Anal. Chem. 1997, 69, 2129-2135
Furnace Atomization Plasma Ionization Mass Spectrometry R. E. Sturgeon* and R. Guevremont*
Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R9
Development and initial characterization of FAPES as an ion source for elemental mass spectrometry is presented. The source is configured with an integrated contact cuvette in which a 40 MHz He plasma is sustained at atmospheric pressure. A differentially pumped interface consisting of a stainless steel sampling tube (0.5 mm i.d.) and tandem skimmer cone serves to sample the plasma from the open end of the cuvette. Mass spectra, characterizing plasma species with the furnace at ambient and elevated temperatures, are relatively structureless and show no evidence of He+, He2+, or carbonaceous ions. Transient ion signals, generated during the atomization of a number of analytes introduced in solution form, reveal that the plasma contains sufficiently energetic species to ionize elements having ionization potentials as high as 10.45 eV (iodine). Significant ionization does not occur in the absence of the plasma, nor in the extraction interface. Using Pb as a test element, acceptable isotopic abundances can be obtained, with an estimated absolute limit of detection of 10 pg (2 ng/mL relative). Future directions are discussed. With the likely exception of laser excited atomic fluorescence in the graphite furnace,1 atomic mass spectrometry offers detection power that is generally superior to that of optical methods. This technique is also synonymous with providing broad elemental coverage, wide dynamic range, isotope abundance information, and the capacity to implement one of the five primary (definitive) methods of measurement, isotope dilution mass spectrometry.2 The recent explosive growth in the analytical use of atomic mass spectrometry can be directly traced to the availability of reliable instrumentation for plasma source MS, specifically ICPMS. Development and characterization of sample introduction systems to enhance the overall capabilities and broaden the scope of application of ICPMS currently comprise significant research resources. The tandem coupling of an electrothermal vaporizer (ETV) with ICPMS is one such example.3 This arrangement permits sampling of solids (both solids and slurries of solids4 ), liquids, and gases (electrostatically precipitated particles from gases and volatile forms of metals such as their hydrides sequestered onto the furnace surface5 ), as they can be volatilized and transported to the ICP for subsequent atomization and (1) Sjostrom, S. Spectrochim. Acta Rev. 1990, 13, 407-465. (2) Moody, J. R.; Epstein, M. S. Spectrochim. Acta, Part B 1991, 46, 15711575. (3) Carey, J. M.; Caruso, J. A. Crit Rev. Anal. Chem. 1992, 23, 397-439. (4) Gre´goire, D. C.; Miller-Ihli, N. J.; Sturgeon, R. E. J. Anal. At. Spectrom. 1994, 9, 605-610. (5) Sturgeon, R. E.; Gre´goire, D. C. Spectrochim. Acta, Part B 1994, 49, 13351345. S0003-2700(96)01198-5 CCC: $14.00 Published 1997 Am. Chem. Soc.
ionization. While not a panacea, ETV sample introduction for ICPMS possesses several attractive features over solution nebulization, including microsampling capabilitity, high efficiency, reduced analyte oxide fractions, enhanced detection power, reduced spectral interferences from sample solvents, and the potential for control of some matrix interferences, as well as direct element speciation through selective volatilization.6 Transport losses7 constitute one of the primary disadvantages of this tandem source arrangement. Whereas analyte introduction efficiency remains higher than that achieved with pneumatic nebulization, losses can occur at several locations in the conduit between the furnace and the ICP.8 While adding a physicochemical carrier to the system often serves to alleviate this problem,7,9 this approach is not optimal, as it may result in contamination and, above all, is not well-enough understood at present to be applied with impunity.10 If the intervening connection between the ETV and the ICP were to be eliminated, transport losses in the line and the gas sampling valve would become insignificant and the robustness of the approach enhanced. Hoffmann et al.11 attempted to achieve this by coupling the output of a furnace atomic nonthermal excitation spectrometry (FANES) graphite