Radiant energy – Ionic separation or analysis – With sample supply means
Patent
1990-12-03
1991-11-26
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
With sample supply means
250296, 250298, B01D 5944, H01J 4900
Patent
active
050685341
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a mass spectrometer in which ions are generated from a sample by means of a microwave or inductively coupled plasma (MIP or ICP, respectively).
Mass spectrometers having an ion source comprising such a plasma can be used for the determination of the elemental composition of a sample dissolved in a solution. In such a spectrometer, the solution is introduced by means of a nebulizer through which a controlled flow of inert gas is passed. This gas is subsequently introduced into the plasma. Several types of plasma have been described, the most common ones being an inductively coupled plasma similar to those used in atomic emission spectroscopy, or a microwave plasma (see, for example, Gray, Spectrochimica Acta, 1985, vol. 40B (10-12) pp 1525-37 and Douglas and Houk, Prog. Analyt. Atom. Spectrosc. 1985, vol 8, 1-18).
In prior spectrometers a quadrupole mass analyzer is employed, interfaced to the plasma by a molecular beam sampling system. The plasma is generated adjacent to a cooled first (or sample) cone containing in its apex a small hole leading to a first evacuated region. A second (or skimmer) cone, also having a hole at its apex, is located downstream of the first cone and divides the first evacuated region from a second evacuated region in which the quadrupole mass analyzer is situated. The second cone and the first evacuated region comprise a conventional pressure reduction stage. Conventionally the holes in the cones are aligned with the axis of the quadrupole mass analyzer, so that ions generated in the plasma pass through them into the mass analyzer. Various arrangements of electrostatic lenses are used to maximize the transmission of ions from the plasma into the analyzer.
The majority of the ions formed are singly charged ions of each of the elements present in the discharge, so that a mass spectrometer with such a source is a valuable instrument for determining the elemental composition of a sample, especially of inorganic materials such as metallic alloys or geological samples.
When the plasma is situated adjacent to the first (or sample) cone, a boundary layer of cooler gas is formed adjacent to its surface. This has a number of deleterious effects on the spectrum, increasing the background peaks and resulting in the formation of unwanted molecular species which cause spectral interferences. It also tends to cause arcing between the plasma and the cone, because the cool gas acts as an electrical insulator (see, e.g, EP 112004), allowing the potential of the plasma to rise to an undefined value.
If the hole in the cone is made larger, the boundary layer is "punctured" by virtue of the increased flow of gas through the aperture, so that arcing and the formation of the molecular species are reduced. However, not all workers report success with this approach. For example, Douglas, in EP 112004, describes an alternative arrangement for a RF ICP involving a grounded variable tap on the load coil which is adjusted to minimize the voltage swing in the plasma. Douglas claims that the boundary layer cannot be completely eliminated by enlarging the hole in the cone, and that the problem of the potential of the plasma being undefined results in "residual arcing" in any practical ICP MS instrument. See also Douglas and Houk (ibid).
A number of workers have investigated the variations in the potential of the plasma and other attempts have also been made to control it. Gray, Houk and Williams (J. Anal. Atom. Spectrom, 1987, vol.2, pp 13-20) measured the potential at various points in a typical plasma and report voltages of between -10 and +30v relative to ground with a grounded sample cone and RF coil. The actual voltages are certainly higher than the measured values because of the cooling of the plasma caused by the probe. Olivares (PhD Thesis, Iowa State University, 1985) attempted to measure the energy of ions produced in an inductively coupled plasma and found a similar range of values as the potential measurements of Gray, dependent on the plasma conditions. The higher than exp
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Bradshaw Neil
Sanderson Neil E.
Anderson Bruce C.
VG Instruments Group Limited
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