Anal. Chem. 1994,66, 3208-3216
Modification of a Commercial Electrothermal Vaporizer for Sample Introduction into an Inductively Coupled Plasma Mass Spectrometer. 1 Characterization M. M. Lamoureux,t D. C. GrOgoire,'~*C. L. Chakrabartl,t and D. M. Goltzt Department of Chemistry, Ottawa-Carleton Chemical Institute, Carleton University, Ottawa, Ontario, Canada K1S 5B6, and.Geologica1 Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K I A OE8
Modifications to a commercial graphite furnace were made for its use as a sample introduction device with an inductively coupled plasma mass spectrometer (ICPMS) and to allow simultaneous measurement of the atomic absorption and mass spectrometric signals. The effect of the internal Ar carrier gas flow, the total Ar carrier gas flow, and the vaporization temperature on the integrated ion intensity for Ag was studied. An absolute limit of detection of 0.23 pg was obtained for Ag when vaporization took place from a graphite platform in the presence of sodium chloride chemical modifier. The utility of simultaneously measuring electrothermal atomization atomic absorption (ETAA) and ETV-ICP mass spectrometric signals was demonstrated by investigating the interference of MgCl2 on the determination of Mn by ETAAS. This study provided direct evidence for preatomization loss of Mn when vaporized in the presence of MgC12. The use of electrothermal vaporizers (ETV) as sample introduction devices for plasma source spectroscopy has increased considerably since the report by Nixon et al.,I who used a tantalum filament vaporizer interfaced to an inductively coupled plasma atomic emission spectrometer (ICP-AES). Electrothermal vaporization sample introduction devices have since found use in ICP-AES,2-8 and interest for their application in inductively coupled plasma mass spectrometry (ICPMS) has grown9-l6since it was first used by Gray and Date.17 + Carleton University. t Geological Survey of Canada. (1) Nixon, D. E.; Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974,46,21&213. (2) Schmertmann, S. M.; Long, E. S.;Browner, R. F. J . Anal. At. Spectrom. 1987, 2, 687-693. (3) Kumamaru, T.; Okomoto, Y.;Matsuo, H. Appl. Spectrosc. 1987, 41, 918920. (4) Dittrich, K.; Berndt, H.; Broekaert, J. A. C.;Schaldach, G.; T61g, G. J. Anal. At. Spectrom. 1988, 3, 1105-1 110. ( 5 ) Barnes, R. M.; Fodor, P. Spectrochim. Acta, Part B 1983, 38, 1191-1202. (6) Aziz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim. Acta. Part B 1982, 37, 369-379. (7) Gunn, A. M.; Millard, D. L.; Kirkbright, G. F.Analyst 1978,103,1066-1073. (8) Millard, D. L.; Shan, H. C.; Kirkbright, G. F. AMIyst 1980, 105, 502-508. (9) Park, C. J.; Hall, G. E. M. J. Anal. At. Spectrom. 1987, 2, 473-480. (10) Shibata, N.; Fudagawa, N.; Kubota, M. AMI. Chem. 1991, 63, 636-640. (11) Hulmston, P.; Hutton, R. C. Spectroscopy 1991, 6, 35-38. (12) Tsukahara, R.;Kubota, M. Spectrochim. Acta, Parr B 1990, 45, 779-787. (13) Shen, W.L.; Caruso, J.A.; Fricke, F. L.;Satzger, R. D.J. Anal. Ar.Spectrom. 1990, 5,451455. (14) GrCgoire, D. C. Anal. Chem. 1990.62, 141-146. (15) Gregoire, D. C. J. Anal. At. Spectrom. 1988, 3, 309-314. (16) Park, C. J.; Hall, G. E. M. J . Anal. At. Spectrom. 1988, 3, 355-361. (17) Gray. A. L.; Date, A. R. Analyst 1983, 108, 1033-1050.
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Up to now, designs of electrothermalvaporizers for ICPMS were basedon twoapproaches: (1) minimizationof the transfer distance, i.e., the distance from the vaporization surface to the ICP torch, and (2) minimization of sample condensation and adhesion to transfer line walls. Matusiewicz et a1.18 minimized the transfer distance (17 cm) by mounting an HGA 500 graphite furnace to an ICP in a vertical position for AES measurements. The modifications to the furnace were simple, but sample introduction was difficult because the furnace was turned on its side. Evans et al.I9 interfaced a modified HGA 500 graphite furnace to an axially mounted microwave-induced plasma (MIP) system. The graphite furnace was connected directly to the microwave cavity, and this design eliminated recondensation of sample on transfer line walls. The transfer distance was minimized, but the modifications were complicated, and the integrated design of the system made switching to other sample introduction devices difficult. Greater attention has been paid to the development of ETV systems using the second design approach. In general, these devicesuse either commercial graphite furnaces or ETV devices specifically designed to optimize the sample transport efficiency by minimizing sample condensation on the transfer line walls. The underlying principle behind minimizingsample condensation at low analyte vapor concentrations involves the formation of clusters resulting from collisions among vaporphase species that serve as condensation nuclei. These clusters continue to grow until they condense into particles. The ideal behavior for analyte vaporized from ETV devices involves rapid self-nucleation to form particles that are large enough to be transported efficiently by a carrier gas through a transfer line but small enough to avoid coagulation and deposition on transport surfaces.20 Shen et al.I3 and Carey et a1.21made a relatively simple modification to an HGA 300 graphite furnace and used the modified furnace with an ICPMS. They replaced the end windows of the furnace with laboratory-built stainless-steel adapters. The rear adapter had a stainless-steel tube sealed at the base, allowing the carrier gas flow to be introduced tangentially. The front adapter had a fitting with an O-ring (18) Matusiewicz, H.; Fricke, F. L.; Barnes, R. M. J. Anal. At. Spectrom. 1986, 1, 203-209. (19) Evans, E. H.; Caruso, J. A.;Satzger, R. D. Appl. Specrrosc. 1991,45, 14781484. (20) Kantor, T. Spectrochim. Acta, Part B 1988, 43, 1299-1320. (21) Carey, J. M.; Evans, E. H.; Caruso, J. A.; Shen, W. L. Spectrochim. Acta, Part B 1991.46, 1711-1721. 0003-2700/94/0386-3208$04.50/0
0 1994 American Chemical Society