furnace directly to the central channel of the plasma torch through a short (presumably graphite) linear interface. Nonlinear calibration curves, characteristic of transport loss,12 were still observed. Use of a combined or integrated tandem source13 would also eliminate the transport loss phenomenon. In such an arrangement, an ETV capable of vaporization/atomization and ionization of analyte could replace the ICP itself. Furnace atomization plasma emission spectrometry (FAPES) potentially provides such a source.14,15 Characteristic of all “enclosed” graphite furnaces, the vaporization/atomization efficiency is high;16 the combination of an rf (He) plasma sustained at atmospheric pressure inside the furnace provides optical detection limits comparable to those of graphite furnace atomic absorption spectrometry,17 as the nonLTE plasma possesses high excitation (3500 K) and ionization (6) Gre´goire, D. C. Trends Appl. Spectrosc. 1993, 1, 313-324. (7) Ediger, R. D.; Beres, S. A. Spectrochim. Acta, Part B 1992, 47, 907-922. (8) Sparks, C. M.; Holcombe, J. A.; Pinkston, T. L. Appl. Spectrosc. 1996, 50, 86-90. (9) Hughes, D. M.; Gre´goire, D. C.; Chakrabarti, C. L.; Sturgeon, R. E.; Byrne, J. P.; Goltz, D. M. Spectrochim. Acta, Part B 1995, 50, 425-440. (10) Gre´goire, D. C.; Sturgeon, R. E. Spectrochim. Acta, Part B 1993, 48, 13471364. (11) Hoffmann, E.; Lu ¨ dke, Ch.; Scholze, H. J. Anal. At. Spectrom. 1994, 9, 12371242. (12) Kantor, T. Spectrochim. Acta, Part B 1988, 43, 1299-1320. (13) Borer, M. W.; Hieftje, G. M. Spectrochim. Acta Rev. 1992, 14, 463-486. (14) Liang, D. C.; Blades, M. W. Spectrochim. Acta, Part B 1989, 44, 10591063. (15) Sturgeon, R. E.; Willie, S. N.; Luong, V. T.; Berman, S. S.; Dunn, J. G. J. Anal. At. Spectrom. 1989, 4, 669-672. (16) Frech, W.; Baxter, D. C. Spectrochim. Acta, Part B 1990, 45, 867-886.
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997 2129
Figure 1. Schematic of the FAPIMS instrumentation.
temperatures (5000 K).18 Although their intensities are weak in comparison to the corresponding atom lines,17 emission signals from the ionic lines of several metals have been observed in the FAPES source, confirming that analyte ionization occurs. It thus remains to explore the potential of FAPES as an ion source for MS. This study reports on the initial characterization of an instrument comprising an integrated contact cuvette-based FAPES source and a research prototype of a commercial Sciex ELAN 6000 mass spectrometer. EXPERIMENTAL SECTION Apparatus. All studies were conducted with a water-cooled FAPES source19 based on an integrated contact cuvette (ICC) pyrolytic graphite-coated graphite furnace of a design similar to that described by Ballou et al.20 A coaxial 1 mm o.d. graphite rod, serving as the powered electrode, traversed the length of the ICC and terminated flush with the end of the tube. Radio frequency (rf) power was supplied to the electrode from a Model RF10L 40 MHz generator and AM-5 matchbox (RF Power Products, Voorhees, NJ). A “blocking” network consisting of capacitors and inductors was placed in the rf line to permit both measurement and control of the dc bias on the powered (center) electrode. The FAPES workhead was mounted on a 15 cm o.d. aluminum feedthrough flange, which also served to provide access ports for furnace power, purge gas, water cooling, rf connection, and temperature feedback. A schematic of this arrangement is depicted in Figure 1. The unit was enclosed within a plexiglass cylinder (12 cm i.d. × 20 cm long), which at one end formed a gas-tight seal to the aluminum flange via an O-ring, and at the other end was closed with a removable end-cap containing a port to permit exit of the purge gas. This cylinder was additionally fitted with a Swagelok port directly above the furnace injection hole to permit dosing of samples with use of an adjustable microliter pipet. A 1 cm long × 1 cm i.d. side port, aligned with (17) Sturgeon, R. E.; Willie, S. N.; Luong, V. T.; Berman, S. S. Anal. Chem. 1990, 62, 2370-2376. (18) Sturgeon, R. E.; Willie, S. N.; Luong, V. T. Spectrochim. Acta, Part B 1991, 46, 1021-1031. (19) Sturgeon, R. E.; Willie, S. N.; Luong, V. T.; Dunn, J. G. Appl. Spectrosc. 1991, 45, 1413-1418. (20) Ballou, N. E.; Styris, D. L.; Harnly, J. M. J. Anal. At. Spectrom. 1988, 3, 1141-1143.
