Control of magnetic sector mass spectrometer magnet

Abstract

A control system for controlling a magnet of a magnetic sector mass spectrometer comprises a magnetic field sensor for sensing the magnetic field of the magnet and generating an output representative thereof; a set point generator configured to generate an output representative of, or related to, a desired magnetic field of the magnet; and a digital controller configured to receive a variable digital input signal from the output of the magnetic field sensor and a set point digital input signal from the output of the set point generator, and to generate a digital output from which is derived a control signal for controlling a current to the magnet so as to control the magnetic field thereof. The control system is arranged to apply to the digital controller a selected one of a plurality of different controller settings, in accordance with the desired magnetic field of the magnet.

Claims

1. A control system for controlling a magnet (70) of a magnetic sector mass spectrometer (10), comprising: a magnetic field sensor for sensing the magnetic field of the magnet (70) and generating an output representative thereof; a set point generator (210) configured to generate an output representative of, or related to, a desired magnetic field of the magnet (70); and a controller (130) configured to receive a variable input signal from the output of the magnetic field sensor and a set point input signal from the output of the set point generator (210), and to generate an output from which is derived a control signal for controlling a current to the magnet (70) so as to control, in turn, the magnetic field thereof; and a processor; wherein the magnetic field sensor for sensing the magnetic field is an ion detector (120) for detecting ions passing through the magnetic sector mass spectrometer (10), the ion detector (120) being configured to generate a detector output signal representative of the quantity of ions incident upon the detector (120); and wherein the processor is arranged to receive the detector output signal, and configured to calculate a controller setting based upon the detection of ions at the detector (120), and wherein the control system is arranged to apply to the controller (130) a selected one of a plurality of different controller settings calculated by the processor based upon the detection of ions at the detector (120), in accordance with the desired magnetic field of the magnet (70).

2. The control system of claim 1, wherein the control system is configured to control the parameters of the mass spectrometer (10), so as to align an edge of a mass spectral peak of an ion species with the ion detector (120), and to determine, by perturbing the mass spectrometer parameters when the peak edge is aligned with the ion detector (120), controller settings suitable for the particular magnetic flux density of the magnet (70) generated in accordance with the corresponding magnet current control signal.

3. A magnetic sector mass spectrometer (10) comprising: an ion source (20) arranged to generate a beam of ions having a mass to charge ratio m/z; an ion accelerator arranged to accelerate ions to a potential U.sub.o; a control system according to claim 1; a magnet (70) under the control of the control system, arranged to divert the accelerated ions along a circular path in accordance with m/z, U.sub.o and the magnetic flux density within the magnet (70).

4. The magnetic sector mass spectrometer (10) of claim 3, further comprising a second ion detector (500, 510) also positioned downstream of the magnet but spatially separated in a transverse direction perpendicular to the direction of travel of the ion beam; wherein the ion beam comprises ions of first and second mass-to-charge ratios, the processor being configured to control the parameters of the mass spectrometer so as to align an edge of a mass spectral peak of ions of the first mass-to-charge ratio with the ion detector (520) which is sensing the magnetic field, whilst ions of the second mass to charge ratio are directed toward the second ion detector (500, 510).

5. A method of controlling a magnetic field generated by a magnet (70) in a magnetic sector mass spectrometer (10), comprising: sensing the magnetic field of the magnet (70) and generating a sensor output representative thereof; generating a set point signal representative of, or related to, a desired magnetic field of the magnet; applying one of a plurality of different controller settings to a system controller, the controller setting which is applied being selected in accordance with system parameters of the magnetic sector mass spectrometer (10); at the system controller (130), receiving, as a first input, a variable input signal derived from the sensor output, and, as a second input, the set point signal, and generating a control signal output which is determined by the first and second inputs and the particular controller setting applied to the system controller (130); and controlling the current supplied to the magnet (70) by a magnet power supply (80), based upon the control signal output, so as, in turn, to control the magnet field generated by the magnet (70), further comprising: generating ions of a first mass to charge ratio (m/z).sub.1; accelerating the ions of mass to charge ratio (m/z).sub.1 to an energy z.sub.1U.sub.o, where U.sub.o is an electrical potential applied to the ions; generating a magnetic field, having a magnetic flux density B.sub.m, in the magnet; directing ions of mass (m/z).sub.1 into the magnet (70), where they follow a curved path; and detecting, with an ion detector (120), those ions that have followed a curved path within the magnet of radius r.sub.m defined by
r.sub.m=B.sub.m.sup.1 (2.U.sub.o.(m/z).sub.1).sup.1/2; and adjusting the parameters of the magnetic sector mass spectrometer (10) so as to cause an edge of a mass peak corresponding with the ions of mass (m/z).sub.1 to align with the ion detector (120) for detecting a change in an ion intensity at the ion detector (120) corresponding to a change in the magnetic field for sensing the magnetic field of the magnet (70).

6. The method of claim 5, wherein the adjusting step comprises adjusting the accelerating potential U.sub.o so as to align the peak edge with the ion detector (120).

