Abstract:
A relay control for an ion pump used in a FTICR MS. When it is desired to admit new sample gas into the vacuum system, the high voltage that powers the ion pump is removed from the pump electrode by opening the normally closed relay contact. A normally open contact relay contact is then closed to ground the pump electrode and discharge any stray capacity. The sample valve is opened momentarily to admit new sample gas and after the ions are formed and trapped in the ion cell in the vacuum system the pump is restarted.

Description:
FIELD OF THE INVENTION 
     This invention relates to a mass spectrometer (MS) which uses the Fourier transform ion cyclotron resonance (FTICR) technique to determine the mass of ions and more particularly to the control of the ion pump used in the vacuum system of the MS. 
     DESCRIPTION OF THE PRIOR ART 
     When a gas phase ion at low pressure is subjected to a uniform static magnetic field, the resulting behavior of the ion is determined by the magnitude and orientation of the ion velocity with respect to the magnetic field. If the ion is at rest, or if the ion has only a velocity parallel to the applied field, the ion experiences no interaction with the field. 
     If there is a component of the ion velocity that is perpendicular to the applied field, the ion will experience a force that is perpendicular to both the velocity component and the applied field. This force results in a circular ion trajectory that is referred to as ion cyclotron motion. In the absence of any other forces on the ion, the angular frequency of this motion is a simple function of the ion charge, the ion mass, and the magnetic field strength: 
     
       
         ω=qB/m  Eq. 1 
       
     
     where: 
     ω=angular frequency (radians/second) 
     q=ion charge (coulombs) 
     B=magnetic field strength (tesla) 
     m=ion mass (kilograms) 
     The FTICR MS exploits the fundamental relationship described in Equation 1 to determine the mass of ions by inducing large amplitude cyclotron motion and then determining the frequency of the motion. The first use of the Fourier transform in an ion cyclotron resonance mass spectrometer is described in U.S. Pat. No. 3,937,955 entitled “Fourier Transform Ion Cyclotron Resonance Spectroscopy Method And Apparatus” issued to M. B. Comisarow and A. G. Marshall on Feb. 10, 1976. 
     The ions to be analyzed are first introduced to the magnetic field with minimal perpendicular (radial) velocity and dispersion. The cyclotron motion induced by the magnetic field effects radial confinement of the ions; however, ion movement parallel to the axis of the field must be constrained by a pair of “trapping” electrodes. These electrodes typically consist of a pair of parallel-plates oriented perpendicular to the magnetic axis and disposed on opposite ends of the axial dimension of initial ion population. These trapping electrodes are maintained at a potential that is of the same sign as the charge of the ions and of sufficient magnitude to effect axial confinement of the ions between the electrode pair. 
     The trapped ions are then exposed to an electric field that is perpendicular to the magnetic field and oscillates at the cyclotron frequency of the ions to be analyzed. Such a field is typically created by applying appropriate differential potentials to a second pair of parallel-plate “excite” electrodes oriented parallel to the magnetic axis and disposed on opposing sides of the radial dimension of the initial ion population. 
     If ions of more than one mass are to be analyzed, the frequency of the oscillating field may be swept over an appropriate range, or be comprised of an appropriate mix of individual frequency components. When the frequency of the oscillating field matches the cyclotron frequency for a given ion mass, all of the ions of that mass will experience resonant acceleration by the electric field and the radius of their cyclotron motion will increase. 
     An important feature of this resonant acceleration is that the initial radial dispersion of the ions is essentially unchanged. The excited ions will remain grouped together on the circumference of the new cyclotron orbit, and to the extent that the dispersion is small relative to the new cyclotron radius, their motion will be mutually in phase or coherent. If the initial ion population consisted of ions of more than one mass, the acceleration process will result in a multiple isomass ion bundles, each orbiting at its respective cyclotron frequency. 
     The acceleration is continued until the radius of the cyclotron orbit brings the ions near enough to one or more detection electrodes to result in a detectable image charge being induced on the electrodes. Typically these “detect” electrodes will consist of a third pair of parallel-plate electrodes disposed on opposing sides of the radial dimension of the initial ion population and oriented perpendicular to both the excite and trap electrodes. Thus the three pairs of parallel-plate electrodes employed for ion trapping, excitation, and detection are mutually perpendicular and together form a closed box-like structure referred to as a trapped ion cell. FIG. 1 shows a simplified diagram for a trapped ion cell  12  having trap electrodes  12   a  and  12   b ; excite electrodes  12   c  and  12   d ; and detect electrodes  12   e  and  12   f.    
