Abstract:
A method of configuring a Faraday detector in a mass spectrometer is described. The mass spectrometer defines a central ion beam axis I, and the Faraday detector is moveable relative to the central ion beam axis I. The Faraday detector includes a detector arrangement having a detector surface, and a Faraday slit defining an entrance for ions into the detector arrangement, the Faraday detector having an axis of elongation A which extends through the Faraday slit. A width of the Faraday slit is chosen and the angle α between the axis of elongation, A, of the Faraday detector, and the central ion beam axis I is adjusted. This prevents admittance of incident ions into the detector cup of the Faraday detector, outside of a maximum admittance angle γ defined between the axis of elongation A of the Faraday detector and a direction of incidence, B, of ions, at the Faraday detector, where α and/or γ is selected according to the criterion that ions entering the detector arrangement should strike the detector surface at a location which prevents secondary electrons generated thereby from exiting the Faraday detector via the Faraday slit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit under 35 U.S.C. §119 to British Patent Application No. 1514536.0, filed on Aug. 14, 2015, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to the configuration of detectors and slits in a multi-collector isotope ratio mass spectrometer such as a sector field mass spectrometer for high resolution analysis of elemental and molecular species. 
       BACKGROUND TO THE INVENTION 
       [0003]    Quantitative analysis of elemental and molecular species, and often an isotopic ratio of species, is a key interest in many fields of science. For instance, accurate and quantitative determination of elemental and molecular species finds application in environmental, science, material sciences, life science and geology. 
         [0004]    A fundamental challenge for accurate and precise quantitative mass spectrometry of molecular and elemental species is the interference between a species of interest and another species having the same nominal mass. One example of a problematic interference is that of isotopologues within a sample having the same nominal mass. For example, in the analysis of methane, 13CH4+, 12CH3D+ and 12CH5+ all have a nominal mass of 17 but an exact mass that differs as a consequence of nuclear mass defect. 
         [0005]    In order to permit discrimination between interfering species, e.g. same nominal mass isotopologues, a mass spectrometer having relatively high mass accuracy is necessary. One such device, sold by Thermo Finnigan under the brand name Neptune™, is described in Weyer et al, International Journal of Mass spectroscopy, 226, (2003) p 355-368. The Neptune™ device is a double focusing multiple collector inductively coupled plasma (MC-ICP) mass spectrometer and may be used to determine isotopic fractions of atomic and polyatomic ions. The detector chamber of the mass spectrometer is equipped with a plurality of Faraday collectors. Ions are spatially separated by the mass analyzer in accordance with their mass to charge ratio. Each Faraday collector is precisely aligned with respect to atomic and polyatomic ions of a particular nominal mass. The Faraday collectors are each provided with an entrance slit. In use, the parameters of the mass analyzer are adjusted so that ions of different masses are scanned across the slit. With suitably high resolution, ion species of the same nominal mass but different true masses can be separately detected. 
         [0006]    Our co-pending application no. GB1514471.0, filed on even date, describes a double focusing gas isotope ratio mass spectrometer (GIRMS) developed by Thermo Fisher Scientific under the name 253 Ultra™. The device has a multiple collector positioned at the focal plane of a double focusing magnetic sector mass analyser. High, medium and low resolution can be selected automatically using a switchable spectrometer entrance slit. The device is capable of resolutions up to several tens of thousand. 
         [0007]    The multiple collector comprises a fixed axial collector which is a dual mode detector having a Faraday cup and a high sensitivity ion counting detector (SEM). The multiple collector also carries 8 moveable detector platforms mounted as 4 platforms on each side of that fixed axial collector. Each moveable detector platform is equipped with a Faraday detector and can also carry a compact discrete dynode (CDD) ion counting detector. In total, the multiple collector can thus carry 9 Faraday detectors (the axial detector plus 8 more, located 4 each side of the axis) and 8 CODs (again, 4 each side of the axial Faraday detector). 
         [0008]      FIG. 1  shows an ideal high resolution scan across the slit of a Faraday collector in a double focusing gas isotope ratio mass spectrometer such as the 253 Ultra™ described above. The presence of “steps” at the shoulders of the main peak is analytically interesting since it may permit identification of different isotopologues or other distinct species. 
         [0009]      FIG. 2  shows a scan across a Faraday detector slit with a first signal artefact that can sometimes be observed, when the mass spectrometer is operated at high resolutions up to, for example, 40,000. The artefact is labelled  1  in the figure. As may be seen, the artefact is proximal to the shoulder of the peak, where analytically interesting information may be present. Thus the presence of the artefact  1  in  FIG. 2  is undesirable. 
         [0010]      FIG. 3  shows a high resolution scan across a Faraday detector slit having a second signal artefact, labelled  2  in the figure, which can also sometimes be observed. Again, artefact  2  is found at the end/shoulder of the main peak, and its presence can reduce or completely mask the ability to detect any analytically significant peak information that would otherwise be seen at the peak shoulders. 
         [0011]    The present invention seeks to identify and address problems with Isotope Ratio mass spectrometers such as the GIRMS and MC-ICP MS, that result in the various unwanted artefacts described above. 
       SUMMARY OF THE INVENTION 
       [0012]    The inventors have identified various difficulties arising from the multiple collector arrangement described above. 