7. The method of claim 5, further comprising: perturbing the parameters of the magnetic sector mass spectrometer (10) once the edge of the mass peak has been aligned with the ion detector (120), deriving, from the applied perturbation, a first static field controller setting for the system controller (130) for application to the system controller (130) for the magnetic flux density B.sub.m; and applying that derived first static field controller setting to the system controller (130) for subsequent detection of ions at that magnetic field B.sub.m.

8. The method of claim 5, further comprising: accelerating the ions of mass to charge ratio (m/z).sub.1 to an energy z.sub.1U.sub.1 where U.sub.1U.sub.o; generating a magnetic field, having a magnetic flux density B.sub.n, in the magnet (70), where B.sub.mB.sub.n; directing ions of mass (m/z).sub.1 into the magnet (70) where they follow a curved path; and detecting, with the ion detector (120), those ions that have followed a curved path within the magnet of radius r.sub.n defined by r.sub.n=B.sub.n.sup.1 (2.U.sub.1.(m/z).sub.1).sup.1/2; wherein adjusting the accelerating potential U.sub.1 so as to align the peak edge with the ion detector (120) and wherein the controller settings for B.sub.n are different to the first static field controller settings for B.sub.m.

9. The method of claim 8, further comprising: perturbing the parameters of the magnetic sector mass spectrometer (10) once the edge of the mass peak has been aligned with the detector (120); deriving, from the applied perturbation, a second static field controller setting for the system controller (130) for application to the system controller (130) for the magnetic flux density B.sub.n; and applying that derived second static field controller setting to the system controller for subsequent detection of ions at that magnetic field B.sub.n.

10. The method of claim 8, further comprising determining a transitioning field controller setting for a transition from B.sub.m to B.sub.n; and applying that transitioning field controller setting to the system controller (130) for subsequent detection of ions when transitioning from B.sub.m to B.sub.n.

11. The method of claim 5, further comprising: generating ions of a second mass to charge ratio (m/z).sub.2; accelerating the ions of mass to charge ratio (m/z).sub.2 to an energy z.sub.2U.sub.1, where U.sub.1 is an electrical potential applied to the ions; generating a magnetic field, having a magnetic flux density B.sub.n, in the magnet (70), directing the ions of mass (m/z).sub.2 into the magnet (70), where they follow a curved path; and detecting, with the ion detector (120), those ions that have followed a curved path within the magnet (70) of radius r.sub.n defined by r.sub.n=B.sub.n.sup.1 (2.U.sub.1.(m/z).sub.2).sup.1/2; wherein: adjusting the system parameters of the magnetic sector mass spectrometer (10) so as to cause an edge of a mass peak corresponding with the ions of mass (m/z).sub.2 to align with the detector (120), B.sub.nB.sub.m and further wherein the static field controller settings for the system controller are different for B.sub.n and B.sub.m.

12. The method of claim 11, further comprising determining a transitioning field controller setting for a jump in magnetic flux density between B.sub.m and B.sub.n, and applying that transitioning field controller setting to the system controller (130) for subsequent detection of ions when transitioning between B.sub.m and B.sub.n.

13. The method of claim 12 further comprising: applying the said transitioning field controller setting determined for a jump from B.sub.m to B.sub.n, when the controller (130) subsequently controls the magnet (70) so as to change the magnetic flux density from B.sub.m to B.sub.n, and applying the second static field controller setting when the magnetic flux density is subsequently to be maintained at B.sub.n once B.sub.n has been reached, and wherein the second static field controller setting differs from the transitioning field controller setting.

14. The method of claim 5, further comprising: generating ions of a second mass to charge ratio (m/z).sub.2; accelerating the ions of mass to charge ratio (m/z).sub.2 to an energy z.sub.2U.sub.1, where U1 is an electrical potential applied to the ions; generating a magnetic field, having a magnetic flux density B.sub.n, in the magnet, directing the ions of mass (m/z).sub.2 into the magnet (70), where they follow a curved path; and detecting, with the ion detector (120), those ions that have followed a curved path within the magnet of radius r.sub.n defined by r.sub.n=B.sub.n.sup.1 (2.U.sub.1.(m/z).sub.2).sup.1/2; wherein r.sub.mr.sub.n but U.sub.o=U.sub.1, so that ions of (m/z).sub.1 travel along a different path to ions of (m/z).sub.2 and wherein setting the parameters of the magnetic sector mass spectrometer (10) so that ions of the first mass to charge ratio (m/z)i, are aligned with the ion detector (520) which is sensing the magnetic field, whilst ions of the second mass to charge ratio (m/z).sub.2 align with a second ion detector (500, 510) spatially separated from the first ion detector.

15. A method of determining controller settings for a controller (130), wherein the controller (130) controls a magnetic field generated by a magnet (70) in a magnetic sector mass spectrometer (10) by measuring the magnetic flux density of the magnetic field with a magnetic field sensor, the method comprising the steps of; generating ions having a mass to charge ratio m/z; accelerating the ions of mass to charge ratio m/z; generating a magnetic field, having a magnetic flux density B.sub.m, in the magnet (70); directing ions of mass to charge ratio m/z into the magnet (70), where they follow a curved path towards an ion detector (120) which is the magnetic field sensor measuring the magnetic flux density; adjusting the parameters of the magnetic sector mass spectrometer (10) so as to cause an edge of a mass peak corresponding with the ions of mass to charge ratio m/z to align with the detector (120); perturbing the parameters of the magnetic sector mass spectrometer (10) once the edge of the mass peak has been aligned with the detector (120); deriving, from the applied perturbation, a first static field controller setting for the system controller (130) for application to the system controller (130) for the magnetic flux density B.sub.m; and applying that derived first static field controller setting to the system controller (130) for subsequent detection of ions at that magnetic field B.sub.m.

