Abstract:
An electronic circuit and a method of generating an electrical discharge for an ionization detector system. The electronic circuit includes a transformer with a primary portion and a secondary portion. The circuit and method produce an electrical discharge across a set of electrodes. The discharge is stable over time and has relatively low peak currents associated therewith.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to devices that may be used to generate and control electrical discharges in ionization sources of analytical devices.  
           [0003]    2. Description of the Related Art  
           [0004]    Gas chromatography devices can separate a gas mixture into the mixture&#39;s component gases and, after the separation, can quantify each component gas. A detector  10  used for analyzing a component gas is illustrated in FIG. 1A. The type of detector  10  illustrated in FIG. 1A is a discharge ionization detector that has previously been disclosed in U.S. Pat. No. 4,975,648 to Lawson et. al., the contents of which are incorporated herein by reference.  
           [0005]    The detector  10  illustrated in FIG. 1A includes a housing  20  that has, formed within it, a discharge chamber  30 , and ionization chamber  40 , and an aperture  50  that connects the discharge chamber  30  and the ionization chamber  40 . Also formed within the housing  20  are a surrounding gas inlet  60  that leads to the discharge chamber  30 , a sample inlet  70  that leads to the ionization chamber  40  and a sample outlet  80  that also leads to the ionization chamber  40 .  
           [0006]    Within the discharge chamber  30  are a pair of spark-generating electrodes  90 . One of the spark electrodes  90  has a small ball at the end thereof, while the other spark electrode  90  has a sharpened tip. Each of the spark electrodes  90  is connected to a separate pin  100  that supports the electrode  90  attached to it at a spatial location within the discharge chamber  30 .  
           [0007]    Each of the pins  100  is contained within a separate sheath  110  that protrudes from both sides of a sealing flange  120 . The sealing flange  120  can be screwed into or otherwise fixed to one end of the housing  20 .  
           [0008]    Outside of the housing  20  and wrapped around each sheath  110  is a separate insulating plug  130 . Each plug  130  leads to a separate wire  140  and each of the wires  140  is electrically connected to the same electronic circuit  150 .  
           [0009]    The electronic circuit  150  provides electrical current to each of the spark electrodes  90  during operation of the detector  10 . The timing, duration and intensity of the sparks created between the electrodes  90  is controlled by the electronic circuit  150 .  
           [0010]    A collector electrode  160  and an emitter electrode  170  are position within the ionization chamber  40  of the detector  10  and are held in place via a bottom flange  180  that is fitted into the housing  20 . A pair of wires  190  connect to the collector electrode  160  and the emitter electrode  170 , respectively, and lead to a pair of electrical couplings  200 . The wires  190  provide current to the collector electrode  160  and emitter electrode  170  when the detector  10  is in operation.  
           [0011]    During operation, a surrounding or carrier gas, such as helium, is allowed to flow into the discharge chamber  30  through the surrounding gas inlet  60 . The spark electrodes  90  are then provided with current from the electronic circuit  150  and are placed in close enough proximity to generate an electrical arc or spark across the electrodes  90 . The electrical spark causes the surrounding gas to discharge photons and metastables at a characteristic energy level.  
           [0012]    The photons and metastables then travel through the aperture  50  of the housing  20  and into the ionization chamber  40  that is filled with a gas that has been separated by the gas chromatography apparatus and that has been flowing into the ionization chamber  40  through the sample inlet  70 . The photons and metastables then mix with and interact with the separated sample gas, cause electrons to be generated in the ionization chamber  40 , cause a current to form between the collector electrode  160  and the emitter electrode  170 , and allow for the concentration of the separated gas to be determined.  
           [0013]    In order for the detector  10  to operate properly, the electrical discharges between the spark electrodes  90  are preferably chosen to be very stable. Instability in the discharges can cause serious deterioration of the analytical measurements being performed in the detector  10 . Such deteriorations can include shifts or oscillations in the analytical measurement. Hence, the detector  10  shown in FIG. 1A is generally attached to an electronic circuit  150  that attempts to drive the discharge while enhancing the stability of the discharge.  
