Radiant energy – Ionic separation or analysis
Patent
1992-03-31
1993-06-29
Berman, Jack I.
Radiant energy
Ionic separation or analysis
250282, 250283, 250397, H01J 4902
Patent
active
052237117
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an improved method and an improved apparatus for the analysis of samples by plasma source mass spectrometry, and particularly to inductively coupled plasma mass spectrometry (ICPMS) and to microwave induced plasma mass spectrometry (MIPMS).
In ICPMS and MIPMS a sample is ionized in a plasma torch and subsequently analyzed by mass spectrometry to determine its elemental or isotopic composition. The sample, dissolved in a solution, is introduced to the torch as an aerosol carried on a flow of inert gas where it passes into a plasma, usually maintained by induction in another flow of inert gas of the same type, typically argon at atmospheric pressure. In ICPMS the plasma is generated by electromagnetic induction from a coil disposed around the torch and energized by radio-frequency current. In MIPMS the plasma is induced in the gas in a microwave cavity coupled to a microwave energy source.
ICPMS has been reviewed, for example, by: A. R. Date and A. L. Gray in Analyst, 1983, 108, pages 159 to 165; D. J. Douglas and R. S. Houk in Progesss in Analytical and Atomic Spectroscopy, 1985, 8, pages 1 to 18; R. S. Houk in Analytical Chemistry, 1986, 58(1), pages 97A to 105A; and R. S. Houk and J. J. Thompson in Mass Spectrometry Reviews 1988, 7, pages 425 to 461. MIPMS is described by D. J. Douglas and J. B. French in Analytical Chemistry, 1981, 53, pages 37 to 41.
In ICPMS and MIPMS, the sample ions pass from the atmospheric pressure ion source, through one or more intermediate vacuum stages to a vacuum chamber where they are analyzed according to mass by a quadrupole filter. The means for detecting the mass-filtered ions usually comprises an electron multiplier either of the discrete dynode or channel type, as described in the aforementioned reviews, although a scintillator type detector is reported by L. Q. Huang et al in Analytical Chemistry, 1987, 59 pages 2316 to 2320. N. Jakubowski et al in Spectrochimica Acta, 1988, 43B, pages 1 to 10, and the International Journal of Mass Spectrometry and Ion Processes, 1986, 71, pages 183 to 197 report the use of an electron multiplier and a Faraday cup for ion detection. In such prior instruments, wherever an electron multiplier is present, it is usual to take precautions to limit extraneous influences such as visible or ultra-violet radiation, or neutral particles, to which such detectors are known to be sensitive as described by F. Nakao in the Review of Scientific Instruments 1975, 46(11), pages 1489 to 1492. Such precautions usually comprise positioning the multiplier off-axis, or alternatively, or additionally, putting a `photon-stop` on-axis to prevent line-of-eight travel from the plasma to the electron multiplier.
SUMMARY OF THE INVENTION
Despite the success of induced plasma mass spectrometry, and particularly of ICPMS, as a technique for analyzing dissolved solids there remain certain improvements that can be made, as will be described below.
It is an object of this invention to provide an improved method for the analysis of a sample in solution by induced plasma source mass spectrometry, and particularly to provide an improved method of ICPMS or MIPMS. It is a further object to provide an improved ICP or MIP mass spectrometer. Further objects are the provision of an improved method and an improved apparatus for ion detection in mass spectrometry.
According to one aspect of the invention there is provided a method for the mass spectrometric analysis of a sample, comprising: inducing a plasma at substantially atmospheric pressure in a gas; introducing said sample to said plasma and therewith ionizing at least part of said sample to form sample ions; transmitting at least some of said sample ions into an evacuated chamber having a mass filter disposed therein, and, by means of said mass filter, selecting sample ions within a selected mass range; characterised by detecting, substantially without charge multiplication, at least some of said mass-selected ions by means of an ion detector, positioned on a straight axis extending through said
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Sanderson Neil E.
Tye Christopher T.
Berman Jack I.
Fisons plc
Nguyen Kiet T.
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