Langmuir Probe Potential Measurements for Reduced-pressure Inductively Coupled Plasma Mass Spectrometry

Abstract

A floating Langmuir probe was used to measure the apparent dc offset potential of a reduced-pressure inductively coupled plasma near a substitute sampling orifice of a mass spectrometer. The experimental results demonstrate that the dc offset potential causes the secondary discharge at the sampling orifice. The plasma potential is in the range +3.5 to +20 V and varies with the plasma operating conditions. The manner by which a water-cooled torch is shielded has a substantial effect on the plasma potential, then the secondary discharge. The measured values of the potential give a good explanation for the enhanced capacitive coupling effect in reduced-pressure ICP-MS reported previously.

References

1. X. Yan, T. Tanaka and H. Kawaguchi, Appl. Spectrosc., 1996, 50, 182.
2. E. H. Evans and J. A. Caruso, J. Anal. At. Spectrom., 1993, 8, 427.
3. T. M. Castillano, J. J. Giglio, E. H. Evans and J. A. Caruso, J. Anal. At. Spectrom., 1994, 9, 1335.
4. E. H. Evans, W. Pretorius, L. Ebdon and S. Rowland, Anal. Chem., 1994, 66, 3400.
5. X. Yan, T. Tanaka and H. Kawaguchi, Spectrochim. Acta, Part B, 1996, 51, 1345.
6. T. Tanaka, K. Yonemura, M. Tanabe and H. Kawaguchi, Anal. Sci., 1991, 7, 537.
7. A. L. Gray, R. S. Houk and J. G. Williams, J. Anal. At. Spectrom., 1987, 2, 13.
8. D. J. Douglas, in Inductively Coupled Plasma in Analytical Atomic Spectrometry, ed. Montaser, A., and Golightly, D. W., VCH, New York, 2nd edn., 1992, ch. 13.
9. H. B. Lim, R. S. Houk and J. S. Crain, Spectrochim. Acta, Part B, 1989, 44, 989.
10. H. B. Lim and R. S. Houk, Spectrochim. Acta, Part B, 1990, 45, 453.
11. E. O. Johnson and L. Malter, Phys. Rev., 1950, 80, 58.