         [0013]      FIG. 4  shows, schematically, a part of a multiple collector  100  for a dual sector mass spectrometer, together with an ion beam  110 . As explained above, the multiple collector  100  comprises a fixed axial collector  120  along with a plurality of moveable collectors ( 130 ). In  FIG. 4 , only some of the moveable collectors ( 130   a,    130   b,    130   c,    130   e,    130   f ) are shown, and for clarity the CDDs have been omitted. As may be seen in  FIG. 4 , the fixed axial collector  120  is positioned upon a central axis I of the ion beam  110 , and a focal plane P extends about the central axis I of the ion beam, at an angle of around 45 degrees thereto. The moveable collectors  130  (along with the fixed axial collector  120 ) are laterally spaced along the focal plane P and each of the moveable collectors  130  are moveable along the focal plane. At least some, optionally all, of the moveable collectors can be mounted on a respective motorized platform. Any moveable collectors that are not mounted on a motorized platform can be moved position by being pushed or pulled by one or more moveable collectors that are mounted on a motorized platform. Typically, every other collector  130  is mounted on a motorized platform. 
         [0014]    The ion trajectories of spatially separated ion species in the beam are not, typically, parallel at the focal plane P. As may be seen in the Figure, separated ions of different ion species (eg different isotopologues) arrive at the focal plane P travelling in different, non-parallel directions. In general terms, the angle between the direction of travel of ions and the central axis I of the ion beam gradually increases with distance away from that central axis I. It is thus desirable to mount the longitudinal axes of the plurality of moveable collectors  130  at different angles relative to the central axis I of the ion beam (or, equivalently, at different angles relative to the focal plane P), in order to reduce the difference in angle between the various incident ion species and the respective longitudinal axes of the Faraday detectors. For example, the longitudinal axis A 1  of the Faraday detector of a relatively outwardly mounted moveable collector (eg, the moveable collector  130   f ) may be aligned at a first angle α 1  relative to the central ion beam axis I. The longitudinal axis A 2  of the Faraday detector of a relatively inwardly mounted moveable collector (eg, the moveable collector  130   e ) may be aligned at a second angle α 2  relative to the central ion beam axis I. Because of the non-parallel ion beam, it is desirable that α 1 &gt;α 2 . 
         [0015]    Each of the finite number of moveable collectors is intended to detect ions across a range of mass to charge ratios. The range of mass to charge ratios that each moveable collector may detect can overlap with the range to be detected by adjacent detectors, but in general terms, each moveable collector  130  is intended to detect ions within a predetermined range of mass to charge ratios, which corresponds with a particular range of incident ion angles (relative to the central ion beam axis I). Each particular ion species will arrive at the focal plane P having its own specific angle of incidence relative to the central axis of the ion beam. Hence, a set of compromise angles is chosen, one for each of the plurality of moveable collectors  130 . The compromise angle that is chosen to mount each moveable collector  130 , lies somewhere between the largest and smallest angles of incidence of ions for that moveable collector  130 . 
         [0016]    Selecting a compromise angle for each of the moveable detector platforms relative to the central beam axis I presents no difficulties in respect of the CDD detectors, because the first dynode of each such CDD lies immediately behind the entrance slit thereof, so that there is a good tolerance to variations in the angle of arrival of incident ions relative to each CDD. However, for the Faraday detectors, it has been found that a much lower range of angles of incidence of ions at the Faraday detectors is acceptable. The apparent reason for this may be understood with reference to  FIG. 5 . 
         [0017]    The Faraday detectors  140   a - 140   h  of the fixed and moveable collectors are of similar construction, and one of them is shown in schematic view in  FIG. 5 . The Faraday detector comprises a cup  200  which is elongate in a direction A. The Faraday detector  140  is, in the embodiment of  FIG. 5 , mounted at an angle α defined as the angle between that longitudinal axis A of the Faraday detector  140  and the central ion beam axis I. 
         [0018]    The cup  200  is provided with a Faraday slit  210  at a first, opening end  220  of the cup  200  facing the incident ion beam. Inside the cup  200  is a graphite insert  230 . In use, ions enter the cup  200  through the Faraday slit  210  and strike the graphite insert  230  resulting in the generation of secondary electrons. The secondary electrons are captured and counted, as will be familiar to those skilled in the art. 
         [0019]    The graphite insert  230  for the Faraday detector  140  is positioned at the inner walls and towards a bottom end  240  of the cup. The Faraday detector  140  also comprises a secondary ion repeller plate  250 , mounted between the graphite insert  230  and the Faraday slit  210 . 
         [0020]    It has been found that the angle, γ, between the direction of travel, B, of ions arriving at a particular one of the Faraday detectors, and the longitudinal axis A of that particular Faraday detector  140 , is important for high resolution analysis. In particular, it is desirable that this “off axis” angle γ is relatively small, so that the ion beam  110  passes through the Faraday slit  210  into the cup  200 , and strikes the graphite insert  230  towards the bottom end  240  of the cup. If the ion beam  110  enters the Faraday detector  140  via the Faraday slit  210  at a relatively larger off axis γ, however, the ion beam strikes the side wall of the Faraday detector away from the bottom end  240  of the cup, as shown in  FIG. 5 . This results in the generation of secondary electrons (labelled e− in  FIG. 5 ) closer to the Faraday slit  210 . If the secondary electrons are generated too close to the opening end  220  of the cup  200 , they may leave the Faraday detector  140  via the Faraday slit  210 , because their energy at the secondary ion repeller plate  250  may be greater than the potential of that secondary ion repeller plate  250 . It is believed that the artefact  1  in  FIG. 2  is a consequence of lost secondary electrons resulting from this off axis incidence of ions at the Faraday detector  140 . 
