Mass Spectrometer Connected To A Gap Chromatograph Through A Valved Molecule Separator

Abstract

An instrument for analyzing organic chemical compounds is provided. The instrument comprises a gas chromatograph which via a molecule separator is connected to the ion source of a mass spectrometer. The separator consists of a number of series connected evacuating stages, each stage being connected to a vacuum pump. Each stage is provided with an input jet nozzle and a coaxial output nozzle, a slide being insertable between these nozzles in the first stage so as to make it possible to switch off the flow through the separator.

Description

The present invention relates to an instrument for analyzing organic chemical compounds, the instrument comprising a gas chromatograph, a molecule separator and a mass spectrometer, the molecule separator being connected between the output of the gas chromatograph and the input of the ion source of the mass spectrometer, the separator including at least one separating stage consisting of an evacuation chamber connected to a vacuum pump, a jet input nozzle via which molecules in gas phase are supplied from the gas chromatograph and an output nozzle coaxial with the jet nozzle.

In a combination instrument as defined above, so called molecule separators are used for separating a carrier gas, usually helium, from the specimen to be analyzed and for reducing the pressure in the ion source of the mass spectrometer.

Different kinds of separators are used for this purpose, the so called Becker-Ryhage-Separator however having several advantages in comparison to the others. In this separator the enrichment is high, the loss of specimen is small, the dead volume is small and the memory effect is negligible. The Becker-Ryhage-Separator is generally made in two stages. In the first stage the pressure is reduced to 0.001 mm. Hg by an oil diffusion pump. In this way approximately 99 percent of the carrier gas is evacuated while at least 75 percent of the specimen remains. The pressure will then be further reduced at the ion source of the mass spectrometer.

The above described separator is described in detail by the inventor for instance in Analytical Chemistry vol. 36., No 4, Apr. 1964 pages 759-764. The principle of the separator is described in the German Pat. No. 105 2,955 (See FIG. 1).

By using a conventional molecule separator several problems do however arise. For changing the gas chromatograph column the following steps have to be performed: (a) The connection between the separator and the ion source of the mass spectrometer has to be switched off or otherwise the current to the filament of the ion source has to be switched off. It is however not desirable to change the ionization conditions and thus a tap is generally mounted between the separator and the ion source; (b) The oil diffusion pump connected to the second separator stage is switched off; (c) The mechanical vacuum pump connected to the first separator stage is switched off; (d) The pressure in the separator is raised by means of a blow cock connected between the mechanical vacuum pump and the first separator stage; (e) Then the gas chromatograph column is exchanged; (f) The system is evacuated by connecting first the mechanical vacuum pump and then the oil diffusion pump; (g) When the pressure has been reduced sufficiently the tap of the ion source is opened. The above described procedure is very lengthy, especially the evacuation. The insertion of a tap between the separator and the ion source does further imply and increase of the dead volume of the system which in turn affects the separation obtained in the gas chromatograph. Furthermore the separator and the connection to the ion source has to be heated and it is of course desirable that the heated part is as small as possible. Although the pressure after the separator is very low and the velocity of the molecules is high a certain risk for degradation in the tap and the connecting pipes arises because of thermic and catalytic effects.

The above discussed drawbacks can be avoided by using an instrument designed in accordance with the present invention, the essential features of which are defined in the attached claims.

The invention will now be described in detail with reference to the enclosed drawings in which:

FIG. 1 shows a two-stage separator according to the invention provided with a switching-off arrangement in the form of a slide;

FIG. 2 shows another embodiment of a two-stage separator according to the invention in which the second separator stage has been combined with the ion source.

In FIG. 1 reference 1 denotes a mounting flange for connection of a gas chromatograph G, reference 2 denotes the first separator stage, reference 3 denotes the second separator stage and reference 4 denotes the mounting flange of the ion source of the mass spectrometer M. The input jet nozzle of the separator stages are denoted 5a and 5b respectively and the output nozzles are denoted 6a and 6b respectively. Reference 7 denotes the slide by means of which the gas flow can be switched off or at least be prevented from passing into the second separator stage. Reference 8 denotes a bellows which controls the position of the slide 7 and reference 9 denotes a knob for moving the bellows. The slide can be set into two positions 11a and 11b, which positions are adjusted by means of a screw 10. References 12 and 13 denote connections leading to a mechanical vacuum pump and an oil diffusion pump respectively.

