Distorted field radio frequency ion source

Details Distorted field radio frequency ion source Distorted field radio frequency ion source

1997-06-17

1999-03-02

Pascal, Robert

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

31511181, 250426, H01J 4912

Patent

active

058775935

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a radio frequency (rf) ion source and in particular to a glow discharge source capable of low power operation over a range of pressures, including atmospheric, in air.
2. Discussion of Prior Art
There exists considerable interest in the development of an ion source which is capable of operating under similar conditions to the commercially available electron impact ion source but which is more versatile and more robust than that source. The electron impact ion source is widely used in vapour analysis systems in which it is coupled to a mass spectrometer. In this source ionising particles in the form of electrons are emitted from a heated tungsten wire into a low pressure cavity, which is evacuated to pressures in the region of 10.sup.-4 to 10.sup.-3 Torr. The electrons in this cavity are accelerated by both electric and magnetic fields to an energy where impact of an electron with a sample molecule causes ionisation of that molecule. The electron impact ion source has the disadvantages that it cannot operate at high pressures and that it tends to bum out in oxygen rich environments, making the source unsuitable for use in analysis systems which operate in air at or close to atmospheric pressure.
Additionally, this source has the further disadvantage that it lacks versatility of use since it is effectively limited to the production of positively charged ions in a relatively energetic ionisation process (so called `hard` ionisation) and usually has associated with it sample molecule fragmentation.
There also exists considerable interest in the development of an ion source capable of operating efficiently at atmospheric pressure with air as the discharge gas in which the plasma is maintained and of interfacing with commercially available mass spectrometers. This would allow for the direct sampling of air in order to monitor for the presence of impurity gases, given off for example from some drugs or explosives such as TNT, RDX and PETN.
One known device, which can operate in air at atmospheric pressure, is that described by Zhao and Lubman (Analytical Chemistry Vol 64, No 13, pages 1427-1428 and Vol 65, No 7, pages 866-876) and comprises an insulated tungsten rod driven electrode, of 0.04" diameter and ground at the end to a sharp tip which is the operative end at which a plasma discharge can occur. This electrode is coupled to an rf source and extends into a grounded 1".times.0.8" (diameter) brass cell which forms an effective "plate" electrode. In use the plasma discharge occurs between the operative end of the rod and the cell walls. The sample, ions from which are to be produced and detected, is introduced into the sample-carrying discharge gas as a liquid and carried by the gas into the brass cell where it is ionised. This device however requires a power supply capable of providing the relatively high forward power of approximately 16 Watts (W) to induce the formation and maintenance of a plasma in air at atmospheric pressure. This has the disadvantage that the power supply is relatively costly and bulky.
Furthermore, even at this relatively high forward power this ion source produces only soft (low energy) ionisation and therefore cannot substitute for the electron impact ion source. If hard (high energy) ionisation is needed, then a higher power rf source would be required. This would compound the aforementioned disadvantage since to provide a hard ionising source a power supply which is capable of providing even higher forward powers than those discussed above will be necessary. Moreover, since the plasma generated by the Lubman ion source is stable only over a limited rf range of 125-375 Kilohertz (KHz), then a further disadvantage is that a relatively large ion energy distribution is likely to result which would effectively reduce the resolution of any analysis system incorporating a mass spectrometer. This is because the energy gained from the rf electric field by the ionised particles is, in part, dependent on t

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patent: 3686683 (1972-08-01), Powers et al.
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patent: 4682026 (1987-07-01), Douglas
Instruments And Experimental Techniques, vol. 26, No. 5, 1983 New York US, pp. 1188-1190, M S Abdugabbarov et al. "Ion source attachment to the MI-1201 mass spectrometer for analysing secondary ions".
Analytical Chemistry, vol. 64, No. 13, 1 Jul. 1992 pp. 1426-1433, XP 000295869 Jianguo Zhao et al "Liquid Sample Injection Using An Atmospheric Pressure Direct Current Glow Discharge Ionization Source."

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