16. A method of determining controller settings for a controller (130), wherein the controller (130) controls a magnetic field generated by a magnet (70) in a magnetic sector mass spectrometer (10) by measuring the magnetic flux density of the magnetic field with a magnetic field sensor, the method comprising the steps of: generating ions; accelerating the ions towards the magnet (70); generating a first magnetic field, having a first magnetic flux density B.sub.m, in the magnet (70); directing ions into the magnet (70), where they follow a curved path towards an ion detector (120) which is the magnetic field sensor measuring the magnetic flux density; adjusting the magnetic field in the magnet (70) to a second magnetic flux density B.sub.n, at which an edge of a peak of ions is aligned with the ion detector (120); perturbing the parameters of the magnetic sector mass spectrometer (10) once the edge of the mass peak has been aligned with the detector (120) at the second magnetic field B.sub.n; deriving, from the applied perturbation, a changing field controller setting for the system controller (130), for application to the system controller (130) when the magnetic flux density changes from B.sub.m to B.sub.n; and applying that derived changing field controller setting to the system controller (130) for subsequent changes in the applied magnetic field between B.sub.m and B.sub.n.

17. A method of controlling a magnetic field generated by a magnet (70) in a magnetic sector mass spectrometer (10), comprising: generating ions of a plurality of ion species; accelerating the generated ions with an electrical potential U.sub.0; directing the accelerated ions into the magnet (70), the magnet (70) being configured to generate a magnet field of flux density B so as to cause ions entering it to be deflected along a curved path; and detecting ions exiting the magnet (70) at an ion detection arrangement (120); wherein ions of a first sacrificial ion species have a mass to charge ratio (m/z).sub.1 and follow a first curved path within the magnet (70) whilst ions of a second, analyte ion species different to the sacrificial species have a mass to charge ratio (m/z).sub.2 and follow a second curved path, different to the first curved path, within the magnet (70); and further wherein the ion detection arrangement has a plurality of spatially separated detectors (500, 510, 520); the method further comprising: adjusting one or more of the flux density B, the acceleration potential U.sub.0 and/or the position of a detector or detectors (500, 510, 520) in the ion detection arrangement (120), so that ions of the sacrificial species are directed toward a first detector (520) in order that the edge of a mass peak representative of those ions of the sacrificial species is aligned with the first detector (520), whilst ions of the analyte ion species are directed toward a second detector (500, 510), spatially separated from the first detector (520), so that a mass peak representative of those analyte ions is generally aligned with the second detector (500, 510) away from the mass peak edges; and controlling the magnetic flux density B by monitoring changes in the detector intensity at the peak edge of the sacrificial mass, so as to maintain the alignment of the analyte ions with the said second detector (500, 510).

18. A magnetic sector mass spectrometer (10) according to claim 3: the ion source (20) is arranged to generate a beam of ions containing a plurality of ion species each having a mass to charge ratio (m/z).sub.i; the ion accelerator is arranged to accelerate the ions in the ion beam to a potential U.sub.o; the ion source (20) is arranged to generate ions of a first sacrificial ion species having the first mass to charge ratio (m/z).sub.1 which follow a first curved path within the magnet (70), and to generate ions of a second, analyte ion species different to the sacrificial species having the second mass to charge ratio (m/z).sub.2 which follow a second curved path, different to the first curved path, within the magnet (70), and further wherein the ion detection arrangement (120) has a plurality of spatially separated detectors (500, 510, 520); the control system being configured to adjust one or more of the flux density B, the acceleration potential U.sub.0 and/or the position of a detector or detectors (500, 510, 520) in the ion detection arrangement (120), so that ions of the sacrificial species are directed toward the ion detector (520) which is sensing the magnetic field in order that the edge of a mass peak representative of those ions of the sacrificial species is aligned with the ion detector (520), whilst ions of the analyte ion species are directed toward the second detector (500, 510), spatially separated from the ion detector (520) which is sensing the magnetic field, so that a mass peak representative of those analyte ions is generally aligned with the second detector (500, 510) away from the mass peak edges; and further wherein the ion detector (520) which is sensing the magnetic field, has an output signal representing intensity at the peak edge of the sacrificial mass, from which the said variable input signal for the controller (130) is generated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may be put into practice in a number of ways, and some preferred embodiments will now be described by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic arrangement of a prior art control arrangement for controlling the magnetic flux density in a magnetic sector mass spectrometer;

(3) FIG. 2a shows a plot of magnetic flux density, B, versus magnetic field strength, H, for 1006 Steel;

(4) FIG. 2b shows a plot of magnetic flux density, B, versus magnetic field strength, H, for stainless steel

