Plasma mass spectrometer

Details Plasma mass spectrometer Plasma mass spectrometer

1990-07-20

1991-09-24

Berman, Jack I.

Radiant energy

Ionic separation or analysis

With sample supply means

250281, 250282, H01J 4904

Patent

active

050515845

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a mass spectrometer in which a sample is ionized in a plasma, e.g. an inductively-coupled or microwave-induced plasma, in which ions characteristic of the elements comprised in the sample are formed.
Mass spectrometers having a plasma ion source comprising an inductively-coupled or microwave-induced plasma may be used for the determination of the elemental composition of a sample dissolved in a solution. Typically the solution is nebulized to produce an aerosol comprising droplets of the solution in an inert gas (e.g. argon) which is fed to a plasma torch. In the case of an inductively-coupled plasma, a coil of a few turns is disposed around the torch and fed with up to 2 kW of radio-frequency electricity (usually at 27 or 40 MHz), which generates a plasma in which ions characteristic of the elements comprised in the sample are formed. In the case of a microwave induced plasma, the end of the plasma torch is inserted through a cavity typically energized with up to 1 kW at 2.3 GHz, with a similar result.
In order to mass analyze the ions formed in the plasma, the torch is positioned so that the plasma is formed adjacent to a cooled sampling cone containing a hole in its apex, through which pass at least some of the ions to be analyzed, entrained in the plasma gas, into an evacuated region. A skimmer cone, also containing a hole in its apex, is disposed downstream of the sampling cone, with which it cooperates to form a molecular beam interface leading into a second evacuated region containing a mass analyzer, typically a quadrupole, and ion detector. In order to increase the efficiency of transport of ions through the region between the skimmer cone and the mass analyzer, an electrostatic lens system is conventionally provided to focus the ions emerging from the hole in the skimmer on the entrance aperture of the mass analyzer. Generally, a "photon-stop" is provided on the central axis of the lens system to prevent photons generated by the plasma from reaching the mass analyzer and increasing the noise level. Typically the lens system generally comprises a "Bessel-box" arrangement with the photon stop on the axis of the lens system in which the electrodes are biased so that at least some of the ions pass around the stop. Such a lens arrangement may also function as an energy analyzer. However, because the pressure immediately downstream of the hole in the skimmer is quite high, the motion of the ions in this region tends to be dominated by collisions with gas molecules rather than by the relatively weak electrostatic field present inside the skimmer, so that the transmission of ions to the analyzer is inefficient. Consequently in prior ICPMS systems the design of the sampling cone-skimmer interface has followed conventional molecular beam practice because the motion of the ions in this region is largely controlled by the flow of the very large excess of neutral molecules through the skimmer. The behaviour of these systems is well established and the parameters for optimum generation of a collimated beam are well established. See for example, Campargue, R, J. Phys. Chem, 1984, vol. 88 pp 4466-4474 and Beijerinck, HCW, Van Gerwen, RJF, et al, Chem. Phys, 1985, vol. 96 pp153-173. These theories predict that a skimmer comprising a cone of external and internal included angles of approximately 55.degree. and 45.degree., respectively, provides optimum transfer efficiency and that any departure from these angles causes a marked reduction in efficiency.
Thus typically, in conventional ICP mass spectrometers the skimmer is positioned to sample from the "zone of silence" between the sampling cone and the estimated position of the Mach disk, and its external and internal included angles are typically 55.degree. and 45.degree. respectively. Similarly, the pressure in the region between the sampling cone and the skimmer is maintained in the region 0.1-2.0 torr, in the region where the "Campargue-type" skimmer theory would be expected to apply.
In prior ICP mass spectrometers, a variety of inter

REFERENCES:
patent: 3616596 (1971-11-01), Campargue
patent: 4121099 (1978-10-01), French et al.
patent: 4358302 (1982-11-01), Dahneke
patent: 4740696 (1988-04-01), Osawa et al.
patent: 4760253 (1988-07-01), Hutton
Campargue, J. Phys. Chem., 1984, 88, pp. 4466-4474.
Beijerinck et al., Chem. Phys., 1985, 96, pp. 153-173.
Beauchemin. et al., Spectrochimica Acta, 1987, 42B, pp. 467-490.
Gregoire, Spectrochim Acta, 1987, 42B, pp. 895-907.
Kawaguchi et al., Anal. Sci., 1987, 3, pp. 305-308.
Gregoire, Appld Spectros., 1987, 41(5), pp. 897-903.
Longerich et al., Spectrochim Acta, 1987, 42B, pp. 101-109.
Hausler, Spectrochim Acta, 1987, 42B, pp. 63-73.
Houk et al., Anal. Chem., 1980 52, pp. 2283-2289.
PCT pub. No. 89/12313 (VG Instruments).
Randall et al., Rev. Sci. Instrum., 1981, 52 pp. 1283-1295.
Olivaries, PhD Thesis (part), Amer. Univ., 1985.
Gentry et al., Rev. Sci. Instrum., 1975, 46, p. 104.
Campargue et al., Proc. AIAA Conj. Ratefried Gas Dynamic, Jul. 1976, Snowmass at Aspen, USA, pub. 1977, pp. 1033-1045.

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