Anal. Chem. 1997, 69, 485-489
Space Charge Evaluation in a Plasma-Source Mass Spectrograph Thomas W. Burgoyne, Gary M. Hieftje,* and Ronald A. Hites
Department of Chemistry and School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405
Space charge effects, and the matrix interferences they cause, are problems in inductively coupled plasma mass spectrometry (ICPMS). It has previously been observed that these deleterious space charge effects are not significantly present in sector-field instruments, a fact that has been attributed, but not demonstrated, to the high accelerating potentials they commonly employ. To examine the significance of space charge in our plasma-source mass spectrograph (which operates at only moderate accelerating potentials) and in other sector instruments, a graphite disk was placed ∼7 cm behind the skimmer. An inductively coupled plasma was operated for 17 h while a 0.01 mM multielement solution was introduced. This disk was then analyzed by spatially resolved laser ablation ICP time-of-flight MS. Second vacuum-stage acceleration appears to be an important factor that governs the elemental distribution within the ion beam. The ion beam width at m/z 208 is one-third of its width at m/z 7 using an accelerating potential of 800 V; at an accelerating potential of 4000 V, the ion beam width does not vary with mass. Olivares and Houk1 and Tan and Horlick2 provided early evidence that the sampled ion beam from an inductively coupled plasma that leads to a mass spectrometer (ICPMS) was not a true representation of the plasma composition. Reportedly, space charge effects are the major culprit in producing these nonspectroscopic interelement interferences (“matrix effects”).3 Coulombic interactions in the extracted beam produce a force away from the central axis of the ion beam. The lighter ions are moved a greater distance away from the center of the ion beam than the heavier ions, resulting in a mass bias in the ion beam cross section. This effect produces a mass bias in the mass spectra and an influence of heavier ions on the signals from lighter ones. Several excellent references exist that discuss space charge in some detail.4-9 (1) Olivares, J. A.; Houk, R. S. Anal. Chem. 1985, 57, 2674-2679. (2) Tan, S. H.; Horlick, G. J. Anal. At. Spectrom. 1987, 2, 745-763. (3) Gillson, G. R.; Douglas, D. J.; Fulford, J. E.; Halligan, K. W.; Tanner, S. D. Anal.Chem. 1988, 60, 1472-1474. (4) Hutter, R. G. E.; Harrison, S. W. Sylvania Technol. 1949, 2, 2-6. (5) Pierce, J. R. Theory and Design of Electron Beams; D. Van Nostrand: New York, 1954. (6) Hutter, R. In Focusing of Charged Particles; Septier, A., Ed.; Academic Press: New York, 1967; Vol. II, pp 3-22. (7) Nagy, G. A.; Szilagyi, M. Introduction to the Theory of Space-Charge Optics; MacMillan Press: London, 1974. (8) Dahl, P. Introduction to Electron and Ion Physics; Academic Press: New York, 1973. (9) Szilagyi, M. Electron and Ion Optics; Plenum Press: New York, 1988. S0003-2700(96)00673-7 CCC: $14.00
© 1997 American Chemical Society
Several studies aimed at investigating space charge in ICPMS have been conducted. Tanner10 calculated ion trajectories for a quadrupole ICPMS by taking into account space charge. Tanner’s model revealed a bias toward high masses in plasma-source mass spectrometers (PSMS). In research more applicable to the present study,11-13 ion deposition experiments were conducted to elucidate ion behavior in the second vacuum region of a PSMS. In general, these experiments were performed by placing a target behind the second vacuum-stage aperture (the skimmer) and depositing the extracted ion beam onto the target for a period of time. The target was then analyzed by other means, to produce spatially resolved elemental profiles of the ion beam. A review of these ion deposition experiments reveals what has been accomplished as well as what might be further learned. Li et al.11 presented the relevant space charge theory and described ion deposition experiments that utilized as a source a microwave plasma torch (MPT). The extracted ions were deposited onto a stainless-steel target and subsequently analyzed by a scanning electron microprobe. Evidence of space charge in the second vacuum region was obtained, in agreement with theory. Chen and Houk12 discussed the ion sampling process and described ion deposition experiments.14,15 Their deposition target was an array of 1.59-mm graphite rods. After deposition, each rod was placed in a nitric acid solution, which was then analyzed by ICPMS. Though only two elements were investigated, the effect of space charge was examined as a function of matrix concentration and of potential on both the photon stop and lens. Last, Chen and Farnsworth13 deposited an ion beam onto a Ni mesh and analyzed the deposits by X-ray fluorescence spectroscopy. Their experiments suggest that factors other than the spatial characteristics of the ion beam affect the spatial distribution on the target; caution was urged in the interpretation of these types of experiments. From this overview, there seems to be no definitive answer on how to conduct ion deposition experiments to elucidate ion beam profiles. The above space charge studies were all conducted on quadrupole PSMS systems, and little information is available that is applicable to magnetic sector PSMS.16 The mass bias and mass(10) Tanner, S. D. Spectrochim. Acta 1992, 47B, 809-823. (11) Li, G.; Duan, Y.; Hieftje, G. M. J. Mass Spectrom. 1995, 30, 841-848. (12) Chen, X.; Houk, R. S. Spectrochim. Acta B 1996, 51, 41-54. (13) Chen, Y.; Farnsworth, P. B. Ion Deposition Experiments as a Tool for the Study of Spatial Distribution of Analyte Ions in the Second Stage of an Inductively Coupled Mass Spectrometer. Spectrochim. Acta, in press. (14) Merkle, B. D.; Kniseley, R. N.; Schmidt, F. A. J. Appl. Phys. 1987, 62, 10171021. (15) Hu, K.; Houk, R. S. J. Vac. Sci. Technol. 1996, 14, 370-373. (16) Turner, P. J. In Applications of Plasma Source Mass Spectrometry; Holland, G., Eaton, A. N., Eds.; The Royal Society of Chemistry: Cambridge, UK, 1991; pp 71-78.
