Anal. Chem. 1994,66, 4375-4381
Evaluation of an Automated Thermospray Interface for Coupling Electrothermal Atomization Atomic Absorption Spectrometry and Liquid Chromatography C. Bendicho* Universidad de Vigo, Campus de Ourense, Departamento de Qulmica Anantica y Alimentaria, As Lagoas s/n, 32004 Ourense, Spain
An automated thermospray interface is used to couple an
electrothermal atomizer and a liquid chromatograph for speciation studies. The thermosprayinterhce allows the periodical deposition in real time of the eluent from the chromatographic column into a graphite tube equipped with a curve-shapedplalform. The time required between consecutive measurements is drastically reduced since the chromatographic eluent is dried on impact onto the preheated platform at 150 "C. A total thermal program time of 40 s is feasible (15 s for the furnace cycle plus 25 s for cooling down). This thermal program time is shorter when compared to that used for conventional sample introduction in a graphite atomizer (e.g., 85 s). Relative standard deviationvalues of about 6%are reached for an arsenate solution when integrated absorbance measurements are used. The applicabilityof the interhce is shown for speciation of arsenate, arsenite, and dimethylarsinate using reverse-phase isochratic separations. The coupling of different separation techniques with atomic spectrometric detectors constitutes the most suitable approach to carrying out speciation studies. Among separation techniques, high-pressure liquid chromatography (HPLC) has emerged as a powerful tool for speciation, since it allows the handling of both volatile and nonvolatile compounds and therefore shows a wider applicability as compared to other chromatography techniques. HPLC can be easily connected to continuous atomization systems such as flames and plasmas, since the eluent flow rate closely approaches the uptake rate of the conventional nebulizers used for sample introduction. However, these couplings share the same drawbacks that are inherent to conventional sample introduction systems in atomic spectroscopy (i.e., low nebulization efficiency) for sample introduction.' Additionally, undesirable effects such as dispersion and peak distortion frequently occur, which degrades further the detection lik~it.~PSince the actual concentration range for speciation falls in the parts per billion to sub-parts per billion region, most couplings between HPLC and atomic spectroscopy techniques do not provide the appropriate detection capability. (1) Ebdon, L.;Hill, S.; Ward, R W. Analyst 1987,112, 1. (2) Katz, E.D.;Scott, R P. W. Analyst 1985,110, 253. (3) Laborda, F.; de Loos-Vollebregt, M. T.C.; de Galan, L. Spectrochim. Acta 1991,468,1089. (4) Ebdon, L;Hill, S.; Walton, A P.; Ward, R W. Analyst 1988,113, 1159.
0003-2700/94/0366-4375$04.50/0 0 1994 American Chemical Society
Improvement in the performance of these couplings can be achieved by using postcolumn hydride generation for species forming volatile hydrides4or thermospray introd~ction.3~~ Graphite furnace atomic absorption spectrometry (GF-AAS) fulfills the detection capability requirements, but the discontinuous nature of a conventional electrothermal atomizer makes it troublesome when used in combination with continuous flow systems. These atomizers are designed to accept low sample volumes, which have to be dried, pyrolyzed, and atomized before the measurement can be obtained. This clearly departs from the concept of monitoring continuously the chromatographic eluent. Several attempts have been made with the interface between a graphite atomizer and a liquid chromatograph, many of them using indirect couplings. Early couplings were based on a collection of the chromatographic effluent at a fixed time in autosampler cups6 or using flow-through cell^.^,^ The indirect couplings (e.g., collection in autosampler cups) do not provide real-timechromatograms, only a small fraction of the total effluent emerging from the column is sampled, and, more importantly, optimization of the chromatographic process is troublesome to perform. On-line couplings require the atomizer to be maintained at atomization temperatures for the time necessary to monitor the whole chromatogram. Such an approach represents a direct coupling, but dilution of the analyte in the vapor phase takes place, and therefore, the residence time in the absorption volume of the furnace is small. In addition, specially designed atomizers have to be used to avoid overheating the furnace housing? A few couplings of HPLC and electrothermal atomizers have been carried out using sophisticated injectors controlled by a microprocessor?JO These interfaces can be considered as direct couplings in which only a fraction of the chromatographic effluent is monitored (i.e., pulsed mode operation). Therefore, the resolution of the chromatographic peak depends on the time interval between consecutive measurements. (5) Roychowdhury, S. B.; Koropchak, J. A Anal. Chem. 1990,62, 484. (6) Brinchan, F. E.; Blair, W. R; Jewett, K L; Iverson, W. P.J.Chmmatogr. 