Radiant energy – Ionic separation or analysis – Methods
Patent
1995-03-14
1996-05-07
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
Methods
250288, 250305, H01J 4926, H01J 4944, B01D 5944
Patent
active
055148689
DESCRIPTION:
BRIEF SUMMARY
This invention relates to mass spectrometers. It is particularly useful in mass spectrometers employing a plasma ion source such as an inductively-coupled plasma (ICP) or a microwave-induced plasma (MIP) source. Such instruments typically use a quadrupole mass filter but magnetic-sector mass filters are also used. The invention is applicable to both types.
ICP and MIP mass spectrometers are characterized by low background noise and high sensitivity and may have detection limits in the parts-per-trillion region across the mass range. However, with present instruments, a limiting factor on the detection limit at certain masses (or strictly mass-to-charge ratios) is the presence of an unwanted background signal at the mass to be monitored. Such spectral interferences (usually known as isobaric interferences) may be due to isotopes of two or more different elements having approximately the same mass, to charged molecular species for example ArO.sup.+, Ar.sub.2.sup.+ or oxide ions, or to doubly-charged species appearing at the same mass-to-charge ratio as a singly-charged ion.
Researchers in ICP mass spectroscopy have devoted considerable effort to establishing the mechanism of formation of these interfering molecular species in the hope that their formation can be reduced. For example, Vanhaecke, Vandecasteele, et al, in Mikrochim. Acta. 1992, vol. 108 pp 41-51 investigated the effect of various instrumental parameters (such as lens voltages, nebulizer and torch flow rates, etc) but were unable to form any general conclusions as to how isobaric interferences could be reduced. Similar work has been reported by Wang, Shen and Sheppard et al (J. Anal. Atomic Spectrom. 1990 vol 5 pp 445-449). Rowan and Houk (Appld. Spectrosc. 1989 vol 43 (6) pp 976-) teach the use of an instrument comprising two quadrupole mass analyzers in series. In this instrument the first quadrupole is not used as a mass analyzer but rather as an "RF only" quadrupole in which the pressure is deliberately kept quite high, at least in the region where the ions enter it. Molecular ions entering the first quadrupole are confined by the RF field and undergo collisions with the gas molecules therein and are lost by scattering. Atomic ions, having much smaller collisional cross-sections will however undergo fewer collisions and be transmitted to the mass analyzer. This arrangement was found to reduce some of the molecular ion interferences.
In order to further understand the processes by which different types of ions are formed in the plasma, several workers have attempted to measure the energy distribution of ions entering the mass analyzer. Peter and Hoffer (J. Vac. Sci. Technol. 1987, vol A5 (4) pp 2285) report experiments carried out to determine the energy of various ions formed in an ICP using an energy filter combined with a quadrupole mass analyzer. However, the authors did not measure the energy distributions of molecular ions formed in the plasma, and merely report the results obtained without further comment as to how the information reported may be usefully employed.
A more detailed investigation of energy distributions was reported by Chambers and Hieftje (Spectrochim. Acta, 1991 vol 46B (6/7) pp 761-784). The authors used a quadrupole mass analyzer combined with a three-grid retarding potential energy filter and report the energy distributions of a variety of ions generated in an ICP. Theories are proposed concerning ion transport processes but no measurements are described on molecular ions and no conclusions are drawn concerning distinguishing between interfering ion species in an ICPMS instrument.
It is the object of the invention to provide a method for the elemental analysis of a sample by ICP or MIP mass spectrometry in which isobaric interferences from molecular and multiply charged ions are reduced. It is a further objective to provide apparatus for carrying out such a method.
In accordance with these objectives the invention provides a method of determining the elemental composition of a sample by plasma mass spectroscopy comp
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Anderson Bruce C.
Fisons plc
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