Discharge ionization source

Details Discharge ionization source Discharge ionization source

2000-06-02

2002-06-18

Berman, Jack (Department: 2881)

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

C250S288000, C250S324000, C250S325000

Reexamination Certificate

active

06407382

ABSTRACT:

BACKGROUND OF THE INVENTION
Atmospheric pressure ionization mass spectrometry and ion mobility spectrometry are techniques that detect the presence of, and identify the composition of, ionizable chemical species in a flowing gas stream. This is accomplished by submitting the ions to an ion analyzer where the ions are separated according to characteristic properties of the ions. For atmospheric pressure ionization mass spectrometry, the ion analyzer is a mass filter that uses a combination of electromagnetic fields to determine the charge-to-mass ratios of the ions. For ion mobility spectrometry, the ion analyzer is a drift tube that uses a constant or oscillatory electric field to determine the mobility of the ions. Although the ion analyzer for atmospheric pressure ionization mass spectrometry works under vacuum conditions (<10
−3
Torr) and the ion analyzer for ion mobility spectrometry works under atmospheric pressure conditions (the definition of mobility only requires that the pressure be greater than approximately 10
−3
Torr), they share the common feature that the ions are generated under atmospheric pressure conditions. The ions created in an atmospheric pressure ionization source of a mass spectrometer are interfaced to the vacuum of the mass spectrometer through an ion sampling pinhole or orifice.
Morning, et al. in a paper entitled “New Picogram Detection System Based on Mass Spectrometer with an External Ionization Source at Atmospheric Pressure” published in Analytical Chemistry, Vol. 45, No. 6, 1973, pp. 936-943, demonstrated that chemical species can be ionized in air or nitrogen using a radioactive source. Beta particles released by the
63
Ni radioactive source create reactant ions that subsequently attach to the chemical species of interest to create product ions. In a paper entitled “Subpicogram Detection System for Gas Phase Analysis Based upon Atmospheric Pressure Ionization API) Mass Spectrometry” published in Analytical Chemistry, Vol. 46, No. 6, 1974, pp. 706-710, Homing, et al. further demonstrated that the radioactive source can be replaced with a discharge ionization source. Spangler, et al. in a final technical report entitled “Nonradioactive Source Development for the XM22 Automatic Chemical Agent Alarm and Auxiliary Equipment” prepared for the U.S. Army in 1992, demonstrated that the negative ions generated by such a discharge ionization source can differ from those generated by a radioactive source. This difference leads to differences in ionization capabilities for selected groups of ionizable compounds.
From the point of view of building commercial hardware, a discharge ionization source is preferable to a radioactive source because of the liabilities associated with broadly distributing radioactive materials. This preference is causing various manufacturers to replace the radioactive source with other sources such as a corona discharge source. For example, U.S. Pat. No. 4,023,398 describes an atmospheric pressure ionization mass spectrometer that uses a radioactive tritium foil for ionization. The foil was later replaced by a point-to-plane discharge in U.S. Pat. No. 4,121,099. Electronics were provided to apply a high potential between the discharge needle and the pinhole.
U.S. Pat. Nos. 3,626,182 and 4,712,008 further disclose the use of a radioactive source for ionization of sample in a linear ion mobility spectrometer. The radioactive
63
Ni foil occupies the inner diameter of a guard ring that is otherwise used to electrically bias the cell. A similar source is disclosed in U.S. Pat. No. 5,420,424 to ionize sample in a transverse-field ion mobility spectrometer. This source was replaced by a discharge ionization source at the University of Toronto and evaluated by Karasek and Kim of the University of Waterloo. In their report entitled “Study of Technology Relating to Plasma Chromatography Sensing Tubes” that was submitted to the Canadian Government (DREV) in 1980, Karasek and Kim noted that an ionization source of the type described in U.S. Pat. Nos. 4,023,398 and 4,121,099 does not work in an ion mobility spectrometer. Insufficient ions passed through the pinhole to produce a measurable signal in the ion mobility spectrometer. Later work by Spangler, et al., as described in the final technical report entitled “Nonradioactive Source Development for the XM22 Automatic Chemical Agent Alarm and Auxiliary Equipment” submitted to U.S. Army in 1992, showed that the ion current could be increased if the pinhole of U.S. Pat. Nos. 4,023,398 and 4,121,099 was eliminated and replaced with a grid. Operation of the resulting point-to-grid discharge, however, was hampered by the need to use excessively high potentials to create positive ions (leading to burnt electrodes), and incorrect ionization chemistry for the negative ion mode of operation. The incorrect negative ion chemistry was attributed to secondary reactions that occurred in the hot plasma.
Taylor, et al. in U.S. Pat. No. 5,684,300 replaced the point-to-grid discharge with a point-to-target discharge. Their target electrode was the internal surface of a bias ring that was otherwise used to bias the IMS cell. Consistent with the observation of Spangler, et al., they found that considerably higher potentials were needed to establish and maintain the discharge. Unlike Spangler, et al., Taylor, et al. had the ability to generate higher potentials (up to 10 kilovolts), and used electrodes more tolerant towards the ion energies produced by these potentials. Their source was more reliable, but pumped an excessive amount of energy (albeit for a short period of time) into the discharge gap. The excessive amount of energy lead to a requirement for delayed sampling of ions to preserve ion chemistry.
Finally, Spangler et al. in a presentation entitled “Low Energy Glow/Corona Discharge Ionization Source for Ion Mobility Spectrometry,” delivered to the 7
th
International Conference on Ion Mobility Spectrometry, Hilton Head, SC in 1998, disclosed a point-to-point discharge ionization source that removed the previous limitations associated with negative ionization. The discharge was generated between two electrodes positioned across the diameter of the IMS cell. Because the discharge was a dc discharge produced by a power supply with limited current producing capabilities, the discharge was unstable and generated severe noise in the ion mobility spectrum.
Hyne in U.S. Pat. No. 3,848,202; LeMay in U.S. Pat. No. 3,940,710; and McLellan in U.S. Pat. Nos. 4,412,333, 4,748,635 and 4,556,981 disclose a three-electrode discharge ionizer that is more stable than a two-electrode discharge ionization source. The third electrode pre-ionizes the gas between the anode and cathode. The configuration is similar to that disclosed by LaFlamme in an article entitled “Double Discharge Excitation for Atmospheric Pressure CO
2
Lasers,” published in The Review of Scientific Instruments, Vol. 41, No. 11, 1970, pp. 1578-1581, and is a variation of the spark gap commonly used to control the operation of lasers (see U.S. Pat. No. 4,481,630). The idea behind the arrangement is that it is easier to break down a narrower gap than a wider gap (a consequence of Paschen's curve); and that once charge is created in a gap, a lower electrical potential is needed to break down the gap. Thus if a third electrode is placed in close proximity to the cathode and biased with the same potential as the anode, the gap between the third electrode and cathode breaks down first, followed by a discharge across the main discharge gap between the anode and cathode. This concept is utilized in U.S. Pat. No. 5,684,300 where the third electrode is biased with a potential opposite the corona discharge electrode to control the duration of the discharge, as well as the quantity of the ions generated. Other possible functions for the third electrode are to serve as a second anode or cathode (depending on polarity) as described in U.S. Pat. No. 5,684,300, or to act as a control electrode similar to that described by Kaibyshev, et al. in a p

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