2130
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
the cross-sectional end of the ICC furnace, permitted access to the mass spectrometer “sampling” tube. The cylinder could be overpressurized (10 cm water) with a flow of He gas, as provision was made to connect the system to a manometer and gas flow meters. The furnace was heated by a Perkin-Elmer HGA-500 power supply. The temperature feedback controller for this unit was interfaced to a remote photodiode which monitored the temperature of the ICC, thereby providing “maximum power” heating capability. A testbed mass spectrometer which was a research prototype of the Perkin-Elmer Sciex Elan 600021 was used. This unit features increased pumping speed on the mass analyzer chamber (Leybold Heraeus Model 360 turbo pump). A number of modifications to the interface were undertaken, primarily required as a result of pumping a He plasma as opposed to Ar. The instrument’s interface roughing pump was replaced with a Model EH500A mechanical booster (Roots) pump and E1M80 backing pump (Edwards High Vacuum, Wilmington, MA), having a combined effective pumping speed of 400 m3 h-1. A Model PRM10 Pirani gauge with Model 503 controller (Edwards) was mounted about 15 cm from the interface to provide a continuous measure of the pressure in this region. A conventional nickel skimmer cone having a 0.88 mm diameter aperture was installed with an O-ring backing and the shadow stop removed. The sampler cone was replaced by a modified unit whose cone tip had been machined down to the flat threaded base, into which a 1/8 in. brass Swagelok union was brazed. This arrangement is schematically illustrated in Figure 2. That portion of the union protruding through the back side of the cone was removed to reveal the original tapered interior surface. A stainless steel tube, 6.5 cm long × 1.6 mm o.d. × 0.5 mm i.d., inserted through the Swagelok union, protruded about 5 mm through the back side and was seated with use of a 1/8-1/16 in. PTFE reducing ferrule. When threaded into the stainless steel interface of the mass spectrometer plate (with an O-ring seal), the tube terminated about 6 mm from the tip of the skimmer cone orifice. In operation, the exposed end of the sampler tube was capped with a 6.5 mm o.d. × 1.4 cm long cylinder of boron nitride (grade (21) Tanner, S. D. J. Anal. At. Spectrom. 1995, 10, 905-922.
Table 1. Data Acquisition Parameters full spectruma replicate time, ms dwell time, ms scanning mode sweeps/reading readings/replicate no. of replicates points/spectral peak resolution a
Figure 2. Schematic of the sampling interface.
HBC, Advanced Ceramics Corp., Cleveland, OH), containing a 6 mm long × 0.5 mm i.d. channel for sampling the plasma. Experiments were conducted with the tube positioned 0.2-2 mm from the center electrode of the FAPES source. This was achieved by feeding the sampler tube through the side arm of the plexiglass chamber and forming a seal to it with use of a flexible rubber septum. This arrangement provided flexibility in the independent movement of the FAPES source so as to permit optimization of the sampling position of the tube relative to the plasma. Some experiments were undertaken with the BN cap replaced with one made of Macor (as illustrated in Figure 2), designed to accommodate both the primary 0.5 mm i.d. channel sampling the plasma and a second channel, perpendicular to the first, located approximately 8 mm downstream from the end of the cap. This side channel was fitted with a 1/16 in. PTFE line to enable delivery of a flow of He gas, saturated with mercury vapor, directly to the interface, downstream of the plasma. Introduction of a flow of mercury-saturated helium facilitated investigations aimed at identifying the source of ionization. Measurements of gas flows were undertaken with bubble flowmeters. At an overpressure of 10 cm of water, a He flow of approximately 10 L/min (minimum) was admitted to the plexiglass chamber; an estimated 2 L/min is drawn through the sampling tube. The interface pressure was maintained at approximately 0.6 Torr. When the Macor end-cap was used to admit He through the side channel downstream of the plasma, a free flow of 200 mL/min He augmented a controlled 20 mL/min stream of mercury-saturated He. The latter was controlled with a needle valve on a 1/16 in. PTFE line, exiting a mercury reservoir through which He had been directed. A tee in this line permitted delivery of this mercury stream to either the Macor side arm or the sampling port on the plexiglass chamber, from which it was directed into the sample dosing hole of the ICC. In this manner, a steady-state ion signal for mercury could be monitored, the source of which was either the FAPES plasma or a location within the sampler tube interface. In operation, the ion optic chamber pressure could be typically maintained at 5 × 10-3 Torr, whereas that of the analyzer chamber was 5 × 10-5 Torr. The base pressure of the latter was 2 × 10-7 Torr. It should be noted that these measurements were taken from the ELAN ion gauges, which are calibrated to respond to
10 10 1 1 1 3-10
transientsb
transient spectrumc
10 10 peak hop 1 1 100-200 1 normal
1 1 1 1 15 10
Figures 3, 4, and 10. b Figures 8 and 9. c Figures 5, 6, and 7.
