Ion gun and mass spectrometer employing the same

Details Ion gun and mass spectrometer employing the same Ion gun and mass spectrometer employing the same

1995-08-15

1996-10-08

Berman, Jack I.

Radiant energy

Ionic separation or analysis

With sample supply means

250281, 250423R, 250287, H01J 3726

Patent

active

055634109

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to ion guns and mass spectrometers. Mass spectrometers offer many benefits for the analysis of unknown gases, either for composition or for trace contaminants, however they have previously been regarded as complex and expensive. The subject of this patent application is a new design ion gun and of mass spectrometer that is relatively simple and compact which should extend the usage of mass spectrometers into new areas.
Mass spectrometers start by vaporising a sample, if not already in the gas phase, and ionising atoms or molecules in the resulting gas to form ions. These atomic or molecular ions are then manipulated by means of electric or magnetic fields, within a vacuum to prevent collisions with ambient gas molecules, in such a way that ions of different masses may be distinguished and their abundance measured. As each element has a different and unique mass the resulting "mass spectrum" may often be relatively easily interpreted in terms of concentrations of different elements. When molecular ions are involved the interpretation may be more complex because a single compound may give rise to several mass peaks due to fragmentation, however there exist databases of mass spectra for most compounds of interest. In particular there is a large body of mass spectral data [(NBS/EPA (USA) MS library (44,000 electron impact mass spectra)] associated with ionisation by means of electron impact.
By comparison with other analytical techniques, for example infra red spectroscopy, mass spectrometry has great advantages because of its applicability to a wide range of compounds together with its high specificity. Unlike most other techniques mass spectrometry allows different isotopes of the same element to be distinguished. It is also particularly well suited to use with a primary separation technique such as gas chromatography, as proposed by G. Matz et al, Chemosphere 15 (1986) p2031.
Mass spectrometers for gas analysis generally consist of a source of ions, a spectrometer where separation according to the mass-to-charge ratio takes place and an ion detector. All mass spectrometers have an evacuated chamber so that the mean free path of the ions of interest is much longer than their intended path within the spectrometer. There are various schemes for separating ions according to their mass-to-charge ratio and because the charge is generally known (e.g. the removal of a single electron) this equates to separation by mass. Most spectrometers effectively act as mass filters, arranging that only ions at, or near to, a certain mass complete the journey from ion source to detector. Examples of this technique are the magnetic or electrostatic sector instruments and Wein filter spectrometers which disperse the ions in space and either have a position sensitive detector or, more usually, a mass selecting aperture or slit. Quadruple spectrometers also work as a narrow bandpass filter, being arranged so that only ions of certain mass to charge ratio have stable trajectories and hence reach the detector. These filter type mass spectrometers can be used to create a mass spectrum by ramping the electric or magnetic fields in such a way that the mass detected is scanned through the range of masses of interest. When a signal from the detector has been collected throughout the range a mass spectrum may be plotted. Clearly when using this method only a small fraction of the ions created in the source actually reach the detector. Other types of mass spectrometer can in principle detect all the ions created in the source. Two examples are the ion trap and the time-of-flight mass spectrometer.
A number of factors affect the suitability of a particular spectrometer for a particular application: the constraints that it places on the source, such as range of ion energies accepted and the permissible physical source size; the ability to resolve small differences in mass; the transmission efficiency from source to detector; the range of masses covered and the complexity, and hence co

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Mamyrin et al., The mass-reflection, a new nonmagnetic time-of-flight mass . . . , Jul. 1973, pp. 45-48.
Oakey et al., An Electrostatic Particle Guide for High Resolution Charged . . . , 1967, pp. 20l-228.
Matz et al., Fast, Selective Detection of TCDD Using the Mobile Mass Septectrometer MM 1, 1986, pp. 2031-2034.

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