Abstract:
A mass spectrometer comprises an ion source which produces an ion beam from a substance to be analysed and a detector to detect a quantity of ions incident thereon. The detector includes two elements ( 16, 18 ) each of which detect a part of the quantity of ions and an attenuation device attenuates the quantity of ions reaching one of the detector elements. At least one of the detector elements ( 16, 18 ) is connected to a time to digital converter (TDC) to allow counting of the ions and at least one of the detector elements is connected in parallel to both a time to digital converter (TDC) and an analogue to digital converter (ADC).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/070,118 filed Aug. 14, 2002 U.S. Pat. No. 6,864,479, entitled “High Dynamic Range Mass Spectrometer,” which is a national stage application under 35 U.S.C. §371 of PCT Application No. PCT/GB00/03332, filed Aug. 31, 2000, entitled “High Dynamic Range Mass Spectrometer,” which claims the priority benefit of United Kingdom Patent Application No. 9920711.0, filed Mar. 9, 1999, which applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a high dynamic range mass spectrometer, preferably although not exclusively of the time of flight kind. 
     2. Description of the Prior Art 
     Time of flight (TOF) mass spectrometers are often used for quantitative analysis of substances. In these applications of a TOF mass spectrometer, it will be necessary to be able to accurately determine the concentration of a substance based upon a detected ion signal. In a TOF mass spectrometer, the ion signals which are to be detected are usually fast transients and can be measured by analogue to digital conversion using a transient recorder or by ion counting as a function of time using a time to digital converter (TDC). Use of a TDC is generally preferred because it can be more difficult to obtain accurate quantitative results using a transient recorder. The use of ion counting is further preferred in an orthogonal acceleration TOF because the signals to be measured tend to be small and the ion count rates are low. Ion counting using a TDC involves the TDC detection the presence of a signal at the detector in excess of predetermined threshold. If the signal detected is in excess of a predetermined threshold then this is deemed to be indicative of the presence of an ion at the detector and the TDC, after detection of the above threshold signal, increments a counter to count the ions. 
     However, a problem arises with a time to digital converter when this is used to count ions in intense ion beams because most TDC&#39;s can only detect on event in a finite small time window. This means that where a TDC is used, it is not normally possible to distinguish between a single ion being detected and a multiplicity of ions being detected at the same time. This arises because a TDC cannot distinguish between different magnitudes of signal, only whether the detected signal exceeds the predetermined threshold. Accordingly, a counter connected to the TDC will only be incremented once upon detection of an above threshold signal regardless of its magnitude and therefore in the case of intense ion beams an accurate quantitative measurement cannot be made. This means that mass spectrometers incorporating such ion counters usually require there to be less than or equal to one ion per signal pulse of any substance to measured. It also means that for a single TDC there will be a relatively low dynamic range. 
     Attempts have been made to provide a mass spectrometer which uses one or more TDC&#39;s to count ions and in which the dynamic range can be extended for better quantitative measurements. 
     Thus for example, U.S. Pat. No. 5,777,326 discloses a TOF mass spectrometer in which the incoming ion beam is spread so as to be capable of being detected by three or more detectors. The signal at each detector is detected by a respective TDC and the signal from each TDC is subsequently added together. However, the problem with this type of arrangement is that simply spreading the beam over a number of detectors does not affect the intensity of the beam to a sufficient extent to significantly enhance dynamic range without a very large number of TDC&#39;s. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alternative form of mass spectrometer in which ion counting can be used to cover a wide dynamic range using a small number of TDC&#39;s. 
     Thus and in accordance with the present invention therefore there is provided a mass spectrometer comprising an ion source to produce ions from a substance to be detected and detector means to detect a quantity of ions incident on said detections means wherein the said detection means includes at least two detector elements, each of which elements detect at least a part of said quantity of ions from the ion source and attenuation means which acts to attenuate the quantity of ions reaching at least one said detection element. 
     With this arrangement it is possible to measure the quantity of ions with and without attenuation which means that both single and multiple ion detections can be quantified more accurately and a high dynamic range for the mass spectrometer can be achieved. This is achieved by parallel acquisition or interleaved acquisition of signal from ion beams with significant attenuation at one detector element and almost no attenuation at another. 
     Preferably each detector element comprises a separate plate anode. Each detector element may be connected via an amplifier to a time to digital converter (TDC) to allow counting of detected ions. 
     Although the discussion has been in terms of using TDC acquisition it will be appreciated that the same principle of attenuation of signal to other detector elements could also be applied to extension of dynamic range using analogue-to-digital conversion (ADC) or combinations of TDC and ADC. 
     The detector elements may be disposed one behind the other relative to the ion source or alternatively may be disposed one above the other in a plane extending generally perpendicular to the direction of ion travel. In the case where the detector element is disposed one behind the other, an earthed member preferably a wire or grid may be provided between the elements to minimize capacitative coupling between these elements. 