Ar, with the result that there is a bias present and the actual pressures are higher than quoted. Various potentials were applied to components of the interface using independent power supplies: typically -40 V on the aperture plate separating the ion optic and analyzer chambers, +10 V on the gold-plated cylinder lens, and +30 V on the sampler tube. All were adjusted for optimum response. Reagents. Industrial grade He (99.995% pure, Air Products Ltd., Nepean, ON, Canada) was used throughout. Stock solutions (1000 mg/L) of metals and anions were prepared by dissolution of their nitrate or chloride salts and the sodium analogues of the halogens, respectively, in high-purity deionized distilled water (DDW) obtained from a NanoPure system (Barnstead/Thermolyne Corp., Boston, MA). Working solutions were prepared by serial dilution of the stocks with DDW. Procedure. The optimum distance between the end of the sampling tube and the tip of the skimmer cone was established which permitted sufficient vacuum in the analyzer chamber to operate the instrument without tripping preset factory limits. Voltages applied to the various interface components were subsequently optimized during successive repetitive atomization events utilizing lead as a test element. The optimum position of the sampling tube with respect to both its distance from the FAPES source and location within the cross section was also investigated. Mass spectral scans were undertaken in the range 1-250 u at various applied rf powers with and without heating the ICC tube. In each case, a steady-state temperature was established before acquiring data. Table 1 summarizes data acquisition parameters used for both mass spectral scans and recording signal transients during the atomization of analytes. For the latter, 5 µL volumes of test solution were pipetted into the ICC furnace, whereupon the solution was dried by applying a thermal program consisting of a 10 s ramp to 100 °C, followed by a hold for 10 s; the residue was “ashed” at 250 °C by applying a 10 s ramp and 10 s hold. The rf power was then turned on (typically 100 W, 0 W reflected) and the high-temperature atomization cycle manually initiated. “Maximum power heating” or a 1 s ramp to 1500-2500 °C was used, depending on the analyte under study. Data acquisition was manually triggered approximately 2 s prior to the atomization event. Files were exported from the ELAN signal graphics program for subsequent numerical evaluation (peak height and area) with in-house software. Where possible, at least two isotopes of a given element were monitored simultaneously with a proximal background mass. Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
2131
RESULTS AND DISCUSSION Consideration of the relationship between gas flow rate through a sampling orifice and the physical properties of a sampled plasma shows that a significantly smaller aperture is required for He plasmas as compared to those operated in Ar; for a 3000 K gas kinetic temperature, a 0.5 mm orifice will extract 2 L/min He.22 As this may be the maximum gas kinetic temperature encountered in the FAPES source18 (during the high-temperature atomization cycle), it is clear that an even smaller orifice may be required when the ICC is operated at room temperature if pumping requirements are to be kept realistic. If the bulk plasma is to be sampled, then the minimum orifice diameter should not be less than 0.14 mm; otherwise, contributions from the cooler boundary layer become significant.22 Room temperature sampling of the FAPES plasma (less than 1000 K) was possible with a 0.5 mm diameter sampler tube in the present case because of the decreased conductance imposed by the length of the tube and the possible slightly off-axis transmission of gas between the skimmer and the tube exit. Despite this, it was necessary to carefully adjust the distance between the skimmer tip and the terminus of the tube in the sampler cone body to achieve conditions under which the pressure in the analyzer chamber would permit operation of the spectrometer, irrespective of the enhanced interface pumping capability. While estimations of the position of the Mach disk arising with such a tubular sampler are likely to be in error (using a simple calculation relating position to orifice diameter and pressure in the interface23 ), an interface pressure of 0.6 Torr suggests this to occur 11.9 mm downstream of the sampler. Accordingly, the skimmer must be located upstream of this point, and, experimentally, it is placed 6 mm from the end of the sampling tube. With this arrangement, the interface capillary extracts He from the ICC furnace at a flow rate of 2 L/min. Use of He as a plasma gas for mass spectrometry