seal to which a poly(tetrafluoroethy1ene) (PTFE) tube was connected and attached to the base of the ICP. The dosing hole of the graphite furnace was sealed with a graphite plug. The carrier gas entered the rear adapter and flowed through the graphite tube, sweeping out the analyte vapor toward the ICP torch. An important feature was the provision for the use of an external sheath gas (argon) as a coolant gas flow immediately downstream of the graphite furnace tube with the object of promoting rapid nucleation before the analyte vapor was carried to cool surfaces. The use of this cooling system resulted in a small “memory effect”, which required three blank firings for its complete removal. The analyte transport efficiency was improved by the use of this coolant flow; however, there was no report on the quantitative aspects of the analyte transport efficiency. A similar design (without the coolant flow) was investigated for AES3 Specifically designed apparatus for sample introduction into the plasma are generally based on the ETV device constructed by Park et al.22 These devices usually consist of one of the following vaporization surfaces: a graphite rod? a graphite strip23or cup;Z4a rhenium filament;g.22or a tungsten strip.12 The vaporization surface is connected to two electrodes and is enclosed inside a glass or quartz envelope. The vaporization surface is heated to incandescence by passing electrical current through it. A carrier gas (usually argon) sheaths the internal surface of the glass or quartz envelope. This sheath gas prevents the analyte vapor from condensing on the envelopesurface and allows transportation of the analyte particles to the ICP torch. There are two major problems with the above design: (1) the analyte loss by condensation on the wall cannot be totally eliminated,2.24.25and (2) there can be excessive dilution of the analyte vapor by the volume of the argon gas contained in the envelope, resulting in decreased sensitivity.12 In effect, all the ETV devices discussed above have suffered from one or a combination of the following problems: (1) complex modifications to the hardware; (2) excessive dilution of sample vapor by the carrier and sheath gas; (3) analyte loss from condensation at cool surfaces. The objective of this work was to develop an ETV device that would operate as an efficient sample introduction system for ICPMS, while at the same time allow for the measurement of atomic and molecular absorption signals. The transport of analyte vapor through the dosing hole of a graphite tube furnace to the ICP torch is an approach that has not been investigated by others. The benefits of such a system are reported here. To date, all ETV devices have been designed to permit only one measurement mode such as the monitoring of a mass spectrometric signal or an atomic absorption/emission signal. The new ETV design reported here allows the atomic or molecular absorption signal and the mass spectrometric signal to be measured simultaneously. The combined AAS and ICPMS data can be used to investigate important problems ~
(22) Park, C. J.; Van Loon, J. C.; Arrowsmith, J.; French, J. B. Can. J . Spectrosc. 1981, 32, 29-36. ( 2 3 ) Gregoire, D.C.; Lamoureux, M.; Chakrabarti, C.L.; Al-Maawali, S.;Byrne, J. P. J . Anal. At. Spectrom. 1992, 7, 519-585. (24) Ng, K. C.; Caruso, J. A. Appl. Spectrosc. 1985, 39, 119-126. (25) Park, C. J. Feasibility Study of an Electrothermal Vaporizer/Inductively Coupled Plasma/Mass Spectrometry System. Ph.D. Thesis, University of Toronto, 1985; pp 35-38, 125.
&f Isomeric view
Figure 1. Schematic diagram of the modified Perkln-Elmer HOA 768 graphite furnace.
in electrothermal atomic absorption spectrometry (ETAAS), such as chloride matrix interferences.
EXPERIMENTAL SECTION Apparatus. The inductively coupled plasma mass spectrometer used for this work was a Perkin-Elmer Sciex Elan 5000. A standard (Perkin-Elmer) demountable ICP torch made of one-piecequartz tubing and equipped with an alumina injector tube was used throughout this work. For the measurement of major ions such as chloride, an offset voltage was applied to one of the ion lenses of the mass spectrometer via the “OmniRange” facility. This was done to effectively reduce the sensitivity of the ICPMS. The ion intensity of a selected mass-to-charge ratio could then be reduced to a level within the linear dynamic range (&lo6 counts s-’) of the detector. Optimization of the ICP mass spectrometer and plasma operating parameters was done using solution nebulization sample introduction and aqueous standards. Reoptimization of these parameters on switching to ETV sample introduction was unnecessary with the exception of small adjustments (f50 mL min-1) to the carrier argon gas flow. Modifications of the graphite furnace, Model HGA 76B (Perkin-Elmer) were made as shown in Figure 1. Photographs of the modified furnace are shown in Figure 2 with the graphite furnace opened (Figure 2a) and closed (Figure 2b). The furnace brass body was redesigned such that a brass lip extended about 4 mm from the interior face of each brass contact electrode. All surfaces of the furnace brass body were chrome-plated. The interior of each brass contact electrode was further coated with carbon by means of a vacuum coating device normally used to preparesamples for scanning electron microscopy. This carbon coating reduced the ICPMS background signals of Cr, Cu, and Fe, by preventing the erosion of the chrome plating during the high-temperaturevaporization step. A high-temperature silicon O-ring was fitted insideeach brass lip. From a quartz tube of 50-mm o.d., a ring of 15-mm width was cut and a hole of 8-mm diameter was drilled at the center. A male ball joint (No. 10) made of quartz was fused to the ring at the hole location (called a “spout” in Figure l), and a female ball joint (No. 10) made of quartz was clamped against the male ball joint. The quartz ring was positioned between the two brass contact electrodes, which were then clamped together. The O-rings located between the quartz Ana&tical Chemistry, Vola66, No. 19, October 1, 1994
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Flgure 3. Block diagram of the simultaneous ETAAS-ICPMS spectrometer system.