(5) FIG. 3 shows a schematic arrangement of a magnetic sector mass spectrometer having a control system embodying the present invention;

(6) FIG. 4 shows a circuit diagram of a preferred embodiment of the control system of FIG. 3;

(7) FIG. 5 shows, schematically, a field sensor arrangement suitable for use with the control system of FIGS. 3 and 4;

(8) FIG. 6 shows, in schematic block diagram format, an alternative arrangement of control system to that shown in FIG. 4;

(9) FIG. 7 shows a mass spectral peak as a plot of detector signal intensity vs magnetic flux density for ions of a particular mass to charge ratio; and

(10) FIG. 8 shows an arrangement of ion detectors suitable for use with the magnetic sector mass spectrometer of FIG. 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(11) Referring first to FIG. 3, a schematic arrangement of a magnetic sector mass spectrometer 10 is shown. The magnetic sector mass spectrometer has an ion source 20 for generating a beam of ions 30. The ions in the ion beam 30 are accelerated by electrostatic plates 40 forming accelerating ion optics, indicated generally in FIG. 3 by reference numeral 50. The ion beam 30 passes through a slit 60 and into a magnet 70. The magnet 70 is an electromagnet driven by a current source 80. A magnetic field is thus formed within the magnet bore.

(12) As explained in the Background section above, and as will be familiar to those skilled in the art, ions entering the magnet 70 will follow a circular path having a radius r.sub.m defined as r.sub.m=B.sup.1 (2.U.sub.0. (m/z)).sup.1/2, where U.sub.0 is the accelerating potential applied to the ions in the accelerating ion optics 50, B is the magnetic flux density in the magnet 70, and (m/z) is the mass to charge ratio of ions of a particular species.

(13) The consequence of this is that, for a given accelerating potential and magnetic field, different ion species in the ion beam 30 will follow different paths (that is, curved paths of different radii), depending upon their m/z. Relatively lighter ions will follow a more tightly curved path indicated by dotted line 100 whilst relatively heavier ions will follow a less tightly curved path indicated by dotted line 90. Neither will exit the magnet as a consequence. Ions of a particular m/z will however have a radius of curvature that results in ions of that species in the ion beam 30 exiting the magnet along path 110. These ions then strike a detector 120 which may, for example, contain one or more Faraday cups and/or secondary electron multipliers (SEMs). In the schematic embodiment of FIG. 3, a single detector 120 is shown for simplicity. The detector output may be stored, exported to a screen 125, and/or, as will be explained below, in particularly preferred embodiments of the invention, used to tune the settings of a controller 130.

(14) The mass spectrometer 10 is under the control of a control system. Its purpose and function will be described in detail below. In brief, however, the control system provides a closed loop feedback control for the magnet 70. This is achieved by monitoring/measuring the magnetic flux density within the magnet 70. To do this, a magnetic field sensor 140 is positioned within the bore of the magnet 70. The preferred configuration and constitution of the magnetic field sensor 140 is set out in further detail in connection with FIG. 5 below.

(15) The magnetic field sensor 140 provides a variable input to a controller 130. The variable input to the controller 130 is representative of the magnetic flux density at the sensor location within the magnet 70. A set point input to the controller is provided, either from an external set point generator (not shown in FIG. 3) or pre-programmed within the hardware or firmware of the controller 130 itself. The controller 130 compares the variable input from the magnetic field sensor 140 with the set point (target magnetic flux density) and then generates a digital controller output that is converted to an analogue signal at DAC stage 220, amplified with amplifier 230, and fed to the magnet current source 80. Control of the current supplied to the magnet windings thus in turn controls the magnetic flux density within the magnet 70.

(16) As with any feedback loop, the arrangement of FIG. 3 has a transfer function. In a preferred embodiment, the controller 130 uses Proportional-Integral-Differential (PID) control, and the controller settings (for example, the proportional gain, the integral time constant and/or the differential time constant) are selected and applied in accordance with the parameters of the mass spectrometer 10, such as, particularly, the chosen magnetic flux density B within the magnet 70 for a particular ion species, a jump between two magnetic flux densities, and so forth.

(17) As will also be seen in FIG. 3, a signal is also tapped off the magnet coil windings and fed back to the controller 130 via an analogue damping section 240, to provide a damping effect upon the driver signal for the current source 80. This will be explained in further detail in connection with FIG. 4 below.

(18) In the following description, it is to be understood that the controller 130 is primarily designed to control the magnetic field in the magnet 70, that is, to permit rapid, accurate adjustments between different magnetic flux densities (so that multiple ion species can be examined separately using the same or a similar location for the detector 120), and also to hold the magnetic flux density in the magnet 70 extremely steady and accurate whilst measurements of ions of a particular species are taken. Nevertheless, it is self-evident from the equation above that the acceleration potential U.sub.0 applied to the electrostatic plates 40 of the accelerating ion optics 50 must also be known and accurately set. U.sub.0 may also vary between experimental analyses, so as to allow a wide range of ion species to be analyzed. However, it is not preferred to vary U.sub.0 whilst carrying out analysis, since variations in U.sub.0 can alter the focusing of the ions in the ion beam 30 at the plane of the detector 120. Thus, in the following description, unless otherwise discussed, the accelerating potential U.sub.0 can be treated as fixed during a particular controller calibration/tuning or subsequent ion analysis experiment.