offers several advantages over Ar,24 not the least of which are the enhanced spectral selectivity arising from the near-monoisotopic low mass of He and the presence of higher energy species which should augment the detection capability of difficult-to-ionize elements. Although no definitive excitation or ionization mechanisms for He plasmas have been deduced,25 it is likely that high-energy species such as He+, He2+, Hem*, and fast electrons can participate in Penning ionization, charge exchange, and electron impact ionization processes.25-27 Since it has been estimated that the power available in the central channel of an Ar ICP is on the order of only 100 W28 at 1 kW forward power, it is not unreasonable to expect that the power density within the FAPES source is comparable to that of an ICP when operated at a forward rf pwer of 100 W, as maintained here. Hence, there is an a priori expectation that FAPIMS should function satisfactorily as an ion source. Detection of emission from excited ionic states in the FAPES serves as evidence of this.17 Background Mass Spectrum. Figure 3 presents a typical background mass spectrum obtained with the ICC furnace at room (22) Zhang, H.; Nam, S.-H.; Cai, M.; Montaser, A. Appl. Spectrosc. 1996, 4, 427435. (23) Douglas, D. J.; French, J. B. J. Anal. At. Spectrom. 1988, 3, 743-748. (24) Nam, S.-H.; Zhang, H.; Cai, M.; Lim, J.-S.; Montaser, A. Fresenius J. Anal. Chem. 1996, 355, 510-520. (25) Brandl, P. G.; Carnahan, J. Spectrochim. Acta, Part B 1994, 49, 105-115. (26) Beenakker, C. I. M. Spectrochim. Acta, Part B 1977, 32, 173-187. (27) Houpt, P. M. Anal. Chim. Acta 1976, 86, 129-138. (28) Olesik, J. W. Anal. Chem. 1996, 68, 469A-474A.
2132 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
Figure 3. Mass spectrum illustrating background with ICC at room temperature.
temperature and a 100 W rf power. Apart from a large signal at masses corresponding to hydrogen and its isotopes, there are no other significant spectral features; this, in itself, is of interest. The intense groupings above m/z 200 are due to Hg+ (m/z 198,199, 200, 201, 202, and 204), Hg(OH)+ (displaced 17 Da higher), and Hg(OH)2+ (34 Da higher). These species are artifacts of the experimental system and were present as impurities within the plexiglass chamber, earlier introduced in some experiments (described below). The mass spectral background of a dry He MIP has been reported to be free of spectral interferences above m/z 40, with the most abundant ions being 8He2+, 16O+, and 14N+.29 Their absence from this spectrum cannot be attributed to a lack of sensitivity of the detection system. It is likely that these species are short-lived (possibly due to electron-ion recombination) and not efficiently transported through the tubular interface, or that they do not play the role of major species in this plasma. Thus, the mass spectra may not accurately reflect the ionic composition of the plasma due to postsampling reactions. Somewhat noteworthy is that Montaser does not observe He+ and He2+ with an atmospheric pressure He ICP operated at 500-700 W power.30 Similarly, Wu et al.31 note rather feeble intensities from these ions compared to other major background species when using a helium microwave plasma torch source at atmospheric presure. It is also of interest that intense bands from CO+ and N2+ have been observed in emission from the FAPES source17 but are absent from the mass spectrum. With the present configuration, the FAPES source is likely well-shielded from atmospheric entrainment, and thus significant signals from nitrogen and water adducts observed by others are not seen. The relatively structureless spectrum, averaging a few hundred counts per mass unit, is opportunistic for atomic MS applications. This count rate is likely biased high; considering that the data were acquired using a dwell time of 10 ms, the spectrum really reflects only background noise in random counting events. Figure 4 illustrates the background spectrum under the same conditions when the ICC furnace is held at a steady-state temperature of 1500 C. No significant change is evident, apart from the increase in intensities for the mercury species. This may be ascribed to increased availability of mercury due to its desorption from heated surfaces within the enclosure. It was necessary to acquire this high-temperature scan relatively quickly (15 s), in order to minimize heating of the plexiglass housing. Consequently, only three points were used in the (29) Olsen, L. K.; Caruso, J. A. Spectrochim. Acta, Part B 1994, 49, 7-30. (30) Montaser, A. Personal communication, 1996. (31) Wu, M.; Duan, Y.; Jin, Q.; Hieftje, G. M. Spectrochim. Acta, Part B 1994, 49, 137.