         [0021]    To address this, in accordance with a first aspect of the present invention, there is provided a method of configuring a Faraday detector in a multiple collector of a mass spectrometer, as defined in claim  1 . The invention also extends to a multiple collector being under the control of a controller configured with a computer program which, when executed, carries out that method, so as to configure the/or each Faraday detector. 
         [0022]    Aspects of this invention thus provide for an arrangement in which the peak in the Faraday detector(s) has a flat top, that is, the artefact resulting from lost charges is not present. This is achieved by, for example, selecting the Faraday collector angle (α)—for example, iteratively—and/or reducing the Faraday slit width, for a given spectrometer entrance slit width, to a size where the artefact-causing effect is removed, while still retaining an optimum ion transmission into the Faraday detector(s). Preferably, where a single Faraday collector angle (α) is adjusted or set for a respective Faraday detector, the Faraday collector angle (α) is so adjusted or set that ions entering the detector arrangement strike the detector surface at a location which prevents secondary electrons generated thereby from exiting the Faraday detector via the Faraday slit no matter where along the focal plane the Faraday detector is positioned (a “compromise” angle). 
         [0023]    In a preferred embodiment, a compromise angle between the longitudinal axis of each of a plurality of Faraday detectors, and the central ion beam axis at each of the respective plurality of Faraday detectors, may be identified, for example iteratively, such that the artefact  1  is removed for all of the Faraday detectors, no matter where along the focal plane each detector is placed. Because of the divergence of the ion beam at the focal plane, each Faraday detector may have its own respective (fixed) compromise angle different from the compromise angle of the other Faraday detectors. For example, the compromise angle of a first Faraday detector relatively closer to the central fixed axial collector may be smaller than the compromise angle of a second Faraday detector relatively more distant from that fixed axial collector, in a direction transverse to the ion beam travel direction. 
         [0024]    In the case that a compromise angle can be identified, and which is suitable to avoid the problems of lost charges right across the allowed range of movement of a particular one of the Faraday collectors, then this may be determined during initial setup of the instrument. Then, the Faraday collector orientation relative to the focal plane P (or, equally, relative to the central axis I of the ion beam, upon which the fixed axial collector is mounted)—that is, a determined compromise angle that addresses charge loss across the range of movement of the Faraday collector—can be fixed during instrument calibration. Having a fixed compromise angle for a given Faraday detector simplifies the mechanical support required by the moveable collector upon which it is mounted, since the Faraday detector is then only required to be moveable in a direction generally parallel with the focal plane P. It may be that no solution is identifiable to provide a (fixed) compromise angle for one, some or even all of the Faraday detectors, which results in the removal of the artefact from the or each of the detectors, across the full range of movement of the or each particular Faraday detector. In that case, the angle of one, some or all of the Faraday detectors relative to that of the fixed axial collector (or equally relative to the focal plane or central beam axis, I) may be adjustable. In other words, the angle of at least one, optionally all, of the Faraday detectors can be mechanically changed with its position along the focal plane. For example, one or more of the Faraday detectors may be pivotally mounted upon a rail or support that extends in a first direction substantially parallel to the focal plane. Then, the Faraday detector may be moved closer to, or further away from, the central axis I of the ion beam, along that first direction. Pivotal mounting of the (or each) Faraday detector also then allows rotation of the Faraday detector about an axis perpendicular to the first direction. This permits the angle of the longitudinal axis of the Faraday detector relative to the focal plane, and thus relative to the central beam axis I, to be adjusted. In that case, a controller may be configured to control both the movement of the moveable collector (which includes the Faraday detector) along the first direction, while simultaneously controlling the direction (that is, the angle) of the longitudinal axis of the Faraday detector relative to the focal plane and the central beam axis I. Put another way, the controller controls both the movement of the Faraday detector along a line, as well as rotation about an axis perpendicular to that line so that, as the spacing of the Faraday detector relative to the central fixed axial collector changes, the angle of the longitudinal axis of the Faraday detector relative to that fixed axial collector changes. Thus, as the Faraday detector moves along the focal plane (to allow it to detect ions of different mass to charge ratios), the longitudinal axis of the Faraday detector may be maintained more or less parallel with the incident ions of that mass to charge ratio. In this manner, the problems of lost charges are ameliorated or resolved. 
         [0025]    Instead of a single pivotal mounting of a Faraday detector relative to a single rail or the like (where the rail preferably extends in a direction substantially parallel to the focal plane), the, or each, Faraday detector could instead be journalled upon first and second spaced non-parallel rails. Then, as the Faraday detector moves along the rails, the changing separation between the rails will result in a change in angle of the longitudinal axis of the Faraday detector relative to the focal plane and the central beam axis I. In one embodiment, the first and second supporting rails may each be linear, so that the rate of change of the spacing between them is constant. This results in a constant rate of change of angle of the longitudinal axis of each Faraday detector, as a function of position of the Faraday detector relative to the central ion beam axis I. Alternatively, one or both of the support rails may be curved so that there is a non-linear (non-constant) change in the angle of the longitudinal axis relative to the separation between the Faraday detector and the central ion beam axis I. Still further, parts of the first and second rail supports may be parallel with each other, while other parts of the rails are non-parallel, eg, curved. This allows a constant angle of the longitudinal axis relative to the focal plane P to be maintained over a first part of the movement of the Faraday detector along the first direction, while, over a second part of the movement of the Faraday detector along that first direction, the relative angle between the focal plane P and the longitudinal axis of the Faraday detector may change, eg under computer control. 