           [0014]    [0014]FIG. 1B illustrates an electronic circuit  150  according to the related art that contains a resistor R, a first electrode  240 , a second electrode  250 , and a high voltage direct current (DC) power source  400 . However, the DC discharges driven by the circuit  150  illustrated in FIG. 1B are unstable due to uncontrolled wandering of the space charge present in the discharge area over time.  
           [0015]    In order to enhance the stability of the discharges compared to the circuit  150  illustrated in FIG. 1B, related art circuits  150  such as the one illustrated in FIG. 1C have been employed and have been disclosed in U.S. Pat. No. 5,153,519 to Wentworth et. al., the contents of which are incorporated herein by reference. The circuit  150  illustrated in FIG. 1C includes a resistor R, a first electrode  240  and a second electrode  250 . According to such related art circuits  150 , short, periodic, DC pulses  410  are used to produce discharges across the spark electrodes  90 . However, the DC pulses generated by the circuit  150  illustrated in FIG. 1C results in discharge peak currents that are far greater than the average current.  
           [0016]    Larger peak currents can cause deterioration and damage of the surface of the cathode spark electrode  90 , particularly when noble gases with larger atomic masses are employed as the surrounding gas. Hence, the high peak currents generated by the circuit  150  illustrated in FIG. 1C require large cathode areas and large cross-sectional discharge areas.  
           [0017]    Such large-area configurations are disfavored because they do not enable high the gas atoms to achieve high linear velocities between the discharge chamber  30  and the ionization chamber  40 . Low linear velocities allow sample gas at high concentrations to diffuse into the discharge chamber  30  and quench the discharge. Hence, the detector&#39;s  10  sample dynamic range is not optimized, as further discussed in U.S. Pat. No. 6,037,179 to Abdel-Rahman, the contents of which are incorporated herein by reference.  
           [0018]    To summarize, the electronic circuit  150  illustrated in FIG. 1B and discussed above leads to instabilities in the DC discharges observed between the spark electrodes  90 . On the other hand, the electronic circuit  150  illustrated in FIG. 1C requires high peak currents to effectuate ionization, can cause damage to the electrodes  90 , and requires a large discharge cross-sectional area.  
         BRIEF SUMMARY OF THE INVENTION  
         [0019]    According to one embodiment, an electronic circuit that includes a first electrode for electrical connection to an ionization detector system, a second electrode for electrical connection to an ionization detector system, and a transformer electrically connected to the first electrode and to the second electrode for creating a spark between the first electrode and the second electrode.  
           [0020]    According to another embodiment, a method of generating an electrical discharge for an ionization detector system that includes providing a first electrode and a second electrode, each electrically connected to an ionization system, providing a transformer electrically connected to the first electrode and the second electrode, inputting a DC voltage into the primary portion of the transformer, and generating a discharge current between the first electrode and the second electrode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The invention will be described by way of example, in the description of exemplary embodiments, with particular reference to the accompanying drawings in which:  
         [0022]    [0022]FIG. 1A illustrates a discharge ionization detector according to the related art.  
         [0023]    [0023]FIG. 1B illustrates an electronic circuit powered by a high-voltage direct current (DC) power supply. This electronic circuit has been used in detectors according to the related art to generate sparks between the spark electrodes of the detector.  
         [0024]    [0024]FIG. 1C illustrates an alternate electronic circuit according to the related art wherein high-voltage, short-duration, DC pulses are used to generate sparks between the spark electrodes of a detector.  
         [0025]    [0025]FIG. 2A illustrates an embodiment of an electronic circuit according to the present invention, using a transformer and a resistor electrically connected to the secondary portion of the transformer.  
         [0026]    FIGS.  2 B- 2 E illustrate the waveforms of various signals monitored within the electronic circuit illustrated in FIG. 2A.  
         [0027]    [0027]FIG. 3A illustrates another embodiment of an electronic circuit according to the present invention, in which two resistors and a diode are electrically connected to the secondary portion of the transformer.  