     As the coherent cyclotron motion within the cell causes each isomass bundle of ions to alternately approach and recede from a detection electrode  12   e ,  12   f , the image charge on the detection electrode correspondingly increases and decreases. If the detection electrodes  12   e ,  12   f  are made part of an external amplifier circuit (not shown), the alternating image charge will result in a sinusoidal current flow in the external circuit. The amplitude of the current is proportional to the total charge of the orbiting ion bundle and is thus indicative of the number of ions present. This current is amplified and digitized, and the frequency data is extracted by means of the Fourier transform. Finally, the resulting frequency spectrum is converted to a mass spectrum using the relationship in Equation 1. 
     Referring now to FIG. 2, there is shown a general implementation of a FTICR MS  10 . The FTICR MS  10  consists of seven major subsystems necessary to perform the analytical sequence described above. The trapped ion cell  12  is contained within a vacuum system  14  comprised of a chamber  14   a  evacuated by an appropriate pumping device  14   b . The chamber is situated within a magnet structure  16  that imposes a homogeneous static magnetic field over the dimension of the trapped ion cell  12 . While magnet structure  16  is shown in FIG. 2 as a permanent magnet, a superconducting magnet may also be used to provide the magnetic field. 
     Pumping device  14   b  may be an ion pump which is an integral part of the vacuum chamber  14   a . Such an ion pump then uses the same magnetic field from magnet structure  16  as is used by the trapped ion cell  12 . An advantage of using an integral ion pump for pumping device  14   b  is that the integral ion pump eliminates the need for vacuum flanges that add significantly to the volume of gas that must be pumped and to the weight and cost of the FTICR MS. One example of a mass spectrometer having an integral ion pump is described in U.S. Pat. No. 5,313,061. 
     The sample to be analyzed is admitted to the vacuum chamber  14   a  by a sample introduction system  18  that may, for example, consist of a leak valve or gas chromatograph column. The sample molecules are converted to charged species within the trapped ion cell  12  by means of an ionizer  20  which typically consists of a gated electron beam passing through the cell  12 , but may consist of a photon source or other means of ionization. Alternatively, the sample molecules may be created external to the vacuum chamber  14   a  by any one of many different techniques, and then injected along the magnetic field axis into the chamber  14   a  and trapped ion cell  12 . 
     The various electronic circuits necessary to effect the trapped ion cell events described above are contained within an electronics package  22  which is controlled by a computer based data system  24 . This data system  24  is also employed to perform reduction, manipulation, display, and communication of the acquired signal data. 
     When a new sample gas is introduced into an ion pumped vacuum system, the pumping action momentarily dislodges small amounts of the previously pumped sample. This introduces error into the measurement. A custom built ion pump designed to reduce this effect will provide improved performance. Such a pump is, however, expensive. An alternative and lower cost solution is provided by the present invention which quickly disables the ion pump prior to opening the valve that admits new sample gas into the vacuum system. Since the pump is disabled before the new sample gas is admitted into the vacuum system the previously pumped sample cannot be dislodged. The speed with which the pumping action of the ion pump is suspended allows the FTICR MS to have a relatively high sampling rate and essentially no errors. 
     SUMMARY OF THE INVENTION 
     The present invention is a mass spectrometer that includes an ion pump and a circuit for controlling said ion pump. The circuit includes a first switch connected between a source of electrical power and the ion pump. The first switch is closed when the ion pump is pumping. The circuit further includes a second switch connected between ground and the ion pump. The second switch is open when the ion pump is pumping. The circuit also includes means for controlling the opening and closing of the first and second switches that is responsive to a signal indicative that the ion pump is to be turned off. The means opens the first switch before the second switch is closed. 
     The present is also a mass spectrometer that includes an ion pump and a circuit for controlling the ion pump. The circuit includes a first switch connected between a source of electrical power and the ion pump. The first switch is closed when the ion pump is pumping. The circuit also includes a second switch connected between ground and the ion pump. The second switch is open when the ion pump is pumping. The circuit further includes a first device connected to the first switch and a second device connected to the second switch. The first and second devices are responsive to a signal indicative that the ion pump is to be turned off for opening the first switch before the second switch is closed. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a simplified diagram for a trapped ion cell. 
     FIG. 2 shows a block diagram of a typical FTICR MS. 
     FIG. 3 shows a functional block diagram for the ion pump relay control circuit of the present invention. 
     FIG. 4 shows a simplified schematic for the ion pump relay control circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 3, there is shown a functional block diagram for the ion pump relay control circuit  30  of the present invention. Circuit  30  includes a source  32  of high voltage, typically in the order of 6.5 KV, that powers the ion pump  14   b . The output of source  32  is connected through the series combination of a switch S 1 , in the form of a normally closed (NC) relay, and a resistor R 1  to junction  34 . The NC relay S 1  delivers the voltage of source  32  to the ion pump  14   b . Junction  34  is connected to ground through the series combination of a resistor R 2  and a switch S 2  in the form of a normally open (NO) relay. The ion pump  14   b  has stray capacitance Cs, which is shown in dashed lines in FIG.  3 . 