1977,15, 493. (7) Brinchan, F. E.; Jewett, K L;Iverson, W. P.; Irgolic, K J.; Ehrhardt, KC.; Stockton, R A J Chromatogr 1980,191,31. (8) Nygren, 0.; Nilsson, C.A; Frech, W. Anal. Chem. 1988,60, 2204. (9) Stockton, R A;Irgolic, K J. Int. J Environ. Anal. Chem. 1979,6, 313. (10) Haswell, S.J.; Stockton, R A; Bancroft, K C. C.; O'Neill, P. 0.;Rahman, A; Irgolic, K J. J. Autom. Chem. 1987,9, 6.
Analytical Chemistry, Vol. 66, No. 23, December 1, 1994 4375
Though thermospray interfaces were originally designed for sample introduction into a mass spectrometer,ll recently they have been widely used as a sample introduction system for flame and plasma atomizers.12J3 Thermospray vaporization has been successfully used for sample introduction into a commercial electrothermal atomizer.14J5 When the aerosol formed upon heating makes an impact on the wall of a graphite tube, a considerable savings in time is achieved, which results in shorter furnace thermal programs. In this work, the use of an automated thermospray interface for the coupling of a commercial atomizer and an HPLC system is described. The interface is evaluated for speciation of three As species using ion-pairing chromatography. EXPERIMENTAL SECTION
Reagents. All chemicals were of analytical grade. Water was obtained from a Milli-Q water purifier. Stock solutions (1000 pg mL-l) of As were prepared in deionized water from sodium arsenate, sodium arsenite (Merck), and dimethylarsinic acid (Nuka). For the reversephase separations, a mixture of 95%water and 5%methanol buffered with ammonium citrate (2 mM, pH = 6.8) was used as a mobile phase. The ion-pairing reagent was tetrabutylammonium bromide (5 mM) . Nickel nitrate (5000 pg mL-l) and palladium nitrate (1000 pg mL-l) were used as matrix modifiers when required. Mobilephase solutions as well as arsenium standards were filtered through a 0.45pm filter prior to use. Instrumentation. (a) HPLC System. A Model 510 solvent delivery pump (Waters) was used in combination with an eightport injection valve @ionex) and a reversephase chromatography column (Waters). The chromatographic column consisted of a Delta Pak C18-100 A,with dimensions of 3.9 mm diameter and 150 mm length. The stationary phase is formed by spherical particles of 5 pm diameter. The high-pressure pump was modified by using plastic material in several components in order to avoid potential sources of contamination. To load the sample into the injection valve 200 pL volume loops were used. (b) ThermosprayInterface. The thermospray interface used in this work has been previously evaluated for sample introduction into a graphite furnace14J5as well as for inductivelycoupled plasma atomic emission ~pectrometry.~J~ It consists of a fused silica capillary (0.1 mm i.d., 0.2 mm 0.d.) inserted into a stainless steel tube (0.5 mm i.d., 1.6 mm 0.d.). The steel tube is electrically heated with a current of about 20 mA, which is regulated by a phase angle temperature controller. The temperature is controlled at the entrance of the vaporizer by a Cr-Al thermocouple attached to the input end. Another thermocouple attached near the exit of the vaporizer permits temperature measurements to be taken at this point, thereby preventing the capillary from overheating. A steel cover surrounds the outlet end of the stainless steel capillary so that the vapor cannot enter the space that exists between the two capillary tubes and cause fluctuations in the tip temperature. Two electrical connectors are welded to the stainless (11) Blakley, C. R; McAdams, M. J.; Vestal, M. L.J. Chromatogr. 1978,12, 574. (12) Choi, D. S.; Robinson, J. W. Spectrosc. Lett. 1989,22, 69. (13) Peng, R; Tiggelman, J. J.; de Loos-Vollebregt,M. T. C. Spectvochim. Acta 1990,45B,189. (14) Bank, P. C.; de Loos-Vollebregt, M. T. C.; de Galan, L. Spectrochim. Acta 1988,43B, 983. (15) Bank, P. C.; de Loos-Vollebregt,M. T. C.; de Galan, L. Spectrochim. Acta 1989,44B, 571.
