Device for collecting ions in a mass spectrometer

The invention relates to a device for collecting ions, in particular in an isotope mass spectrometer. At least one secondary electron multiplier (SEM) is provided, this being constructed in the manner of a card, namely box-like and with a thickness which is low in comparison with its length and width. The SEM is held in a frame. In addition, a flat flexible printed circuit board with a plurality of parallel conductors is connected to the SEM. The flexible printed circuit board permits the positioning of the SEM in order to match the ion paths to be sampled.

RELATED APPLICATIONS

This application claims priority to German Patent Application Serial No. 102 38 347.2, filed on Aug. 16, 2002, the disclosure of which is hereby incorporated by reference in its entirety.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to a device for collecting ions, in particular in a mass spectrometer, having at least one secondary electron multiplier (SEM), the SEM being formed in the manner of a card, specifically substantially box-like with a low thickness in relation to the length and width.

BACKGROUND OF THE INVENTION

In a brochure entitled “Triton Neptune, Multicollector Mass Spectrometers for High Precision Isotope Ratio Determination” published by Thermo Finnigan MAT GmbH, Bremen, Germany, 2001, various constituent parts of mass spectrometers are explained. Inter alia, a multicollector as an ion collector having a large number of Faraday cups is illustrated. The Faraday cups are of rectangular and flat disc-like design, so that a plurality of Faraday cups can be arranged beside one another at short intervals in the ion path. For measurements of only low ion currents, in particular in connection with isotope ratio determination, SEM are preferably needed. These must be matched as well as possible to the existing system.

SEM having approximately the size and the dimensions of a cheque card are known. The SEM are somewhat smaller than the Faraday cups.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device for collecting ions in a mass spectrometer, which can be used and adapted as flexibly as possible. The device according to the invention is characterized in that the SEM is held in a frame. The frame can have approximately the dimensions of the Faraday cup, which would be otherwise used. The SEM is insertable and removable from the frame. Accordingly, the frame is preferably somewhat thicker than the SEM. As a result of using the frame, the external dimensions of the SEM are matched to the dimensions of other constituent parts of the device. At the same time, the frame permits easy replacement of the SEM. In this way, the SEM can be held in the frame merely by being wedged in.

In one embodiment of the invention, provision is made for the SEM to have an entry opening for the ions on a narrow side, for the frame to have a passage opening for the ions on a narrow end side, the inlet opening and the passage opening being aligned with each other, and for the SEM to be at least partially inserted into the frame on a further narrow side of the latter which is located transversely with respect to the narrow end side. The frame is upright with large-area, upright walls. The SEM is inserted into the frame from above. The passage opening of the frame is located at its front end side, while the large-area upright walls of the frame are aligned substantially parallel to the ion flight path. The greatest length of the frame likewise extends parallel to the ion flight path. By contrast, the greatest dimension of the SEM extends parallel to the insertion direction (insertion of the SEM into the frame).

Preferably, and independently of the inventive features mentioned previously, a flat flexible printed circuit board having a plurality of parallel lines is connected to the SEM. The flexible printed circuit board is also referred to as a strip conductor or flexible conductor track. Similar flexible conductor tracks are used in ink jet printers for the electrical drive of the print head. In the present case, the flexible printed circuit board initially extends in the plane of the SEM. In this plane, the individual lines including signal lines are located beside one another and parallel to one another at intervals. An end of the flexible printed circuit board opposite to the SEM is connected to electrical contacts, which lead onward. By means of the use of the flexible printed circuit board in the manner described, the space available for the electrical connections is optimally utilized. The influence on the signals resulting from high voltages, which are naturally present is minimized. If a plurality of SEM are arranged beside one another, nevertheless far-reaching possible adjustments for the individual SEM are provided for matching to the ion paths to be sampled. The flexible printed circuit boards are also preferably insulated electrically on only one side, for cost reasons, namely by the base material. Applied to the latter are individual conductor tracks, preferably without additional insulation. Adjacent flexible printed circuit boards cannot short-circuit one another since there is always an insulated side opposite a side provided with electrical conductors. Electric lines of adjacent SEM never come to lie opposite one another.

