Abstract:
An ion detector ( 27 ) for use in a time-of-flight mass spectrometer ( 1 ) is disclosed. The ion detector ( 27 ), which has an extended dynamic range, comprises at least one microchannel plate electron multiplier ( 42, 32 ) together with first and second collection electrodes ( 36, 38; 39 ). The second collection electrode ( 36, 38 ) is arranged such that in receives more electrons per ion entering the ion detector ( 27 ) than the first collection electrode ( 39 ). In one embodiment the collection electrodes ( 36, 38; 39 ) are formed in an array consisting of a larger plate-like collection electrode ( 36, 38 ) and a smaller plate-like collection electrode ( 39 ). In an alternative embodiment a second microchannel plate electron multiplier is arranged between the first collection electrode ( 46 ) and the second collection electrode ( 47 ). In this embodiment the first collection electrode consists either of a grid electrode ( 42 ) or more preferably of a wire electrode, and the second collection electrode consists of a plate-like electrode ( 43 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the national stage of International Application No. PCT/GB99/00248 filed Jan. 25, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a time-of-flight mass spectrometer and its associated ion detection system. It provides apparatus for detecting ions in a time-of-flight mass spectrometer, and methods of operating that apparatus, which result in improved performance at a lower cost when compared with prior spectrometers. 
     In a time-of-flight mass spectrometer, a bunch of ions enters a field-free drift region with the same kinetic energy and the ions temporally separate according to their mass-to-charge ratios because they travel with different velocities. Ions having different mass-to-charge ratios therefore arrive at a detector disposed at the distal end of the drift region at different times, and their mass-to-charge ratios are determined by measurement of their transit time through the drift region. 
     Prior detectors for time-of-flight mass spectrometers comprise an ion-electron converter followed by an electron multiplying device. In some embodiments, ions strike a surface of the multiplying device to release electrons and a separate converter is not provided. Because the detector must respond to ions leaving the whole exit aperture of the drift region, it is conventional to use one or more microchannel plate electron multipliers as the multiplying device. A collector electrode is disposed to receive the electrons produced by the microchannel plates and means are provided to respond to the current flow so generated and produce an output signal. The chief difference between such a detector and the similar device conventionally used with magnetic sector, quadrupole or quadrupole ion-trap spectrometers is the electronic signal processing, which must produce signals indicative of the transit time of the ions as well as the number arriving in any particular time window (corresponding to one or more mass-to-charge ratios). This data must be generated and read out before the next bunch of ions can be admitted into the drift region, so that detector speed is an important determinant of the repetition rate, and hence the sensitivity, of the entire spectrometer. 
     The earliest detectors for time-of-flight spectrometers comprised a DC amplifier connected to the collector electrode and an analogue-to-digital converter (ADC) for digitizing the output of the amplifier. Usually, this arrangement was used with time-slice detection whereby the amplifier was gated to respond only to ions arriving within a certain time window (typically corresponding to one mass unit). The time window was moved (relative to the time of entrance of ions into the drift region) during repeated cycles of operation so that a complete mass spectrum was eventually recorded. An improvement involved the use of several amplifiers and ADC&#39;s arranged to simultaneously record a different time window. Nevertheless, many cycles of the spectrometer are still required to record a complete mass spectrum and the repetition rate of the spectrometer is severely limited by the time taken for the analogue-digital conversion in each cycle. Digital transient recorders (for example, as described in U.S. Pat. Nos. 4,490,806, 5,428,357 and PCT applications WO94/28631 and WO95/00236) have been devised to efficiently process the digital data produced by the ADC, but, particularly in the case of time-of-flight mass analyzers for the analysis of ions from continuous (as opposed to pulsed) ion sources, these do not represent a cost-effective solution to the problem of achieving a high repetition rate. 