4376 Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
H
A
M
L
I
Figure 1. Coupling of GF-AAS and HPLC. (A) HPLC pump. (B) Sample. (C) Injection valve, 200 pL loop. (D) Silica capillary, 100 pm internal diameter. (E) HPLC column. (F) Thermospray holder. (G) Thermospray vaporizer. (H) Temperature controller. (I) Sequence controller. (J) Vacuum chamber. (K) Electrothermal atomizer. (L) Graphite furnace programmer. (M) Buffer.
steel tube for power supply. The vaporizer is enclosed in a metallic probe for electrical isolation and mounted onto a holder that can move backward and forward into the furnace housing by way of a pneumatic actuator. When no sample is to be introduced into the furnace, the outlet end of the vaporizer is kept in a vacuum chamber made of plastic and located in front of the furnace housing. This chamber is necessary to prevent liquid and vapor from entering the furnace. The operation of the thermospray interface is controlled by a sequence controller (Omron, Scy-PO). Every individual action can be programmed independently using one of the 12 channels available. The sample is loaded onto the sample loop in the injection valve and metered in the chromatographiccolumn. The action is performed by two pneumatic valves. The first pneumatic actuator moves the probe containing the vaporizer forward or backward into the furnace housing, depending on the action required (Le., whether the chromatographic eluent is going to be introduced into the graphite furnace or not). A second pneumatic actuator moves a plastic shield located at the back of the vacuum chamber upward or downward, so that the outlet end of the vaporizer can be introduced into the furnace. The effluent that comes out the column is then introduced through the thermospray vaporizer and injected for a period of 2 s (timer accuracy is about f 1 0 ms) in the graphite tube. The start of the thermal program is initialized by the sequence controller. The program switches off when the furnace cycle is completed. A repetitive sequence is programmed onto the sequence controller that permits sampling of the chromatographiceffluent, actioning of the dif€erent pneumatic actuators, and a delay time for the furnace to cool down as many times as required to collect the chromatographic peaks. The direction of flow in the injection valve is changed once the chromatogram is developed to permit the loading of a new sample if required. The system configuration is shown in Figure 1. (c) Graphite Furnace Atomizer. A Perkin-Elmer absorption spectrophotometer, Model 1100,is fitted with an HGA-500graphite furnace. For the coupling to HPLC, the atomizer was slightly modified. First, the dosing hole located in the contact cylinders is enlarged so that sample introduction can be performed parallel to the furnace plate. When a thermospray is introduced into the furnace a common problem arises, i.e., the vapor formed upon heating the liquid can condense on the optical windows located on the side of the furnace house, hence causing irreproducible absorbance measurements and high background. To avoid this
Table 1. Influence of the Distance between the Platform Surface and the Capillary Tip
instance from platform (mm)
peak area (A*s)
0.5 1.5 2.5 3.5 4.5 5.5
0.491 0.520 0.586 0.508 0.514 0.508
Figure 2. Curve-shaped platform used for sample deposition in the coupling of HPLC- ETA-AAS.
