer surface temperature and on nebulizer air temperature measured at outlet (without carrier liquid flow)

unit, the heater assembly presented in Fig. 1 was mounted on the laboratory bench and temperatures at various points were monitored using a thermocouple. The variations in apparent temperature of the liquid jet emerging from the capillary (water, 1 ml min-I, 1.0 cm from the tip) and the back-pressure (pump head) as a function of voltage applied to the liquid heater are reported in Fig. 2(a). The nebulizer gas flow and heating were not operational during these measurements. Water temperature increased in proportion with the applied voltage up to a break point, where phase transition occurred. At this point, steam rapidly expanded and partially recondensed into a fine and cool (25-30 "C) mist. The expected lowering of water viscosity with increas- ing temperature caused an advantageous drop in back- pressure, which allowed the use of fragile HPLC columns and reduced stress on pump and valve seals.

Preliminary experiments indicated that additional input of thermal energy via the nebulizer air improved the AAS signal. Therefore, the gas heater was permanently con- nected between the air line and the AAS nebulizer gas inlet. Heating of the nebulizer itself prevented it from acting as a heat sink for the superheated liquid channelled through it via the silica capillary. Temperatures of the nebulizer surface and of the expanding air stream (measured 1.0 cm from the nebulizer outlet) as a function of applied voltage are shown in Fig. 2(6). No liquid was pumped through the nebulizer during these measurements. The Joule-Thomp- son effect caused a sharp decrease in air temperature upon expansion at the nebulizer outlet.

The response surface predicting the effect of applied voltages on integrated absorbance obtained from 0.97 pg of lead is presented in Fig. 3. The predictive model was obtained by multiple regression of the experimental results (integrated absorbance versus applied voltage). The model

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662 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JUNE 1993, VOL. 8

v)

0

0,

ul

=. 2.0

1.6

g 1.2 n m 0.8 '0 2 0.4 2 c, g o C -

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Fig. 3 Response surface plot predicting the combined effects of liquid and gas heater voltages on integrated absorbance of 0.97 pg of lead in the FI-thermospray mode

was validated by simple linear regression of observed versus predicted integrated absorbance (coefficient of correlation, r= 0.99 12; slope = 1 .OOO). The response surface indicates that the minimum liquid heater voltage required to obtain a thermospray was around 13 V. This corresponds [Fig. 2(a)] to a temperature of water inside the capillary of around 90-100 "C. From this point, the effect of the two heating elements on absorbance were additive. Maximum analyti- cal signal was obtained at an approximate setting of 17 V for the liquid heater and 37 V for the gas heater, where the resistive wire became red hot (about 900 "C). At these optimal settings the back-pressure was about 130 x 1 O4 Pa and the AAS nebulizer gas surface temperature was about 200 "C.

Both heating elements were proportioned in order to melt before overheating the nebulizer assembly. The use of a more powerful gas heater resulted in extreme temperatures which initiated a pyrolysis of the nebulizer seals. The high linear velocity of the liquid phase (8.8 m s-l) limited the residence time in the 5 cm liquid heater to only 6 ms at a flow rate of 1 ml min-'. This short residence time required the use of a high resistance heating coil as it ensured sufficient transport of thermal energy to the capillary. Excessive thermal energy in the liquid heater caused premature vaporization of the pressurized water inside the capillary, increasing the back-pressure and promoting a rapid corrosion of the silica, which eventually burst. The use of a 50 pm i.d. capillary appeared to be a fair compromise in order to keep the back-pressure at a moderate level and maintain a smooth thermospray effect. A 100 pm i.d. capillary did not induce a sufficiently high back-pressure to prevent internal vaporization, which was reflected by a sputtering effect and an uneven thermospray.

The thermal energy carried by the heated nebulizer air (120 "C) was absorbed by the endothermic thermospray expansion process. The spray resembled the optimum 'dry aerosol' described in LC-MS app1i~ations.I~ The additional signal enhancement provided by the gas heater (Fig. 3) was attributed to a mechanism distinct from the thermospray effect itself. This temperature-dependent mechanism most probably implies analyte desolvation; a process which has been described earlier in the heated mixing-chamber approach. l 7 Thus, combined thermospray-desolvation pro- cesses mght explain the additive effects of the two heating systems as seen in Fig. 3.

