Abstract:
A mass spectrometer generates an ion current, accelerates it and passes it through a mass filter to be received by a detector which outputs a signal according to the intensity of this ion current. A control unit serves not only to carry out mass spectrometry experiments on a specified reference sample under specified conditions but also to judge the level of degradation of the detector from both the intensity values of output signals from the detector during each of these experiments and the standard deviation of these measured intensity values.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a mass spectrometer and more particularly to a mass spectrometer provided with means for monitoring the level of degradation of its detector. The invention also relates to a method of monitoring the level of degradation of the detector of such a mass spectrometer. 
     In a mass spectrometer, a sample to be analyzed is initially ionized in an ionization chamber (herein referred to as the “ion source”). Ions of different kinds are normally generated in the ion source and are then accelerated by an ion lens to enter a mass filter comprising, for example, a quadrupole such that only the ions having a specified mass-to-charge ratio are allowed to pass through the filter and are detected by a detector. 
     Secondary electron multiplier tubes are the most commonly used type of detectors used with a mass spectrometer. A secondary electron multiplier tube is a detector adapted to output an electric signal with intensity according to the number of incident electrons by making use of a metal which emits a larger number of secondary electrons than the number of incident ions thereon with energy greater than a specified value. In general, members which are made of such a metal are arranged in a plurality of stages such that the number of secondary electrons will increase in a step-wise fashion and those secondary electrons emitted from the metal member of the last stage are taken out as the electric signal. A specified voltage difference is applied between each mutually adjacent pair of metal members when ions are being detected but the ion-electron multiplication factor (the ratio between the number of emitted electrons and that of the incident ions) will naturally change if this voltage is varied. 
     These metal members become degraded due to pollution by ions whenever a sample analysis is carried out, and this affects the ion-electron multiplication factor. One of the prior art methods for checking the level of degradation of a secondary electron multiplication tube has been to introduce a specified amount of a reference sample is introduced into the mass spectrometer while the applied voltage (to the metal members) is set at a specified level and to detect the ions generated from this reference sample as described above. By comparing the detected intensity of the output signal from the secondary electron multiplier tube with its initial value when the tube was still new, one can determine the condition of the degradation. 
     This method of judging the condition of degradation is not truly trustworthy. For example, although the condition of degradation is the same, the detected intensity of the output signal from the secondary electron multiplication tube will be lower if a different component of the mass spectrometer such as its ion source is degraded because this will be adversely affecting the efficiency of generating ions. In other words, one cannot determine with a method as described above if a drop in the intensity of the output signal is due to the degradation of the secondary electron multiplier tube itself or that of some other component. As a result, one may end up carrying out a wasteful maintenance work although the cause of the drop in the signal intensity is elsewhere by mistakenly believing that the cause was in the detector. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention in view of the problem as described above to provide a mass spectrometer having means for monitoring the condition or the level of degradation of its detector comprising a secondary electron multiplier tube. 
     It is another object of this invention to provide a method of monitoring the level of degradation of a detector such as a secondary electron multiplier of a mass spectrometer. 
     A mass spectrometer embodying this invention, with which the above and other objects can be accomplished, may be characterized, not only as generating an ion current, accelerating it and passing it through a mass filter into a detector adapted to output a signal according to the intensity of this ion current, but also as comprising a control unit which serves to carry out mass spectrometry experiments on a specified reference sample under specified conditions and to judge the level of degradation of the detector from both the intensity values (for example, the numbers of ions detected per unit time by the detector) of output signals from the detector during each of these experiments and the standard deviation of these measured intensity values. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a schematic block diagram of a mass spectrometer embodying this invention; and 
     FIG. 2 is a graph for showing the relationship between output signal intensity and detection frequency. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While an ion current of a constant intensity is being received by a detector, its detector will serve to output a signal depending upon this intensity of the ion current but the intensity of this output signal is not completely constant but includes some fluctuations. If the output signal of the detector is sampled many times while it is receiving an ion current of a constant intensity, the distribution curve, representing the relationship between the intensity of output signal (I) and the frequency of signal detection at each intensity level (ρ) will be as shown in FIG. 2, peaking at a certain central intensity value. It is to be reminded, however, that the curve in FIG. 2 is drawn in a schematic fashion with its spread drawn in an exaggerated fashion for the convenience of description. 
     Let us now consider a situation where the multiplication factor of the detector has dropped because of the degradation of the detector itself. In such a situation, the fluctuation in the intensity of the output signal will drop also at the same rate as the drop in the intensity of the output signal. Thus, the ratio between the fluctuation and the intensity will hardly change in spite of any change in the multiplication factor. If the drop in the intensity of the output signal is due to some other cause such as the drop in the intensity of the ion current occasioned by the degradation of the ion source, by contrast, the fluctuation in the output signal will drop at a rate which is different from that of the drop in the intensity, the fluctuation generally dropping at a slower rate than the intensity of the output signal. In summary, the ratio between the fluctuation and the intensity of the output signal will change if the output signal drops by some other cause. The present invention is based on this observation. 
     The invention is described next by way of an example with reference to FIG. 1 which shows a mass spectrometer  10  of this invention as having an ion source  11 , an ion lens  12 , a mass filter (a quadrupole)  13  and a secondary electron multiplier tube serving as a detector  14  enclosed inside a vacuum container  15 . A device  16  for introducing a reference sample is disposed outside this vacuum container  15  and is connected to the ion source  11  through a tube  18  with a valve  17  therein. The ion source  11 , the ion lens  12 , the quadrupole  13  and the detector  14  are all connected to a control unit  20  to which is also connected a memory device (such as a hard disc drive)  21 . The control unit  20  and the memory device  21  may be formed by installing a specified program and a device driver in a commonly used personal computer. Preliminarily set in the memory device  21  are condition setting data on the conditions of measurement for the purpose of adjustment, such as the ionization voltage for the ion source  11 , the accelerating voltage for the ion lens  12 , the voltage to be applied to the detector  14  and the direct-current voltage to be applied to the quadrupole  13 , a high-frequency voltage and its frequency. In the example being described here, the direct-current voltage to be applied to the quadrupole  13 , the high-frequency voltage and its frequency are preliminarily set such that only ions with a specified mass-to-charge ratio can pass through the quadrupole  13 . 
     The level of degradation of the detector  14  is checked as follows. First, the user places a reference sample inside the aforementioned reference sample introducing device  16  and operates an input device (not shown) such as the keyboard of a personal computer to transmit to the control unit  20  a command to start the adjustment. Upon receiving this command, the control unit  20  reads out the aforementioned adjustment data stored in the memory device  21  and controls the operations of the ion source  11 , the ion lens  12 , the quadrupole  13  and the detector  14  on the basis of these data. If the valve  17  is opened thereafter, the reference sample inside the device  16  begins to flow through the tube  18  into the ion source  11 . After a time period which is sufficiently long for stabilizing the flow rate of the reference sample into the ion source  11 , the control unit  20  samples the output signal continuously for a specified number of times. If each sampling time is 100 μs and the specified number of times is 100, the control unit  20  will be required to monitor the output signals from the detector  14  for a period of 10 seconds, measuring the intensity of the output signal from the detector  14  at the rate of once every sampling time. The intensity data thus obtained are sequentially stored either in another storing means not shown in FIG. 1 or the memory device  21  shown in FIG.  1 . 
     After the specified number of sampling has been completed, the control unit  20  reads out the stored intensity data and obtains therefrom the average intensity and the standard deviation, as well as their ratio (hereinafter referred to as the “deviation-to-average ratio”). These numerical data are also stored in the memory device  21  so as to serve as the data for determining the detector degradation. Collection of such data for determination of detector degradation is preferably carried at a specified frequency such as once for every analysis, once every day or once per week. 
     When the mass spectrometer  10  is adjusted for the second time, or at the time of any subsequent adjustment, the data from the previous adjustment are already stored in the memory device  21 . Thus, the control unit  20  can retrieve data stored earlier and to thereby determine the current level of degradation of the detector  14  by comparing the current data with such earlier data. 
     The method of determining the level of detector degradation is explained next by way of examples with reference to Tables 1 and 2 which show results of data obtained as described above from two mass spectrometers A and B, respectively, each adjusted in four experiments. Table 1 for mass spectrometer A shows that the average intensity of the detector decreases with time but hardly any change is observed in the deviation-to-average ratio. Thus, it may be concluded that the lowering in the intensity of the output signals from the detector of mass spectrometer A is due to the degradation of the detector itself. Table 2 shows, on the other hand, that the deviation-to-average ratio increases as the average intensity drops. This indicates that the lowering of the intensity of the output signals is due not only to the degradation of the detector itself but also to some other factors. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Average 
                 Standard 
                 Deviation-to- 
               