problem, the furnace housing is kept at 80 "C by recirculating water heated in a bath. Second, in contrast to the original design by Bank et al.,15no vacuum system is used inside the furnace to prevent vapor condensation on the optical windows. This vacuum is thought to cause fast removal of excess vapor produced during sample introduction, and, in turn, analyte losses can occur in the vacuum line when no instantaneous drying of the droplets takes place. Therefore, the internal gas (argon) is preheated before entering the furnace so that vapor can be efficiently removed. Curve-shaped platforms, specially designed for thermospray deposition (Figure 2), were used instead of the conventional rectangular-shaped platforms. In this case, the platform remains in front of the dosing hole. Optimization of the Thermospray Interface. The performance of the interface was evaluated using a 0.1 pg mL-l A s 0 solution. For preliminary optimization experiments,the interface was operated in continuous mode, i.e., neither injection valve nor chromatographic column was used, and the sample was delivered directly by the HPLC pump to the thermospray vaporizer. Typically, liquid flow rates in the range of 0.2-0.6 mL min-l were used. RESULTS AND DISCUSSION
Influence of the CapillaryTip Temperature. The thermospray vaporizer temperature is adjusted so that a stable spray is formed at the outlet end. A critical aspect in the operation of the thermospray vaporizer is the formation of droplets at the silica capillary tip which impairs precision of the sample introduction. This phenomenon was observed when using low vaporizer temperatures and silica capillary tubes with an internal diameter larger than 0.2 mm. Figure 3 shows the integrated absorbance for 2 ng of A s o . At a temperature below 150 "C, the sample
100
150
deposition is incomplete. At a temperature above 200 "C, a decrease in the integrated absorbance is observed, which can be attributed to analyte losses. This behavior is also displayed by As(I1I) and DMA, and it is believed to be caused by decreased nebulization efficiency. Influence of the Platform Temperature. The influence of the platform temperature for sample deposition is presented in Figure 4. For A s o , a steady integrated absorbance is observed between 100 and 350 "C. Integrated absorbance drops from a temperature above 350 "C as a result of uncontrollable boiling processes occurring on the surface of the platform. The Ni matrix modifier mixed with the As solution was used to determine whether the absorbance decrease was due to the volatilization of As or to the loss of sample droplets when they impact on the hot graphite surface. As can be seen, integrated absorbance decreases at a temperature of about 450 "C, which means that Ni does not modify to a large extent As stabilization during the deposition process. For As(I1I) and DMA, losses start at temperatures significantly lower than for A s 0 . Integrated absorbance drops at 200 "C for As(I1I) and 180 "C for DMA. It can be concluded that the effective deposition of the three As species requires deposition temperatures below 180 "C. Influenceof the CapillaryTip Distance fiom the Platform. Whereas thermospray deposition on the wall of a graphite tube is almost independent of the capillary distance from the wall, deposition on the platform needs to be optimized. Table 1shows the influence of the distance between the platform surface and the capillary tip. No critical influence is seen in the region from 0.5 to 5.5 mm when the 100 pm diameter capillary and an introduction time of 2 s are used. When an introduction time longer than 2 s was used at a pump flow rate of 0.6 mL min-l, a decreasing integrated absorbance is observed with increasing
200
250
TllERMOSF'RAY TEMPERATI IRK. OC
Figure 3. Influence of the capillary tip temperature on the integrated absorbance for an As(V) solution. Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
4377
I
PLATFORM TEMPERATURE, 0C Figure 4. Influence of the deposition temperature on As(V) (0,without matrix modifier; 0 , with nickel nitrate as matrix modifier), As(ll1) without matrix modifier), and DMA (0,without matrix modifier).