(19) Having provided an overview of the feedback system and its position and function within the magnetic sector mass spectrometer 10, a particular preferred embodiment of the controller 130 and associated components will now be described. Methods for tuning/calibrating the settings for the controller 130 will then be described in a separate section.

(20) Preferred Configuration of the Controller 130

(21) A circuit diagram illustrating a particularly preferred arrangement of the control system of FIG. 3 is shown in FIG. 4. In FIG. 4, components common to FIG. 3 are labelled with like reference numbers.

(22) The controller 130 of FIGS. 3 and 4 is constituted by a microcontroller to which a set point (target magnetic flux density) is supplied from a Field Programmable Gate Array (FPGA) 210. The magnetic field sensor 140 also supplies a variable input to the controller 130 as explained above in connection with FIG. 3.

(23) The controller 130 is digital. The set point generated by the FPGA 210 is a first digital input to the digital controller 130, and the second input to the digital controller, from the magnetic field sensor, is also digital. The magnetic field sensor is in preference a magnetorestive or Hall sensor which generates an analogue output. This is converted to a digital signal using an ADC (not shown in FIG. 4). In preference, the ADC is located immediately adjacent the magnetic field sensor (or even upon/forms a part of the sensor) in order to minimize the effect of temperature variations upon the magnetic field stability. This will be explained further in connection with FIG. 6 below.

(24) The controller 130 provides a PID response to the difference between the digital set point input and the digital (digitized) variable input from the magnetic field sensor. The PID settings for the controller 130 are, in an important feature of the arrangement, not fixed. Instead, a plurality of different PID settings is employed, with the specific PID setting employed at a given time being dependent upon parameters of the magnetic sector mass spectrometer 10. For example, the PID settings applied to the controller 130 may be related to one or more of the present magnetic flux density in the magnet 70, the new end set point of the magnetic flux density when a different ion species is to be detected, the magnitude of the jump between the two differing flux densities, and so forth. The specific PID settings for a particular set of mass spectrometer parameters may be obtained through calibration/tuning at a particular set of mass spectrometer parameters, for example in a manner to be described below, and may be stored as a matrix of values in a memory (not shown in FIG. 4), which may form a part of the controller 130, or may be physically discrete from the controller 130 instead. Moreover, although a matrix of value for the PID parameters of the controller 130 can be stored in the form of a lookup table, in other arrangements, a functional relationship can be derived, for example deterministically, so that for any set of mass spectrometer parameters, PID settings can be obtained via that functional relationship.

(25) The output of the controller 130 is a digital control signal for controlling the magnet current, which is derived from the difference between the set point and measured inputs with the appropriate PID parameters applied. That digital output is converted into an analogue signal by a digital to analogue conversion (DAC) stage 220. The analogue output of the DAC stage 220 is received by an amplification module shown generally at 250 and formed of an operational amplifier 230 that in turn provides an analogue output to a power stage which forms a part of the magnet current source 80.

(26) An analogue damping section 240 is also provided as a feedback between the magnet coils, the DAC stage 220 and the non-inverting input of the operational amplifier 230. The analogue damping section includes an additional field coil at the magnet 70 providing a feedback to the operational amplifier 230 for setting the magnet current in the power stage of the magnet current source 80. The analogue damping section 240 also includes switchable high or low damping; this allows for a faster control of transients, whereas the digital control with longer cycle times (<=5 ms) eliminates drift.

(27) In order to achieve maximum stability of the magnetic flux density experienced by ions as they traverse the magnet 70, it is desirable that the resolution of the analogue output of the DAC stage 220 is very high, so that the current to the magnet coils is in turn very accurately controlled. One preferred way of implementing this desideratum is to combine two single DACs to form the DAC stage 220. As may be seen in FIG. 4, the DAC stage 220 is formed of a first, 20 bit DAC 220a, along with a second, 12 bit DAC 220b. The analogue output of each DAC 220a, 220b is combined along with the fed back analogue damping signal from the analogue damping section 240 to form the non-inverting input to the operational amplifier 230.

(28) As an alternative to the use of two DACs 220a, 220b, the DAC stage 220 could instead achieve the desired degree of resolution by using a very fast switching between two values, if the switching time is much faster than the time constant of the power stage in the magnet current source 80.

(29) Although in FIG. 4, the DAC stage 220 is shown separate from the controller 130 and the operational amplifier/current source 80, it will of course be understood that the DAC stage 220 can form a part of the controller 130 or, alternatively, a part of the amplification stage.

(30) The approach to magnetic flux density control illustrated by the arrangement of FIGS. 3 and 4, wherein the controller 130 is digital and so are the inputs to it along with the control signal output, means that only a small part of the system control takes place in the analogue domain, where temperature sensitivity may be an issue. In particular in the arrangement of FIG. 4, the only potential sources of temperature drift are in the (analogue) amplification module 250 and at the magnetic field sensor 140. As to the amplification module, temperature fluctuations that might influence the magnet current are measured indirectly by the magnetic field sensor, and are thus fed back and controlled.

