JP2006189299A - Gas chromatograph mass spectrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas chromatograph mass spectrometer capable of removing fundamentally He<SP>*</SP>which is a main cause of a background originated in He. <P>SOLUTION: An ionization chamber 32 disposed in a vacuum chamber 31 of an MS part 30 is provided with an exhaust aperture, and He introduced into the ionization chamber 32 and He<SP>*</SP>generated in the ionization chamber 32 are discharged in the direction different from the taking-out direction of signal ions, by utilizing a pressure difference generated by a vacuum pump 38 connected to the exhaust aperture. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a gas chromatograph mass spectrometer.

  As shown in FIG. 3, a gas chromatograph mass spectrometer (hereinafter abbreviated as GCMS) includes a gas chromatograph part (hereinafter abbreviated as GC part) 10 for temporally separating components mixed in a sample, The interface unit 20 has a configuration in which a mass analysis unit (hereinafter abbreviated as MS unit) 30 for separating and detecting components separated by the GC unit 10 based on the mass number (mass / charge) is connected. ing. The MS unit 30 includes an ionization chamber 32 in which an ionization electron source (filament) 33 for ionizing a sample is disposed, and a QMF (Quadrupole-mass-filter, quadrupole mass) that separates the ionized sample based on mass. A mass separator 36 such as a filter) and a detector 37 that detects ions separated by the mass separator 36. Ions generated in the ionization chamber 32 are extracted from the ionization chamber 32 by the potential difference between the ionization chamber 32 and the extraction electrode 34 and enter the mass separator 36 via the ion transport optical system 35.

The sample is mixed with a rare gas (usually using He) as a carrier gas, introduced into the GC unit 10, separated by the column 12 of the GC unit 10, and introduced into the ionization chamber 32 of the MS unit 30. Normally, the sample is ionized in the ionization chamber 32 by a 70 eV electron beam. At this time, the He gas flowing into the ionization chamber 32 together with the sample is also ionized and excited. When He is ionized, He ⁺ (ionization energy: 24.6 eV) is generated, but when He is excited to a metastable state with a higher energy level than the ground state and a long lifetime, He ^* (metastable helium , 2 ¹ S He ^* excitation energy: 20.61 eV, 2 ³ S He ^* excitation energy: 19.82 eV).

He ⁺ or He ^* generated from the carrier gas He becomes a background component that cannot be ignored when performing high-sensitivity analysis, and causes a decrease in the S / N ratio. However, of He from the background, due to He ^+, by providing a suitable electromagnetic field, He ⁺ it is possible to remove before reaching the ion detector 37.

On the other hand, since He ^* is electrically neutral, it cannot be removed by an electromagnetic field. He ^* remaining in the vacuum chamber 31 behaves as a carrier for carrying energy and interacts with surrounding atoms / molecules to cause an ionization phenomenon. For example, when a metal surface atom and He ^* interact, He ^* gives an electron to the metal surface atom and becomes He ⁺ . Such a phenomenon is called resonance ionization. Further, when an insulator such as residual gas (X) in the apparatus interacts with He ^* , the partner atom (molecule) is ionized to generate X ⁺ . Such a phenomenon is called Penning ionization. That is, the background due to the He ^*, not only caused by the He ^* itself reaches the detector 37, with He ⁺ or X ⁺ is a detector 37 that is generated from the He ^* secondarily Some are caused by being detected.

Note that metastable states also exist in rare gases other than He, and the same problem arises when these are used as a carrier gas for GCMS. However, among metastable rare gases, the internal energy of He ^* is the largest, and ionization is likely to occur due to interactions with other atoms and molecules.

In order to reduce such background, an off-axis GCMS in which the optical axes of the optical elements constituting the apparatus are partially offset from the same straight line has been developed ( Patent Document 1). This is because the optical axis is shifted between the ionization chamber and the mass separator or between the mass separator and the detector, and the straight traveling He ^* is deviated from the ion optical axis to the mass separator or detector. By preventing entry, the background caused by He ^* is suppressed.