(A,
0.8
0.6
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2
2$
0.4
2 a
5
0
0.2
u
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500
1000
1500
ASHINC; Tl:MPI-.M'I'IIRE, OC Figure 5. Influence of the ashing step on As(V) (0),As(ll1) (0)and DMA (A).
deposition distance from the platform. The partial deposition of the sample on the wall, which occurs in these conditions, was confrmed by the presence of two peaks in the atomic absorption signals. Muence of the Ashing Step. The need for an ashing step has to be considered to establish whether this step should be retained or, if it can be omitted. The removal of the ashing step can provide two advantages when electrothermal atomizers are coupled to a liquid chromatograph. First, the possibility of analyte loss is reduced. This is critical when volatile species are involved (e.g., As species). Second, the total time for the thermal program is shortened, and, therefore, a larger number of individual samples can be monitored along the chromatographic separation. Figure 4378 Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
5 shows the ashing curves for 1.2 ng of AsO, 1.1ng of As(III), and 1.1 ng of DMA. In all cases, a deposition temperature of 150 "C was used. The hold time for the ashing step was 10 s. As can be seen, A s 0 displayed the lowest volatility. Integrated absorbance decreases at a temperature of 600 "C for DMA, which is the most volatile species. The influence of the Ni matrix modifier was studied for AsO. This matrix modilier increases considerably the thermal stability of As (V) up to temperatures of about 1300 "C. No dfierences in integrated absorbance are observed for A s 0 when a matrix moditier is used in comparison to those obtained without the use of a matrix moditier, provided that the ashing temperature is maintained below 900 "C. It appears that in all cases the ashing step can be omitted without
the deteriorationof sensitivity. The integrated absorbance values are virtually the same for the three As species when an atomization temperature of 2500 "C is used. Thermal Program. In contrast to the on-line interface between an electrothermal atomizer and a liquid chromatograph, a direct coupling operated in the pulsed mode allows the use of small sample volumes and gas stop for atomization. However, the number of individual measurements carried out while the chromatograph is running should be as high as possible to describe the actual peak concentration profile. Therefore, the thermal program should be kept to a minimum, to increase the total number of measurements. Bearing in mind this prerequisite, the time required for each step of the thermal program was optimized accordingly. A time of 6 s for drying (1 s ramp, 5 s hold time) was seen to be sufficient for achieving an effective deposition of the sample at 150 "C. This time is much shorter than that used for conventional sample introduction at room temperature, since the sample is dried on impact on the preheated platform. When the ashing step was skipped, background was detected at the beginning of the atomization step, which indicated that water vapor remained partially in the absorption volume of the graphite tube. Therefore, a short step at a temperature of 300 "C was included between the deposition and the atomization step (1 s ramp, 5 s hold time) to remove the water vapor. The sample was then atomized at 2500 "C for 3 s. No clean-out step was seen to be required. According to this thermal program, a total time of 15 s is obtained. When the furnace is operated at these conditions, a repeatability expressed as a relative standard deviation of 6.3% was reached by measuring the integrated absorbance. Further shortening of the time between consecutive measurements was mainly limited by the time required for the cooling-down of the platform. Temperature measurements of the platform were made in the range of 2500 "C and 80 "C using an optical pyrometer and a Cr-AI thermocouple. The experimental decay curve of temperature versus time indicated that at least 25 s was required for the platform to reach 150 "C. It should be noted that faster cooling-down of the wall takes place. In addition, external cooling by blowing gas permits a fast cooling of the graphite tube,1° although it is less successful when the platform is to be cooled as a result of the bad thermal contact with the wall. For a sample introduction time of 2 s and a total thermal program of 40 s, 5%of the total chromatographiceluent is sampled. Further shortening of the thermal program can be achieved when the sample is deposited at higher temperatures. A s 0 can be deposited at 300 "C without losses, which permits the use of a cooling-down time of 20 s. However, the more volatile As species are lost at this temperature, unless a matrix modifier is used. Another possibility to increase the total amount of chromatographic eluent that is actually sampled is to use longer introduction times. For the above mentioned introduction time, the fraction of eluent sampled is 20 pL when the HPLC pump is operated at 0.6 mL min-l. Figure 6 shows the saving in time achieved with the use of thermospray deposition and omitting both preheating of the platform and the ashing step. A conventional thermal program for As (Figure 6a) including dry step (31 s), ashing step (21 s), atomization step (3 s), and clean-out (4 s) and a 25 s coolingdown (e.g., when using platform atomization) requires about 85 s. This total time is shortened to about 70 s when thermospray deposition is used and the system is operated in flow injection
3000
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I
2000
3
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1000
20
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2000
-
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I
40
60
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1000
2Irl c
0
20
40
60
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20
40
60
80
3000 I ;
w'
2000
-
c 5 4
1000
0
5c TIME. s Figure 6. Thermal program for (A) conventional sample introduction, (B) thermospray deposition and flow injection introduction, and (C) thermospray deposition in the coupling of HPLC and ETA-AAS.