         [0026]    Thus it will be understood that it is possible to combine the two concepts of a fixed compromise angle for the Faraday detectors, and a variable angle for the Faraday detectors. Depending upon the amount of the ion beam spread, for example, it may be necessary or desirable that only some of the moveable Faraday detectors have a variable angle relative to the focal plane of the ion beam or the central beam axis I. In particular, relatively outwardly located Faraday detector(s) (eg, the detector in the moveable collector  130   f ) may be mounted upon a curved or otherwise non-linear support/rail, while relatively inwardly positioned Faraday detector(s) (eg the detector in the moveable collector  130   e ) may be positioned at a fixed angle with respect to the central fixed axial collector. 
         [0027]    For example, a multiple collector may comprise N Faraday detectors (N may be 9, for example) of the N Faraday detectors, a central Faraday detector might be fixed in a position defining a transverse axis, and having a detector body that is presented at a first angle relative to the focal plane of the incident ion beam. A first group of M Faraday detectors of the N in total (M&lt;N) may be positioned laterally of the central Faraday detector, and may be relatively moveable along the focal plane of the incident ion beam so as to adjust the separation, along that focal plane, between them or at least two of the, M Faraday detectors, but where however the angle between each of the M Faraday detectors remains fixed, preferably at a respective previously identified compromise angle. 
         [0028]    A second group P of Faraday detectors, however (P is also &lt;N, and, preferably, P+M+1=N) may also be relatively moveable with respect to the central fixed Faraday detector/the focal plane, but may have a variable angle relative to the focal plane as they move laterally. Those P Faraday detectors, for example, may even have a fixed angle relative to the focal plane over a first range of movement in the transverse direction, while having a variable angle relative to the focal plane over a second range of movement in the lateral direction. Generally each of M and P can be a number from 0 to N−1, provided P+M+1=N) 
         [0029]    A multiple collector for an Isotope Ratio mass spectrometer in accordance with claim  10  is also provided. 
         [0030]    A further problem that has been identified by the inventors is sometimes observed when carrying out higher resolution scans. It is thought that the artefact  2  shown in  FIG. 3 , at the edges of the peak, is the result of an electron cloud which is formed when the ions strike the edges of the Faraday slit. This electron cloud pulls down the intensity vs mass scan. In lower resolution scans, although the ions incident at the slit entrance may create an electron cloud, any negative effects of such an electron cloud on the detector output tend not to be observable because the edges of the peak tend to rise and fall relatively slowly. However in higher resolution scans, particularly those in the Ultra-253 instrument where resolution may be up to 40,000, the peak edges tend to be steeper, so that the effect of the electron cloud can then become apparent. 
         [0031]    In order to address the second problem, a multiple collector for an isotope ratio mass spectrometer is provided, in accordance with claim  13 . Using such a slit shape in the multiple collector suppresses the secondary electron cloud at the slit edges and thus removes the negative dips at the shoulders of the scan. The use of this slit shape is applicable both to Faraday detectors and also to CDDs within the multiple collector; in particular it has been found that the electron cloud generated adjacent to a slit with parallel sides is present in both such types of detector. Using the modified slit shape of aspects of this invention is thus of benefit in removing artefacts arising in the outputs of both the Faraday detector(s) and the CDDs. 
         [0032]    The invention also extends to an isotope ratio mass spectrometer, such as a double focusing MC-ICP-MS, a double focusing gas isotope ratio MS or the like, the isotope ratio mass spectrometer comprising an ion source, a magnetic and, optionally an electric sector for selection of ions of species of interest, and a multiple collector as defined above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention may be put into practice in a number of ways and some specific embodiments will now be described by way of example only and with reference to the accompanying drawings in which: 
           [0034]      FIG. 1  shows an ideal high resolution scan across the slit of a Faraday collector in an isotope ratio mass spectrometer; 
           [0035]      FIG. 2  shows a high resolution scan across a Faraday detector slit of a isotope ratio mass spectrometer having a first signal artefact; 
           [0036]      FIG. 3  shows a high resolution scan across a Faraday detector slit of an isotope ratio mass spectrometer having a second signal artefact; 
           [0037]      FIG. 4  shows, schematically, a part of a multiple collector for a dual sector mass spectrometer, including a plurality of Faraday detectors; 
           [0038]      FIG. 5  shows, schematically, a section through one of the Faraday detectors of  FIG. 4 ; 
           [0039]      FIG. 6  shows a schematic plan view of a double focusing gas isotope ratio mass spectrometer having a multiple collector including a fixed collector mounted on a central beam axis, and moveable collectors, each of which comprises a Faraday detector, mounted around the central beam axis; 
           [0040]      FIG. 7  shows a schematic plan view of one of the moveable collectors of  FIG. 6 , in two positions each at a common angle relative to the central beam axis; 
           [0041]      FIG. 8  shows a schematic plan view of one of the moveable collectors of  FIG. 6 , in multiple positions each of which is at a different angle relative to the central beam axis; 
           [0042]      FIG. 9  shows a schematic plan view of one of the moveable collectors of  FIG. 6 , illustrating an embodiment of the present invention; 
           [0043]      FIG. 10  shows a schematic plan view of one of the moveable collectors of  FIG. 6 , illustrating an alternative embodiment of the present invention; 
           [0044]      FIG. 11  shows a schematic sectional view through the end of a Faraday detector, having Faraday slits configured in accordance with the prior art; and 
           [0045]      FIG. 12  shows a schematic sectional view through the end of a Faraday detector, having Faraday slits configured in accordance with a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0046]    Referring first to  FIG. 6 , there is shown a schematic representation of a double focusing gas isotope ratio mass spectrometer  10 . Ions are generated at the ion source  20  which is powered by power supply  30  connected via connectors  31 ,  32 . Via one or more ion optical devices (not shown), the ions are accelerated and passed through an electrostatic analyzer (ESA)  40  which assists in focusing the ion beam and selecting ions of the required energy. The ions next enter a focusing quadrupole  50  to further focus the ion beam. On exiting the focusing quadrupole, the ion beam passes through an exit aperture defined in a mask  60 , and then onwards through a magnetic field applied at an electromagnetic sector  70 . 