Figure 2. Modified Perkin-Elmer HGA 76B graphite furnace: (a) open position; (b) closed position.
ring and brass lips allowed for the interior of the furnace to be sealed from the outside. The interface between the ETV unit and the ICP torch consisted of a 75 X 0.5 cm (i.d.) Teflon tube with a three-way stopcock located midway between the ETV and the torch assembly. The stopcockwas used to prevent the introduction of air into the transfer line during insertion of sample solution into the graphite tube. The Ar carrier gas flow was regulated using a mass-flow controller provided on the ICPMS equipment and was divided in two streams by a plastic Y-connector fitted with gas flowmeters (Matheson, Model 602). One Ar gas stream was connected to the external sheath gas entry port of the modified furnace while the other was connected to the internal purge gas entry port of the modified furnace. This configuration allowed for changes in internal purge gas flow rate without changing the total flow of the Ar carrier gas reaching the argon plasma. An HGA 500 (Perkin-Elmer) graphite furnace power supply was used with the modified ETV system. Pyrolytically coated graphite tubes (Perkin-Elmer, Part No. 09 1504) wereused throughout. Laboratory-made single-cavityplatforms were made of anisotropic pyrolytic graphite. Integrated analyte ion intensity (peak area) rather than ion count rate (peak height) was used for calibration purposes because of its insensitivity to variable vaporization kinetics, especially when results obtained at different heating rates were compared. Thus, the use of integrated ion intensity allows for comparison between results obtained by use of vaporization from the graphite tube wall and those obtained by use of a platform vaporization surface. The simultaneous measurement of the atomic or molecular absorption signals and the mass spectrometric signal required additional equipment. Figure 3 is a schematic diagram of the simultaneous measurement system. The system operates as a single-beam spectrometer for both atomic and molecular absorption spectrometries. The sample is vaporized in the furnace, and the atomic and/or molecular vapor is carried 3210
Anal'icalChemistry,
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away by the argon carrier gas from the furnace to the ICPMS. Silver and manganese hollow cathode lamps (Perkin-Elmer) were operated at 10 and 15 mA, respectively. A deuterium lamp (Perkin-Elmer) was operated at 20 mA and was used for background correction. The deuterium lamp was positioned in place of the hollow cathode lamp and the background signal recorded under the same experimental conditions as were used for atomic absorption measurements. The gain on the detector was adjusted prior to sample vaporization such that the output d.c. current of the detector for the deuterium background correction set-up was about the same as was used for atomic absorption measurements. Lenses (12.5-cm focal length) were positioned to focus light from the radiation source at the center of the graphite furnace and to transmit light from the center of the furnace to the entrance slit of the monochromator. The entrance slit was set at 200 pm for all experiments, which provided a spectral bandpass of 0.7 nm. The resonance lines used for Ag and Mn AAS measurements were 328.1 nm and 279.5 nm, respectively. Resonance radiation was isolated by a 0.5-m Ebert-mount grating monochromator, 50 X 50 mm grating, ruled with 638 lines mm-l (Varian Techtron). The monochromator was fitted with a Hamamatsu R216 photomultiplier tube. The hollowcathode lamp power supply and the lock-in amplifier were laboratory-made and were synchronously modulated at 843 Hz. The detector response was set at 22 ms. Transient atomic or molecular absorption signals were recorded on a digital oscilloscope (Nicolet, Model 4094). Standards and Reagents. A 1000 pg mL-l stock solution of Ag was prepared by dissolving0.1579 g of AgNO3 (99.99%, Anachemia) in ultrapure water. The solution was then transferred quantitatively into a 100.00-mL calibrated flask that contained 0.2% (v/v) H N 0 3 (Ultrex, Baker). A series of standard solutions in the range 0.2-200 ng mL-l were prepared by serial dilution of the 1000 pg mL-l Ag stock solution. All standard solutions contained 0.2% (v/v) H N 0 3 (Ultrex, Baker). A 1000 pg mL-l stock solution of NaCl was prepared by dissolving 0.1000 g of NaCl (99.9%, AnalaR, BDH Chemicals) in 100.00 mL of ultrapure water. This solution was further diluted such that a 5-pL volume contained 100ng of NaCl. This solutionwas used as a chemical modifier for silver determinations. A stock solution of 400 pg mL-l aluminium was prepared by dissolving 0.4000 g of aluminium powder (99.99%, SPEX) in a minimum volume of 1+1 HCl (Ultrex, Baker). The resulting solution was diluted to 1000.00 mL with 0.2% (v/v) HN03. A 1000 pg mL-l stock solution of manganese was prepared by dissolving 0.5026 g of
Table 1. ETV-ICPMS Operating Conditions for Evaluation of Modifled EN and Dotermlnatlon of Ag
Heating Program" step drying pyrolysis temp, "C ramp time, s hold time, s
120 10 30
c
1m
vaporizationb 2200/variable/2400
500 10 20
1 5
Mass Spectrometer and Plasma Conditions sampler skimmer r.f. power, kW reflected power, W auxiliary Ar flow rate, L min-I coolant Ar flow rate, L min-I dwell time, ms measurements per m / z integration time, s
nickel, 1.14-mm orifice nickel, 0.89" orifice 1.05 5 0.9 15 30 1 10
0
100
200 300 400 500 Internal f l o w / d min"
600
700
Figure 4. Effect of internal Ar flow on the Integrated Ion lntenslty of 250 pg of Ag vaporized from the tube surface.