mode (Figure 6b). In spite of the time saved for drying, the introduction of the sample from the sample loop requires an additional time, so that all the sample is quantitatively transferred into the graphite tube.14J5 A fraction of this time is spent in preheating the platform to the selected deposition temperature. The omission of the pyrolysis step and the use of cooling only until the platform is at the deposition temperature (e.g., 150 "C) provide further saving of time, and a total time of about 40 s is feasible (Figure 6c). Characteristic Masses for A s o ,As(III), and DMA. The characteristic masses (mol pg/0.0044 A$ for A s 0 , As(III), and DMA using thermospray deposition as well as manual deposition at room temperature have been determined. The results are presented in Table 2. When the sample is manually introduced and dried, similar mo values are observed for A s 0 and DMA, but significantly higher mo values are observed for As(I1I) when no matrix modifier is used. When Pd is used as a matrix m o a e r , the mo values are similar for A s 0 and DMA, which are in turn very similar to those obtained without the matrix modifier. The mo for As(I1I) is improved when this species is atomized in the presence of Pd and it approaches those observed for the other Analytical Chemistry, Vol. 66,No. 23, December 1, 1994
4379
0.5
A 5.33
8.66
12
15.33
18.G6
22
RETENTION TIME. min. Flgure 7. Chromatogram obtained for As(V) (0.25 pg), As(lll) (0.18 pg), and DMA (0.27 pg) using ion-pairing chromatography. The sample containing the three As species was loaded into a 200 mL loop. The thermal program shown in Table 3 is used. The deposition time is 2 s,and the pump flow rate is 0.2 mL min-I. ~
~~~
Table 2. Characteristic Masses (mpg10.0044 A%) for Different As Compounds Using Both Manual Introduction and Thermospray Deposition
As species no modifier Pd modifiep thermosprayb literature 14 13 18 17 (ref 16) As 0 As @I) DMA
24 15
16 14
22 18
For the Pd modiiier experiment, 20 p L of lo00 pg mL-l Pd solution were introduced by micropipet, dried, and pyrolyzed at lo00 "C for 20 s. A 2-s deposition time at a pump flow rate of 0.6 mL min-' was used. Neither matrix modifier nor pyrolysis step was used. The deposition temperature was kept at 150 "C. (I
Table 3. Thermal Program Used for the Coupling of Electrothermal Atomlzatlon AAS and Liquid Chromatography
step" t ("C) ramp (s) hold time (s) flow rate (mL/min) read
1
2
3
150 1 5 300
300 1 5 300
2500 0 3 0 b
Step 2 was used for faster removal of water vapor before atomization. Reading in the spectrophotometerwas activated at this step. (I
As species. For thermospray deposition, neither matrix modzer nor the pyrolysis step was used. As can be seen, the mo values for the three As species are very similar, and, moreover, they are in close agreement to the value of 17 pg reported in the literature for As.16 In view of these results, the validity of the approach (16) Slavin, W. Graphite Furnace AAS. A source book; The Perkin Elmer Cop.:
1984.
