Mass spectrometer ion source

Abstract: The ionization chamber (1) of an ion source for use with a mass spectrometer is seperated from an ion lens system (3) while the chamber (1) is in contact and directly coupled with an interface block (7) in which an embedded heater (9) serves to heat both the interface block (7) and the ionization chamber (1).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ion source section of a mass spectrometer for ionizing a gaseous sample introduced. 2. Description of the Prior Art A gas chromatograph (GC) is provided for separating a gaseous mixture into its various constituent parts. A mass spectrometer (MS) is provided for analizing a gaseous sample to determine its molecular structure. If the output of a gas chromatograph is coupled to the input of a mass spectrometer, the combined instrument is known as a gas chromatograph/mass spectrometer (GC/MS). FIG. 3 shows a conventional ion source section of a mass spectrometer coupled to a gas chromatograph. The ion source section comprises an ionization chamber 1, an ion extraction electrode 2, an ion lens system 3, and a quadrupole electrode 4 for a quadrupole mass spectrometer. A sample gas is introduced into the ionization chamber 1, vertically to the plane of the drawing, through a sample introduction inlet 1h. A filament 1f is provided outside the ionization chamber 1 for injecting electrons into the ionization chamber 1 through an electron injection aperture 1e so that the sample molecule is ionized within the ionization chamber 1. An electron trap 1t is provided for capturing electrons passing through the ionization chamber 1. FIG. 4 is a rear side of the apparatus of FIG. 3, showing a heater 5 embedded within the ionization chamber 1 for maintaining the sample gas introduced into the chamber 1 at a predetermined temperature. A column 6 for the gas chromatograph is coupled to the ionization chamber 1 through an interface block 7. The interface block 7 is a metal block attached to an outer housing 8 of the mass spectrometer. An introduction aperture 7h coupled to the column 6 of the gas chromatograph passes through the interface block 7. A heater 9 is embedded within the interface block 7. A temperature sensor 10 is provided for generating a control signal to keep the interface block 7 at a predetermined temperature. Because the ionization chamber 1 injects ion to the mass spectrometer portion, grounded, the chamber 1 should have a voltage different from the ground. Therefore, it is necessary to electrically isolate the ionization chamber 1 and the interface block 7 with introducing the gas. For this purpose, an isolator 11 having a through-hole is interposed between the interface block 7 and the ionization chamber 1. The above-structured ion source section of the mass spectrometer has the following problems. Since the sample molecules are attached to the inside wall of the ionization chamber 1 so that the resultant mass spectra may make a ghost, it is necessary to clean the inside of the ionization chamber 1 some times. However, the ionization chamber 1 is integrated with the ion lens system 3 and a bolt 3b. So, in order to remove the ionization chamber 1, the ion lens system 3 must be resolved so that it is very troublesome to reassemble the ionization chamber after cleaning. It is preferred that the ionization chamber 1 and the interface block 7 be kept at the same temperature. The isolator 11 is interposed between them, which has a low thermal conductivity. It is, therefore, impossible to heat the ionization chamber 1 by the conducting heat from the interface block 7. The heater must be provided individually in both the interface block 7 and the ionization chamber 1, which is rather difficult to continuously keep them at the same temperature. For this reason, further, the ionization chamber 1 is difficult to be disassembled and reassembled for cleaning. EP-A-O 298 597 discloses a gas chromatograph/mass spectrometer having only one heater, and therefore is simpler in construction as the above described device. This reference discloses a thermal coupler 14 which is a screw connecting a ion source section and an interface. The removal of the ion source section from the interface takes much more labor and time compared with the present invention since not only the thermal coupler 14 but also flange means 48, 54 and 58 have to be unscrewed. Furthermore, the ion source section 32 is united to with the quadrupole section (ion lens system) 34 by the thermal divider section 36. Since the ion source section can never be separated from the ion lens system the cleaning of the ionisation chamber is difficult. SUMMARY OF THE INVENTION To resolve the above problem, it is an object of the present invention to provide an improved ion source of a mass spectrometer suitable for cleaning and heating the ion source. Briefly described, in accordance with the present invention, an ionization chamber is separated from an ion lens system while the ionization chamber is in contact with an interference block so that they are directly coupled in which a heater embedded in the interface block functions to heat both the interface block and the ionization chamber. With the arrangement of the present invention, since the ionization chamber is separated from the ion lens system and is directly coupled to the interface block, the ionization chamber can be removed without disassembling the ion lens system. It becomes easier to remove and reassemble the ionization chamber for cleaning it. The interface block and the ionization chamber are directly coupled without any isolator therebetween, thermal conductivity therebetween is so good that the heater in the interface block is sufficient to heat them at the same temperature. Temperature control is very simple. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: FIG. 1 shows a vertical sectional view of an ion source according a preferred embodiment of the present invention; FIG. 2 shows a cross sectional view of the ion source of FIG. 1, taken along the line I-I; FIG. 3 shows a vertical sectional view of a conventional ion source; and FIG. 4 shows a cross sectional view of the ion source of FIG. 3, taken along the line I-I. DESCRIPTION OF THE PREFERRED EMBODIMENT FIGs. 1 and 2 show a preferred embodiment of the present invention. Referring to FIG. 1, an ionization chamber 1 is of the type for ionizing according to electron impact. Electrons emitted from a filament 1f are injected into the ionization chamber 1 through an electron injection aperture 1e to ionize the sample molecules introduced through a sample introduction inlet 1h. An electron trap 1t is provided for capturing electrons passing through the ionization chamber 1. An ion source is a space covered by the ionization chamber 1 and an ion extraction electrode 2. The ion extraction electrode 2 is a part of an ion lens system 3 while the ionization chamber 1 is separated from the ion lens system 3. The ion lens system 3 is constructed such that an ion extraction electrode holder 2H and a plurality of lens electrodes 31, 32, and 33 are integrated by a bolt 3b through electrically isolated spacers. A quadrupole electrode 4is an electrode for a quadrupole type mass spectrometer. Referring to FIG. 2, an interface block 7 is provided for connecting the ion source and a column of the gas chromatograph. The interface block 7 is attached to cover the opening on a side of an outer wall 8 of the mass spectrometer via a screw 12. Along the central axis of the interface block 7, a sample introduction aperture 7h penetrates to which the column 6 of the gas chromatograph is connected. A projection 7p is formed at the center of a side of the interface block 7 being within the mass spectrometer, said projection 7p being penetrated by the sample introduction aperture 7h. The projection 7p of the interface block 7 is coupled to a cavity formed around the sample introduction inlet 1h on a side of the ionization chamber 1 to thereby combine them. Since the ionization chamber 1 can rotate around the sample introduction inlet 1h and cannot be positioned only with the simple coupling by the projection 7p, there is provided another projection 7p' in line with the projection 7p so that the ionization chamber 1 is coupled with the two projections 7p and 7p' to be positioned. The projection 7p' is shown as a samll circle in FIG. 1. A press pole 13 is provided for pressing a side of the ionization chamber 1 to the inner side of the interface block 7. A thermal insulator 13a is disposed at the tip of the press pole 13. A screw 13b at the inner side of a knob is rotated for pressing the ionization chamber 1 leftward of the drawing of FIG. 2 through the press pole 13. A heater 9 is embedded in the interface block 7, a temperature sensor 10 is provided for temperature control, and a thermal insulator 7m is positioned. Although there is no heater in the ionization chamber 1, it is directly coupled to the interface block 7 so that the ionization chamber 1 is heated by the conducting heat from the heater 9. Here, there are provided a couple of heater and temperature sensor, satisfactorily. The interface block 7 and the ionization chamber 1 are both made of stainless. To clean the ion source, referring to FIGs. 1 and 2, a rear lid 8a on an outer wall 8 of the mass spectrometer is opened and the press pole 13 is returned backward. Then, the ionization chamber 1 is manually moved rightward in FIG. 2 so that the coupling of projections 7p and 7p' of the interface block 7 to the ionization chamber is released. Then, the ionization chamber 1 is outside the outer wall 8 of the mass spectrometer to clean it. Here, an ion extraction electrode 2 as a part of the ion source remains in the ion lens system 3. In this preferred embodiment the ion extraction electrode 2 is coupled to an ion extraction electrode holder 2h positioned leftmost in the ion lens system 3 in FIG. 1. The ion extraction electrode 2 is put out of the outer wall 8 of the mass spectrometer, independently on the removal of the ionization chamber 1. According to the present invention, the ionization chamber 1 and the interface block 7 are directly in contact with each other to thereby be thermally integrated. The interface block 7 is coupled to the column 6 of the gas chromatograph, the voltage of the interface block 7 is the same as the ground voltage of the outer wall 8 of the mass spectrometer, so that the voltage of the ionization chamber 1 is the ground also. A DC bias voltage is applied to the quadrupole electrode to provide an acceleration voltage between the ionization chamber 1 and the quadrupole electrode to inject the ions to the mass spectrometer portion. Thus a DC and high frequency voltage is applied to the quadrupole electrode at the center of the DC bias voltage. As described above, in accordance with the present invention, the cleaning of the ionization chamber can be possible by removing the ionization chamber without disassembling the ion lens system, which is very simple. Since the interface block and the ionization chamber are directly in contact with each other, the heater embedded in the interface block enables the heat of the ionization chamber via thermal conductivity.It is unnecessary to provide a specific heater or a specific temperature sensor for the ionization chamber. Here, thermal uniformityis improved with less numbers of the necessary elements and low cost. The rate of malfunction of the heater and the temperature control system becomes low. The heater is provided only in the interface block so that the break of the heater can be repaired only by changing the heater outside the vacuum. The maintenance is thus simple. While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein.

Claims: An ion source for a mass spectrometer comprising: an interface block (7) connected to a sample source, an ionization chamber (1) ionizing a sample from the sample source introduced through said interface block, a heater (9) for heating said interface block and said ionization chamber, characterized in that said interface block is directly connected to said ionization chamber by a projection (7p) inserting in a dent, and said ionization chamber is fixed by an independent press means (13) pressing said ionization chamber towards said interface block, and wherein said ion source further comprises an ion lens system (3) provided independently on said ionization chamber.

Inventors: KAMESHIMA NORIO, KAMESHIMA, NORIO
Applicants: SHIMADZU CORP, SHIMADZU CORPORATION
Application year: 1991
Main classification: H01J49
IPC codes: H01J49, G01N30