Figure 6. Mass spectral scan acquired during the atomization of Cu. Figure 4. Mass spectrum illustrating background with ICC at 1500 °C.
Figure 7. Mass spectral scan acquired for Hg (introduced continuously into the source) during ramp heating to 2400 °C. Figure 5. Mass spectral scan acquired during the atomization of Cd.
software to define each peak (as opposed to the 10 points used for Figure 3), with the result that visual “resolution” is severely degraded and the isotopic sets are blended into “bands” rather than discrete peaks. Analyte Detection. A number of different analyte cation and anion solutions were individually dosed into the ICC and vaporized/atomized into a 100 W plasma. Transient signals were recorded for at least two isotopes, where possible, as well as a proximal background mass. In this manner, the identity of the signal detected could be easily confirmed if the correct isotope ratios were obtained. The species tested and detected in this manner had ionization potentials ranging from 3.89 to 11.84 eV and included Cs, Rb, K, Na, Pb, Mg, Cu, Sb, Cd, Zn, Se, S, Hg, I, P, and Br. These elements permitted a cursory examination of the system to be undertaken in an effort to define the limitations of the plasma with respect to ionization capability. No signals from elements having ionization potentials above 10.5 eV could be detected with the present arrangement. It is not clear at this time what “buffering” agent is present in the plasma to induce this limitation, or if it is an artifact of the sampling process. Further studies are required to elucidate this. Figures 5-7 illustrate some of the mass spectral scans generated in the immediate vicinity of interest for several of the above elements. Cadmium, copper, and mercury have been selected for illustration, as they collectively present a cross section of elements with disparate volatility (Hg > Cd > Cu) and ionization potential (10.43, 8.99, and 7.74 eV for Hg, Cd, and Cu, respectively), which serve to characterize the performance of the system. In each case, the deposited solution of the element was dried at 110 C, “ashed” at 300 °C for 5 s, and then volatilized into a 100 W plasma by ramp heating over 1 s to 2400 °C, followed by a hold at this temperature for 4 s. Figure 5 shows the typical isotope
abundance pattern for Cd. It must be noted that, since these data present the summed intensities for several scans of the spectral window (each scan requiring about 1 s), during which time the element population transient evolves, the resulting isotopic ratios cannot be expected to be correct. Clearly, rapid acquisition of data for only two or three isotopes would produce a more accurate pattern, or, alternatively, simultaneous detection of all isotopes achieved with time-of-flight approaches would be preferable. Figure 6 displays the mass spectral scan for the atomization of copper under the same conditions as for Cd. Clearly evident are the 63 and 65 isotopes of Cu which, despite the unfavorable data acquisition, produce a 63/65 ratio only 6% different from that expected. It is clear from the above that the background under and in the immediate vicinity of these isotopes is free of any significant interference, and it is possible to tentatively identify signals from other elements such as 56Fe, 58Ni, and 64Zn, 66Zn, and 68Zn which were likely present as impurities in the dosed copper solution. Figure 7 presents a spectral scan for mercury. In this case, the analyte was continuously introduced via the gas phase throughout the duration of the heating transient. It should be noted, however, that accurate isotope ratios still cannot be expected during such multiple scans since the source temperature, and hence plasma conductance through the transfer tube, vary with time (temperature). Nevertheless, the isotopic fingerprint is evident. The contribution of thermal processes to the production of the ion signal was estimated by comparison of response from the atomization of several easily ionized elements (EIEs) in the presence and absence of the rf plasma. With an atomization temperature of 1500 °C, no signal was obtained for Rb (800 ng) in the absence of a plasma. With the same atomization program, more than 3 × 106 ions/s were registered with the plasma on. Using an atomization temperature of 2000 °C (1 s ramp), signals could be obtained for Rb but not for Na (500 ng) in the absence of the plasma. Earlier studies have demonstrated that thermal Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
2133
Figure 8. Signal transient for atomization of 1 ng of lead.