         [0028]    FIGS.  3 B- 3 D illustrate various signals monitored within the electronic circuit illustrated in FIG. 3A. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    [0029]FIG. 2A illustrates an electronic circuit  150  according to a first embodiment of the present invention wherein an alternating current (AC) source is used to generate a current across the spark electrodes  90 . According to certain embodiments, the AC source can also include a DC component.  
         [0030]    The electronic circuit  150  illustrated in FIG. 2A includes a step-up transformer  205  with a primary portion  210  that includes a primary coil  212  and a secondary portion  220  that includes a secondary coil  222 . Each coil  212 ,  222  in FIG. 2A contains a different number of loops, with the primary coil  212  containing more loops than the secondary loop  222 . However, the configuration of FIG. 2A is not limiting of the present application and coils  212 ,  222  with numbers and ratios of loops different from what is illustrated are also within the scope of the present invention.  
         [0031]    The primary portion  210  includes two TTL conjugated clock inputs CK 1 , CK 2 , that each lead to one of the open collector buffers U 1 . a,  U 1 . b,  in the circuit  150  and to a set of electronic devices including a set of resistors R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , switching diodes, CR 1 , CR 2 , 5V DC external voltages, feedback capacitors, C 1 , C 2 , and power field effect transistors (FET) with built-in diodes Q 1 , Q 2 .  
         [0032]    Resistor R 10  is a sense resistor sufficiently small enough to prevent significant voltage drop across it. However, resistors R 11  and R 13  have larger resistance values than resistor R 10  and draw very little current from the power supply. When the resistance values are chosen such that R 11 /R 12 =R 13 /R 14 , which is usually the case, the output voltage of the monitor V mon =(R 12 /R 11 )*R*I in . In other words, the monitored voltage V mon  is directly proportional to the input current I in .  
         [0033]    The two TTL conjugated clocks CK 1 , CK 2 , can have frequencies that can be chosen to be on the order of between 1 kHz or less to 16 kHz or more. The feedback capacitors C 1 , C 2 , and associated electronic components dampen the fly-back action of the transformer  205  and also protect against the virtual short circuit should the power FETs Q 1 , Q 2 , ever conduct at the same time to produce opposing and canceling fluxes in the transformer  205 .  
         [0034]    Also included in the primary portion  210  of the transformer  205  is a lead to an input voltage V in  that produces current I in  across a resistor R 10 . This input voltage V in  is typically chosen to be a regulated DC voltage. The lead from the input voltage V in  can be electrically connected to a series of resistors R 10 , R 11 , R 12 , R 13 , R 14 , and an operational amplifier U 2 , contained in a current monitor section  230  of the circuit  150  wherein the input current I in  can be monitored as V mon . The input voltage V in  then can assist in powering the secondary portion  220  of the transformer  205 .  
         [0035]    The secondary portion  220  of the transformer  205  illustrated in FIG. 2A includes a ground, a single resistor R 7 , and two probes V t , V disch , located on either side of the resistor R 7 . The secondary portion  220  of the transformer  205  also contains a first electrode  240  and a second electrode  250  across which an electrical arc or spark may be formed when the circuit is in operation.  
         [0036]    FIGS.  2 B- 2 E illustrate several graphs of signals monitored as a function of time in the electronic circuit  150  illustrated in FIG. 2A. FIG. 2B illustrates wave forms that represent the voltage levels of the conjugated clocks CK 1 , CK 2 , as a function of time and shows that the clocks CK 1 , CK 2 , are cycled between “on” and “off” values at regular intervals such that one clock CK 1 , CK 2 , is always in the “on” position.  
         [0037]    [0037]FIG. 2C illustrates the voltages monitored at position V t  within the secondary portion  220  of the transformer  205 . This graph shows a maximum voltage V out , a minimum voltage −V out , and also shows that a time lag exists as the voltage switches between these extreme values.  
         [0038]    [0038]FIG. 2D illustrates the discharge voltage V disch  as the circuit  150  operates. The maximum discharge voltage V disch  peaks at V hi  after each occurrence of a circuit switch. V disch  then attains a steady state plateau V ss  that can be on the order of between 200 and 300 volts. The small difference between V hi  and V ss  in the circuit illustrated in FIG. 2A can be attributed to the fact that some of the sample gas in the ionization chamber  40  remains ionized as V disch  switches polarity.  