RESULTS AND DISCUSSION Optimization of Operational Parameters. The performance of the modified graphite furnace was evaluated using the following parameters: Ar flow rate within the graphite tube, total flow of Ar delivered to the ICP torch, and vaporization temperature. Calibration curves and detection limits were obtained for Ag vaporized under various experimental conditions. Silver was chosen as the test element because its mechanism of atomization is simple and well-known. It is generally accepted that Ag atoms are formed by direct vaporization of condensed silver.2G28The experimental and operating conditions used for this study are given in Table 1.
Internal Flow. The total Ar flow delivered to the ICP torch was set at 1 L min-l on the mass flow controller of the ELAN 5000. The internal and external Ar flows delivered to the ETV were varied independently in a manner such that the total flow of Ar delivered to the ICP torch remained at 1 L min-l for all experiments. Figure 4 shows the change in the integrated ion intensity for the vaporization of 250 pg of Ag while the internal flow was varied. The maximum integrated ion intensity of Ag was obtained at an internal flow of 450 mL min-l. At internal flows lower than 450 mL min-I, Ag atoms may be lost because of diffusion of Ag atoms toward the cooler end of the tube. The strong temperature gradient that exists along the tube length29J0allows Ag atoms to diffuse toward the ends of the tube and favors recondensation of Ag at the ends of the tube. This effect is expected to be important only for a low internal convective flow, such as 50 mL min-l, where analyte transport by diffusion is of the same order as convective flow in the opposite direction. A second reason accounting for decreased integrated ion intensity at low internal flow may be due to Ag vapor loss to vaporizer and transport tube surfaces as a result of poor self-nucleation and particle formation. Dean and Snook3I measured the atomic absorption signal of Ag at different vertical positions above a graphite rod used as an ETV and showed that Ag persisted as atoms for a distance of up to 2 cm for an Ar flow of 1 L m i d . Discrete atoms are more reactive than clusters or aggregates of atoms, and these may react with and/or be adsorbed onto surfaces with which they come into contact. Optimum self-nucleationallows better mass transport efficiency and, therefore, an increased integrated ion intensity. The cooler external Ar gas within the quartz ring and outside the graphite tube furnace serves to cool hot Ag vapor leaving the graphite tube, and there should be an optimum ratio of internal to external flow for a given total flow of Ar gas that provides rapid cooling promoting self-nucleation.20 For the modified ETV system studied here, the optimum ratio of internal (450 mL min-1) to external flow (550 mL min-') for Ag was calculated to be 0.8 18. Total Flow. The effect of varying the total Ar flow on the integrated ion intensity of 250 pg for Ag is shown in Figure
(26) Frech, W.; Lundberg, F.; Cedergreen, A. Prog. Anal. AI. Specrrosc. 1985,8, 257-270. (27) Smets, B. Specrrochim. Acto, Part E 1980, 35, 33-42. (28) Rowston, W. B.; Ottaway, J. M. Analyst 1979, 104, 645659.
(29) Findlay, W. J.; Zdrojewski, A.; Quickert, N. Spectrosc. Lerr. 1974, 7,63-72. (30) Slavin, W.; Myers, S.A.; Manning, D. C. Anal. Chim. Acra 1980,117,267273. (31) Dean, J. R.; Snook, R. D. J. Anal. A I . Spectrom. 1986, I , 461-465.
Sample volume, 5 pL. b Vaporization temperature of 2200 OC used for studies of the effect of the Ar flow (both internal and total flow) on the Ag+ signal; "variable" temperatures used for studying the effect of vaporization temperature on the Ag+ signal; vaporization temperature of 2400 "C used for calibrationcurves and for quantitativedeterminations of Ag.
manganese metal (99.94%, Fisher Scientific) in 10.00 mL of 1 + 1 HNO3 (Ultrex, Baker) followed by dilution to 500.00 mL with ultrapure water. A 5% (mass/v) magnesium chloride stock solution was prepared by dissolving 10.6700 g of MgC1206H20(99.4%, Baker Analyzed Reagent) in 100.00 mL of ultrapure water. A solution of 0.02 pg mL-l manganese plus 1% (mass/v) magnesium chloride in 1% (v/v) HCl (Ultrex, Baker) was prepared by serial dilution of the manganese and the magnesium chloride stock solutions with ultrapure water. Five-microliter volumes of test solution were deposited into the graphite furnace with an Eppendorf micropipet. The pipet was inserted through the quartz spout and the graphite tube dosing hole, and the test solution was deposited on the graphite tube wall directly below the dosing hole. The transfer line was then reconnected to the ETV and the heating program begun. The three-way stopcock was switched to the "ICP" position following a 10-s flush period to remove air from the transfer line.
Analyticel Chemistry, Vol. 66, No. 19, October 1, 1994
3211
F
1m
.B 3
B
ij
3
1
2oooo
tI 500
.
l
.
I
.
I
.
~
.
~
.
l
.
I
600 700 800 900 loo0 1100 1200 Total flow/ml m i d
Flgure 5. Effect of the total Ar flow on the integrated ion intensity of 250 pg of Ag vaporized from the tube surface.