     An alternative detection system for time-of-flight mass spectrometers is based on ion counting. In these methods, a signal due to a single ion impact on the detector is converted to a digital boolean value, “true” (which may be represented by a digital “1”) in the case of an ion impact, or “false” (e.g, a digital “0”) if there has been no ion impact. Various types of timers and/or counters are then employed to process the digital data produced. For example, a counter associated with a particular time window may be incremented whenever a signal is generated in that time window. Alternatively, the output of a timer, started when an ion bunch enters, may be stored in a memory array whenever the detector generates a “true” signal. The advantage of an ion-counting detector over an analogue detector is that variations in the output signal of the electron multiplier due to a single ion impact, which may be ±50% or more, are effectively eliminated because each signal above the noise threshold is treated identically. Further, an ion counting detector does not suffer from the additional noise inevitably produced by the ADC incorporated in an analogue detector system, and is also faster in operation. Consequently, the contribution of noise to the overall ion count is reduced and a more accurate ion count is achieved, particularly in the case of small numbers of ions. The disadvantage is that the digital signal representing an ion impact must be processed very quickly, before the next ion arrives at the detector, if that ion is to be detected. In practice, all detectors have a deadtime immediately following an ion impact, during which they cannot respond to an ion impact. This limits the number of ions which can be detected in a given time, so that a dynamic range of the detector is also limited. Corrections can be made to the detector output to compensate for the effects of deadtime (see, for example, Stephen, Zehnpfenning and Benninghoven, J. Vac. Sci. Technol. A, 1994 vol 12 (2) pp 405-410), and in corresponding EP patent application claiming priority solely from GB 9801565.4 filed Jan. 23, 1998 (Agents Ref: 80.85.67750/004), but even when such corrections are carried out the detector dynamic range still effectively reduces the performance of a time-of-flight mass spectrometer with such a detector. 
     An improved ion-counting detector for time-of-flight mass spectrometry has been described in general terms by Rockwood at the 1997 Pittsburgh Conference, Atlanta, Ga. (paper No 733), and is available commercially from Sensar Larsen-Davis as the “Simulpulse” detector. According to information published by Sensar Larson-Davis it comprises a large number of individual equal-area anodes, each of which is provided with a digital pulse generating circuit which is triggered by the arrival of an ion at its associated anode. The anodes are disposed in a wide-area detector so that they are all equally likely to be struck by an ion exiting from the drift region. Consequently, simultaneous (or near-simultaneous) ion strikes are most likely to occur on different electrodes and the effect of detector deadtime is greatly reduced. The data from each of the anodes is summed into an 8-bit digital word representative of the ion intensity at any particular time, and the value of that word and its associated time is stored in a digital memory. However, such a detector is obviously complicated and expensive to manufacture. 
     An electron multiplier ion detector for a scanning mass spectrometer which has two modes of operation to extend its dynamic range is disclosed by Kristo and Enke in Rev. Sci. Instrum. 1988 vol 59 (3) pp 438-442. This detector comprises two channel type electron multipliers in series together with an intermediate anode. The intermediate anode was arranged to intercept approximately 90% of the electrons leaving the first multiplier and to allow the remainder to enter the second multiplier. An analogue amplifier was connected to the intermediate anode and a discriminator and pulse counter connected to an electrode disposed to receive electrons leaving the second multiplier. The outputs of the analogue amplifier and pulse counter were electronically combined. A protection grid was also disposed between the multipliers. At high incident ion fluxes, the output signal comprised the output of the analogue amplifier connected to the intermediate anode. Under these conditions a potential was applied to the protection grid to prevent electrons entering the second multiplier (which might otherwise cause damage to the second multiplier). At low ion fluxes, the potential on the protecting grid was turned off and the output signal comprised the output of the pulse counter. In this mode the detector operated in the single ion counting mode. In this way the detector was operable in a low sensitivity analogue mode using the intermediate anode and a high sensitivity ion counting mode using both multipliers and the pulse counter, so that the dynamic range was considerably greater than a conventional detector which use only one of these modes. 
     Other prior art teaching of electron multipliers with means for extending the dynamic range includes a simultaneous mode (i.e., pulse counting and analogue) electron multiplier taught in U.S. Pat. No. 5,463,219. U.S. Pat. No. 4,691,160 teaches a discrete dynode electron multiplier having two final collector electrodes of different areas, each connected to a separate analogue amplifier and selectable by means of a manually operated switch. Soviet Inventors Certificate SU 851549 teaches the disposition of a control grid between two channelplate electron multipliers, the potential on which can be adjusted to control the gain of the assembly. GB patent application 2300513 A teaches a similar control grid disposed between certain dynode sheets in an electron multiplier comprising a stack of such sheets, and which is especially suitable for a photomultiplier tube. Prior art disclosed in U.S. Pat. No. 4,691,160 also comprises a continuous dynode electron multiplier having two collector electrodes, one of which is capable of receiving electrons from a dynode disposed prior to the final dynode so that the multiplier has less gain. 