         [0039]    [0039]FIG. 2E illustrates the discharge current I disch  that flows between the first electrode  240  and the second electrode  250  illustrated in FIG. 2A when the circuit  150  is in operation. Two steady state plateaus of current are illustrated, one at a value of I ss  and the other at a value of −I ss . The steady state plateau of the discharge current I disch  is set by the formula: I ss =(V out −V ss )/R 7 .  
         [0040]    The circuit  150  illustrated in FIG. 2A does not experience the space charge fluctuations that are associated with circuits  150  powered by DC voltage, sources such as the circuit in FIG. 1B. Also, the circuit illustrated in FIG. 2A does not require the large discharge peak currents seen when using a pulsed DC source, such as the circuit of FIG. 1C.  
         [0041]    [0041]FIG. 3A illustrates another embodiment of the present invention wherein the electronic circuit  150  includes a transformer  205 . The primary portion  210  of the transformer in FIG. 3A can be identical to the primary portion  210  of the transformer  205  illustrated in FIG. 2A. However, the secondary portion  220  of the electronic circuit  150  illustrated in FIG. 3A has a different implementation.  
         [0042]    The secondary portion  220  according to the embodiment illustrated in FIG. 3A includes a ground connection, two resistors R 7 , R 8 , a high-voltage diode CR 3 , a first electrode  240  and a second electrode  250 . The resistor R 8  and the high-voltage diode CR 3  are positioned in a parallel configuration and the resistor R 7  is electrically connected in series with the parallel configuration. A voltage V t  is monitored between the coils of the secondary portion  220  of the transformer  205  and the resistor R 7 .  
         [0043]    FIGS.  3 B- 3 D illustrate graphs of the wave forms of various signals monitored within the circuit illustrated in FIG. 3A as a function of time. The graph in FIG. 3B illustrates the V t  voltage monitored between the secondary portion  220  coil and resistor R 7 . As V t  switches between a maximum voltage of V out  and a minimum voltage of −V out , the switch in value is not instantaneous and a time delay is shown.  
         [0044]    The discharge voltage V disch  across the first electrode  240  and the second electrode  250  is shown in FIG. 3C. The small difference between the high voltage, V hi , seen and the steady state voltage, V ss , can, as above, be attributed to ionized gas molecules between pulses.  
         [0045]    [0045]FIG. 3D shows that the addition of the resistor R 8  and the high-voltage diode CR 3  in the circuit  150  illustrated in FIG. 3A results in a modulation over time of the current flowing between the first electrode  240  and the second electrode  250 . As shown, two steady state current plateaus I ss1  and −I ss2  exist.  
         [0046]    When either the collector electrode  160  or the emitter electrode  170  illustrated in the device in FIG. 1A is the main source of ionization in the ionization chamber  40 , the circuit in FIG. 3A is preferred. This preference is due to the fact that the circuit in FIG. 3A saves power while maintaining the same level of ionization. The magnitudes of the two steady state current plateaus I ss1 , and −I ss2  can be determined by the following equations:  
           I   ss1 =( V   out   −V   ss )/ R   7   
           I   ss2 =( V   out   −V   ss )/( R   7 + R   8 )  
         [0047]    Because the first electrode  240  and second electrode  250  illustrated in FIGS. 2A and 3A are electrically connected to the spark electrodes  90  illustrated in FIG. 1A, the transformer-based electronic circuits  150  enhances stability of the electrical discharges across the spark electrodes  90 . The circuits illustrated in FIG. 2A and 3A also avoid the large peak currents that allow the use of smaller discharge cross-sectional areas and higher discharge gas linear velocities for linearity enhancement. Further, the embodiments of the present invention discussed above include current monitors  230  that monitor the average current drawn from the discharge input supply and therefore provide additional data concerning the state of the discharge.  
         [0048]    The foregoing detailed description has been given for understanding exemplary implementations of the invention only and no unnecessary limitations should be understood therefore as modifications will be obvious to those skilled in the art without departing from the scope of the appended claims and their equivalents.