excitation in the ETV is very inefficient for populating states having energies higher than 4 eV.32 Even in such cases, care was taken to introduce the analyte into an environment having the highest vapor temperature possible, using such techniques as two-step isothermal furnaces, contoured tubes, platforms, and probes. These observations are consistent with the above mass spectral measurements in that the ionization potential of Rb is 4.2 eV and that of Na is 5.1 eV. The effect of plasma power was briefly examined using a suite of alkali metals as probes. In general, signal intensities increased in an apparent exponential manner as power was increased from 20 to 120 W. However, no background ions of nearby mass were monitored, with the result that the exact dependence of net intensity is unknown at this time. Whereas rf power may significantly influence the degree of analyte ionization, this cannot be deconvoluted from possible accompanying changes in the sampling position as the plasma expands or contracts with changing power. Sampling at various positions over the cross section of the source results in significant variation in the ion intensity. The system does not currently have the capability of achieving reproducible positioning necessary to permit any laterally resolved data to be acquired. Figure 8 illustrates the results obtained for the atomization of 1 ng of lead at 1500 °C. A 100 W plasma power was used. The transients are double-peaked in appearance, likely a consequence of the existence of two major desorption sites from within the ICCsthese being direct from the wall of the heated tube (appearing early in time) and a slightly delayed population whose source is probably the initially cooler center electrode, which acts as a L’vov platform in this system. The same phenomenon has been observed when the FAPES source is operated in the emission mode.33 The isotopic abundances, calculated from the areas under each of the single transients, are 52.6, 24.7, and 22.6% for m/z 208, 206, and 207, respectively. These are in satisfactory agreement with the natural abundances of 52.4, 24.1, and 22.1%, respectively. It is clear from the signal for 210 u that there is no significant background in this region of the spectrum. A search for Pb2+ and Pb(OH)+ was unsuccessful. No other molecular species of any of the elements examined were specifically addressed, so the extent of formation of hydrated metal ions and hydroxides in this plasma is not known at this time. Such species have been detected in He ICPMS.22,24 Figure 9 illustrates the signals obtained for the atomization of a DDW blank under identical conditions. Again, the isotopic ratios (32) Baxter, D. C.; Frech, W. Spectrochim. Acta Rev. 1995, 50, 655-706. (33) Imai, S.; Sturgeon, R. E. J. Anal. At. Spectrom. 1994, 9, 493-500.
2134 Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
Figure 9. Transient background for atomization of DDW blank for lead.
confirm that the signals correspond to the detection of lead, with no significant contribution from spurious background. As these studies were not conducted in a clean room atmosphere, contamination is problematic. Based on the collection of 10 replicate signals, an estimate of the limit of detection was made, yielding a 3σblank limit of detection (LOD) of 10 pg. This figure can clearly be improved by reducing the level of contamination and modifying the sampling interface to permit more efficient ion collection. An improvement of 100-fold is required to achieve the same detection power as that currently offered for Pb in our laboratory using ETV-ICPMS technology. Suffice to state that it serves to indicate the potential of this approach to trace multielemental MS. Detection limits for other elements were not estimated at this time. Apart from the usefulness of such figures of merit as the LOD, the robustness of the source is of paramount importance. Interferences due to EIEs are severe in both FANES34 and FAPES35,36 emission sources, with as little as 2 µg of NaCl causing suppression of signals in FANES. Although rf FAPES devices are generally less affected by the presence of EIEs,36 there are reports that even sub microgram amounts of NaCl perturb signals.35 It is speculated that EIEs alter the excitation conditions, although the exact cause of this effect is unknown.36 Atomization of 1 ng of lead in the presence of 50 µg of NaCl resulted in an average recovery of 60 ( 5% (n ) 3) using FAPIMS; no attempt was made to prevolatilize any of the salt using an ashing temperature higher than 250 °C. This is significant, since this small degree of signal suppression could previously only be achieved using a palladium modifier and atomization from a platform when emission measurements were undertaken.37 The reason for the apparent robustness of FAPIMS is unknown at this time. Source of Ionization. Attempts were made to determine the location of ionization in the system, as the question arises whether ion formation occurs in the primary plasma or as a consequence of the extraction process, i.e., is the source in the low-pressure interface region, or does it arise due to a discharge between the FAPES and the extraction tube? It is clear from the above that (34) Falk, H.; Hoffmann, E.; Ludke, Ch. Prog. Anal. Spectrosc. 1988, 11, 417480. (35) Hettipathirana, T. D.; Blades, M. W. J. Anal. At. Spectrom. 1993, 8, 955960. (36) Imai, S.; Sturgeon, R. E. J. Anal. At. Spectrom. 1994, 9, 765-772. (37) Sturgeon, R. E.; Willie, S. N.; Luong, V. T.; Berman, S. S. J. Anal. At. Spectrom. 1991, 6, 19-24.