1600
1800
ZOO0 2200 Temperature/'C
2400
2600
Flgure 6. Effect of the vaporization temperature on the integrated ion intensity of 100 pg of Ag vaporized from the tube surface.
5. The ratio of internal to external flow was set to theoptimum value of 0.8 18 for a total Ar flow of 1 L m i d . Figure 5 shows a maximum integrated ion intensity for Ag at 1000mL m i d . The effect of the total carrier gas flow on the ICPMS signal is well d o c ~ m e n t e d ~ ~ J ~Changing J2. the carrier gas flow in effect varies the sampling depth or position of the argon plasma relative to the sampling cone of the ICPMS. The optimum argon gas carrier flow rate corresponds to an ICP sampling position where the highest concentration of analyte ions exists in the plasma. As discussed above, the low integrated ion intensity for Ag obtained at low total argon flow may also be due to a change in the Ag nucleation process occurring within the ETV. Vaporization Temperature. Figure 6 shows the variation in the Ag integrated ion intensity as a function of the vaporization temperature. The internal and total Ar gas flow rates were set at 450 mL min-I and 1 L min-I, respectively. The maximum integrated ion intensity of Ag was obtained at a vaporization temperature of 2400 OC. Kantor20 reported that there is a minimum analyte vapor concentration below which self-nucleation will not occur. At low vaporization temperatures, the rate of vaporization of Ag may be too low to produce a Ag vapor concentration sufficient to allow adequate self-nucleation. At a temperature of 2400 OC, the rate of vaporization of Ag provided a high enough vapor(32) Vaughan, M.-A.; Horlick, G.; Tan,
s. H. J . A n d . At. Spectrom. 1987, 2,
165-772.
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phase concentration of the metal to promote efficient selfnucleation. However, if the vaporization temperature is increased above the optimum, the two following processes could occur separately or in concert: first, the rate of vaporization of Ag above 2400 OC could be such that the Ag vapor concentration would exceed a threshold value for optimum self-nucleation,resulting in the formation of particles sufficiently large to coalesce and deposit on transport surfaces in spite of large carrier gas second, the temperature of Ag atoms leaving the furnace could be so high that these atoms would require more time, or a greater distance, to cool before efficient nucleation could take place. This may cause the loss of Ag on the cool surfaces located along the transit path to the ICP. In both cases, a loss of Ag to the transport surfaces would result in a decrease in the integrated ion signal. This effect is analogous to ETAAS studies when atomization temperature are increased. In ETAAS, the atomization efficiency and the diffusional loss of analyte are increased to a different degree when the atomization temperature is increased, resulting in the existenceof an optimum temperature at which the difference between the rates of atom formation and of loss (by diffusion) is a maximum. Similarly, the optimum vaporization temperature of 2400 OC represents the temperature at which the difference between the rate of vaporization and loss of analyte (presumably by adsorption or by reaction with the transport surfaces15) is a maximum. CalibrationCurues. Calibration curves were obtained using background-corrected signals from the vaporization of analyte from both the tube surface and a graphite platform. Five microliters of Ag standard solution covering a concentration range of from 0.2 to 200 ng mL-' was used. When NaCl chemical modifier was used, 5 p L of a 20 pg mL-I NaCl solution (100 ng) was deposited onto thesurface of the graphite platform following the addition (and drying) of Ag solution. Sodium chloride was selected for evaluation as a chemical modifier because it has been shown to be beneficial for the determination of Ag by ETV-ICPMS.33 A total Ar flow rate of 1 L min-l, an internal Ar flow rate of 400 mL min-I, and a vaporization temperature of 2400 OC were used. These experimental conditions gave the greatest sensitivity for Ag using the modified furnace. Other experimental and operating conditions are given in Table 1. Thecalibration curves for silver, shown in Figure7, obtained without addition of NaCl (curves A and B, Figure 7) were not linear. Curvature of calibration curves in the low-mass range (0-50 pg of Ag) was observed for vaporization of Ag alone both from the graphite tube and from the graphite platform. The calibration curve for Ag obtained with the addition of 100 ng of NaCl (curve C, Figure 7) was linear over the entire mass range covered and had a slope of 1039 counts pg-l with an intercept of zero. There was only a slight improvement in the sensitivity for Ag vaporized alone from the graphite platform surface (curve B, Figure 7) compared to Ag vaporized alone from the graphite tube surface (curve A, Figure 7). Ediger and be re^^^ and Grdgoire et al.34 have reported curvature in calibration curves for several analytes. They attributed this nonlinearity to a mass transport effect. The (33) Ediger, R. D.; Beres, S. A. Appl. Specrrosc. 1992, 47, 901-922. (34) Gregoire, D. C.; AI-Maawali, S.;Chakrabarti, C. L. Spectrochim. Acto. Port B 1992, 47, 1123-1132.
400000
-
Y)
U
e +d
1
m
I
250
0
500
750
lo00
Mass of Ag/pg
Flgure 7. Callbratlon curves for Ag: (A) tube surface vaporization; (B) platform vaporlzatlon; (C) Ag vaporized from platform surface wlth 100 ng of NaCI.
