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Method and apparatus for plasma generation

Method and apparatus for plasma generation description/claims

This application is a continuation of U.S. patent application Ser. No. 11/471,854 (filed 21 Jun. 2006), which is a continuation of U.S. patent application Ser. No. 10/894,861 (filed 19 Jul. 2004), which is a continuation-in-part of U.S. patent application Ser. No. 10/215,251 (filed 7 Aug. 2002), the entire contents of which are incorporated herein by reference, which claims priority to and the benefit of the following applications: U.S. Provisional Patent Application 60/310,902 (filed 8 Aug. 2001); U.S. Provisional Patent Application 60/335,219 (filed 25 Oct. 2001); U.S. Provisional Patent Application 60/340,815 (filed 12 Dec. 2001); and U.S. Provisional Application 60/388,052 (filed 12 Jun. 2002). The U.S. patent application Ser. No. 10/894,861 also claims priority to and the benefit of the following applications: U.S. Provisional Patent Application 60/488,019 (filed 17 Jul. 2003); U.S. Provisional Application 60/498,163 (filed 27 Aug. 2003); U.S. Provisional Patent Application 60/498,093 (filed 27 Aug. 2003); U.S. Provisional Patent Application 60/518,367 (filed 8 Nov. 2003); and U.S. Provisional Patent Application 60/520,284 (filed 14 Nov. 2003).

This application also incorporates herein by reference the entire contents of U.S. patent application Ser. No. 10/462,206 (filed 13 Jun. 2003).

The invention relates to an ionization source, and more particularly, in one embodiment, to a plasma generator for atmospheric gas discharge ionization.

Creation of ionized particles is a useful tool for many applications, such as for ignition of lasing or to assist chemical analysis, among other uses. In some equipment, high energy radioactive sources of alpha or beta particles are employed for the ionization process. However, because of the potential health hazard and need for regulation, wide-spread use of equipment using radioactive ionization sources has been limited. And even though smoke alarms use radioactive sources, the amount of ionization is low, and they still require government regulation.

There are several ionization methods that avoid radioactive sources. Corona discharge is a source of non-radioactive ionization. It provides high energy in a compact package. However, this process is not stable and can contaminate the sample with metal ions or NOx, as would interfere with analytical results. Furthermore, the generated ion species depends upon the applied voltage.

RF discharge ionization reduces some of these disadvantageous effects. RF discharges are subdivided into inductive and capacitive discharges, differing in the way the discharge is produced.

Inductive methods are based on electromagnetic induction so that the created electric field is a vortex field with closed lines of force. Inductive methods are used for high-power discharges, such as for production of refractory materials, abrasive powders, and the like.

Capacitive discharge methods are used to maintain RF discharges at moderate pressures p˜1-100 Torr and at low pressures p˜10−3-1 Torr. The plasma in them is weakly ionized and non-equilibrium, like that of a corona discharge. Moderate-pressure discharges have found application in laser technology to excite CO2 lasers, while low-pressure discharges are used for ion treatment of materials and in other plasma technologies.

Another ionization process is UV ionization. This process is sometimes referred to as atmospheric pressure photo-ionization (APPI). In low pressure conditions, photo-ionization involves direct interaction of photons with samples, forming positively charged molecular ions and free electrons. At elevated pressure conditions, the situation is not so simple and the ionization process for sample molecules can include a sequence of gas phase reactions, the details of which depend on the energetic properties of initially formed ions and free electrons (due to direct photo-ionization) and on the nature of the ambient gas.

One disadvantage of UV ionization is that it provides low to moderate ionization energies. This limits the types of molecules that can be ionized. As well, sometimes APPI can give unexpected results. The photons are typically generated in a tube, with the photons passing through a window, and this window material affects efficiency. Also, the surfaces of the UV devices can become contaminated or coated from the ionization product, which can degrade device performance or output intensity. As well, the UV tubes can be delicate and fragile, and hence are generally not suitable to operation in harsh environments or in applications requiring a significant amount of manual handling.

The invention, in various embodiments, addresses the deficiencies in the prior art by providing reliable non-radioactive ionization sources for various applications. More particularly, in one aspect, the invention provides a capacitive discharge apparatus for generating a stable plasma at pressures including at or around atmospheric pressure.

According to one embodiment, a plasma ionization source of the invention, also referred to as a plasma generator or a plasma ionizer, includes at least two plasma electrodes spaced by an ionization gap. In one practice of the invention, a system includes a capacitive gas discharge plasma generator for generating a plasma and a sample ionizer for ionizing a sample, with the sample ionizer being enabled by the plasma. In one embodiment, the gas is air and the plasma is formed at substantially atmospheric pressure and generates positive and negative ions substantially concurrently.

According to one advantage, the invention reduces or eliminates creation of ions from electrode material, and/or creation of other by-products, which may contaminate plasma ionization and which may impact further processes, such as sample ionization and analysis.

In various practices of the invention, the electrodes may or may not be protected from the plasma. In some embodiments, the electrodes are outside of the plasma environment or are otherwise isolated from the plasma to achieve a clean and more stable plasma. This favorably impacts downstream sample analysis.

In various practices of the invention, plasma formation may be immediate to sample ionization or may be physically separated from sample ionization. Sample ionization may or may not occur in the plasma. Separation of plasma formation from sample ionization results in cleaner, more reliable, and more stable sample ionization. This also favorably impacts downstream sample analysis.

In one practice of the invention, the plasma is formed in a gas flow channel and at least one of the plasma electrodes is protected from contact with the plasma. One or more of the plasma electrodes may be located external to the gas flow channel. Alternatively, at least one of the plasma electrodes may have an associated material layer for protecting the surface of the electrode(s) from destructive contact with the plasma. In one embodiment of the invention, the material layer includes a low or non-conductive material (e.g., an insulator or dielectric) for isolating one or more of the plasma electrodes. In one implementation, the invention employs a dielectric of high permittivity material to enlarge spacing between the metal electrodes of the plasma generator, while still achieving tight, effective gap spacing. This embodiment achieves a plasma with well-defined, temp-controlled emission qualities.

In various implementations, electrode surface protection reduces and/or prevents erosion of electrode surfaces, and/or limits and/or prevents ion contamination of the plasma. Thus, according to one advantage, a plasma generator of the invention is able to ionize a wide range of compounds for practical analytical applications, without contaminating the ionized sample.

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