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dependent interferences that have been observed in sector ICPMS seem to be minimal;17 it has been suggested that the relative lack of a mass bias or interference in these instruments is a result of the high ion beam acceleration that these instruments employ.18 To date, no information is available concerning the shape and composition of the sampled ion beam from the skimmer in the second pressure stage of a sector-field plasma-source mass spectrometer. Furthermore, nothing is known about how the ion beam changes as a function of the accelerating potential in the second pressure region. Although theory dictates that space charge effects should be minimal when high accelerating potentials are employed (which is the expected reason why space charge in sector instruments is minimal),4-9 this expectation has never been experimentally verified in a plasma-source mass spectrometer. To determine the effect of space charge, ion deposition and analysis experiments were performed by changing the accelerating potential in the second vacuum-stage region. For this study, all experiments were conducted on a simultaneously detecting Mattauch-Herzog mass spectrograph, intended for coupling to a plasma source for multielement analysis.19 It is a compact instrument (∼80 cm in length), and it is designed to detect a complete atomic mass spectrum in two mass windows that straddle but avoid argon. The mass spectrograph currently operates at an acceleration potential of 1000 V. This accelerating potential lies between the high acceleration common in sector mass spectrometers and the nominal acceleration common in quadrupole mass spectrometers. The goals of this study were threefold: first, to improve the current methodology in ion deposition/space charge experiments; second, to determine the element-specific ion beam profile in our plasma-source mass spectrograph; and third, to determine the effect of accelerating potential on that profile. EXPERIMENTAL SECTION Ion Implantation. The sector-field plasma-source interface has been described in some detail previously,20 so only the information pertinent to the present study will be given here. In place of the third-stage aperture, a 2.54-cm-diameter graphite disk was placed 3.8 cm from the skimmer aperture (see Figure 1). The graphite disk was located roughly in the same location as the third-stage aperture. This graphite disk was held in place by a modified Cajon fitting. The disk was maintained at ground potential, and acceleration of the ion beam was accomplished by setting the desired potential on the source or the first pumping stage. The acceleration region in the second-stage vacuum region is circularly symmetrical about the central axis of the ion beam. All other operating conditions are detailed in Table 1A. This experiment was conducted at two different second-stage accelerating potentials: 800 and 4000 V. Under usual operating conditions, the mass spectrograph signal is optimized at an 800-V accelerating potential (first-stage interface at 1000 V). Thus, part of this experiment was designed to represent the typical operating (17) Taylor, P. D.; DeBievre, P.; Walder, A. J.; Entwistle, A. J. Anal. At. Spectrom. 1995, 10, 395-398. (18) Feldmann, I.; Tittes, W.; Jakubowski, N.; Stuewer, D.; Giessman, U. J. Anal. At. Spectrom. 1994, 9, 1007-1014. (19) Burgoyne, T. W.; Hieftje, G. M.; Hites, R. A. The Design and Performance of a Plasma Source Mass Spectrograph. J. Am. Soc. Mass Spectrom., in press. (20) Burgoyne, T. W.; Hieftje, G. M.; Hites, R. A. Reducing the Energy Distribution in a Plasma-Source Sector-Field Mass Spectrometer Interface. J. Anal. At. Spectrom., submitted for publication.
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Figure 1. Mechanical drawing of the ion implantation experiment in second vacuum region of sector-field ICPMS interface. The hatched areas are stainless steel (type 304). The ICP is extracted through two conductance-limiting apertures, the sampler and skimmer, and the extracted ion beam is implanted onto a graphite disk (which has replaced the third conductance-limiting aperture). Acceleration of the ion beam occurs between the first-stage vacuum region (high potential) and the graphite disk (ground). Table 1. Experimental Conditions (A) Ion Implantation (on Sector-Field Mass Spectrograph) ICP forward power 1.25 kW ICP reflected power 5W torch gas (Ar) flows outer 14.0 L/min intermediate 1.06 L/min central 1.00 L/min nebulizer Meinhard spray chamber (room temp) Scott-type first vacuum-stage pressure 0.9 Torr second vacuum-stage pressure (3-5) × 10-4 Torr sampler aperture diameter 0.7 mm skimmer aperture diameter 0.6 mm (B) Graphite Disk Analysis (on TOF Mass Spectrometer) laser YAG (Q-switched) wavelength 1064 nm laser focal-spot diameter 300 µm laser pulse width 8 ns laser pulse energy 300 mJ ICP forward power 1.35 kW ICP reflected power