(31) The other temperature sensitive part of the control system is the magnetic field sensor 140 (which produces an analogue signal output) and the ADC (not shown in FIG. 4) that converts the analogue magnetic flux density value to a digital signal. As explained above, in preference, the ADC forms a part of the field probe so that the location of any potentially problematic temperature variations is strictly confined. Although digital magnetic field sensors are available, they are unable to provide the necessary degree of digital resolution and, moreover, are unsuitable for the magnetic flux densities contemplated in the mass spectrometer FIG. 3.

(32) FIG. 5 shows, in schematic block form, an arrangement for the magnetic field sensor 140 that seeks to address temperature instability at that magnetic field sensor 140.

(33) In the arrangement of FIG. 5, the analogue output of the magnetic field sensor 140 is amplified by sensor amplifier 300 and then converted from an analogue to a digital signal with ADC 310. The digital output of the ADC 310 is filtered at filter 320 and the filter digital output provides the variable digital input signal to the controller 130.

(34) It is desirable to carry out filtering of the magnetic field sensor output after it has been converted into a digital signal by the ADC 310. Digital filtering provides, when employed correctly, a better combination of smoothing and time resolution than may be achieved using resistor/capacitor (RC) type analogue filters.

(35) A particularly preferred arrangement of filter 320 is an infinite impulse response filter, because that requires fewer data points to filter, compared with FIR filters. A particularly preferred implementation may be a Chebyshev type I filter, as such a filter is very sharp and has no ripple in the stop band. A sharp filter is desirable because the time resolution is maintained to a high degree.

(36) As an alternative to the foregoing, an elliptic filter might be employed instead.

(37) The magnetic field sensor 140 may be Hall sensor, a magnetoresistive sensor, an AMR sensor, a GMR sensor, a CMR sensor or a TMR sensor, for example. Particularly preferred is a magneto-resistive sensor in the form of field plates, which is a magnetoresistive sensor cut from a single crystal. Such field plates have a high dynamic range compared to Hall sensors. Also, Hall sensors tend to suffer from the problem of popcorn noise and thermal load due to the perpendicular Hall current. The result of a relatively flat characteristic curve is that relatively high Hall currents are in turn needed. On the other hand, single crystal Hall effect sensors may provide an acceptable improvement over traditional Hall sensors.

(38) Returning to FIG. 5, temperature control of the magnetic field sensor 140, the sensor amplifier 300 and ADC 310 is achieved by the use of a temperature sensor 330. The output signal of the temperature sensor may be employed, in a simpler arrangement, directly to control the ADC 310: in particular the input of the temperature sensor 330 is employed by the ADC 310 to correct the analogue magnetic field sensor signal using a known signal/temperature relation.

(39) The preferred manner of temperature control, however, is as shown in FIG. 5. A temperature controller 340 receives the signal from the temperature sensor 330 as a first, variable input. A set point input is also supplied to the temperature controller 340. The temperature controller 340 produces a control output signal that is used to control a heater 350 so that a closed loop thermal control is achieved.

(40) In addition to, or instead of, a heater 350, a cooler such as a Peltier cooler may be employed.

(41) It is desirable, in order further to optimize temperature control, that each of the temperature sensitive components in FIG. 5 (all of the components save for filter 320) be contained within a closed housing (shown as dotted line 360), with thermal insulation inside.

(42) Turning now to FIG. 6, an alternative embodiment of a control system is shown in schematic block form. Components common to FIGS. 3, 4 and 6 are labelled with like reference numbers. Furthermore, for the avoidance of repetition, those parts of the control system common to FIGS. 3, 4 and 6 will also not be described again in detail.

(43) In FIG. 6, a digital set point value is generated by the FPGA 210. This provides a first, set point digital input to the controller 130. The magnetic field sensor 140 provides a digital, variable input signal to the controller 130. The output of the digital controller 130, as previously, is converted to an analogue signal that is then amplified in the amplifier module 250 for generation of a demand current for the coils of the magnet 70.

(44) In the arrangement of FIG. 6, however, a further input to the controller 130 is provided from the detector 120 (see also FIG. 3). The detector output is either from a Faraday cup or from an SEM. A data logger 400 processes the output of the ion detector 120. That processed output may either be used directly to provide a further control to the controller 130, or may be pre-processed using a computer or other microprocessor 410. The purpose of using the output of the ion detector 120 as a further input to the controller 130 will be explained in the following section, which concerns the calibration/tuning of the controller 130.

(45) Calibration/Tuning of the Settings of the Control System

(46) Techniques for calibrating or tuning the controller settings for the control system shown in FIGS. 3, 4 and 6 will now be described. By calibrating or tuning is meant the process of determining optimal constants for the control system at a variety of different magnetic flux densities. In the embodiments of FIGS. 3, 4 and 6, the controller 130 employs PID control, and the following techniques result in the derivation of the proportional gain and the integral and differential time constants for different flux densities and also when attempting to transition rapidly between two different magnetic flux densities.

(47) The calibration techniques all rely upon the sharpness of the edge of a mass spectral peak in a typical magnetic spectrum. A typical such peak is shown in FIG. 7, which is a plot of intensity at the ion detector 120 (arbitrary units) as a function of magnetic flux density for a magnetic sector mass spectrometer having an instrument resolution of 21500.