One channel input of the SEM is preferably earthed. Accordingly, the channel output is connected to high voltage, in particular to about 2000 V.

The frame can preferably be connected to a holder by its narrow underside. The frame is held in a defined position by the holder or can be inserted into the holder in a defined position. The holder itself can be adjusted transversely with respect to the ion flight path, so that the frame can be set to a defined ion flight path.

The frame preferably has holding means on its narrow upper side to connect the frame to a guide means. This results in further possible adjustment in interplay between the holding means and the guide means (or a plurality of guide means).

According to a further embodiment of the invention, a plurality of SEM are provided with frames, the frames being held on at least one common guide means and being capable of being positioned relative to one another at defined intervals on the latter. Accordingly, the guide means permit a plurality of frames to be combined into a group, it being possible for defined positions to be assumed within the group. At least one frame of the group is preferably provided with a holder, so that the result is that a group with a plurality of SEM (in corresponding frames) is held only by one frame. The guide means provided are preferably two rods, onto which the individual frames are threaded with appropriate holding means.

In addition, at least one Faraday cup is preferably provided, whose external dimensions correspond to those of the frame. The Faraday cup is a specific ion collector, which can be provided in addition to the SEM.

According to a further embodiment of the invention, groups are formed which contain either at least one Faraday cup and otherwise at least one SEM, or which contain more than one SEM, at least one Faraday cup or SEM in a group being connected to a holder by its narrow underside, and the Faraday cups and SEM within the same group being arranged on one or more common guide means via holding means on the upper side and being capable of being positioned relative to one another. Accordingly, by adjusting one holder, an entire group can be displaced, in particular transversely with respect to the running direction of the ion beam. Adjustments of the positions within a group are carried out by moving the holding means along the guide means.

A mass spectrometer is also a constituent part of the invention, in particular an isotope mass spectrometer, having one or more devices according to the invention, preferably having a multicollector.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1shows a secondary electron multiplier (SEM)11inserted into a frame10. The frame is of substantially rectangular design with a length LR in the direction of a longitudinal mid-axis12and a width BR extending transversely with respect thereto. The frame10has a thickness DR which is very low as compared with the width BR or length LR and which is oriented at right angles to the plane of the figure. Corresponding directional indicators for the aforementioned dimensions comprising length LR, width BR and thickness DR are likewise shown in FIG.1. Overall, the dimensions of the frame are comparable with those of a somewhat thicker cheque card.

The frame10has a narrow end side14provided with a passage opening13, a narrow end side15opposite the former, a narrow underside16, a narrow upper side17and large-area side walls18,19lying parallel to the plane of the figure, see also FIG.6. The designations upper side and underside also relate to the envisaged arrangement of the frame10in an isotope mass spectrometer.

The SEM11likewise has a format similar to a cheque card, namely with a length LS along a longitudinal mid-axis20, a width BS and a thickness DS. The latter extends at right angles to the plane of the figure. The longitudinal mid-axis20runs in the plane of the figure but at right angles to the longitudinal mid-axis12of the frame10. The thickness DS is somewhat smaller than the thickness DR of the frame10.

The SEM11is plugged into the frame10from above, that is to say, in the area of the narrow upper side17. A corresponding receiving opening in the frame10is designated by the number21. The SEM11inserts into the frame10over about ⅔ of its length and has a bell-like inlet opening22, a meandering channel23which adjoins the said inlet opening22, an insulating ceramic body24and electrical terminals25. The construction of such an SEM of cheque-card size with a continuous dynode is known in principle. The ions entering the inlet opening22, each time they collide with walls of the channel23, knock out secondary electrons which, in turn, separate further secondary electrons during the collision. There is an amplification of about 107at a channel output26here. In the present case, a high voltage of about 2000 V is applied to the channel output26, while a channel input27is earthed. The inlet opening22of the SEM11is aligned with the passage opening13of the frame10and is arranged on a narrow long side28of the SEM11.