         [0047]    The exit aperture at mask  60  has different possible widths which determine the resolution of the ion beam. As the aperture allows only a portion of the focused ion beam to pass, selection of an aperture having a larger area or wider slit allows a greater portion of the ion beam (in other words, a larger number of ions) to pass through into the magnetic field, and so provides a more sensitive measurement. However, a small area or narrower aperture can be useful to reduce ion optical aberrations, thereby delivering improved resolution for the measurement, albeit at the expense of some sensitivity. 
         [0048]    Within the magnetic mass analyzer at the electromagnetic sector  70 , the applied magnetic field causes a change of direction or a deflection of the ions. Ions of greater mass are deflected less than ions with smaller mass, causing a spatial separation of the ions according to their mass-to-charge ratios. The separated ions exit the magnetic mass analyzer  70  and pass into the detector chamber  80 . A multiple collector  100  including a plurality of Faraday detectors and conventional differential detectors (CCD) are arranged within the detector chamber  80 . The general arrangement of the detectors is as described above in connection with  FIG. 4  in particular, in that there is a fixed axial collector  120  having a Faraday detector, together with 8 further moveable collectors (mounted 4 each side of the fixed axial collector), each of which moveable collector may be provided with a Faraday detector and a CDD (not shown in  FIG. 6 ). 
         [0049]    The Faraday detectors  140  are arranged along the focal plane P of the ion beam in order to receive each species of spatially separated ions simultaneously. The operation of the mass spectrometer  10  and the collection of data may be controlled by a computer  90  having a control module and analysis module. 
         [0050]      FIG. 7  shows a highly schematic, simplified plan view of one of the moveable Faraday detectors  140   f  within the detector chamber  80 , in first and second positions. No particular significance is to be attached to the identity of the particular Faraday detector chosen for description here; the invention in several of its preferred embodiments is equally applicable to any of the moveable Faraday detectors, and indeed may be applicable in part to the fixed axial detector as well, as will become apparent from the description that follows. It is also to be appreciated that  FIG. 7  is not drawn to scale; indeed, some dimensions have been exaggerated for a better understanding of the principles involved. 
         [0051]    The Faraday detector  140   f  itself is constructed in the manner described above in connection with  FIG. 5 , and so the details of it (cup, graphite insert, Faraday slit etc) will not be repeated here for brevity. 
         [0052]    In the arrangement of  FIG. 7 , the longitudinal axis A of the moveable Faraday detector  140   f  is mounted at a fixed angle α relative to the central ion beam axis. Axes parallel to the central ion beam axis I and intersecting the longitudinal axis A of the moveable Faraday detector are marked in  FIG. 7 , as I′ and I″ for the two positions of Faraday detector shown. 
         [0053]    The Faraday detector  140   f  shown in  FIG. 7  is capable of movement along the axis C-C′ which extends parallel with the focal plane P, that is, the axis of movement of the Faraday detector  140   f  is preferably at or around 45 degrees to the central ion beam axis I. The Faraday detector  140   f  may be moved using a driver motor or the like (not shown), along a rail or other linear support which extends along the direction C-C′ (also not shown in  FIG. 7 ). In this manner, the Faraday detector may be positioned at a plurality of positions, only two of which are shown in  FIG. 7 , so as to align with ions arriving from the electromagnetic sector device  70  at different positions along the focal plane P in accordance with their mass to charge ratio. Manual or mechanical movement of the Faraday detector  140   f  is of course possible as well or instead. 
         [0054]    The ion beam  110  is not parallel at the focal plane, but rather at least somewhat fan shaped so that ions of different mass to charge ratios diverge from one another at that focal plane P. The angle α of the Faraday detector is, on the other hand, fixed. This means that, at the opening in the Faraday slit  210  of the Faraday detector  140   f,  the “off axis angle” between the incident ions and the longitudinal axis of the Faraday detector  140   f  differs between the two positions of the Faraday detector shown in  FIG. 7 . In general terms, because of the fan shaped ion beam, the off axis angle reduces as the Faraday detector moves towards the central ion beam axis I, and increases as it moves away from it. 