     With switch S 1  normally closed and switch S 2  normally open the ion pump  14   b  is energized and can continue to maintain the vacuum in the chamber  14 . When it is desired to have a new sample gas enter into the chamber  14 , the ion pump should be stopped and the stray capacitance Cs, should be discharged before the leak valve  18  is opened to admit the new sample gas. The ion pump relay control circuit  30  causes NC relay Si to break before NO relay S 2  makes when it is desired to have a new sample gas enter into chamber  14 . 
     The breaking of NC relay S 1  removes the voltage of source  32  from the ion pump  14   b  and the subsequent making of NO relay S 2  causes stray capacitance Cs to discharge. When the ion pump  14   b  is turned on, circuit  30  causes NO relay S 2 , which is closed when the pump is off, to open before NC relay S 1 , which is open when the pump is off, is closed. 
     Referring now to FIG. 4, there is shown a simplified schematic for circuit  30 . The input  30   a  of circuit  30  has a signal, labeled as Pump Off in FIG. 4, which is at zero volts when the pump  14   b  is pumping. The signal at input  30   a  is connected through an inverter  36  to junction  38 . Junction  38  is connected through a resistor R 3  to a positive voltage +V2 and to one electrode of a transistor Q 1  which is associated with NC relay S 1 . The signal at input  30   a  is connected through an inverter  40  to a junction  42 . Junction  42  is connected through a resistor R 4  to the positive voltage +V2 and to one electrode of a transistor Q 2  which is associated with NO relay S 2 . The transistors Q 1  and Q 2  each have another electrode connected to ground. A capacitor C 1  is connected between transistor Q 2  and junction  42  to thereby form a Miller integrator circuit. 
     NC relay S 1  is held closed by a permanent DC magnet  48 . The coil  44  of NC relay Si is connected between a positive voltage +V1 and the third electrode of transistor Q 1 . A diode D 1  is also connected across coil  44 . The coil  46  of NO relay S 2  is connected between the positive voltage +V1 and the third electrode of transistor Q 2 . The series combination of a diode D 2  and a Zener diode D 3  is connected across coil  46 . 
     NC relay S 1  is connected to source  32  and through resistor R 1  to junction  34  and ion pump  14   b . NO relay S 2  is connected to ground and through resistor R 2  to junction  34  and ion pump  14   b . The type for transistors Q 1  and Q 2  are both selected so that both transistors are off when pump  14   b  is pumping. This ensures that the ion pump  14   b  will continue to pump even if the signal at input  30   a  is lost as long as there is a source of power for the positive voltage, +V2, connected to the transistors Q 1  and Q 2  and the positive voltage +V1 connected to relay coils  44  and  46 . 
     When the ion pump  14   b  is to be stopped, the amplitude of the Pump Off signal at input  30   a  becomes negative. As was described in connection with FIG. 3, when the ion pump  14   b  is to be stopped, NC relay S 1  should be opened before NO relay S 2  is closed. Circuit  30  accomplishes this result by turning on transistor Q 1  faster than it turns on transistor Q 2  when the amplitude of the Pump Off signal becomes negative. Transistor Q 2  turns on slower than transistor Q 1  because of the Miller integrator circuit. Therefore, when ion pump  14   b  is to be stopped, circuit  30  first turns on transistor Q 1  to open the NC relay S 1  to thereby disconnect voltage source  32  from the ion pump and then turns on transistor Q 2  to close the NO relay S 2  to thereby ground the stray capacitance Cs of the ion pump. 
     When ion pump  14   b  is to be turned on again, the Pump Of f signal returns to zero volts. Transistors Q 1  and Q 2  are both turned off quickly. The NO relay, S 2  which was closed when the ion pump  14   b  is off, however, opens quicker than the NC relay S 1 , which was open when the ion pump is off, closes. This is because of Zener diode D 3  which is connected to the voltage +V1 connected to coil  46 . In one embodiment for circuit  30 , the voltage +V1 connected to coils  44  and  46  was +12 Volts. In that same one embodiment for circuit  30 , relays S 1  and S 2  were both reed relays to minimize arcing and the resistance of resistors R 1  and R 2  were 100 Kohms to limit the current in circuit  30  if both switches S 1  and S 2  are closed and also to protect the relay points from pitting. 
     It is to be understood that the description of the preferred embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.