               
                   
                 Adjustment 
                 Intensity 
                 Deviation 
                 Intensity Ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 First 
                 800,000 
                 800 
                 0.001 
               
               
                   
                 Second 
                 700,000 
                 700 
                 0.001 
               
               
                   
                 Third 
                 400,000 
                 420 
                 0.00105 
               
               
                   
                 Fourth 
                 100,000 
                 110 
                 0.0011 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Average 
                 Standard 
                 Deviation-to- 
               
               
                   
                 Adjustment 
                 Intensity 
                 Devaition 
                 Intensity Ratio 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 First 
                 800,000 
                 800 
                 0.001 
               
               
                   
                 Second 
                 700,000 
                 800 
                 0.00114 
               
               
                   
                 Third 
                 400,000 
                 800 
                 0.002 
               
               
                   
                 Fourth 
                 100,000 
                 400 
                 0.004 
               
               
                   
                   
               
             
          
         
       
     
     The example described above is not intended to limit the scope of the invention. Many modifications and variations are available within the scope of this invention. For example, a standard value of the deviation-to-intensity ratio may be preliminarily specified in order to automatically identify an abnormal change in the ratio. A display device such as a display screen of a personal computer may be used in such an application, although not shown in FIG. 1, such that the control unit  20  may serve to cause a warning message displayed thereupon when the calculated ratio exceeds the preliminarily specified reference ratio value. In the example of Tables 1 and 2, if the reference ratio value is set to 0.002, mass spectrometer B will be outputting such a warning message at the fourth adjustment.