0
1
2
3
4
5
6
7
8
Time16
Flgurs 8. ETV-ICPMS signal pulse for Ag Vaporized from a graphite platform: (A) 100 pg of Ag; (B) 100 pg of Ag plus 100 ng of NaCI.
greater the mass of analyte, the better was the transport efficiency because larger masses condensed rapidly into transportable particles which reduced the probability of condensation on transport line surfaces. Grbgoire et a1.34 showed a curvature of the analytical calibration curve for Ag in the low-mass range (0-4ng of Ag) when Ag was vaporized from a graphite strip ETV. A small curvature of the calibration curve for Ag in the low-mass range (0-50 ng of Ag) was observed in this work with the modified furnace for vaporization from both platform and tube surfaces. The addition of NaCl on the platform with Ag greatly improved the sensitivity for Ag compared to Ag vaporized alone. Figure 8 shows typical lo7Ag+signal pulses for 100 pg of Ag volatilized with and without 100 ng of NaCl from a platform and clearly shows the enhancement in the lo7Ag+ ICPMS signal when NaCl is added as a chemical modifier. The improvement in sensitivity caused by the addition of a chemical modifier has been reported for many elements using various chemical modifiers.14J5J3s34 Ediger and Beres,33 and Grbgoire et suggested that chemical modifiers act as physical carriers, resulting in improved transport efficiency of analyte from the vaporization surface to the ICP. The relatively large amount of chemical modifier that is covaporized with analyte condenses into particulate material more rapidly than the analyte. This allows the analyte vapor to condense onto these particles and be transported efficiently
to the ICP rather than being lost onto the cool surfaces of the ETV. This mechanism is consistent with those of Kantor20 and Gr6goire,15 who underlined the possible vapor-phase interactions that might increase the transport efficiency of the analyte to the ICP. Ediger and be re^^^ reported a 2-fold enhancement in the Ag sensitivity upon the addition of 100 ng of NaCl to 100 pg of silver for vaporization from a graphite tube. These authors used a Perkin-Elmer HGA 600MS graphite ETV, where the sample was transported from the middle of the tube furnace through the end cone and the transfer line to the ICP. An adapter replaced the furnace end window and a Teflon transfer line connected the adapter to the ICP. The small difference in sensitivity between vaporization from the tube surface and the platform surface should allow a direct comparison of the results of this work with those reported by Ediger and be re^.^^ We have obtained a 4.6-fold enhancement in the sensitivity, for 100 pg of Ag with 100ng of NaCl as the chemical modifier, which is 2 times better than that obtained by Ediger and be re^^^ using similar experimental conditions. The improvement in the sensitivity for Ag, when NaCl is used as a chemical modifier, may be attributable to a more efficient nucleation of Ag onto NaCl particles in the modified ETV compared to the HGA 600MS graphite ETV. The addition of sodium chloride to Ag also improved the repeatability of determination as reported by GrCgoire et a1.34 The mean value of the relative standard deviation (RSD) for the vaporization of from 10 to 250 pg of silver was reduced from 19.0 to 12.8% RSD when NaCl was added to silver. Absolute limits of detection determined for Ag vaporized alone from the tube surface, for Ag vaporized alone from the platform surface, and for Ag plus 100 ng of NaCl vaporized from the platform surface were 1.7, 1.6, and 0.23 pg, respectively. Comparison of ETAAS and ETV-ICPMS Signal Pulses. In order to correlate simultaneous measurement of ETAAS and ETV-ICPMS signals,various factors must first be assessed such as the difference in the appearance time of analytes measured using ETAAS and ETV-ICPMS, the relative peak broadening, and the relative distortion of scale. A prerequisite to this investigation was to verify that the carrier gas flow was not a turbulent flow, but was rather a laminar flow. For an Ar flow of 1 L min-I, and a transfer line of 75 cm X 0.5 cm i.d., the Reynolds number was found to be 341, indicating laminar flow. When vaporizing highly organic material at a temperature of 2200 OC, we observed that the sample vapor leaving the furnace is confined into a column of gas of diameter equal to the diameter of the spout. Further, the small stream of smoke produced flows out of the graphite furnace directly through the opening of the quartz spout and is confined in the vertical direction. This observation supports the conclusion that a laminar flow exists within the ETV device with the experimental conditions and instrumental setup used. Appearance Time. Figure 9 shows simultaneous recordings of atomic absorption (Figure 9b) and mass spectrometric signals (Figure 9a) for 250 pg of Ag vaporized from the graphite tube. The internal flow was set at 150 mL min-l and the total flow at 1 L min-l. All other experimental conditions are given in the experimental section and in Table 1. The appearance time, 71, is defined as the time at which AAS or Analytical Chemism, Vol. 66, No. 19, October 1, 1994
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Table 2. EN-ECP-MS Operating Condltlona for Time Scale Evaluation Studies
80000
step temp, OC ramp time, s hold time, s
d 0
drying
pyrolysis
vaporization
120 10
1200 10 10
2250
40
Sample vol, p L mass of A1 deposited, fig carrier gas flow rate total, min-1 internal, min-1 mass spectrometer and plasma conditionsb dwell time, ms OmniRange setting, for Z7Al+
105
2 5 2' 1 .o
0.05 30 5.0
Taken as the nitrate. *The same as shown in Table 1 except when stated otherwise. 0
1
2
3
4
5
6
7
Time/s Fbure 0. Analyte signal pulses for the vaporization of 250 pg of Ag with simuttaneaus measurement by ETAAS and ICPMS: (a) ICPMS signal; (b) AAS slgnal. Vaporlzatlon from the tube surface at 2400 OC. fwhm, full wldth at haif-maximum.