4380 Analytical Chemistry, Vol. 66, No. 23, December 1, 1994
used for atomization of As species separated by liquid chromatography is acceptable, considering the nonsignificant differences of sensitivity obtained compared to the conventional procedure. Speciation of A s o ,As(III), and DMA. The separation of the three As species considered in this work was carried out using an ion-pairing chromatographic column. The thermal program is shown in Table 3. The HPLC pump was operated at a flow rate of 0.2 mL min-I, so that the dispersion of the system was increased. This condition was seen to be necessary to collect an appropriate number of fractions during the chromatographic running and thus avoid missing the chromatographic peak. Whereas good separations are reported for As on ionexchange columns with phosphate buffers, the addition of phosphates is seen to cause spectral interference on As.17 Ammonium carbonate buffers have been recommended since they require minimum thermal pretreatment and do not cause interference.18 For reversephase separations, sodium heptanesulfonate? tetraheptylammoium nitrate, and tetrabutylammonium phosphate' have seen used as ion-pair reagents. The use of these reagents usually required Zeeman background correction? Tetrabutylammonium bromide (5 mM) was used in this work as an ion-pair reagent, since no background interference is caused at the concentration used. For a mobile phase formed by 95%water and 5%methanol, good separation was observed for A s 0 and As(III), but poor resolution was observed for AsOII) and DMA. The addition of a citrate buffer at low concentration (2 mM) was found to be helpful to resolve the two peaks. A chromatogram run in these conditions required less than 25 min to resolve the three As species (Figure 7). The quantification of each As compound was carried out by summation of all individual peak areas obtained during elution of the correspondent chromatographic peak, according to the a p (17) Letoumeau, V. A; Joshi, B. M.; Butler, L. C. At. Spectrosc. 1987, 5, 145. (18) Woolson, E. A; Aharonson, N. 1.Assoc. 4R: Anal. Chem. 1980, 63, 523.
proach established by Woolson and Aharonson.16 Only integrated absorbance values higher than Sfold the standard deviation of the blank were considered in these calculations. The chromatographic detection limits, defined as 3-fold the standard deviation of the blank, were 5,8, and 15 ng/mLfor AsUII), AsO,and DMA, respectively. Detection limits are improved by a factor of 1550-fold when compared to those obtained using a flow-through cup for discontinuous coupling between HPLC and ETA-AAS.l8 Better detection limits can be achieved using HPLC-ICPMS. A detection liiit of 2 ng/mL was reported for AsaII) and arsencbetaine1gafter chromatographic separation and ICPMS detection. CONCLUSIONS An automated thermospray vaporizer was used as an interface for coupling electrothermal atomization AAS and liquid chromatography in As speciation studies. The chromatographic eluent (19) Beauchemin, D.; Siu, IC W. M.; McLaren, J. W.; Berman, S S J Anal. Atom. Spectrom. 1989,4, 285.
can be sampled every 40 s with good reproducibility and sensitivity using deposition of the thermospray on a preheated platform. The sensitivity for thermospray deposition is comparable to that obtained by manual introduction of the sample. Further shortening of the thermal program, although it is mainly limited by the coolingdown step, would permit the monitoring of larger eluent fractions, thereby improving chromatographic detection limits.So far, the results shown for speciation of Aso,AsUII), and DMA are promising and would permit the extensive application of this interface to other speciation studies. ACKNOWLEDGMENT The author thanks the Delft University of Technology (The Netherlands) for allowing the use of the instrumentation and Dr. M. T. C. de Loos-Vollebregt for helpful discussions. Received for review December 8, 1993. Accepted July 1994.a
7,
Abstract published in Advance ACS Abstracts, October 1, 1994.
Analytical Chemistry, Vol. 66, No. 23,December 1, 1994
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