     The attenuation means may be performed by at least one of the detector elements and in this case the at least one detector element is adapted to allow a proportion of incident signal to pass through the element without being detected. The adaptation may comprise a plurality of perforations or other apertures in the element. Alternatively a separate attenuation device may be provided between the ion source and the detector elements which acts to reduce the number of ions reaching at least one of said elements or at least a part thereof. In these circumstances the attenuation device may comprise a perforated plate. 
     Preferably, in the case where the attenuation means is formed by a perforation of the detector element, the cross-sectional area of the perforations compared to the total cross-sectional area of the plate is approximately 1 to 100. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will now be described further by way of example and with reference to the accompany drawings of which: 
         FIG. 1  shows a schematic version of a prior art form of mass spectrometer; 
         FIG. 2  shows a schematic version of one embodiment of mass spectrometer in accordance with the present invention; 
         FIG. 3  shows a variation on the embodiment show in  FIG. 2 ; 
         FIG. 4  shows a schematic version of a second embodiment of mass spectrometer in accordance with the present invention; 
         FIG. 5  shows a schematic version of a third embodiment of mass spectrometer in accordance with the present invention; 
         FIG. 6  shows a schematic version of a fourth embodiment of mass spectrometer in accordance with the present invention; and 
         FIG. 7  shows a schematic version of a fifth embodiment of mass spectrometer in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the drawings, there is shown in  FIG. 1  a schematic representation of one standard form of prior art mass spectrometer detector. The spectrometer  10  comprises an ion source (not shown) which produces an ion beam from a substance to be analyzed. The ion beam is directed by conventional means onto a pair of microchannel plates  11 , 12  (hereinafter referred to as a chevron pair) which generates secondary electrons due to the collision of the ions in the ion beam with the material of the plates  11 , 12  in the microchannels. Secondary electrons generated are detected by a single plate anode  13 , the detected signal is amplified in an amplifier  14  and is passed to a time to digital converter (TDC) (not shown) which detects detected signals over a predetermined threshold and increments a counter to count these above threshold signals. 
     This form of mass spectrometer suffers from the problem that if an above threshold signal is detected by the TDC, the counter will be incremented only once regardless of the magnitude of the signal in exceeding the threshold. Thus even if the signal is of such a magnitude as to constitute more than one ion being detected, the counter will still only be incremented once. The TDC cannot distinguish between different magnitude above threshold signals. This means that the mass spectrometer is very inaccurate when used for quantitative measurements of intense signals. 
     One form of mass spectrometer in accordance with the present invention is shown in schematic form in  FIG. 2 . In this arrangement, the ion beam generated by the ion source (not shown) is also incident on a chevron pair  11 , 12  as with the embodiment of  FIG. 1 . The ion beam strikes the microchannel plate  11  and causes the ejection of secondary electrons from the surface of the microchannels. The secondary electrons cause the ejection of further secondary electrons as they accelerate through the microchannels in the plates  11 ,  12  which results in an electron beam which emerges from the chevron pair  11 , 12  being essentially an amplified signal version of the incoming ion beam. The secondary electron beam then strikes a first anode  16  for detection. The first anode  16  is perforated in order that some of the secondary electrons pass through the first anode  16  without being detected. The remainder of the secondary electrons strike the first anode  16  and are detected. For detection purposes, the first anode  16  is connected to an amplifier  14  and to a time to digital converter (not shown) the output of which increments a counter (not shown) as previously explained. Those secondary electrons which pass through the perforations  17  in the first anode  16  strike a second anode  18  placed substantially immediately behind the first anode  16  and are detected. The secondary anode is connected to a second amplifier and a second time to digital converter, the output of which increment a counter in the same manner as mentioned above. 
     It will be appreciated that the ratio of the cross-sectional area of the perforations to the total cross-sectional area of the anode can be chosen to give a particular degree of attenuation to the incoming secondary electron beam. 
     Thus, in use, the ion beam is directed onto the chevron pair  11 , 12 . This results in the generation of secondary electrons in the manner mentioned above. These secondary electrons emerge from the chevron pair  11 , 12  and are incident of the first anode  16 . It is thought that by arranging for the cross-sectional area of the perforations in the first anode to be of the order of 1% of the total cross-sectional area of the anode will give the possibility for more accurate quantitative measurements over a large dynamic range, however, it is to be appreciated that the ratio of the cross-sectional area of the perforations to the total area of the anode can be of any desired magnitude in order to give appropriate attenuation characteristics. 
