Abstract:
A differentially pumped mass spectrometer system comprises a mass spectrometer having first and second pressure chambers through which, during use, ions are conveyed along a path. A pump assembly for differentially evacuating the chambers is attached to the mass spectrometer. The pump assembly comprises a housing attached to the mass spectrometer and a cartridge inserted into the housing. The cartridge has a plurality of inlets each for receiving fluid from a respective pressure chamber and a pumping mechanism for differentially pumping fluid from the chambers. The cartridge is inserted into the housing such that the pumping mechanism is inclined relative to the ion path, but with the cartridge protruding into the mass spectrometer to such an extent that at least one of the inlets at least partially protrudes into its respective chamber without crossing the ion path.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a divisional application of application Ser. No. 11/630,729 filed Dec. 21, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a vacuum pump, and in particular to a vacuum pump with multiple inlets suitable for differential pumping of multiple chambers. 
     There are a number of types of apparatus where a plurality of chambers or systems need to be evacuated down to different levels of vacuum. For example, in well known types of mass spectrometer, the analyser/detector has to be operated at a relatively high vacuum, for example 10 −5  mbar, whereas a transfer chamber, through which ions drawn and guided from an ion source are conveyed towards the detector, is operated at a lower vacuum, for example 10 −3  mbar. The mass spectrometer may comprise one or more further chambers upstream from the analyser chamber, which are operated at progressively higher pressures to enable ions generated in an atmospheric source to be captured and eventually guided towards the detector. 
     Whilst these chambers may be evacuated using separate turbo-molecular vacuum pumps, each backed by a separate, or common backing pump, for example a rotary vane pump, it is becoming increasingly common to evacuate two or more adjacent chambers using a single, “split flow” turbo-molecular pump having a plurality of inlets each for receiving fluid from respective chamber, and a plurality of pumping stages for differentially evacuating the chambers. Utilising such a pump offers advantages in size, cost, and component rationalisation. 
     For example, EP-A 0 919 726 describes a split flow pump comprising a plurality of vacuum stages and having a first pump inlet through which gas can pass through all the pump stages and a second inlet through which gas can enter the pump at an inter-stage location and pass only through a subsequent stage of the pump. The pump stages prior to the inter-stage location are sized differently from those stages subsequent to the inter-stage location to meet the pressure requirements of the different chambers attached to the first and the second inlets respectively. 
     However, when mounted to a mass spectrometer in a conventional manner, for example with the axis of the pump, or more particularly, its shaft axis, either parallel to or perpendicular to the plane of the outlet flanges of the mass spectrometer, conductance limitations of such a split flow pump compromise performance in comparison to an arrangement where adjacent chambers are evacuated using a bespoke vacuum pump directly mounted on to the respective chamber. 
     For example, when the pump is orientated with respect to the mass spectrometer such that the shaft axis is parallel to the plane of the outlet flanges, then gas must flow around a right angle bend to enter the pump inlet, which results in a pressure drop and associated loss of pumping speed. When the vacuum pump is orientated with its shaft axis perpendicular to the plane of the inlet of the outlet flange, whilst gas may flow easily into the first inlet, gas must flow around two bends in order to enter the second pump inlet. 
     In EP-A 1 085 214, these problems are reduced by mounting the split flow pump to the bottom of the mass spectrometer such that the shaft axis is inclined at an angle to the plane of the outlet flanges. With this orientation, the gas flows into the inlets by flowing around bends of obtuse angle so that there is little pressure drop between the outlet flanges and the pumping inlets. However, with such an arrangement the overall volume occupied by the mass spectrometer and split flow pump is increased in comparison to an arrangement where the shaft axis is parallel to the outlet flanges. 
     It is an aim of at least the preferred embodiment of the present invention to seek to provide an improved arrangement for the differential evacuation of a multi-chambered system, such as a mass spectrometer. 
     BRIEF SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a system comprising a plurality of pressure chambers and a vacuum pump for differentially evacuating the chambers, the pump comprising a plurality of inlets each for receiving fluid from a respective pressure chamber and a pumping mechanism for differentially pumping fluid from the chambers, wherein the pump protrudes through an opening into the chambers such that at least one of the fluid inlets is at least partially located within its respective pressure chamber, and the longitudinal axis of the pump is inclined to the plane of the mouth of the opening. 
     With such an orientation of the pump relative to the pressure chambers, the conductance of the inlets of the pump can be maximised and high effective pumping speeds can be achieved for a given pumping mechanism. Furthermore, since the pump protrudes into the chambers, the overall volume occupied by the chambers and pump is minimised. 