         [0055]    The Faraday detector  140   f  has a limited range of movement along the axis C-C′. The full range of angles/positions along the focal plane at which the multiple collector  100  of  FIG. 6  can detect incident ions is defined by the maximum separation between the outermost moveable collectors ( 130   f  and  130   h ). Angles and positions between those two extremities are detected using those detectors, one or other of the inwardly positioned moveable detectors  130   a, b, c, e, f, g,  or the fixed axial collector  120 . The angle α, or a derivative of it (for example, an angle measured relative to the focal plane), is chosen so as to avoid incident ions from striking the inner side walls of the Faraday detector  140   f  and generating electrons too close to the Faraday slit  210  so that they are lost rather than captured within the Faraday detector. In particular, in accordance with one aspect of this invention, across the range of movement of a given one of the Faraday collectors  140 , the ion beam enters the Faraday collector at an angle α sufficiently acute that substantially all of the secondary electrons created are captured and detected/counted, rather than being lost from the Faraday detector via the Faraday slit  210 . 
         [0056]    The width of the Faraday slit  210  is preferably reduced to the minimum width that still provides a flat top peak shape for the ions even for the lowest spectrometer resolution setting (using the widest available spectrometer entrance aperture defined in the mask  60 ). In the arrangement shown in  FIG. 6 , the width of the entrance aperture in the mask  60  (and the magnification of the ion optics) determines the width of the Faraday slit  210 . In accordance with embodiments of the invention, therefore, an initial setup procedure may be carried out. The procedure may be carried out either during construction or installation of the mass spectrometer, with the various selected parameters then fixed during subsequent use, or the computer  90  of the mass spectrometer  10  may be programmed to run a setup routine during each startup of the instrument, or may even be programmed to run a calibration at regular or specified intervals during use. 
         [0057]    Setup proceeds as follows. Once the beam line has been correctly aligned with the multiple collector  100  and the fixed axial collector  120 , a Faraday slit width is chosen for a particular one of the Faraday detectors  140 . Choice of the slit width will depend, for example, on the intended use of the particular instrument being configured. For example, the slit width which is optimal or appropriate for detection of high mass ion species (say, Caesium to Uranium ions), may be different to the slit width that is appropriate for carbon based simple molecules (CHx, CO, CO2 etc). 
         [0058]    Next, angles for each of the plurality of moveable collectors, and in particular for each of the Faraday detectors  40 , are identified. Identification of a suitable angle for each Faraday detector  140  proceeds on the basis of finding a solution to the problem of avoiding the artefact  1  shown in  FIG. 2 —that is, finding an angle for each Faraday detector  140 , at which ions are captured deep inside that Faraday detector so that no secondary electrons can escape—for all possible positions along the focal plane P for a particular detector. The angle thus identified is henceforth referred to as the “compromise angle”. 
         [0059]    The geometry and dimensions of the components relevant to this solution are such that theoretical calculation of a suitable angle is impractical. Moreover, the mass spectrometer has a wide range of potential applications, and different applications will require accurate/high resolution detection of particular, different ion species. Each species will arrive at different positions/angles to the focal plane P of the ion beam, so it is not sufficient simply to choose a single, generic Faraday detector angle if the artefact caused by secondary electron loss is to be avoided. 
         [0060]    Instead, the (or at least, a) solution to the problem is determined empirically. A starting point for iterative analysis may be used, based upon previously identified suitable angles for the particular instrument application intended. Iterative identification of the optimum compromise angles may be achieved by using one or more test samples that produce ions of known mass to charge ratios, and in particular ion species similar or identical to those that the instrument is intended to analyse when commissioned into use. 
         [0061]    The ions generated by a test sample or samples are scanned across the Faraday slits of the respective appropriate ones of the Faraday detectors  140 . The resulting scans (eg of  FIGS. 1 and 2 ) are studied, either by a user or through software analysis, to look for artefacts such as artefact  1  shown in  FIG. 2 . If the artefact is present in a scan from a particular Faraday detector, the angle of the longitudinal axis thereof is adjusted relative to the central ion beam axis I to provide a different angle, at which, ideally, the artefact is not present. The process is then repeated for other positions of each Faraday detector  140  across its range of movement until either the artefact  1  is removed for all such positions, or is minimized. 
         [0062]    In practice it may be possible simply to select a first trial angle for the moveable collector relative to the central ion beam axis I, move the moveable collector to one extreme of its range of travel along the focal plane P, carry out the scan described above, and then repeat at the other extreme of the range of travel along the focal plane P. If the artefact  1  is observed in either of the two scans thus carried out, then a new angle for the moveable collector relative to the central ion beam axis I is chosen and the steps above are repeated. The iterations repeat until an angle is found at which the artefacts are not visible in the scan at either end of the range of movement of the particular moveable collector being set up. The reason why it may only be necessary to carry out scans at the extremes of the range of movement of each moveable collector is because of the divergent shape of the ion beam. If the chosen angle for the moveable member solves the problem of secondary electron loss at each extreme, then it must solve the problem at all positions between those extremes. 
         [0063]    The (or an) angle of the longitudinal axis of each Faraday detector relative to the central ion beam axis I/the longitudinal axis of the fixed axial collector  120 , at which the artefact  1  is removed or its presence is minimized, at both ends of the range of travel of a particular moveable collector, is then selected as the compromise angle for that moveable collector. Depending upon various factors, there may be either a relatively narrow or a relatively wide range of angles that solve the problem of secondary electron loss and which could, therefore, be employed as the compromise angle. 