     Therefore, if the area of the perforations represents approximately 1% of the total area of the anode, this means that 1% of the secondary electron beam which is incident on the first anode  16  will pass through that anode without being detected. This means that the intensity of any signal present at the first anode would be reduced by two orders of magnitude if measured at the second anode  18 . Therefore it would be appreciated that with this arrangement, that if for example the first anode  16  can be used to detected signals of a first two orders of magnitude then the second anode, at which the signal has been reduced in intensity by a factor of 100, can be used to detect signals at a second two orders of magnitude. It will be appreciated that this allows much more accurate quantitative analysis of the incoming ion beam since signals which are above threshold will be differentiated according to their magnitude and accordingly if a signal is of such a magnitude as to constitute more than one ion arriving, the present arrangement will detect this and the counters will be incremented by the respective TDC&#39;s by the correct number of ions. It can clearly be seen that this will result in a significant increase in the dynamic range of the mass spectrometer. 
       FIG. 3  shows a variation on the embodiment of  FIG. 2  in which an earthed grid  19  is positioned between the first and second anode  16  and  18 . The earthed grid  19  assists in the minimization of capacitative coupling effects between the two anodes  16  and  18 . 
     Whilst in the embodiments of  FIGS. 2 and 3 , attenuation of the secondary electron signal is carried out by the perforated first anode  16 , attenuation can be carried out in many different ways. 
     Thus for example, as shown in  FIG. 4 , the attenuation can be carried out by wires or a grid placed in front of the first anode  16  to form the second anode  18 . The cross-sectional area of the wire or grid compared to the cross-sectional area of the first plate anode is small such that a large proportion of the incident signal from the chevron pair  11 , 12  passes through the second anode  18  without being detected. As with the other embodiments, the attenuation can be varied by changing the cross-sectional area of the wire or grid to achieve a desired dynamic range. Furthermore, as with the other embodiments, an earthed grid  19  can be placed between the two anodes to minimize capacitative coupling of these anodes. 
     A further alternative is shown in  FIG. 5 . In this embodiment, the first anode  16 , a second anode  18  and, optionally an earthed grid  19 , are constructed as sandwich layers of a printed circuit board  21 . The first anode  16  is formed as a perforated plate attached to a first support layer  22  which I also perforated, the perforations in the first support layer  22  being in register with the perforations in the first anode  16 . Attached to the opposite side of the first support layer  22  is an earthed grid, perforations in the grid also being in register with the perforations in the first support layer  22  and the first anode  16 . Attached to the opposite side of the earthed grid  19  is a second support layer  23  which carries a second anode  18  attached thereto. Fingers  24  of the second anode  18  extend through the second support layer  23  and terminate adjacent to the perforations in the earthed grid  19 . 
     In this embodiment, the attenuation is carried out by the first anode  16  and only a proportion of the secondary electrons reach the fingers  24  of the second anode  18  through the aligned apertures. As in the previous embodiments, the earthed grid  19  minimizes capacitative coupling between the two anodes. 
     A still further alternative is shown in  FIG. 6  in which a separate attenuation element  26  of appropriate form is placed in the ion beam before the ion beam is incident on the chevron pair  11 , 12 . The attenuation element in this embodiment, comprises a perforated plate, and is arranged so as to interfere only with a part of the incoming ion beam and reduces the proportion of that part of the beam which reaches the chevron pair  11 , 12 . In this embodiment, the first anode  16  and the second anode  18  are also provided but they are provided in the same plane extending generally parallel to the longitudinal axis of the chevron pair  11 , 12  as spaced therefrom. Thus the attenuation element attenuates only a part of the incoming ion beam which, after passing through the chevron pair  11 , 12  and generating secondary electrons, is incident on the second anode  18 . The unattenuated part of the incoming ion beam after passing through the chevron pair  11 , 12  is incident on the first anode  16 . Therefore it will be appreciated that the same effect is achieved with the embodiment as is achieved in the other embodiments. 
     It will of course be appreciated that the overall attenuation required may also be achieved by a combination of attenuation of the incident ion beam reaching an area of the microchannel plates detector and attenuation of the secondary electron signal, for example  FIG. 7 . 
     It will further be appreciated that attenuation can be achieved by a combination of restricting the proportion of ion beam reaching a part of the chevron pair  11 , 12  (as in the embodiment of  FIG. 6 ) with a restriction on the secondary electron signal emerging from the chevron pair (as in the embodiment of  FIG. 4 ). An example of an embodiment of this type is shown in  FIG. 7 . In this embodiment, the incident ion beam is attenuated by a perforated member placed before the chevron pair  11 , 12 . Also the secondary electron signal emerging from the Chevron pair  11 , 12  is attenuated by placing a relatively small second anode in front of an relatively large first anode. 
     It will be appreciated that it is the attenuation of the incoming ion beam or the secondary electrons ejected from the chevron pair  11 , 12  which allows the TDC elements to more accurately count incoming ions over a large dynamic range. The use of attenuation means that it is possible to discriminate between different magnitude above threshold signals giving rise to a more accurate quantitative analysis of the incoming ion beam and also giving rise to an extension to the dynamic range of the mass spectrometer. 
     It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only.