Fig. 4 depicts the influence of insertion of the silica capillary into the nebulizer unit on the absorbance of 0.97 pg of lead. Depth of insertion was measured as the distance between the aspiration needle inlet of the nebulizer and the capillary tip. The response remained essentially constant when inserting the capillary from 5 to 8 cm. Over a range of 7 cm (3-10 cm relative to the aspiration needle inlet), the

v) 2.0

2

2 n m 1.0

2 w P 0.5

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\

1.5 m

v)

U c,

c -

0 z 1 .o 6.0 , 11.0

Depth of insertionhm

Fig. 4 Effect of depth of insertion of the silica capillary into the sample aspiration needle of the AAS nebulizer on response to 0.97 pg of lead in the FI-thermospray mode

response varied to a maximum range of 30%. During analytical optimization, the silica capillary was inserted 5.5 cm, whereby its tip was positioned about 2 mm beyond the nebulizer outlet. At insertion depths of less than 4.5 cm, no signal was obtained when the voltage applied to the liquid heater was adjusted below 12 V (at any gas heater voltage). This confirmed that conventional pneumatic nebulization did not contribute to the formation of the fine aerosol.

When using the two optima heater voltages mentioned above and the optimum insertion depth of the capillary, no liquid could be observed in the drainage tube from the mixing chamber of the AAS. This reflected a total conver- sion of 1 ml min-l carrier liquid into an aerosol, which was efficiently transported to the analytical flame.

Performance of Thermospray Nebulization in FI-AAS mode The AAS response to copper obtained by conventional aspiration, by FI and by FI-thermospray using water as carrier is shown in Fig. 5. No observable peak broadening at the baseline or tailing occurred in either of the FI modes. The signal-to-noise (S/N) ratio observed for copper (Fig. 5) was essentially similar to that previously reported for a sophisticated hydraulic nebulization device. I6 The baseline noise remained unchanged in all three modes of sample introduction. Consequently, a comparison of signal-to- noise (S/N) ratios is immediately possible by observing the peak heights of the signals. Results similar to those for copper were obtained also for cadmium and for lead. The S/N ratios obtained in the FI-AAS mode for cadmium, copper and lead are 61, 70 and 53%, respectively, of those obtained by conventional aspiration. This suggests that the sample volume (97 pl) used in the FI-AAS mode was not sufficient to reach maximum absorbance for the given concentration. The FI-thermospray-AAS mode provided a 6.3-6.9 times increase in S/N on a peak height basis and a 6.5-7.7 times signal improvement on an integrated absor- bance basis, relative to the FI-AAS mode. These improve- ment factors are comparable to those reported previously for more complex design^.'^-^*

Statistics associated with the calibration graphs are presented in Table 2. Within the concentration ranges studied, the linear calibration models involving correlation of integrated absorbance with mass of analyte injected were accurate (0.9982trt0.9997). The LODs (three standard deviations of the baseline noise) were 1.9 ng of cadmium, 8.5 ng of copper and 27 ng of lead. All blanks were non- detectable. The average relative standard deviation (for triplicate injections) was 6.7% for concentration levels

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JUNE 1993, VOL. 8 66 3

' 2.5 rnin '

97 ng 0.234

0.208

0.182

0.156

0.130

E

4-

u

: OS1O4 2 0.078

0.052

0.026

0

a

49 ng

w I

t Time- FI-AAS

Conve n tional AAS

T hermospray- FI - AAS

Fig. 5 Composite of AAS responses to copper recorded under various sample introduction modes: conventional suction (5 ml min-l of 1 pg ml-I of copper); conventional FI (97 ng of copper); and thermospray enhanced FI (4-97 ng of copper)

~~ ~~~ ~~~

Table 2 Calibration regression and LODs for cadmium, copper and lead using thermospray-FI-AAS

Element Range/ng Slope* Intercept r LODt/ng Cd 15-48.5 5470 2800 0.9996 1.9

Pb 10-970 250 1070 0.9997 27 c u 4.2-97.0 1790 -555 0.9982 8.5

* Integrated signal per ng of analyte injected. t Calculated as 30 of the baseline noise.

approaching the LODs, 2.6% at 5-10 times the LOD and 0.8% at 50-100 times the LOD.

Determination of Metallothioneins by HPLC-Thermospray- AAS The applied liquid flow rate (1 ml min-l) and volume of the sample loop (97 pl) in the FI-thermospray-AAS mode were selected to allow for the direct application of the optimized thermospray unit as a HPLC-AAS interface. Higher flow rates or larger sample loops would increase the risk of overloading the column or reduce peak-resolution, and were therefore not considered.

The LOD obtained for cadmium justified its use as a marker analyte for the determination of sulfhydryl rich polypeptides such as metallothioneins in biological samples by HPLC-thermospray-AAS, via the cadmium saturation method.I9 A chromatogram of semi-purified horse kidney metallothioneins I and 11 isoforms is presented in Fig. 6 (4.9 pg of total protein injected). The isoforms were separated isocratically with a 15 mmol 1-l TRIS-HC1 buffer adjusted to pH 7.2 1. Under these conditions, the LOD was estimated at 400 ng (as an individual metallothionein isoform). As reported previously, baseline separation can be obtained using 10- 100 mmol 1- TRIS-HC1 buffer gradient.I9 During the optimization of buffer concentration for the separation of metallothioneins, the interface could handle up to 100 mmol 1-l TRIS-HCl buffer without problems, except for a thin deposition of salt in the nebulizer outlet cup which was rinsed with water daily.