     Co-pending PCT patent application claiming priority from GB9801565.4, GB9804286.4, GB9810867.3 and GB9813224.4 and filed simultaneously herewith (Agents Ref: 80.85.67901/003) teaches a time-of-flight mass spectrometer having an ion-counting channelplate detector with two or more collection electrodes of unequal areas and means for automatically selecting data from the most appropriate electrode according to the ion flux at different mass to charge ratios. In this way the dynamic range of the detector is extended by switching to data from a smaller electrode whenever the data from a larger electrode is likely to be inaccurate due to detector deadtime. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a time-of-flight mass spectrometer and a detector therefor, which has a greater dynamic range than most prior apparatus and which is cheaper to manufacture than prior spectrometers and detectors of equivalent performance. It is a further object to provide methods for operating such a spectrometer and detector. 
     According to a first aspect of the present invention there is provided a time-of-flight mass spectrometer. 
     In a preferred embodiment a second electron multiplying means may be provided between the first collection electrode and the second collection electrode to receive electrons which are not collected on the first collection electrode and to generate a greater number of electrons per ion entering the detector at the second collection electrode than at the first collection electrode. 
     Alternatively, both collection electrodes can be disposed to receive secondary electrons from a single electron multiplying means but the first electrode may have a smaller effective area than the second collection electrode so that the second collection electrode receives more electrons per ion entering the detector. 
     The term “effective area” means that area of a collection electrode which actually receives the secondary electrons. Thus the first collection electrode may comprise a grid-like electrode of smaller effective area than the second collection electrode. 
     In an alternative embodiment, the grid-like electrode(s) may be replaced with at least one, preferably a single, wire electrode. 
     The signal processing means associated with each of the collection electrodes may comprise an analogue or a digital (i.e, pulse-counting) system. Preferably, both signal processing means are digital, but in a less preferred embodiment one may be digital and the other may be analogue. 
     Analogue signal processing means may comprise a fast analogue amplifier followed by an A-D converter which outputs a digital signal to the memory means on receipt of a clock pulse. 
     Pulse-counting signal processing means may comprise a discriminator which generates a digital “true” signal to the memory means in response to the arrival of secondary electrons at the collection electrode in the period immediately preceding a clock pulse. 
     Typically, a digital signal processing means is used in association with the second collection electrode to provide the maximum sensitivity. A pulse-counting system of this nature unavoidably suffers from dead-time errors such that immediately following triggering of the discriminator the discriminator is unable to respond for a time, and co-pending PCT patent application claiming priority from GB9801565.4, GB9804286.4, GB9810867.3 and GB9813224.4 and filed simultaneously herewith (Agents Ref: 80.85.67901/003) teaches apparatus and methods for minimizing this problem in a similar detector system for a time-of-flight mass spectrometer. 
     Preferred embodiment of the memory means of the invention may comprise RAM associated with a suitably programmed digital computer or microprocessor. Thus, a spectrometer cycle is started each time a bunch of ions enters the drift region. In the case of an analogue signal processing means, a clock generator may cause the signal processing means to store the digital output of signal processing means in the memory means at a series of transit times corresponding to the ticks of the clock generator during that spectrometer cycle. 
     After all the ions of interest have travelled through the drift region the spectrometer cycle is terminated, a new bunch of ions is generated and a new cycle is started. Data at each clock tick from this and subsequent cycles may then be added to the data previously stored in the memory means for the same transit time value. 
     In the case of a pulse-counting detector a similar arrangement may be adopted, storing for example, a digital “1” at the clock tick immediately following the triggering of the fast discriminator by an ion arrival at the associated collection electrode and accumulating valves at corresponding transit times in subsequent detector cycles. Alternatively, memory may be conserved by storing only each transit time at which an ion triggers the fast discriminator. 
     In accordance with the preferred embodiment of the invention the output means is operative to determine the quantity of ions entering the detector at one or more transit times subsequent to the completion of at least one, and more usually many, spectrometer cycles. 
     The number of cycles during which acquisition takes place will be dependent on the rate at which the mass spectrum is changing and the capacity of the memory means. For example, in the case of a TOF spectrometer used for monitoring fast chromatographic peaks the repetition rate may be 10 kHz and data may be stored for about 0.5 seconds (ie, approximately 5,000 spectrometer cycles) in the memory means before being processed by the output means. Longer time periods and lower repetition rates are more typical for MALDI TOF spectrometers. 
     Once the data from the desired number of spectrometer cycles has been acquired, the output means may generate mass spectral data in the form of the quantity of ions entering the detector at each of one or more transit times. 