When the column of the gas chromatograph should be changed or if it for some other reason is desirable to switch off the gas flow through the separator, the slide 7 is inserted between the nozzles 5a and 6a. The distance between these nozzles is about 0.1 mm. and thus the slide should have this dimension. The nozzle 5a does also have a diameter of about 0.1 mm. The slide does not necessarily have to seal this nozzle completely but a small leakage into the separator housing can be permitted. It has turned out in practice that in spite of this leakage the mechanical vacuum pump provides a pressure which is sufficiently low for making the oil diffusion pump work and for not making it necessary to have the ion source switched off. Even if the pressure in the first separator stage 2 is as much as 1 mm. Hg, instead of the normal value 0.1 mm. Hg, the system works satisfactory. Thus the pumps do not have to be switched off.

As it is sometimes desirable to have a certain leakage into the separator without having any gas flow into the ion source, the slide 7 can be shaped so as to seal only the output nozzle 6a. In this case the slide 7 could be provided with a slot in the surface turned to the input nozzle 5a. This shaping of the slide is suitable when a gas flow through only the gas chromatograph column is wanted, for instance after mounting the column.

In the arrangement according to the present invention the dead volume after the separator is reduced considerably. In the previously used systems the volume of the tap between the separator and the ion source is approximately 0.5 ml. and the connecting pipes have a volume of approximately the same size. If these values are compared with the area of the channel through the separator (0.8 mm..sup.2) and the area of the nozzle (0.008 mm..sup.2) the significance of the reduced dead volume is understood. Of course a reduced volume after the separator means less than a reduced volume before the separator, the pressure conditions being completely different, but a reduced volume is still desirable. The insertion of some sort of tap before the separator is not suitable as this will increase the dead volume. Each person skilled within this field is aware of the drawbacks of such an arrangement, especially when capillary columns are used in the gas chromatograph.

A tap mounted before the separator further makes it more difficult to remove the flange for cleaning the nozzle 5a. The other nozzles do not have to be cleaned so often as the pressure and thus the fouling is less at these nozzles.

In the above described embodiment of the invention the switching off means consist of a slide. The slide can of course be shaped in different ways and the insertion of the slide between the nozzles of the separator can be arranged in different ways. If only a sealing of the output nozzle is wanted it might thus be suitable to make the top of the output nozzle radially movable.

In FIG. 2 a new way of connecting the separator to a mass spectrometer is shown. This connection is made possible by using the present invention. In this figure the references denote the same members as in FIG. 1, the difference being that reference 15 denotes the connection leading to the ion source and references 14a and 14b denote the nozzle of the second separator stage. Reference 16 denotes the connection of the high vacuum system which in this case is the same for the second separator stage and the ion source housing. In this embodiment the dead volume is considerably decreased in comparison with the embodiment of FIG. 1. On the other hand the flexibility will be somewhat reduced.

Claims

1. An instrument for analyzing organic-chemical compounds, the instrument comprising a gas chromatograph, a molecule separator and a mass spectrometer having a housing, the molecule separator being connected between the output of the gas chromatograph and the input of the ion source of the mass spectrometer, the molecule separator including at least one separating stage consisting of an evacuation chamber connected to a vacuum pump, a jet input nozzle from which molecules in gas phase are supplied from the gas chromatograph and an output nozzle coaxial with the jet nozzle, characterized in that the molecule separator comprises a slide element, and means mounting said slide element for movement in a direction transverse to the axis of said input and output nozzles so as to be inserted between the jet nozzle and the output nozzle to close the space therebetween and cut off the gas flow through the molecule separator and

2. An instrument according to claim 1, characterized in that the surface of said slide element is shaped to seal the output nozzle and to provide space for leakage at the jet input nozzle when the slide element is in an

3. An instrument according to claim 1, characterized in, that the number of stages is at least two, the last stage being combined with the housing of the ion source.