Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2001-08-08
2003-07-08
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S281000
Reexamination Certificate
active
06590205
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ionization method for mass spectrometry and a mass spectrometry apparatus, more particularly relates to an ionization method for mass spectrometry based on ion attachment which suppresses the diassociation of a sample gas for mass spectrometry to enable stable ionization over a long period of even an organic compound gas or halogen-based gas and a mass spectrometry apparatus in which such an ionization method is used.
2. Description of the Related Art
In recent years, mass spectrometry has been broadly used for analysis of pollutants and extremely fine amounts of impurities contained in process gas for the production of semiconductors. In mass spectrometry, an ionized sample gas is passed through the inside of an electromagnetic field of a mass spectrometer to cause separation in accordance with mass and the separated components having specific masses of the sample gas are detected and measured by an electron multiplier or other detectors. For mass analysis of a sample gas, the sample gas has to be ionized at a previous stage.
As methods for ionization of the sample gas, in the past, electron impact ionization, atmospheric pressure ionization, and ion attachment ionization may be mentioned.
Electron impact ionization is a method for causing electrons to collide with a sample gas at a high speed and using the impact energy to strip electrons of the sample gas for ionization. Due to the higher impact energy, gas molecules (target analysis substance) themselves making up the sample gas are sometimes split (disassociated) and low molecular weight substances are produced. According to mass spectrometry based on electron impact ionization, fragment peaks of a lower molecular weight than the main peaks are produced when the mass of the gas molecules making up the sample gas are the main peaks respectively.
Atmospheric pressure ionization is the method of using a source of radiation or corona discharge to ionize a gas with a higher ionization potential than the target analysis substance and produce primary ions and causing those primary ions to collide with the target analysis substance to cause an ion/molecule reaction (charge transfer) and ionize the sample gas at pressure (1×10
5
Pa). Since it uses a source of radiation etc. to generate the primary ions, at least the same number of electrons as the primary ions are produced. These electrons rebond with the primary ions to reduce the concentration of the primary ions. When the sample gas is an organic compound etc., an ion/molecule reaction with the primary ions does not easily occur.
Ion attachment ionization is a method of suppressing diassociation of the gas molecules making up the sample gas and of enabling efficient ionization. This ionization method is for example disclosed in Japanese Examined Patent Publication (Kokoku) No. 7-48371. Ion attachment ionization does not only ionize the sample gas effectively just under a high pressure such as atmospheric pressure (1×10
5
Pa) but also can analyze the sample gas under a lower pressure.
The above-mentioned ion attachment ionization has been reported as the system of Hodge in
Analytical Chemistry,
vol. 48, no. 6, p. 825 (1976); the system of Bombick in
Analytical Chemistry,
vol. 56, no. 3, p. 396 (1984); and the system of Fujii et al. in
Analytical Chemistry,
vol. 1, no. 9, p. 1026 (1989),
Chemical Physics Letters,
vol. 191, no. 1.2, p. 162 (1992), and Japanese Unexamined Patent Publication (Kokai) No. 6-11485. Ion attachment ionization has developed as a modification of chemical ionization. On the other hand, ion attachment ionization can suppress the disassociation of a sample gas better than chemical ionization or atmospheric pressure ionization and in particular is effective in mass spectrometry of a polymer organic compound with a low bonding energy etc.
In ion attachment ionization, for example, a metal ion emitter containing an alkali metal salt is heated to ionize the alkali metal and emit ions. These metal ions are caused to gently attach to locations of concentrations of charges of the gas molecules making up the sample gas and substantially ionize the sample gas. An alkali metal is best suited for this ionization. The electrons contributing to this ionization of the metal are trapped inside the metal ion emitter and do not rebond with the metal ions emitted as primary ions, so the concentration of metal ions can be sufficiently enhanced. In ion attachment ionization, an inert gas such as N
2
or Ar is introduced together with the sample gas, and the inert gas and the ionized sample gas are made to collide to quickly strip the excess energy from the sample gas ions and stabilize the sample gas ions. This is because the excess energy produced when the metal ions attach to the sample gas becomes a cause of separating the sample gas ions into the sample gas and metal ions.
In ion attachment ionization, the efficiency of ionization of the sample gas is improved along with the amount of the N
2
, Ar, or other inert gas introduced, that is, along with a rise in the pressure of the region of ionization. This ionization efficiency is seen to improve slightly along with a rise in pressure more than about 100 Pa as a critical pressure, but substantially becomes saturated at such a pressure. That is, under a pressure of over 100 Pa, the ionization efficiency becomes substantially constant.
According to ion attachment ionization, it was not possible to perform mass spectrometry stably over a long period. This is because the sample gas has an effect on the metal ion emitter depending on the type of the sample gas.
For example, when the sample gas is a polymer organic compound, heat decomposition occurs due to the heat transmitted from the heated metal ion emitter. That is, low molecular weight substances are produced. The low molecular weight substances are substantially ionized by attachment of the primary ions, that is, the metal ions. These pass through the mass spectrometer by an electric field and are transported to the detector. In the detector, for detection and measurement of the low molecular weight substances in addition to the target substance for mass spectrometry, fragment peaks are produced on the spectrum showing the results of analysis. Further, the low molecular weight substances deposit on the metal ion emitter and cover and conceal the surface. This reduces the area of the surface from which metal ions can be emitted. Sufficient metal ions can no longer be supplied inside the ionization chamber. As a result, the ionization efficiency of the sample gas falls.
Further, when for example the sample gas is a halogen gas or halogen-based radicals, the components of the emitter and the sample gas chemically react (corrosion reaction) at the surface of the metal ion emitter and cause a change in the performance of the metal ion emitter. In particular, in the case of halogen-based radicals, since a bias voltage is applied to the metal ion emitter, an etching reaction etc. are caused and the metal ion emitter itself is eaten away.
In this way, in the conventional ion attachment mass spectrometry apparatus and ionization method, there were the problems, depending on the type of the sample gas, of the occurrence of fragment peaks, a fall in the concentration of primary ions, and a reduction in the volume of the metal ion emitter itself and therefore the inability to ionize the sample gas stably over a long period and the inability to perform mass spectrometry correctly.
Note that as art related to this problem of the present invention, the atmospheric pressure ionization mass spectrometer disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-310091 may be mentioned. This atmospheric pressure ionization mass spectrometer enables analysis by adopting a double-wall tubular structure making the ion generation part and sample gas introduction part separate chambers at atmospheric pressure (1×10
5
Pa) from the viewpoint of preventing flow of the gas back
Fujii Toshihiro
Nakamura Megumi
Sasaki Tohru
Shiokawa Yoshiro
Anelva Corporation
Gurzo Paul M.
Lee John R.
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