     The output means preferably uses the data associated with the second collection electrode (or the data associated with both the first and second collection electrodes) in order to obtain the maximum sensitivity. 
     However, data associated with the second electrode may be unreliable at certain transit times if the number of ions entering the detector at a particular transit time exceeds a certain limit, for example because of detector dead-time in the case of pulse-counting signal processing means or because of saturation of the A-D converter in an analogue signal processing means. In such circumstances the output means may use data from the first collection electrode alone, which data is less likely to suffer from deadtime or saturation problems. 
     Conveniently, a decision on whether data from the second collection electrode is reliable at any given transit time is made from an examination of the data from the first collection electrode which has been stored in the memory means at the relevant transit time. The relative gains of the detector system of the collection electrodes and their associated signal processing means is known (either by experimental calibration or from the ratio of the areas of different sized collection electrodes) so that a threshold output level may be set in relation to the output of the signal processing means associated with the first collection electrode above which data associated with the second collection electrode should not be used. 
     Preferably the output means comprises a suitably programmed digital computer. 
     According to a second aspect of the present invention there is provided a method of time-of-flight mass spectrometry. 
     It will be appreciated that according to the invention and in contrast to the prior art of U.S. Pat. No. 5,463,219 it is not necessary to provide fast hardware to examine the signals generated by the first collection electrode while the data is being acquired. Instead, the decision as to whether data associated with the second collection electrode is valid is made once all the data from a plurality of ion bunches has been stored in the memory means. Consequently the speed at which data from the collection electrodes can be stored in the memory means is increased. This is especially important in the case of a time-of-flight spectrometer if the rate of generation of ion bunches, and hence the sensitivity of the spectrometer, is not to be degraded. Prior types of dual mode electron multipliers (e.g, that described in U.S. Pat. No. 5,463,219) intended for scanning mass spectrometers require hardware for monitoring the low-gain output signal in order to activate some means of preventing damage to the high-gain section of the multiplier when the ion beam flux exceeds a certain value. However, with a time-of-flight spectrometer this situation does not arise so readily because the number of ions arriving in each bunch will generally be far less than the number likely to cause damage to the multiplier. These prior types of dual-made multipliers are unsuited to use with a time-of-flight mass spectrometer because the presence of the protection system reduces the rate of data acquisition. The present inventors have realized that the limitation on dynamic range in the case of a time-of-flight detector is more likely to be imposed by the limited dynamic range of a sufficiently fast A-D converter, or the dead-time of a pulse-counting system, and not by the possibility of saturation of or damage to the multiplier itself. Thus, the present invention overcomes the limitations of prior dual-mode detectors when used for time-of-flight mass spectroscopy by storing data from the collection electrodes directly in the memory means and requiring no additional real-time processing or slow electronic hardware, and therefore, provides a detector with extended dynamic range which does not require the repetition rate of the spectrometer to be reduced. 
     Preferred variations on the method will be apparent from the discussion presented above in respect of the apparatus of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic drawing of an ICP mass spectrometer; 
     FIG. 2 is a drawing of an ion detector suitable for is use in the invention; 
     FIG. 3 is a drawing of an array of collection electrodes suitable for use in the detector shown in FIG. 2; and 
     FIG. 4 is a drawing of an alternative type of ion detector. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, an ICP mass spectrometer is generally indicated by  1  comprises an ICP torch  2  which generates a plasma  3 . As in conventional ICP mass spectrometers a sample to be analyzed may be introduced into the torch  2  entrained in the torch gas supplies (not shown). Ions characteristic of such a sample are generated in the plasma  3 . The torch  2  is disposed adjacent to a sampling cone  4  which comprises an orifice  5  through which at least some of the ions generated in the plasma  3  may enter a first evacuated chamber  6  which is pumped by a first pump  7 . In agreement with conventional practice there is provided a skimmer  8  which cooperates with the sampling cone  4  to provide a nozzle-skimmer interface. An additional stage of pumping is provided by a second pump  10  connected to a second evacuated chamber  9 . Ions from the plasma  3  exit from the skimmer  8  along an axis  11 , pass through the second evacuated chamber  9  and exit through a third evacuated chamber  13  through an orifice in a conical extraction lens  12  which forms part of the boundary wall between the chambers  9  and  13 . The third chamber  13  is evacuated by a third pump  14 . In accordance with the teachings of EP patent application 0813228 a hexapole rod assembly  15  (containing gas at a pressure of about 10 −2  torr)is provided in the third evacuated chamber  13  to reduce interferences from unwanted species and reduce the energy spread of ions. 