ICPMS signals become visible above the baseline noise. The appearance times for silver in ETAAS and ICPMS were 1.25 and 2.10 s, respectively. Both appearance times remained the same when the internal carrier gas flow was reduced to 50 mL min-I; thus, the ICPMS signal lagged behind the ETAAS signal by 0.85 s. This difference in the appearance time was due to the finite time required to transport Ag vapor from the ETV to the ICPMS (transit time). For ETAAS, the transit time is zero since the graphite tube serves as both the atomization surface and the analysis volume. The transit time for Ag vapor from the ETV to the ICPMS was calculated by assuming that once the analyte vapor entered the mass spectrometer interface, detection was instantaneous. For a total Ar flow rate of 1 L m i d thorough a transfer line of dimension 75 X 0.5 cm i.d., the transit time was calculated to be 0.88 s, which correlated well with the observed difference of 0.85 s between the appearance times of silver measured by ETAAS and ICPMS.
PeakBroadening. Peak broadening caused by the transport of analyte through the transfer line to the ICPMS can be measured by comparing the difference in the full width at half-maximum (fwhm) of each peak. Figure 9 shows that the fwhm of the Ag peak in ICPMS (Figure 9a) is 45% broader than the Ag peak in ETAAS (Figure 9b). The fwhm of the silver peak in ICPMS and in ETAAS was 0.46 and 0.34 s, respectively. At lower internal carrier gas flows, the ICPMS silver peak broadened even more. The fwhm of the ICPMS silver peak was 56% greater than that obtained in ETAAS for an internal carrier gas flow of 50 mL min-l. The broadening was probably due to diffusion broadening of the Ag vapor during its transport in the transfer line and in the plasma. Difference in the Time Scale. The broadening of signal pulses in ETV-ICPMS may limit its use for investigating phenomena closely related in time, such as the appearance of 3214
Analytical Chemistry, Vol. 66, No. 19, October 1, 1994
double Pb peaks3sor the formation of aluminium spikes36in ETAAS. Also, differences between the time scale of the ICPMS and the ETAAS signals can present additional problems when mechanistic studies in ETAAS are being carried out. For example, if an ETAAS signal, composed of two or more distinct events, cannot be correlated with the corresponding ICPMS signal by a common time scale, then conclusions based on the simultaneous measurement of the ETAAS and ICPMS signals could be erroneous. Differences in time scale were investigated by vaporizing 2 pg of A1 as the nitrate at a low heating rate in order to form atomic absorption spikes of aluminium.36 It has been reported that the atomization of relatively large masses (micrograms) of aluminium at low heating rates ( 4 - 2 0 OC s-l) causes A1 atomic absorption spikes to appear on top of a smoothly rising A1 atomic absorption signal. It is not the purpose of this work to explain the occurrence of the A1 spikes although these can be used to assess any difference in time scale between the ICPMS and the ETAAS signals. The experimental conditions used for this study are given in Table 2. Figure 10 shows the simultaneous recording of the ETAAS (Figure lob) and the ICPMS (Figure loa) signals for aluminium spikes. The appearance times, 71, 72, and 73, of the first three peaks in Figure loa, were27.02,40.12,and 51.30f 0.07s,respectively. Similarly, the appearance times, 71, 72, and 73, of the first three peaks in Figure lob, were 26.52, 39.50, and 50.45 f 0.07 s, respectively. The appearance times of the aluminium spikes in the ICPMS correlated well with their ETAAS analogues, and the difference between the appearance times was not greater than the transit time of 0.88 s required for the A1 vapor to travel from the ETV to the plasma of the ICP. Also, the time separating a peak maximum from the next peak maximum, Atmax,was calculated for both AAS (Figure lob) and ICPMS (Figure loa) signals. For the first and second peak maxima, Atmax for ETAAS and ICPMS signals were 13.40 and 12.95 f 0.07 s, respectively; for the second and third peak maxima, Atmaxfor ETAAS and ICPMS were 11.10 and 10.90 f 0.07 s, respectively; and for the third and fourth peak maxima, Atmax for ETAAS and ICPMS were 8.05 and 8.30 f 0.07 s, respectively. The above results clearly (35) McLaren, J. W.; Wheeler, R. C. Analysr 1977, 102, 542-546. (36) L'vov, B. V.;Polzik, L. K.;Romanova, N. P.;Yuzefovskii, A. I. J. Anal. AI. Specrrom. 1990, 5, 163-169.
Table 3. ETAAS Operating Conditlonr for Mn Studios stea
drying drying pyrolysis cool atomize clean temp, O C hold time, s ramp time, s record, s read, s baseline, s gas flow internal (mL min-1)
t....,...... 10 20
0
....... - . . . . * . . . . . . . . . I 30
40
50
60
70
90 40 10
120 20 5
400 3 2
variable 40 10
2500 7
2700 3
1
1
-2 300
300
300
carrier gas flow rate total, 1 min-I internal, mL min-1 sample, vol, p L mass spectrometer and plasma conditions' dwell time, ms r.f. power, kW
0 -1 50
300
300
1 .o 50 5
20 1 .o
Time/s
Figure 10. Analyte signal pulses for the vaporization of 2 pg of AI with simultaneous ETAAS and ICPMS: (a) ICPMS signal; (b) ETAAS signal. Heating rate, 10 OC s-l; Initial temperature, 1650 OC.