         [0064]    Because of the divergence of ions across the ion beam, a compromise angle identified for a first of the detectors, adjacent to the fixed axial detector  120 , (eg the Faraday detector  140   a ) may not be suitable for detectors further away from the fixed axial detector  120  (eg the Faraday detector  140   d ). Therefore, the iterative procedure for empirical determination of a suitable compromise angle may be carried out separately in respect of some or all of the moveable collectors  130 . 
         [0065]    The iterative procedure described above selects but then fixes the angle of the longitudinal axis of each Faraday detector  140  relative to the central ion beam axis I. In other words, once a compromise angle is identified or chosen for a given Faraday detector  140 , that compromise angle is then retained and maintained constant unless and until it is decided to recalibrate the mass spectrometer. The benefit of this is that the arrangement by which each Faraday detector  140  is mounted for movement in the direction C-C′ ( FIG. 7 ) along the rail or support may be relatively simple, reducing cost and complexity. 
         [0066]    As an alternative, however, and as will now be described by reference to  FIGS. 8, 9 and 10 , one, some or all of the Faraday detectors  140  may be mounted so as to be both moveable in a first direction (generally, a direction parallel with the focal plane P of the incident ion beam) and also rotatable about a second axis orthogonal thereto, in order to permit the longitudinal axis of each Faraday detector  140  to present a range of angles relative to the central ion beam axis I. 
         [0067]    Referring first to  FIG. 8 , one of the plurality of Faraday detectors  140   f  is shown respectively in first, second and third positions relative to the fixed axial collector  120 /the central ion beam axis I. As previously, no particular significance is to be attached to the selection of the Faraday detector  140   f  for the following description; the techniques employed are equally applicable to any of the plurality of moveable collectors  130   a - 130   h.  Moreover,  FIG. 8  is not drawn to scale and the angles have been exaggerated to assist with explanation. 
         [0068]    In a first position, wherein the Faraday detector  140   f  is furthest away from the central ion beam axis I in a direction along the focal plane P of the ion beam, the angle α 1  between the longitudinal axis of the Faraday detector relative to the central ion beam axis I is relatively large. In a second position, in which the Faraday detector  140   f  is relatively closer to the central ion beam axis I in a direction along the focal plane P of the ion beam, the angle α 2  between the longitudinal axis of the Faraday detector relative to the central ion beam axis I is smaller than the angle α 1 . In a third position, the Faraday detector  140   f  is relatively closest to the central ion beam axis I in a direction along the focal plane P of the ion beam. Here, the angle α 3  between the longitudinal axis of the Faraday detector relative to the central ion beam axis I is smaller than the angle α 2 . 
         [0069]    As noted previously, ions arriving at the focal plane P are divergent (that is, the beam is somewhat fan shaped at the focal plane P). By allowing the angle α to be changed or adjusted as the Faraday detector  140   f  moves along the focal plane P of the ion beam  110  (not shown in  FIG. 8 ), the relative angle between incident ions and the longitudinal axis of the Faraday detector  140   f  can be reduced or even substantially removed. This in turn permits the artefact  1  shown in  FIG. 2  to be addressed/removed. No single compromise angle is chosen in the arrangement illustrated in  FIG. 8 , but rather a range of angles may be presented between the longitudinal axis of the Faraday detector relative to the central ion beam axis I. This in turn may allow a wider range of Faraday slit widths to be provided; in particular if the angle α between the longitudinal axis of the Faraday detector and to the central ion beam axis I can be adjusted as the Faraday detector moves along the focal plane P, it may be possible to employ a wider Faraday slit width than would otherwise be available if the artefact is to be removed. This in turn may permit a higher instrument sensitivity to be achieved. 
         [0070]      FIG. 9  illustrates, schematically, one possible mechanical arrangement of a moveable collector  130  that permits movement of the Faraday detector  140  both in a linear direction along the focal plane P of the ion beam, and also in a rotational direction about an axis defined through the Faraday detector  140 . Again for clarity purposes, the CDD and other components forming the moveable collector  130  have been omitted. 
         [0071]    As shown in  FIG. 9 , the moveable collector  130  is mounted upon a rail  300  that extends in a direction C-C′ that is generally parallel with the focal plane P of the ion beam, that is, extends in preferred embodiments in a direction that is approximately 45 degrees to the central ion beam axis I. The moveable collector  130  is connected to the rail  300  via a pivotable connector  310  that permits rotation of the Faraday detector  140  in the direction D-D′ marked in the Figure. In the embodiment of  FIG. 9 , the pivotable connector  310  is preferably connected between the rail  300  and a point on the moveable collector  130  at, or near, the latter&#39;s center of mass, for mechanical efficiency. 
         [0072]    The moveable collector  130  may be connected to the computer  90  and may be driven by one or more motors that are under the control of the computer. The motor or motors may drive the moveable collector  130  linearly in the direction C-C′ and also may rotate the Faraday detector in the direction D-D′. For example, a stepper motor could be employed under the control of the computer  90  so as to permit selection of one of a finite number of angles α, depending upon the linear position of the moveable collector  130  upon the rail  300 . The angle α might change linearly with position along the rail  300 , or may change non-linearly, depending upon the specific profile of the ion beam in a direction transverse to the direction of beam travel. Still further, the angle α may be variable across a part of the extent of travel of the moveable collector  130  in the direction C-C′, but fixed (eg, at a predetermined compromise angle) over a different part of that range of travel. 