(48) At both peak edges (leading and trailing edges), the slope of the plot is very high; this means a very small variation in the magnetic flux density value results in a marked effect on the measured intensity. Therefore, measuring the intensity at the peak edge provides an extremely precise way to measure any changes in the magnetic flux density. By determining the slope of the peak at the peak edge, the intensity of the peak can be converted into a magnetic flux density value.

(49) Assuming a linear behaviour between 5% and 95% of the peak height, and for a resolving power of 21500 (the resolving power for the peak in FIG. 7), a change in the magnetic flux density of 1 ppm would result in a change in the measured intensity of 5% of the maximum value of the peak, and 10% of the value at half the peak height.

(50) This technique provides further advantages. Simply measuring the magnetic flux density using the magnetic field sensor 140 provides a useful snapshot of the flux density, but is limited to a measurement only of the flux density in the immediate vicinity of the magnetic field sensor 140. By measuring the magnetic flux density using the peak edge, by contrast, an integral over the area of the magnet covered by the ion beam 30 is measured. This may be particularly advantageous when jumping between first and second flux densities, because the slower processes, e.g. due to the reorganisation of the magnetic domains, and the remanent magnetisation in the magnet core, are dependent on the position inside the magnet gap. This is especially important for small magnetic field values.

(51) For optimal system control, it is desirable to obtain controller settings both for static and dynamic magnetic fields. Mass peak edges can be used to obtain both.

(52) Determining the Controller Settings at a Static Magnetic Flux Density.

(53) The first task is to align the peak edge with the detector 120. The magnetic flux density B and the acceleration potential U.sub.0 are adjusted so that ions of a particular mass to charge ratio are directed towards the detector 120. Once the edge of the peak has been located by adjusting the parameters of the mass spectrometer, a perturbation is applied to the mass spectrometer parameters to introduce slight oscillations within the control system. From the oscillation frequency the time constant of the control loop is determined and, for a PID control algorithm, the I and D settings can be determined. There are many ways of determining the control settings heuristically or otherwise: Ziegler Nichols, Chien, Hrones and Reswick, and others, each of which will be well known to those skilled in the art of closed loop control. Also, the setting for P can be varied until the oscillations are no longer visible in the intensity.

(54) This technique permits the derivation of optimized settings for the controller for the particular magnetic flux density employed in the initial calibration experiment defined above. In particular, the derived settings can be stored for use in any subsequent analysis, in which the magnetic flux density is the same or similar to that at which the settings were derived.

(55) By repeating the analysis at different values of magnetic flux density, an array or matrix of controller settings can be generated, each controller setting being associated with a respective flux density. The matrix can be stored for subsequent use as a look up table or can be used to derive or calculate a functional relationship between B and the controller settings (eg between B and PID).

(56) As explained previously, the equation of motion governing ion movement in a magnetic sector mass spectrometer contains four parameters that can in principle be changed. Of these, however, the radius of the curved path of the ions is essentially fixed in that only ions travelling along a specific path defined by the radius r.sub.m will arrive at the detector 120. This means that, in order to obtain the controller settings for multiple values of B, either the accelerating potential U.sub.0 or the mass to charge ratio of the ions must be changed. In one preferred embodiment, the accelerating voltage is changed. Because, for a given mass, the magnetic flux density is proportional to the square root of the voltage, modifying the accelerating voltage by a factor of 4 is equivalent to a factor of 2 variation in the magnetic flux density.

(57) In order to extend the determined controller settings across a wider range of magnetic fields (i.e. beyond what can be ascertained simply by modifying U.sub.0), different mass to charge ratio ions must be employed. For example, if CO.sub.2 is introduced into the ion source 20, there are fragment masses of considerable intensity at the masses 12, 22, 28, 44. Therefore, with one gas, and different accelerating voltage values U.sub.0, U.sub.1, U.sub.2 . . . , the controller settings for the whole mass range up to mass 44 can be ascertained.

(58) Determining the Controller Settings for a Changing Flux Density.

(59) If the magnet field is modified step wise, and if the final magnetic flux density value is a value where the mass peak is at a raising or falling edge as shown in FIG. 7, the reaction of the system to a stepwise jump can also be determined and optimized. Optimal controller parameters can in particular be ascertained, for example, in accordance with the second rule by Ziegler Nichols, or by other algorithms. Optimizing the controller settings for a jump between two magnetic flux densities allows the speed at which the jump can take place to be optimized in turn. Also, when considering the derivation of controller settings for a dynamic magnetic field, the effect that the magnetic flux densityand the time behavior of itis dependent on the position, and especially the difference between the integral that the ion beam 30 covers and the position of the magnetic field sensor 140, is also calibrated out.

(60) Depending on the starting magnetic flux density value, this technique can be repeated for different jump sizes. Also, in a manner similar to the procedure outlined above, different end values of the magnetic field/flux density can be obtained.