Figure 10. Mass spectrum illustrating background with ICC at room temperature and internal discharge established in interface region.
thermal ionization of analyte contributes insignificantly to the ion population. The low electrical conductivity of He leads to the presence of high electric fields in the plasma and the possibility of secondary discharges being established in the extraction interface, as noted with He ICPs.22,24 To investigate this possibility, a secondary discharge was deliberately established in the lowpressure interface by striking a plasma (likely a corona discharge) between the extraction tube and the grounded interface by application of a high voltage. The precise location of the discharge is unknown: it may occur between the end of the sampler tube and the tip of the skimmer cone, but more likely it is established between the end of the sampler tube and the proximal wall of the grounded sampler base. In any case, remarkably different background spectra were generated with this arrangement, as illustrated by the data in Figure 10. Clearly, a higher energy plasma is present, giving rise to an atomic mass spectrum more characteristic of those observed with He MIPs29 in that intense signals are generated for 8He2+, 12C+, 16O+, 14N+, 40Ar+, and several molecular species likely to be in the sampled gases, such as OH+, H2O+, H3O+, CO+, O2+, and CO2+. Based on this observation, it is clear that, under normal sampling conditions (cf., Figure 3), significant ionization is not taking place in the low-pressure interface. An additional experiment designed to further elucidate the major source of ionization utilized the Macor tee at the end of the sampling tube as a means to introduce mercury vapor in a stream of He directly into the interface via that portion of the sampling tube which bypasses the atmospheric plasma. Signals obtained in this manner were compared with those generated by directing the same mercury saturated He stream into the sample introduction port of the ICC. The latter arrangement resulted in (38) Nam, S.-H.; Masamba, W. R. L.; Montaser, A. Anal. Chem. 1993, 65, 27842790. (39) Myers, D. P.; Hieftje, G. M. Microchem. J. 1993, 48, 259-277.
mass spectra for mercury having intensities less than 1% of those generated by sampling the vapor passing through the FAPES source. Thus, although some ionization occurs in the sampling tube, likely due to the presence of a weak plasma being drawn into the tube itself, its contributions to the resultant signals are insignificant; there is no important source of ionization other than the FAPES itself. Although a BN or Macor insulating tip was placed over the sampling tube, this does not inhibit the formation of a potential developing between the tip and the plasma due to interaction with the rf field. The tip likely floats to some voltage less than the plasma potential, and an energetic discharge at atmospheric pressure can then occur between the FAPES source and the sampling tube, as is the case with He ICPs.22,24,38 Although this could not be ruled out with the present configuration, neither was there any direct evidence of its existence, such as the presence of ions characteristic of the stainless steel, BN, or Macor base materials due to erosion. CONCLUSIONS FAPIMS possesses several attractive features for atomic mass spectrometry which warrant further investigation. Clearly, the detection power remains to be explored, once a more practical interface is constructed. Substitution of a conventional nested cone arrangement in place of the sampler tube should enhance the ion transmission efficiency substantially. Additionally, application of appropriate bias voltages to the interface elements may serve to further improve ion extraction efficiency. A more compact furnace compartment is required in order to eliminate the dead volume of the current plexiglass cylinder such that gas flow rates can be reduced by an order of magnitude. A more complete understanding of the properties of the plasma would be beneficial such that steps may be taken to improve ionization efficiency and robustness. Ionization mechanisms require investigation. As transient signals must be manipulated, multielement application would benefit from time-of-flight detection schemes.39 Extension of FAPIMS to applications in the areas of detection of chromatographic effluent are readily envisioned. ACKNOWLEDGMENT We thank Scott D. Tanner and Perkin-Elmer Sciex for the testbed mass spectrometer used in this study. Received for review November 25, 1996. Accepted March 19, 1997.X AC9611982 X
Abstract published in Advance ACS Abstracts, May 1, 1997.
Analytical Chemistry, Vol. 69, No. 11, June 1, 1997
2135