show that there is no significant difference in the time scale since 7 and Atmaxvalues correlate well between ETAAS and ICPMS peak analogues. This implies that investigations of fundamental processes occurring in ETAAS can be studied by measuring simultaneously the AAS and ICPMS signals, provided that broadening of the ETV-ICPMS signal pulses has no significant impact on the interpretation of the experimental results. Interference of MgClz Matrix on the Determination of Mn by ETAAS. Byrne et a1.37 have investigated the mechanism of chloride interferences with the determination of manganese in ETAAS using a graphite strip ETV coupled to an ICPMS. These authors have reported that manganese is lost during the pyrolysis step because the magnesium chloride matrix undergoes hydrolytic decomposition, and the manganese is carried away from the graphite furnace with the hydrogen chloride gas generated by the hydrolysis reaction. It is possible that the differences in geometry and heating characteristics of a graphite strip ETV and a graphite tube ETV (such as those used in ETAAS) may be such that the graphite strip ETV may not be used to study effects occurring within a graphite tube. The formation and dissipation of atoms from a graphite strip may be different from those occurring in a graphite tube furnace in ETAAS even if the heating characteristics of the atomization surface are identical. The gas-phase temperature and the probability of atoms colliding against the graphite surface leading to secondary reactions in a tube furnace are expected to be different from those in a graphite strip. Therefore, the preatomization loss observed by Byrne et al.37in the ETV-ICPMS experiments may not be comparable to the actual preatomization loss that occurred in the ETAAS experiments. In order to verify that the preatomization loss observed in the ETV-ICPMS was not specific to the ETV unit used, but was rather a genuine effect caused by the hydrolytic decomposition of the MgClz matrix, the interferences by MgClz matrix on the determination of (37) Byrne, J. P.; Chakrabarti, C. L.; Grkgoire, D. C.; Lamoureux, M.; Ly, T. J . Anal. At. Spectrom. 1992, 7, 371-381.
(1
Same as those shown in Table 1 except when stated otherwise.
0
10
20
30 40 Timels
50
1
60
70
Figure 11. Analyte signal pulses for the vaporization of 100 pg of Mn in 1 % (v/v) HCI plus 200 pg of MgClp with simuitaneous ETAAS and ICPMS:(a) ICPMS signal; (b) ETAAS signal. Vaporization temperature, 2500 OC; pyrolysis temperature, 1000 OC.
Mn was investigated using the modified furnace. The modified Perkin-Elmer HGA 76B furnace and the Perkin-Elmer HGA 500 graphite furnace used by Byrne et had identical heating characteristics and were of the same geometry. The experimental and operating conditions used here were the same as those used by Byrne et ale3' and are given in Table 3 as are the experimental and operating conditions for the ICPMS. Figure 11 shows the simultaneous measurements of the ETAAS (Figure 11b) and ICPMS (Figure 1la) signals from 200 pg of Mn in 1% (v/v) HC1 plus 100 pg of MgC12 using a pyrolysis temperature of 1OOO'C. The ICPMS signal (Figure 1la) demonstrates that a large amount Mn is lost during the pyrolysis step. The ETAAS signal (Figure 1 lb) shows no Mn atomic absorption signal during the pyrolysis step other than a small signal (20-26 s) due to background interferences not fully corrected by the deuterium arc background-correction system. This indicates that the Mn must have been lost in Analytical Chemistry, Vol. 66,No. 19, October 1, 1994
3215
molecular form since no atomic absorption signal was detectable during the pyrolysis step. The preatomization loss of Mn in ETV-ICPMS for a pyrolysis temperature of 1000 OC was calculated to be 90.6%,with the remaining 9.4%Mn vaporized during the atomization step. The corresponding preatomization loss in ETAAS for the same pyrolysis temperature wascalculated by taking the ratioof the integrated absorbance for Mn in the 1% (v/v) HC1 plus MgCl2 solution (Figure 4, curve B, from ref 37) and the integrated absorbance for Mn in the 1% (v/v) H N 0 3 solution (Figure 4, curve E, from ref 37). In ETV-ICPMS, the integrated ion intensity for MR in the 1% (v/v) HCl was the same as for Mn in the 1% (v/v) H N 0 3which indicated that there was no interference by HCl on the Mn signal. However, in ETAAS there was an interference by 1%(v/v) HCl on the Mn integrated absorbance (Figure 4, curve E, from ref 37) whereas there was none when 1% (v/v) HNO3 was used as the diluent (Figure 4, curve A, from ref 37). Therefore, it was necessary to calculate the ratio of the integrated absorbance using Mn in 1% (v/v) HNO3 since it represented the total amount of Mn measured from a solution free from chlorideinterferences. The preatomization loss of Mn in ETAAS was found to be 89.7% with the remaining 10.3%Mn vaporized during the atomization step. The Mn preatomization loss of 89.7%in ETAAS correlated well with the Mn preatomizationloss of 90.6%in ETV-ICPMS. Therefore, the preatomization loss of Mn observed in both graphite strip ETV-ICPMS and graphite tube ETV-ICPMS
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Analjltcal Chemlstty, Vol. 68, No. 19, October 1, 1994
corresponded to preatomization loss of Mn observed in ETAAS, and the conclusions that were drawn by Byrne et al.37using a graphite strip are supported by the measurements made using the graphite tube ETV.
CONCLUSION The modified ETV can be used as a sample introduction device for ICPMS, and good limits of detection were obtained for silver. The design of the modified ETV is unique; Le., the sample vapor is extracted from the dosing hole as opposed from the tube end, and this allows the modified ETV to be used in the normal ETAAS configuration. The simultaneous measurement of the ETAAS and ICPMS signals provides the opportunity to investigate common problems in ETAAS, such as preatomization loss of analyte or matrix interferences. ACKNOWLEDGMENT The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support. They also thank Dr. J. C. Hutton of Battelle Pacific Northwest Laboratories for helpful discussion and useful comments while writing this paper. M.M.L. also thanks NSERC for the award of a postgraduate scholarship. G.S.C. publication 47893. Recehred for revlew February 15, 1994. Accepted June 3, 1994.' Abstract published in Advance ACS Abstracts, August 1, 1994.