     After passing through the rod assembly  15  ions pass through an orifice  16  in a wall  17  which divides the third evacuated chamber  13  from a fourth evacuated chamber  18  which contains a time-of-flight mass analyzer. A vacuum pump  19  maintains the pressure in the chamber  18  at 10 −6  torr or better. On entering the evacuated chamber  18  the ions pass through an electrostatic focusing lens  20  and enter an ion pusher  21 , electrodes in which are fed with pulses from a pulse generator  22  in such a way that bunches of ions are repeatedly ejected parallel to an axis  25  into a drift region  24 . In a general sense, therefore, items  1 - 24  comprise an ion source for repeatedly generating bunches of ions. The ion pusher  21  comprises ion accelerating means for causing at least some of these bunches to enter the drift region with substantially the same component of kinetic energy along the axis  25  (which is perpendicular to the ion axis  11 ). This arrangement therefore comprises an orthogonal acceleration time-of-flight analyzer, but a linear arrangement is also within the scope of the invention. The ions leaving the ion pusher  21  travel into the drift region  24  along a trajectory  23 ,(which deviates from the axis  25  because the ions have a finite component of velocity in the direction of the ion axis  11 ), and become separated in time according to their mass to charge ratios. The drift region  24  is a reflecting type analyzer and comprises an electrostatic ion mirror  26  which changes the direction of travel of the ions following trajectory  23  and directs them into an ion detector  27 . Use of the ion mirror  26  both decreases the size of the spectrometer and improves mass resolution but a linear analyzer could be used if desired. 
     Signal processing means  28  and  29  are connected to the collection electrodes in the detector  27  (described below) and their digital outputs are connected to a digital memory means  30 . A digital computer  31  controls the signal processing means  28  and  29  and also the pulse generator  22  which controls the generation of ion bunches. Computer  30  is programmed to cause the pulse generator  22  to repetitively generate bunches of ions and to record the data generated by the signal processing means  28  and  29  for each bunch in the digital memory means  30 , which typically comprises fast RAM. In the case of an analogue signal processing means, the digital output is recorded at a series of transit times relative to the time of generation of the ion bunch until all the ions of interest have entered the detector. 
     For maximum economy of memory usage a portion of memory is set aside for storing the digital output at each of the series of transit times for one ion bunch. The values of the digital output at transit times for subsequent ion bunches are then added to the previously stored values at corresponding transit times in order to produce an averaged value at each transit time taken over the whole series of ion bunches. In the case of pulse-counting signal processing means, computer  30  is programmed to store the time at which an ion bunch is generated and the times at which ion arrivals at the detector triggers the signal processing means, which typically occurs only once for each ion bunch. This is more efficient than storing a boolean value representing the output of a pulse-counting system at each of the transit times at which the output of an analogue signal processing means has to be sampled, but the latter method is within the scope of the invention. 
     During this data acquisition phase the computer  31  merely causes data to be stored in the memory means  30 , and does not need to examine that data in any way. Similarly, no additional hardware responsive to the output of the signal processing means is required for the proper operation of the detector  27 . 
     Once the data from a desired number of ion bunches has been stored in the memory means, the digital computer  31  may access this data and copy it to a disk for subsequent processing, or may carry out that processing in real time, thereby freeing the digital memory  30  to receive data from the next series of ion bunches. During the subsequent processing the computer  31  determines the quantity of ions which entered the detector at each transit time while the ion bunches were generated using the data associated with the second collection electrode, except as provided for below. Computer  31  further applies tests to the data to establish whether data from the second collection electrode is valid, and if not, uses data from the first collection electrode alone. 
     When data from the first collection electrode is used it is multiplied by a factor equal to the ratio of the effective areas (defined above) of the collection electrodes to make it compatible with that from the second collection electrode. 
     Unfortunately, a decision on whether data from the second collection electrode is reliable at any given transit time cannot be made directly on the basis of the observed ion arrival rate at that electrode because the observed rate may be affected by deadtime. For example, the observed rate may fall to zero in the case of an extending deadtime detector subject to a high ion arrival rate. Instead, data from the first collection electrode (which has been stored in the memory means) at the relevant transit times may be used to predict the ion arrival rate at the second electrode, and hence whether data associated with the second electrode is likely to be unreliable. Alternatively, data associated with the second collection electrode may be corrected step-by-step for the effects of deadtime, starting at the beginning of a peak. The magnitude of the correction so generated may then indicate that the ion arrival rate at the electrode later in the peak would be so great that adequate correction would be impossible, in which case data from the first collection electrode alone should be used to characterise the entire peak. 