The receiving opening21in the frame10is provided on the inside with a lower supporting surface29and two lateral supporting surfaces30,31and a compression spring32. The compression spring32acts on a further narrow long side33of the SEM11, opposite the narrow long side28. A narrow lower end side34of the SEM11comes to lie on the lower supporting surface29, which is considerably smaller for this purpose. The narrow long side28rests on the lateral supporting surfaces30,31, above and below the inlet opening22. Overall, the SEM11is positioned as accurately as possible after being inserted into the frame10.

Opposite the lower end side34, the SEM has an upper narrow end side35. In the area of the same, a flexible printed circuit board36is connected to the SEM11. On the flexible printed circuit board36, four lines are printed beside one another on a carrier37made of flexible plastic, or applied in another way, see FIG.4. The individual conductors38,39,40,41are provided for different voltages. The outer conductor38carries the secondary electron voltage. The conductor39lying closest thereto is earthed. The following conductor40carries the ion signal. The last, outer conductor41is connected to a high voltage.

At an end opposite the SEM11, the flexible printed circuit board36has a connecting piece42having four contacts43,44,45,46, which are associated with the conductors38,39,40,41.

The SEM11with frame10is a constituent part of a multicollector, not specifically shown, for a mass spectrometer. In order to record an ion distribution (or for other reasons), a plurality of frames10with SEM11are provided beside one another. In this case, various types of groups can be formed. Firstly, the frames10with SEM11can be arranged with their undersides16on specific holders47.FIG. 5shows a frame10on a holder47. The latter has a cylindrical receptacle48for the frame10. The receptacle48is provided, in a manner not specifically shown, with a specifically configured upper side, so that recesses49on the underside16can be placed on the holder17in an exactly reproducible position.

The holder47is arranged on a carriage50which, in a manner not shown specifically, can be displaced with a movement component transverse to the ion beam. Given a plurality of carriages50each having a frame10, the individual SEM can be positioned independently of one another. An arrangement having a plurality of carriages (but with Faraday cups) is shown in more detail in the company brochure from Thermo Finnigan MAT GmbH cited in the introduction to the description. Reference is made to the entire disclosure of the company brochure.

FIG. 7shows a plurality of frames10arranged with their large-area side walls18,19adjacent. The said frames are threaded onto two guide means51,52constructed as rods. For this purpose, each frame10has holding means53,54on its narrow upper side17. In practical terms, these are hooks with inner contact surfaces55and locking screws56opposite the latter. By means of the aforementioned guide means51,52and holding means53,54, it is possible to connect a plurality of frames10to one another to form a group57and, at the same time, to define the relative arrangement of the frames10within the group57exactly. One of the frames10is mounted on the holder41shown inFIGS. 5 and 6and can be adjusted with the carriage50. Accordingly, the entire group57can be moved with the carriage50. Mobility is also favoured by the aforementioned flexible printed circuit boards36on each frame10.

Instead of the frame10with SEM11, the Faraday cups shown in the aforementioned company brochure can also be arranged on the carriage50or holders47. Their construction can be seen in the company brochure and, in addition, is illustrated in the German Laid-Open Specification DE 198 38 553. In the present case, holding means corresponding to the holding means53,54illustrated inFIG. 5are additionally provided.

The external dimensions of the frames10and of the Faraday cups should largely correspond to one another. This makes it possible to form groups which contain only frames10with SEM11or else at least one Faraday cup and at least one frame10with SEM11, the individual members of a group being held together by the guide means and holding means already mentioned. A group57with a Faraday cup58and four frames10is shown in plan view inFIG. 8, in a similar way to FIG.6. The Faraday cup58is connected electrically by a conductor59to the carriage50but not to the SEM11. In addition, only the Faraday cup58is situated on the holder47. Otherwise, the group is held together by the holding means and guide means51,52. Finally, the Faraday cups can also have flexible printed circuit boards for an electrical connection.