Abstract:
A mass spectrometer, having an electron multiplier tube to serve as an ion detector, includes a voltage source for applying a suddenly rising voltage to the electron multiplier tube while no ions to be detected are being introduced and an inspecting apparatus for detecting the induced voltage generated in the electron multiplier tube when this suddenly rising voltage is applied. The condition of the wiring connections can be determined by analyzing the behavior of the measured induced voltage.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a mass spectrometer having an electron multiplier tube serving as an ion detector. 
     A mass spectrometer usually uses an electron multiplier tube as the ion detector. The electron multiplier tube usually has many metallic electrode plates arranged in a longitudinal array, each having a different voltage applied thereto. As a relatively small number of ions are introduced through an entrance slit and are made to collide with the first of these metallic electrode plates, a larger number of secondary electrons are emitted therefrom and these secondary electrons are then caused to collide with the next metallic electrode plate of the array such that a still larger number of electrons are emitted. This process is repeated many times so as to multiply electrons. 
     After such an electron multiplier tube has been used for a long period of time, the surface condition of its electrode plates changes significantly due to the collisions with ions and electrons and the tube ceases to multiply electrons with the same efficiency as when it was new. The surfaces of the electrode plates are also gradually contaminated by the impurity molecules which remain in the vacuum chamber. In other words, electron multiplier tubes must be regarded as articles of consumption and must be replaced every so often, after having been in use for some time. 
     There are many lines to be disconnected and connected again whenever an electron multiplier tube is replaced, such as high-voltage lines and grounding lined for applying DC bias voltages to the electrode plates, of which there are many, and a signal line through which detection signals are to be outputted. When a new electron multiplier tube is connected, various tests are carried out to look for errors in connections but it sometimes happens that the connections may appear proper at the time of the replacement but that the proper connection is lost by the time the tube is assembled and its operation is actually started. Situations of such an incomplete contact could not be adequately detected according to the prior art technology. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention, in view of this problem of prior art technology, to provide an improved mass spectrometer which, although of a simple structure, allows the testing of the connections between its electron multiplier tube and the surrounding elements under the same conditions of its actual use. 
     A mass spectrometer embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a voltage applying means for applying to its electron multiplier tube a suddenly rising voltage while no ions to be detected are being introduced into the electron multiplier tube and an inspecting means for detecting the induced voltage generated in the electron multiplier tube when this suddenly rising voltage is applied thereto and to determine the condition of the wiring connections on the basis of the measured induced voltage. 
     Since an electron multiplier tube is provided with a source of a high DC voltage (from one to several kilovolts) for applying various bias voltages to the plurality of its electrode plates, such a voltage source may be used as the aforementioned voltage applying means. The induced voltage may be measured from the detected output from the electron multiplier tube. In other words, no new elements are required to form a mass spectrometer of this invention from a prior art mass spectrometer or to make proper wire connections. 
    
    
     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 structural diagram of a mass spectrometer embodying this invention; 
     FIG. 2 is a schematic block diagram of the mass spectrometer of FIG. 1; and 
     FIG. 3 is a waveform diagram which may be obtained typically for testing wire connections of the mass spectrometer according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described next by way of an example. As shown in FIG. 1, a vacuum chamber  1  contains therein an ion source  2  for ionizing the sample molecules, say, by electron bombardment, ion lenses  3  for forming a convergent beam from the generated ions, a mass separator  4  for separating these ions according to their mass numbers and an ion detector  5  for detecting the separated ions. In this example, an electron multiplier tube is used as the ion detector  5 , and a quadrupole filter is shown as the mass separator  4  but a mass separator of a different type may be substituted. The ion source  2 , too, may be of a different type, say, a device employing a chemical ionization process. 
     The electron multiplier tube, which serves as the ion detector  5  in this example, is formed with a plurality of (six according to this example but may be a different plural number) curved electrode plates  10   a-   10   f,  a resistor array  11  for applying different voltages to these electrode plates  10   a-   10   f,  and an electron collector  12  for finally capturing the electrons. A high-voltage line  13  of the electron multiplier tube  5  is pulled out of the vacuum chamber  1  and connected to a high-voltage source  22  through a switch  21 . There is also a grounding line  14  which is similarly pulled out of the vacuum chamber  1  and is grounded. A signal line  15  connected to the collector  12  of the electron multiplier tube  5  is pulled out of the vacuum chamber  1  and is connected to a preamplifier  20 . The output from the preamplifier  20  is transmitted to a signal processor (not shown) for carrying out various signal processing at the time of carrying out measurements and also to a wiring inspection part  25  which comprises a comparator  26 , a time detector  27  and a judging unit  28 . 
     A control unit  23  for the mass spectrometer may be formed with a CPU and functions as follows when carrying out an automatic adjustment process on the voltages applied to the ion lenses  3  and the mass separator  4  with respect to the mass numbers. Firstly, the switch  21  is set in the ON condition while the ions are not being generated, that is, while no heat-generating current is being supplied to the filament (not shown) for generating thermal electrons. The voltage of the high-voltage source  22  may be set appropriately within the range of one to several kilovolts. 
     As the switch  21  is connected when the high-voltage line  13  and the grounding line  14  are connected properly, specified voltages are individually applied to the electrode plates  10   a-   10   f  of the electron multiplier tube  5 , as divided by the resistor array  11 . Since the electron collector  12  is disposed close to the last of the electrode plates ( 10   f ) (as symbolically indicated by capacitance C therebetween in FIG.  2 ), an induced voltage is generated on this collector  12  instantaneously as a voltage is applied to this last-stage electrode plate  10   f,  becoming lower exponentially with time thereafter. If the collector  12  and the preamplifier  20  are connected properly, a similarly varying voltage appears also in the output from the preamplifier  20 . 
     FIG. 3 shows a waveform diagram which may be obtained typically, indicating the changes in the output voltage V 0  from the preamplifier  20 . The comparator  26  of the wiring inspection part  25  serves to compare this output voltage V 0  with a specified threshold voltage V th  and to generate a signal having a rectangular waveform during the period in which the output voltage V 0  exceeds this threshold voltage V th . The time detector  27 , upon receiving this signal with the rectangular waveform, functions to measure the pulse width T of this received signal, and the judging unit  28  determines whether this pulse width T is greater than a specified critical value or not. If the pulse width T is found to be greater than this critical value, a detection signal indicating that the connection is normal is outputted to the control unit  23  which responds by causing a display made on the display device  24  that the connection is normal. 
     If there is any defective connection among the high-voltage line  13 , the grounding line  14  and the signal line  15 , the output from the preamplifier does not undergo any substantial changes and the time detector  27  of the wiring inspection part  25  will detect T=0 and the judging unit  28  will transmit another detection signal to the control unit  23 , indicative of a defective connection. Thereupon, the control unit  23  causes a warning display made on the display device  24 . 
     The invention has been described above by way of only one example, but this example is not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of this invention. For example, the inspection routine described above may be adapted to be carried out automatically while the mass spectrometer is not undergoing any analysis or in response to a specified start signal inputted by the user. The advantage of this invention is that any defective wiring connections to the electron multiplier tube can be detected from outside while the tube is under the same condition as when it will be being used for an actual analysis. Thus, the present invention can prevent the user from continuing the analysis when the wiring connections are less than perfect. Still another advantage of this invention is that an improved mass spectrometer with such capability can be obtained without any large additional components but only with a small addition of a few hardware and software.