     The use of the invention is particularly advantageous where the system under evacuation is a mass spectrometer system, as the inclination of the pump allows the pump to be inserted into the chambers without the pump crossing the path of the ions conveyed within the mass spectrometer. Therefore, in a second aspect, the present invention provides a differentially pumped mass spectrometer system comprising a mass spectrometer having a plurality of pressure chambers through which, during use, ions are conveyed along a path; and a pump for differentially evacuating the chambers, the pump comprising a plurality of inlets each for receiving fluid from a respective pressure chamber and a pumping mechanism for differentially pumping fluid from the chambers, wherein the pump is inclined relative to at least part of the ion path and protrudes into the spectrometer without crossing the ion path but with at least one, preferably each, of the fluid inlets at least partially located within its respective pressure chamber. 
     The pump is preferably in the form of a cartridge inserted into a housing attached to or part of the mass spectrometer such that the cartridge protrudes through a mouth of the housing into said at least one of the chambers. This can provide a relatively simple construction for mounting and aligning the pump relative to the chambers under evacuation, as opposed to an arrangement wherein the pump is integrated into the body of the mass spectrometer. 
     As the pump may be provided in isolation from the mass spectrometer, a third aspect of the present invention provides a pump comprising a housing, a cartridge insertable into the housing, the cartridge comprising a fluid inlet and a pumping mechanism, and means for locating the cartridge within the housing such that a part of the cartridge defining the fluid inlet protrudes from a mouth of the housing, and such that the longitudinal axis of the pumping mechanism is inclined relative to the plane of the mouth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a vertical cross-section through a vacuum pump; 
         FIGS. 2 to 6  are various different external views of the pump; and 
         FIG. 7  is a simplified cross-section illustrating the connection of the assembly to a multi-chambered system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pump  10  comprises a housing  12  having a bore for receiving a cylindrical cartridge  14  containing a pumping mechanism and a plurality of fluid inlets  16 ,  18 ,  20  and a fluid outlet  22 . 
     With reference to  FIG. 1 , the cartridge  14  comprises a multi-component body  24  within which is mounted a drive shaft  26 . Rotation of the shaft  26  is effected by a motor  28  positioned about the shaft  26 . The shaft  26  is mounted on opposite bearings. For example, the drive shaft  26  may be supported by a hybrid permanent magnet bearing and oil lubricated bearing system. 
     The pumping mechanism within the cartridge includes at least three pumping sections  30 ,  32 ,  34 . The first pumping section  30  comprises a set of turbo-molecular stages. In the example shown in  FIG. 1 , the set of turbo-molecular stages  30  comprises four rotor blades and four stator blades of known angled construction. In this example, the rotor blades of the first pumping section are integral with the drive shaft  26 . 
     The second pumping section  32  is similar to the first pumping section  30 , and also comprises a set of turbo-molecular stages. In the example shown in  FIG. 1 , the set of turbo-molecular stages  32  also comprises four rotor blades and four stator blades of known angled construction. In this example, the rotor blades of the first pumping section are also integral with the drive shaft  26 . 
     Downstream of the first and second pumping sections is a third pumping section  34 . The third pumping section  34  is in the form of a molecular drag mechanism, for example, a Holweck drag mechanism. In this example, the Holweck mechanism comprises two rotating cylinders and corresponding annular stators having helical channels formed therein in a manner known per se. The rotating cylinders are preferably formed from a carbon fibre material, and are mounted on a disc located on the drive shaft  26 . In this example, the disc is also integral with the drive shaft  26 . The pump outlet  22  is located downstream from the Holweck mechanism  34 . 
     The cartridge  14  has three inlets  16 ,  18  and  20 . The first, low fluid pressure inlet  16  is located upstream of all of the pumping sections. In this example, the first inlet  16  is substantially orthogonal to the longitudinal axis of the drive shaft  26 , as indicated at  36 . The second, middle fluid pressure inlet  18  is located interstage the first pumping section  30  and the second pumping section  32 . In this example, the second inlet  18  extends about the longitudinal axis of the drive shaft  26 . The third, low fluid pressure inlet  20  may be located, as illustrated, upstream of or, alternatively, between the stages of the Holweck mechanism  34 , such that all of the stages of the Holweck mechanism are in fluid communication with the each of the inlets  16 ,  18 ,  20 . 
     Returning now to the housing  12 , the bore has an inlet formed in the rear surface  38  of the housing  12  and through which the cartridge  14  is inserted into the housing  12 . The inner surfaces  40 ,  42 ,  44 ,  46  of the bore guide the cartridge  14  towards the fully inserted position shown in  FIGS. 1 to 6  as it is inserted into the bore. The end of the bore is profiled as indicated at  48  in  FIG. 1  to define abutment surfaces for engaging the front end of the inserted cartridge  14  and which, with the rear surface  38  of the housing  12 , limit the extent to which the cartridge  14  can be inserted into the housing  12 . 