     Referring next to FIG. 2, an embodiment of an ion detector suitable for use in the invention comprises a pair of microchannel plate electron multipliers  42 ,  32  disposed to receive ions directed towards the detector  27  by the ion mirror  26  (FIG.  1 ). The ion flux is schematically illustrated in FIG. 2 by the arrows  33 . Each ion strikes the front surface of the multiplier plate  42  causing the release of a burst of electrons at its rear surface corresponding to the ion impact. These electrons are received by the front face of the second multiplier plate  32  so that a larger burst of electrons is generated at its rear face. These impact on a collection electrode array  34  and cause signals to be generated by the signal processing means  28 ,  29  which are connected to the electrodes in the array  34 . A power supply  35  maintains a potential difference of approximately 2 kV between the faces of the multiplier plates  42  and  32  as required for their proper operation. A collection electrode array  34  suitable for use in the detector illustrated in FIG. 2 is shown in FIG.  3 . It comprises an insulated substrate  37 , typically of ceramic, on which are coated three electrically conductive electrodes  36 ,  38 , and  39 . Two of these electrodes,  36  and  38 , are connected by the lead  41  and function as a single electrode of area approximately 8 times that of the smaller electrode  39 . Lead  41  also connects the larger (second) composite electrode  36 ,  38  to a signal processing means  28 , and the lead  40  connects the smaller (first) electrode to a signal processing means  29 . This arrangement of electrodes compensates for an inhomogeneous distribution of ion flux represented by the arrows  33 , at least along an axis parallel to the long dimension of the electrode  39 , but other arrangements of electrodes are within the scope of the invention. 
     An alternative embodiment of an ion detector  27  suitable for use with the invention is shown in FIG.  4 . It comprises first and second electron multiplying means  43 ,  44 , each of which comprises a microchannel plate electron multiplier. The channelplates are spaced apart by a series of insulators  45  which also supports a first collection electrode  46 . Electrode  46  comprises a grid having a transparency of about 50% so that it collects approximately half of the electrons leaving the first multiplying means  43  and transmits the remainder to the second electron multiplying means  44 . A second collection electrode  47  is disposed to receive electrons leaving the second electron multiplying means  44 . Power supplies  48  and  49  maintain a potential difference of about 1 kV across each of the channelplates. A third power supply  50  maintains a potential difference of about 200 volts between the rear face of channelplate  43  and the front face of channelplate  44  to ensure electrons are efficiently transmitted between the two. 
     As in the FIG. 2 embodiment, signal processing means  28  and  29  are connected to the first and second collection electrodes  46  and  47  respectively. In a detector of this type, low-gain signals and high-gain signals are available at the collection electrodes  46  and  47  respectively. These signals correspond to the signals at the small and large area collection electrodes  39  and  36 ,  38  of the detector shown in FIG.  2 . 
     A disadvantage of the ion detector shown in FIG. 4 is that the effective area of the grid electrode is strongly dependent on the threshold setting of the discriminator  28 . For the grid electrode the amplitude of the current pulses produced extends over a greater range than those produced by the plate-like electrode  47 , presumably because electrons passing close to the wires comprising the grid but not actually striking a wire induce a signal in the electrode which is smaller than the minimum signal which would be produced by impact of those electrons on a solid electrode. This effect becomes more pronounced as the number of wires comprised in the grid is increased. While it has the effect of allowing the effective area of the grid to be varied by adjusting the threshold of the discriminator  28 , it is more difficult to maintain the ratio of the effective areas of the grid electrode  46  and the plate electrode  47  at a constant value. Consequently, in a more preferred (unillustrated) embodiment of the ion detector the grid electrode  46  (FIG. 4) may be replaced by a single wire stretched across the electrode  47  between the two insulators  45 . Typically a wire 0.5 mm diameter can be used. The range of pulse amplitudes produced by such an electrode is smaller than that produced by a grid electrode but still greater than that produced by the plate electrode, which provides adequate stability of the ratio of the effective areas while allowing some adjustment of that ratio by alteration of the threshold level of the discriminator  28 . Because of this “induction” effect the effective area of the wire is considerably greater than its actual area.