MT 1

I 1 I I

0 2 4 6 8 1 0 Retention time/min

Fig. 6 HPLC-thermospray-AAS chromatogram of horse kidney metallothioneins I and I1 isoforms from a semi-purified commer- cial extract (4.9 pg of total protein); cadmium was monitored as a marker analyte

The HPLC mobile phases containing 0.1 mol 1- I pyridi- nium ion and 0.1 mol 1-l sodium carbonate have proved efficient in ion-exchange separations of arsenic species*O and were tested for possible future use of the thermospray nebulizer in arsenic speciation. While compatible with the pyridinium buffer, the heated portion of the silica capillary deteriorated when exposed to the sodium carbonate buffer. Because exposure to a similar ammonium carbonate buffer did not affect the capillary, the corrosion was attributed to a sodium mediated devitrification process observed previ- ously in heated quartz atomizer^.^ Thus, in the present version, the interface is not compatible with mobile phases that contain sodium at the 0.1 mol 1-' level. However, the capillary did support lower sodium concentrations present in the mobile phase (0.02% NaN,) for several hours without a significant increase in internal diameter as observed by microscopy. Future modifications of this design will include the replacement of the silica capillary by a metal capillary for improved chemical and physical stability.

Conclusions The thermospray nebulizer FI-AAS interface described in this article provided 6-8 times better S/N ratios for cadmium, copper and lead as compared with conventional FI-AAS. This improvement is similar to those obtained using more sophisticated techniques described in the literature. The interface was built from readily available and inexpensive components and represents a truly modu- lar accessory. It is easily connected without any modifica- tions to a standard flame atomic absorption spectrometer. The usefulness of the interface for the coupling of HPLC to AAS has also been demonstrated. Further improvements will include the use of a metal capillary in order to improve mechanical and chemical stability.

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664 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JUNE 1993, VOL. 8

The authors gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada, the Canadian Department of Fisheries and Oceans and the Danish Research Academy. We would also like to thank Marsolais from Perkin-E1mer’ Canada’ for technical support (AAS Model 3 100).

9 Brickmann, F. E., Blair, W. R., Jewett, K. L., and Inverson, W. P., J. Chromatogr. Sci., 1977, 15, 493.

10 Hill, s., Ebdon, L., and Jones, P., Anal. Proc., 1986, 23, 6. 1 1 Browner, F.9 and Boom, A. we, Anal. Chem.7 1983, 56,

786A. 12 Jones, D. R., Tung, H. C., and Manahan, S. E., Anal. Chem.,

1976, 48, 7.

References Ebdon, L., Hill, S., and Ward R. W., Analyst, 1987, 112, 1. Burguera, M., Burguera, J. L., and Alarcon, 0. M., Anal. Chim. Acta, 1988, 214, 421. Welz, B., Xu, S., and Sperling, M., Appl. Spectrosc., 1991, 45, 1433. Blais, J. S., and Marshall, W. D., J. Anal. At. Spectrom., 1989, 4, 271. Blais, J. S., Momplaisir, G. M., and Marshall, W. D., Anal. Chem., 1990,62, 1161. Mornplaisir, G. M., Blais, J. S., Quinteriro, M., and Marshall, W. D., J. Agric. Food. Chem., 1991, 39, 1448. Welz, B., and Schubert-Jacobs, M., At. Spectrosc., 199 1, 12,9 1. Blais, J. S., and Marshall, W. D., J. Anal. At . Spectrom., 1989, 4, 641.

13 Blackley,’ C. R., and Vestal, M. L., J. Anal. Chem., 1983, 55, 750.

14 Robinson, J. W., and Choi, D. S., Spectrosc. Lett., 1987, 20, 375.

15 Robinson, J. W., and Choi, D. S., Spectrosc. Lett., 1989, 22, 779.

16 Weber, G., and Berndt, H., Chromatographia, 1990, 29, 254. 17 Gustavsson, A., and Nygren, O., Spectrochim. Acta, Part B,

1987, 42, 883. 18 Gustavsson, A., Spectrochim. Acta, Part B, 1987, 42, 1 1 1. 19 Lehman, L. D., and Klaassen, C. D., Anal. Biochem., 1986,

153, 305. 20 Hansen, S. H., Larsen, E. H., Pritzl, G., and Cornett, C., J.

Anal. At. Spectrom., 1992, 6, 629.

Paper 2/05 4 8 3 C Received October 13, 1992 Accepted January 8, 1993

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