     As shown in  FIGS. 1 to 6 , the housing  12  is shaped so as to expose the bore at a number of locations to allow fluid to enter the fluid inlets  16 ,  18 ,  20  when the cartridge  14  is in the fully inserted position. In the example shown, the housing  12  comprises a mouth  50  formed in a flanged planar surface  52  of the housing  12 , the flanged planar surface  52  being inclined at an acute angle to the rear surface  38  of the housing, and at an acute angle θ to the longitudinal axis of the bore of the housing. The angle θ may be at any angle between 10° and 80° inclusive, preferably at an angle between 20° and 50° inclusive. In the example illustrated in the figures, θ=27.5°. In the fully inserted position shown in the figures, the longitudinal axis  36  of the pumping mechanism is co-axial with the bore of the housing  12 . 
     In order to locate the cartridge in the fully inserted position, curved members  54 ,  56  defining part of the bore of the housing  12  extend across the mouth  50  of the housing  12 . In this example, the curved members  54 ,  56  are integral with the housing  12 . Alternatively, the curved members  54 ,  56  may be separate members insertable into the housing  12 . The curved inner surfaces  44 ,  46  of the curved members  54 ,  56 , which form part of the bore of the housing  12 , form a seal with the body  24  of the cartridge  14  whilst allowing each of the inlets  16 ,  18 ,  20  to be partially exposed by the mouth  50  formed in the flanged planar surface  52  of the housing  12 . As shown in the figures, part of the first inlet  16  and part of the second inlet  18  project through the mouth  50  of the housing  12 , whilst the third inlet  20  is located just beneath the mouth  50 . 
       FIG. 7  shows the pump  10  attached to an example of a multi-chamber system  60  to be evacuated using the pump  10 . In the example shown, the multi-chamber system  60  is a mass spectrometer system. A high vacuum chamber  62  immediately follows first, (depending on the type of system) second, and third evacuated interface chambers  64 ,  66 ,  68 . The first interface chamber  64  is the highest-pressure chamber in the evacuated spectrometer system and may contain a capillary or sample cone through which ions are drawn from an ion source into the first interface chamber  64 . The second, interface chamber  66  may include a first ion guide for guiding ions from the first interface chamber  64  into the third interface chamber  68 , and the third chamber  68  may include a second ion guide for guiding ions from the second interface chamber into the high vacuum chamber  62 . 
     The flanged planar surface  52  of the pump  10  is attached to the planar, bottom surface  70  of the system  60 , for example by means of bolts or the like. An O-ring located within a groove  72  assists in forming a seal between the surfaces  52  and  70 . As shown in  FIG. 7 , with the pump  10  attached to the system  60 , the cartridge  14  protrudes into the system  60  through an opening  74  formed in the bottom surface  70  of the system such that the first inlet  16  of the cartridge  14  and the first pumping section  30  protrude into the high vacuum chamber  62 , and the second inlet  18  of the cartridge  14  and the second pumping section  32  protrude into the third chamber  68 , and such that the pump  10  is inclined at angle θ to the path  76  of ions conveyed within the system  60  during use to maximise conductance at the inlets of the pump. The extent to which the pump  10  extends into the system  60  is not so great, however, as to cause the pump to cross the ion path  76 . To prevent fluid leakage between the chambers of the system  60  during use, the upper surface  78  of the cross member  54  sealingly engages a conformingly-profiled lower surface of the dividing wall  80  between the high vacuum chamber  60  and the third chamber  68 , and the upper surface  82  of the cross member  56  sealingly engages a conformingly-profiled lower surface of the dividing wall  84  between the second chamber  66  and the third chamber  68 . 
     In use, the first interface chamber  64  is connected to a backing pump (not shown), which also pumps fluid from the outlet  22  of the pump  10 . The backing pump typically creates a pressure within the first chamber of roughly the same order of magnitude as that at the outlet  22  of the pump  10 . Fluid entering each inlet  16 ,  18 ,  20  of the pump  10  passes through a respective different number of stages before exiting from the pump. Fluid pumped through the first inlet  16  passes through both sets  30 ,  32  of turbo-molecular stages in sequence and the Holweck mechanism  34  and exits the pump via outlet  22 . Fluid pumped through the second inlet  18  passes through set  32  of turbo-molecular stages and the Holweck mechanism  34  and exits the pump via outlet  22 . Fluid pumped through the third inlet  20  passes through the Holweck mechanism  34  only and exits the pump via outlet  30 . Consequently, the pump  10  is able to provide the required vacuum levels in the chambers  62 ,  66 ,  68 , with the backing pump providing the required vacuum level in the chamber  64 . In this example, in use the first interface chamber  64  is at a pressure of around 1-10 mbar, the second interface chamber  66  is at a pressure of around 10 −1 -1 mbar, the third interface chamber  68  is at a pressure of around 10 −2 -10 −3  mbar, and the high vacuum chamber  60  is at a pressure of around 10 −5 -10 −6  mbar.