U.S. Pat.No. 3,410,997

  In the off-axis GCMS as described above, in order to connect between optical elements whose optical axes are not arranged on the same straight line, ions having a curved optical axis between them as shown in FIG. It is necessary to provide the transport optical system 40. Such an ion transport optical system 40 has a problem in that the manufacturing cost of the apparatus is increased because the degree of difficulty in design and manufacture is higher than that of an ion transport optical system having a general linear optical axis. . Further, in the ion transport optical system 40 having such a curved optical axis, signal ions may be lost.

Further, according to the off-axis method, although it is possible to remove the He ^* coming proceeds to QMF with directivity, He ^* and which flows while floating, and the straight line collides with He or residual gas He ^* taking different trajectories may pass through the ion transport optical system 40 having the curved optical axis without deviating and cannot be removed. In addition, He ^* that deviates from the ion transport optical system 40 collides with the inner wall of the vacuum chamber to become He ⁺ , or interacts with the residual gas X to generate X ^+, thereby generating these secondary ions. May reach the ion detector.

Therefore, the problem to be solved by the present invention is to provide a gas chromatograph mass spectrometer capable of fundamentally removing He ^* , which is the main cause of the He-derived background, with a simple structure.

A gas chromatograph mass spectrometer according to the present invention, which has been made to solve the above problems, introduces a sample separated by a column in a gas chromatograph section into an ionization chamber disposed in a vacuum chamber of the mass spectrometer section. In the mass spectrometer,
a) an exhaust opening provided in the ionization chamber;
and b) a vacuum pump connected to the exhaust opening.

In general, the ionization chamber of the MS section is generated in the ionization chamber with an opening for introducing a carrier gas containing a sample into the ionization chamber, an opening for passing an electron beam emitted from an electron gun, and the ionization chamber. There are three types of apertures: an aperture for extracting the signal ions toward the mass separator. In the conventional apparatus, the path for discharging He ^* and He from the ionization chamber has only an ion extraction opening, and the ion transport optical system side after the ionization chamber is in a high vacuum, so that He ^* and He It flowed out toward the ion transport optical system. The present invention provides a new opening (exhaust opening) for exhausting He ^* and He in the ionization chamber, and generates a negative pressure to the outside of the ionization chamber by a vacuum pump connected to the exhaust opening. Is. At this time, the signal ions generated in the ionization chamber are extracted by the action of the ion extraction electric field and introduced into the mass separator as before, but electrically neutral He ^* and He are pressures generated by the vacuum pump. Due to the difference, the gas is discharged from the exhaust opening to the outside of the ionization chamber.

  In the gas chromatograph mass spectrometer of the present invention, it is desirable that the exhaust opening be covered with a metal mesh in order to suppress disturbance of the extraction electric field due to the newly provided exhaust opening in the ionization chamber.

As described above, the gas chromatograph mass spectrometer of the present invention discharges He ^* and He generated in the ionization chamber in a direction different from the extraction direction of the signal ions by newly providing a vacuum exhaust system in the ionization chamber. It is possible to reduce the background by preventing He ^* and secondary ions resulting from it from being detected by the detector. In addition, since the He ^* can be fundamentally removed without using an off-axis method, the manufacturing cost can be reduced, and the loss of signal ions, which is a concern when using a curved optical axis optical system, can be reduced. In addition, since the background can be reduced while maintaining the signal intensity, an analysis with a high S / N ratio can be realized in a high sensitivity analysis in which a background derived from He is a problem.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to examples.

[Example]
As shown in FIG. 1, the gas chromatograph mass spectrometer of the present embodiment has a configuration in which a GC unit 10 and an MS unit 30 are connected by an interface unit 20.

  In the GC unit 10, a carrier gas (He gas) having a predetermined flow rate is supplied to a column (capillary column) 12 heated to an appropriate temperature by a column oven 13 through a sample vaporization chamber which is a part of the injector 11. The liquid sample supplied and injected into the sample vaporizing chamber by a microsyringe or the like is immediately vaporized and is carried in the carrier gas flow and sent into the column 12. While passing through the column 12, each component in the sample gas is temporally separated and reaches the outlet thereof, and is introduced from the sample introduction tube 21 into the ionization chamber 32 of the MS unit 30 through the interface unit 20.