         [0073]    It will be understood that the arrangement in  FIG. 9  could be employed in all or just some (as well as none) of the multiple moveable collectors. For example, it may be that moveable collectors  130  relatively closer to the fixed axial collector  120  are provided with a non-pivoting connector between the moveable collector  130  and the rail  300  upon which they move in the linear direction (C-C′). For those moveable collectors, a (single) compromise angle is then chosen for all linear positions of the moveable collector along the rail  300 . Relatively outwardly positioned moveable collectors  130 , however, could be provided with the pivotable connector  310  shown in  FIG. 9 . Such an arrangement may be appropriate where a compromise angle can be found that avoids the artefact  1  ( FIG. 2 ) for an acceptably wide Faraday slit, for ions arriving at the focal plane relatively near to the central axis I of the ion beam, whereas for ions arriving at the focal plane P at relatively distant positions, a single compromise angle may not be suitable to avoid the artefact  1 , without having to use an unacceptably narrow Faraday slit  210 . 
         [0074]      FIG. 10  shows an alternative mechanical arrangement for linear and rotational movement of a moveable collector  130 . In the arrangement of  FIG. 10 , components common to the arrangement of  FIG. 9  are shown with like reference numerals. 
         [0075]    In  FIG. 10 , a moveable collector  130   e  is illustrated, in highly schematic plan view (relative to the mass spectrometer  10  shown in  FIG. 6 ), in first and second positions relative to the central ion beam axis I. Once again the choice of moveable detector  130   e  for exemplifying this embodiment of the invention is not to be considered to be significant. 
         [0076]    In  FIG. 10 , by contrast with  FIG. 9 , the moveable collector  130   e  is mounted, at first and second ends thereof, upon a pair of non-parallel rails  300   a,    300   b.  In particular, a first pivotable connector  300   a  is provided between the moveable collector  130   e  and a first rail  300   a  towards an opening end  220  of the Faraday detector  140   e.  A second pivotable connector  300   b  is provided between the moveable collector  130   e  and a second rail  300   b  towards a bottom end  220  of the cup  200  of the Faraday detector  140   e.  A motor or the like, for example under the control of the computer  90 , may drive the moveable collector  130   e  along the first and second rails  300   a,    300   b  in the direction C-C′. In  FIG. 10 , the first rail  300   a  extends in a direction that is generally parallel with the focal plane P, whereas the second rail  300   b  extends at an angle that is not parallel to that focal plane P. The changing separation between the two rails  300   a,    300   b  in a direction parallel with the central ion beam axis I causes the moveable collector  130   e,  and hence the Faraday detector  140   e,  to rotate about an axis passing through the moveable collector  130   e  and defined in a direction into and out of the page (as viewed in  FIG. 10 ). 
         [0077]    In  FIG. 10 , the two rails  300   a,    300   b  are each linear (though non parallel), so that the separation between the rails changes constantly with distance in the direction C-C′. Other arrangements can be contemplated; for example one or both of the rails may be curved; the two rails may be parallel along a part of their length and non-parallel (straight or curved) along another part of their length; or the rate of separation of the two rails  300   a,    300   b  may be different at different parts of their lengths. 
         [0078]      FIG. 11  shows a schematic sectional view through a prior art Faraday slit  1 . The slit is laser cut and the side walls  2  of the slit  1  are generally parallel. The inventors have identified the artefact  2  shown in  FIG. 3  (dips at the shoulders of the scan) and have posited that these dips are caused by the shape of the slit side walls. In particular, the inventors believe that the artefacts  2  are caused by ions incident upon the slit in  FIG. 11  striking the inner side walls  2  of the slit  1 , resulting in secondary electrons  3  that form an electron cloud at the edges of the slit  1  such that at least some of the electrons are collected by the Faraday detector. This electron cloud at the slit edges is what is believed to pull down the intensity vs. mass to charge ratio in the scan of  FIG. 3 . 
         [0079]      FIG. 12  shows a schematic sectional view through a plate  420 , in which is formed a Faraday slit  210  whose shape is in accordance with a further aspect of the present invention. As seen in  FIG. 12 , the side walls  400  of the slit entrance are formed with a slope so that the slit entrance at a front face  410  of the plate  420  is narrower than the slit opening at a rear face  415  of the plate  420 . In that manner, ions arriving at the front face  410  of the plate  420 , at a range of angles at and around 90 degrees to the front face  410  of the plate  420 , cannot “see” the side walls  400  of the Faraday slit  210 . This shape prevents the formation of secondary electrons as the incident ion beam strikes the inner side walls  400  of the Faraday slit  210 . 
         [0080]    The shaped Faraday slit  210  of  FIG. 12  may be formed using a number of material processing techniques, such as laser cutting, grinding, polishing and so forth. 
         [0081]    Although the side walls  400  shown in  FIG. 12  have a constant slope between the front and rear faces  410 ,  415  of the plate  420 , they do not need to be so. For example, the side wall could be curved—eg, convex—so that the rate of change of separation between the side walls  400  of the Faraday slit  210  increases in a direction from the front face  410  to the rear face  415  of the plate  420 . 
         [0082]    Although some specific embodiments have been described, it will be understood that these are merely for the purposes of illustration and that various modifications or alternatives may be contemplated by the skilled person.