Anal. Chem. 2000, 72, 4435-4440
Reduction of Mass Bias and Matrix Effects in Inductively Coupled Plasma Mass Spectrometry with a Supplemental Electron Source in a Negative Extraction Lens Narong Praphairaksit and R. S. Houk*
Ames Laboratory, U. S. Department of Energy, Department of Chemistry, Iowa State University, Ames, Iowa 50011
Electrons from a heated tungsten filament are created inside the extraction lens and driven out toward the skimmer. These electrons move through the ion path and reduce space charge effects between positive ions in the beam. The ion transmission efficiency is improved by factors of two (for Pb+) to 27 (for Li+). The greater sensitivity improvement for low-mass ions leads to a substantial reduction in mass bias. With the additional electrons, MO+/M+ and M2+/M+ abundance ratios increase but can be minimized with a small reduction in aerosol gas flow rate. No new background ions are observed with this technique. Matrix effects can be significantly diminished when the electron source is operated under the high electron current mode. The mass dependence of matrix-induced suppression of analyte signals is essentially eliminated. Using flow injection analysis to minimize solid deposition, the technique can tolerate Na matrix up to 10 000 ppm (1%) with only ∼15% loss of analyte sensitivity. Space charge repulsion between positive ions has been identified as a major cause of sensitivity losses, mass bias, and matrix effects in ICPMS.1-6 Mass bias is a discrimination against lighter ions (lower kinetic energy) that occurs largely during the process of ion transport. A large variation in the sensitivities of ions of different masses complicates the use of internal standardization and can produce substantial error in isotope ratio measurements. The matrix effect is typically a suppression of analyte ion signal in the presence of concomitant elements.1-4 Recently we introduced a promising technique that can apparently alleviate space charge and matrix effects in ICPMS.7,8 * Corresponding author: (tel) 515-294-9462; (fax)515-294-5233; (e-mail)
[email protected]. (1) Olivares J. A.; Houk, R. S. Anal. Chem. 1986, 58, 20-25. (2) Gillson, G. R.; Douglas, D. J.; Fulford, J. E.; Halligan, K. W.; Tanner S. D. Anal. Chem. 1988, 60, 1472-1474. (3) Tan, S. H.; Horlick G. J. Anal. At. Spectrom. 1987, 2, 745-763. (4) Crain, J. S.; Houk, R. S.; Smith, F. G. Spectrochim. Acta, Part B 1988, 43, 1355-1364. (5) Tanner, S. D. Spectrochim. Acta, Part B 1992, 47, 809-823. (6) Douglas, D. J.; Tanner, S. D. Fundamental Considerations in ICPMS. In Inductively Coupled Plasma Mass Spectrometry; Montaser, A., Ed.; WileyVCH: New York, 1998, pp. 656-666. (7) Praphairaksit, N.; Houk, R. S. Anal. Chem. 2000, 72, 2351-2355. (8) Praphairaksit, N.; Houk, R. S. Anal. Chem. 2000, 72, 2356-2361. 10.1021/ac000590j CCC: $19.00 Published on Web 08/18/2000
© 2000 American Chemical Society
A supplemental electron source, like that in an electron impact ionizer, is added to the base of the skimmer to supply a controllable number of moderate-energy electrons to balance the excess positive charge in the ion beam. These supplemental electrons are added to the key area between skimmer and extraction lens, where space charge repulsion dominates.9 Analyte ion sensitivities, particularly for the lighter elements, improve substantially as a result of the reduced space charge field, and matrix effects are attenuated significantly. These early experiments with supplemental electrons were performed on an ICPMS instrument that used an extraction lens with a positive applied voltage. Although some ICPMS manufacturers employ such a lens,10,11 most commercial instruments operate with a negative voltage on the extraction lens. This paper shows that the same concept can be employed with this other basic type of ICPMS device. The supplemental electrons are made inside the extraction lens and forced out toward the skimmer, so that they travel through the zone where most of the space charge effect occurs. Sensitivity is improved, and mass bias and matrix effects are reduced, with this arrangement. EXPERIMENTAL SECTION Instrumentation. The homemade ICPMS system used throughout this study has been described previously.7,8,12,13 An ultrasonic nebulizer with desolvation was employed. Adding the electrons from the base of the skimmer, as was done in the earlier reports, was not effective with a negative extraction lens. Therefore, the filament that generated the supplemental electrons was moved inside the extraction lens (Figure 1). Two circular holes (3-mm i.d.) are drilled on opposite sides of the perforated cylindrical extraction lens (1.4-cm i.d.) approximately 1.0 cm from the end. A tungsten filament (straight wire, 0.15 mm thick, 2 cm long, R ≈ 0.6 Ω) is strung inside and across the lens through these two holes. Ceramic spacers (1-mm i.d., 2 mm long) are glued onto the holes with a high-temperature-resistant ceramic cement (Sauereisen, Scientific Instrument Services) to isolate this filament (9) Allen, L. A.; Leach, J. J.; Houk, R. S. Anal. Chem. 1997, 69, 2384-2391. (10) Denoyer, E. R.; Jacques, M. D.; Debrah, E.; Tanner, S. D. At. Spectrosc. 1995, 16, 1-6. (11) Douglas, D. J.; French, J. B. Spectrochim. Acta, Part B 1986, 41, 197-204. (12) Hu, K.; Clemons, P. S.; Houk, R. S. J. Am. Soc. Mass Spectrom. 1993, 4, 16-27. (13) Hu, K.; Houk, R. S. J. Am. Soc. Mass Spectrom. 1993, 4, 28-37.
Analytical Chemistry, Vol. 72, No. 18, September 15, 2000 4435
Table 2. Sensitivities and Enhancement Factors for Various Elements Measured in Normal and Electron Modes Sensitivity (cts.s-1/µg.mL-1) analyte ion 7Li+
24Mg+ 27Al+
55Mn+ 59Co+ 75As+ 89Y+
115In+
159Tb+
Figure 1. Schematic diagram of the modified electron source extraction lens and the new ion optical system. S, sampler; SK, skimmer; L1, extraction lens; L2, repeller; L3, photon stop; L4, tapered, perforated lens; L5, lens no. 5; DPP, differential pumping plate; W, tungsten filament; EF, electrical feedthrough; CI, ceramic insulator. Table 1. Typical Operating Conditions plasma conditions forward power reflected power outer gas flow auxiliary gas flow nebulizer gas flow solution uptake rate sampling position sampling orifice skimmer orifice ionizer filament current ion lens voltages (Figure 1) extraction lens lens no. 2 lens no. 3 (Photon stop) lens no. 4 lens no. 5 differential pumping plate quadrupole pole bias
1.3 kW