(61) The system behaviour, and especially the dynamic behaviour, particularly at small magnetic field/flux density/mass values, is dependent upon the position within the magnet 70. This means that, for a wide simultaneous mass range and a low mass, e.g. for determining He.sup.3 and He.sup.4 simultaneously, the controller settings are dependent upon the mass to charge ratio of the ions, or, more exactly, the position of the detector 120 as well. In such highly specialised cases, it may be preferable to determine the controller settings independently for different collector positions.

(62) Determining the Controller Settings for Both Static and Dynamic Fields.

(63) The controller settings obtained by jumping between two magnetic flux densities can be combined with those obtained (separately) from static field measurements. During a jump between two flux densities, in general terms it is desirable that the final flux density (for measuring ions of the final m/z) should be reached as quickly as possible; therefore the controller settings are optimized with this goal in mind. After the jump, the stability of the magnetic flux density is of utmost importance; therefore after having reached the second magnetic flux density, the controller settings can be switched to use those derived instead from the static calibrations for that particular set of mass spectrometer parameters. In a transition period, a combination of the static and dynamic controller settings may be employed, where the long term drift effects determined by the controller settings during a jump are taken into account, but otherwise the settings obtained by the static calibration are employed.

(64) The various different methods of determining the controller settings, both for static and dynamic magnetic fields, may be carried out automatically, without user interaction. Determination of the settings may be repeated e.g. on a yearly or biennial basis and/or after baking the system. It is also desirable to carry out the controller setting determination when the instrument is parameterized for a high resolving power, in order to obtain maximum precision during ion analysis.

(65) Using Peak Edges for Magnetic Flux Density Control.

(66) Turning to FIG. 8, a specific arrangement of ion collectors (eg Faraday cups) forming the ion detector 120 is shown.

(67) Often, not all masses falling into the mass range of the detector 120 are used for measuring isotope ratios. There might be a mass that is not from the analyte gas, e.g. Ar (mass 40) when measuring CO2 (mass 44). Or, alternatively, not all isotopes coming from the analyte gas are of interest for a specific analytical problem. In either case, the ion species that is identified to be present but not of analytical interest can be used as a sacrificial mass for collection in a variable multicollector ion detector as shown in FIG. 8.

(68) As seen in that Figure, the ion detector 120 comprises a plurality of spatially separated ion collectors 500, 510, 520. The relative positions of the ion collectors are adjustable: in particular, the third ion collector 520 position may be adjustable alone, and/or two or all three of the collectors may be relatively moveable. Moreover, in one embodiment, the collector positions may be manually adjusted, whereas in other embodiments, the digital controller or another microprocessor may adjust the position of the ion collectors in order to achieve the effect below.

(69) By adjusting one or more of the accelerating potential, the magnetic flux density, and the absolute/relative collector positions, ions of a first mass to charge ratio (m/z).sub.1 are directed towards a first collector 500. Ions of a second mass to charge ratio (m/z).sub.2 are directed towards a second collector 510. Meanwhile, the third collector, 520, can be positioned in such a way that, during analysis of an analyte, the sacrificial mass, whose mass to charge ratio is (m/z).sub.3, is at the peak edge. As explained above, the signal of this third collector 520 thus provides a very precise way to measure changes in the magnetic flux density of the magnet 70. The other masses (m/z).sub.1 and (m/z).sub.2 are analysed via the first and second collectors 500, 510, as usual.

(70) The signal from the third collector 520 may be fed back into the magnet current source 80 (either electronically or by the data logger 400, or via the computer 410see FIG. 6), in order to stabilize the magnetic flux density further.

(71) There are two cases where this feedback is especially advantageous. The first case is where there is a small shoulder at the edge of the measurement mass. Here, it is mandatory to keep the magnetic flux density as stable as possible: by small changes in the magnetic flux density the shoulder would be left. Stabilizing the flux density by the procedure explained above would increase the accuracy. Because a variation in the flux density of 1 ppm would be equivalent to a variation in the signal intensity of 5% of the peak value, this technique could be used to gain flux density stabilities far into the sub-ppm range.

(72) The second particularly advantageous use of the feedback is when measuring a small intensity with a SEV at the foot of a large peak. Here, it is important that the SEV is protected from the much larger intensity of the neighbouring mass, and this protection can be done using the procedure explained above.

(73) Although FIG. 8 shows two analyte ion collectors, it will be understood of course that other quantities of analyte ion species can be employed (1, 3, 4 . . . ) Moreover, there may be circumstances where 2 or more different sacrificial masses are desirable.

(74) The foregoing detailed description has described only a few of the many forms that this invention may take. For this reason the detailed description is intended by way of illustration and not by way of limitation. Various modifications and additions to the specific embodiments that have been described, will be contemplated by the skilled person. For example, although the embodiments above are described in the context of a digital PID controller, it will be understood that various other controllers are contemplated, such as, but not limited to, a state controller with observer; an interdependent network control; direct synthesis methods; a minimal prototype algorithm; Dahlin's Algorithm; Vogel-Edgar Algorithm; Internal Model Control; Digital Feedforward control; General linear controller; R(s) U(s)=T(s) Ysp(s)S(s) Y(s); R, S, T are polynomials of arbitrary order; Discrete-time linear MISO controllers; Model predictive Control; and/or Fuzzy control.