Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2003-09-16
2004-11-23
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S286000, C250S288000
Reexamination Certificate
active
06822226
ABSTRACT:
This invention relates to corona ionisation sources and in particular to continuous corona ionisation sources used in ion mobility spectrometry.
Ion mobility spectrometers are used in numerous applications such as the detection of narcotics, explosives and chemical warfare agents in air and for environmental monitoring.
A typical ion mobility spectrometer (IMS) such as that described in U.S. Pat. No. 4,777,363 comprises an ionisation source, a reaction region and a drift region. As monitoring/detection takes place at ambient atmospheric pressure, ionisation of the sample gas at atmospheric pressure is required. After ionisation, ions generated from the sample gas are expelled into a drift region where, under the influence of an electric field and collisions with a counter-flowing drift gas, the ions attain a constant velocity before arriving at a collector. The ion mobility spectrum obtained is characteristic of the sample being investigated.
Radioactive materials, such as
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Ni, are traditionally used as the ionisation sources in ion mobility spectrometers. The output from such radioactive sources is highly stable, and, in addition, they are noise and power free. However, radioactive sources have to be handled and disposed of with great care and the exposure of operating personnel to ionising radiation has to be carefully controlled and monitored. Taking all the necessary precautions in relation to the use, transportation, storage and disposal of devices incorporating radioactive sources can therefore prove costly.
Continuous corona ionisation sources have previously been used in IMS systems as an alternative to the use of radioactive sources. The ions produced by an IMS incorporating a continuous corona ionisation source and operating in negative mode (i.e. producing and collecting negatively charged ions) have however been found to differ markedly from the ions produced using a radioactive
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Ni ionisation source. See, for example, B Gravendee and F J de Hoog, J Phys B: At Mol Phys 20, 6337 (1987) for a discussion of the ionic species produced when air is ionised by a continuous corona ionisation source. The major problem is that neutral species formed during the ionisation process react with the initially formed reactant ion species. This results in the formation of unreactive ions which are significantly more stable to reaction than those produced by radioactive
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Ni ionisation sources, and which do not react so readily with sample vapour. Consequently, the sensitivity of IMS systems incorporating continuous corona ionisation sources can be low and it is commonly accepted by those skilled in the IMS field that continuous corona ionisation sources are unsuitable alternatives to traditional radioactive ionisation sources.
Various pulsed corona discharge sources have also been developed and, to a limited extent, these overcome some of the disadvantages associated with the use of continuous corona ionisation sources that are described above. However, pulsed corona discharge sources are expensive, involve complex pulsing, triggering and timing delays and have to be synchronised with gate opening events. As such there is still a need for improvements to the corona ionisation apparatus to improve ion mobility spectral data to make this a viable alternative to radioactive ionisation sources. Furthermore, there remains a need to develop a corona discharge ionisation apparatus, for use with an ion mobility spectrometer, whereby the unwanted side reaction between the neutral species and the reactant ion species, both formed during the initial ionisation stage, is minimised or eliminated. This would ensure that a more efficient reaction is achieved between the initially formed reactant ion species and the sample material, in turn improving the spectral results.
Various ion mobility spectrometers have been disclosed in the prior art. For example U.S. Pat. No. 4,445,038 discloses an apparatus for simultaneous detection of positive and negative ions in ion mobility spectrometry comprising dual drift regions respectively on either side of a centrally located reaction region; U.S. Pat. No. 5,234,838 discloses an ion mobility spectrometer for the analysis of ammonia whereby dimethly methyl phosphonate is added to the carrier gas stream prior to application of the carrier gas stream into the ionisation chamber thus forming clusters with the ammonia which have different drift times; and U.S. Pat. No. 5,283,199 discloses an ion mobility spectrometer for the analysis of chlorine dioxide whereby a controlled quantity of amine is added to the carrier gas stream prior to application of the carrier gas stream into the ionisation chamber thus suppressing the chlorine peak.
Although these documents optionally disclose the use of corona ionisation sources none of the documents specifically address the problem of improvement of the quality of corona ionised ion mobility spectra. More specifically they do not address the problem of how to minimise, or eliminate, the interaction of the neutral and reactant ion species in the ionised gas. Interestingly the apparatus of each of these documents has a configuration such that a certain flow of drift gas, whose primary role is to separate ions in the drift region, may pass from the drift region into the ionisation region of the spectrometer. However, this has the secondary effect of interfering with the flow of material in the ionisation region including sweeping neutral material to the exhaust. Such neutral material may include sample material introduced into the ionisation region and neutral species produced during the ionisation process.
Several problems are associated with configurations such as those described in the prior art. These include that the drift gas may not pass sufficiently closely to the ionisation source to efficiently remove the problematic neutral species formed in the ionisation gas; that the rate of flow of the drift gas into the ionisation region may be inconsistent as a result of having to pass through a shutter grid; that it is not possible to adjust the flow of any drift gas in the ionisation region separately from the flow rate in the drift region; and that the removal of neutral material will result in dilution of the sample material with a corresponding reduction in sensitivity.
Thus there remains the problem of how to efficiently remove the neutral species formed during the ionisation process from the ionised gas thus minimising or eliminating the interaction of the neutral and initially formed reactant ion species. There also remains the problem of how to maximise the efficiency of the reaction between the initially formed reactant ion species and the sample material. Finally there remains the problem of how to achieve this effect without affecting the use of the drift gas to separate ions in the drift region of the spectrometer. It is an object of this invention to mitigate some of the disadvantages, as described above, that are associated with the use of corona ionisation sources.
According to a first aspect the present invention relates to an apparatus for ionisation of a gas comprising a corona ionisation source, a means for flowing gas past the corona ionisation source, and a means for applying an electric field to move any ions produced by the corona ionisation source away from the corona ionisation source, characterised in that the direction of gas flow through the corona ionisation source is substantially different to the direction of the ions in the esectric field.
According to a second aspect, the present invention relates to an ion mobility spectrometer comprising an apparatus for ionisation of a gas as characterised in the first aspect of the invention.
Herein, the term “substantially different” shall be taken to mean a difference between the direction of electric field induced ion flow from the corona ionisation source and the direction of gas flow past the corona ionisation source, such that ions produced by the ionisation source become spatially separated from any neutral species produced by the ionisation s
Bell Andrew John
Ross Stuart Keith
Hashmi Zia R.
Kilpatrick & Stockton LLP
Lee John R.
Russell Dean W.
The Secretary of State for Defence in Her Britannic Majesty&apos
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