  In the MS unit 30, a filament 33 for generating an electron beam is attached to the ionization chamber 32, and sample molecules introduced into the ionization chamber 32 are ionized by the electron beam. The generated ions are extracted to the outside of the ionization chamber 32 by the electric field generated by the extraction electrode 34, converged by the ion transport optical system 35 having a linear optical axis, and in the long axis direction of the QMF (or other mass separator) 36. Introduced into space. A voltage obtained by superimposing a DC voltage and a high-frequency voltage is applied to the QMF 36, and only ions having a mass number (mass m / charge z) corresponding to the applied voltage pass through the space in the major axis direction, and the detector 37 Will be detected. The ionization chamber 32, the ion transport optical system 35, the QMF 36, and the detector 37 are disposed in a vacuum chamber 31 that is vacuumed by a vacuum pump (not shown).

As shown in FIG. 2, the ionization chamber includes a sample introduction opening 32a for introducing a sample from the GC unit 10, beam openings 32b and 32c for passing an electron beam generated from a filament, and an ionization chamber. In addition to the ion extraction opening 32d for extracting the generated ions, an exhaust opening 32e for discharging He ^* and He is provided. The exhaust opening 32e is connected to a vacuum pump 38 provided outside the vacuum chamber 31, and the exhaust opening 32e does not adversely affect the extraction of ions from the ionization chamber 32. Further, a metal mesh 39 is stretched.

In the gas chromatograph mass spectrometer of the present embodiment having the above-described configuration, the signal ion S ⁺ generated in the ionization chamber is extracted from the ionization chamber 32 to the ion transport optical system region by the extraction electric field. He introduced into 32 and He ^* generated when the He is excited by an electron beam are electrically neutral and are not influenced by the extraction electric field, and are discharged from the exhaust opening 32e to the outside of the ionization chamber 32. The

In this way, by evacuating He ^* and He in a different direction from the signal ion S ⁺ , secondary ions such as He ^* and He ⁺ and X ⁺ resulting from this are mixed with the signal ion in the ion transport optical system. Can be prevented and the occurrence of background can be suppressed.

  As described above, the best mode for carrying out the gas chromatograph mass spectrometer of the present invention has been described using examples, but the present invention is not limited to this, and various modifications can be made within the scope of the present invention. Is acceptable. For example, the gas chromatograph mass spectrometer of the present invention is not limited to the case where He is used as a carrier gas as described above, but can be similarly applied to the case where other rare gas is used as a carrier gas.

BRIEF DESCRIPTION OF THE DRAWINGS Sectional drawing which shows schematic structure of the gas chromatograph mass spectrometer which is one Example of this invention. Sectional drawing which shows the structure of the ionization chamber periphery of the gas chromatograph mass spectrometer of the Example. Sectional drawing which shows schematic structure of the conventional gas chromatograph mass spectrometer. The figure which shows the ion transport optical system which has a curvilinear optical axis.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... GC part 11 ... Injector 12 ... Column 13 ... Column oven 20 ... Interface part 21 ... Sample introduction tube 30 ... MS part 31 ... Vacuum chamber 32 ... Ionization room 32a ... Sample introduction opening 32b, 32c ... Beam opening 32d ... Ion extraction opening 32e ... exhaust opening 33 ... filament 34 ... extraction electrodes 35, 40 ... ion transport optical system 36 ... mass separator 37 ... ion detector 38 ... vacuum pump 39 ... metal mesh

Claims (2)

In a gas chromatograph mass spectrometer for introducing a sample separated by a column of a gas chromatograph unit into an ionization chamber disposed in a vacuum chamber of a mass spectrometer unit,
a) an exhaust opening provided in the ionization chamber;
b) a vacuum pump connected to the exhaust opening;
A gas chromatograph mass spectrometer characterized by comprising: The gas chromatograph mass spectrometer according to claim 1, wherein the exhaust opening is covered with a metal mesh.

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