Abstract:
Methods and systems to compensate for distortions created by dynamic voltage applied to an electron multiplier used in mass spectrometry. An electron multiplier has a cathode end accepting ion flow, an opposite emitter end and an interior surface. The electron multiplier produces an electron output from ions colliding with the interior surface. A variable power supply has a voltage output coupled to the cathode end and the emitter end of the electron multiplier. The voltage output changes dynamically to adjust the electron output from the electron multiplier. An anode is located in proximity to the electron multiplier. An electrometer is coupled to the anode in proximity to the electron multiplier to measure the current generated by the electron output. A low pass filter circuit is coupled to the emitter end to the ground of the electrometer to attenuate emitter voltage changes. A bias circuit is coupled to the emitter end to stabilize emitter to anode voltage difference.

Description:
COPYRIGHT 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     TECHNICAL FIELD 
     This disclosure relates generally to mass spectrometry and specifically to a circuit to provide voltage stabilization for anode bias in an electron multiplier. 
     BACKGROUND 
     Mass spectrometry is a widely used analytical technique that measures the mass-to-charge ratio of charged particles from a sample chemical compound. Mass spectrometry has many applications such as determining particle mass, the elemental composition of a sample or molecule, and the chemical structures of molecules, such as peptides and other chemical compounds. In the mass spectrometry process, chemical compounds are ionized to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios. 
     In a typical mass spectrometry system, a sample is loaded onto the mass spectrometer and undergoes vaporization. The components of the sample are ionized by one of a variety of methods, such as exposure to an electron beam, which results in the formation of ions. The ions are separated according to their mass-to-charge ratio in an analyzer by electromagnetic fields. 
     A mass spectrometer includes an ion source, a mass analyzer and an electron detector. The ion source converts gas phase sample molecules into ions. The mass analyzer sorts the ions by their masses by applying electromagnetic fields. The electron detector measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present. 
     A typical electron detector includes an electron multiplier that is a cylindrical tube having a cathode at one end and an anode at the opposite end. The ions are injected at the cathode and an electrical voltage is applied between the cathode and the anode resulting in the ions being exciting and colliding with the interior surface of the cylinder to produce electrons. The collisions create an avalanche of electrons which exit a hole at the opposite end of cylinder and are collected by the anode. An electrometer is connected to the anode to measure the resulting current. The amount of electrons produced is measured by electron gain which is a function of the voltage between the cathode and the emitter nodes. The higher the applied voltage, the more energetic the electrons from the ions are thereby producing more electrons, and therefore the more the gain increases. 
     Conventional electron detectors set the voltage at a fixed value and gain is constant over time. The user may calibrate the electron gain and determine the current at the anode. It is desirable to adjust the voltage over a dynamic range to create a larger signal for measurement purposes. Different ranges of voltages are desirable since the ion strength of the measured compounds changes with the type of compound. The signal ranges are limited in a fixed voltage because of the drift (lower limit) and a high limit set by the saturation point from a certain voltage. 
     Present electron detectors provide dynamic range by determining the highest peak of a signal and for the next scan the electron multiplier is set to an appropriate gain with the peak value. This produces a large range for the detector, but when the voltage between the cathode and anode changes quickly in such dynamic ranging, a transient error signal is introduced into the electrometer through capacitive coupling to its electron collector. A common configuration of electron multipliers is have a bias resistor at the anode (emitter) end that generates, as a result of the channel current flowing through the resistor, a bias voltage for attracting the exiting electrons to the electrometer collector. Since the channel resistance and bias resistor form a voltage divider, changes in the multiplier voltage also result in changes to the bias voltage. Additionally, capacitive coupling through the channel body, from cathode to emitter, also perturbs the emitter potential. Such bias changes may result in minor changes in collection efficiency, but more importantly, the emitter potential changes are capacitively coupled to the electrometer collector. 
     In order to solve such distortions, a zener diode has been connected in place of the integral resistor, which stabilizes the voltage and reduces the error. However zener diodes still have problems that cause additional distortion. Zener diodes do not provide perfect voltage regulation and cause slight voltage changes when the current through the zener diode changes. Further, relatively high voltage zener diodes are required to ensure efficient electron collection by the electrometer. In avalanche mode, zener diodes produce diode noise thereby adding noise to the output signal of the electron multiplier. 
     Therefore it would be desirable to have an electron detector that is capable of dynamic range without distortions from current produced from the parasitic resistance in the electron multiplier tube channel in conjunction with the applied voltage. It would be desirable for a circuit that may correct for distortions that may be integrated with the mechanical components of the electron detector. It would also be desirable to have a circuit to compensate for the transient current generated by capacitive coupling between the cathode and anode electrodes when the channel voltage changes dynamically. It would also be desirable to have a circuit to compensate for distortions from an avalanche voltage for a zener diode used to compensate for voltage distortions. It would be desirable to have a compensation circuit that is small enough to be included in the vacuum around certain parts of the electron multiplier. 
     SUMMARY 
     Aspects of the present disclosure include a system for stabilization of electron multiplier anode bias under a dynamic voltage input. An electron multiplier has a cathode end receiving an ion flow, an opposite emitter end and an interior surface. The electron multiplier produces an electron output from ions colliding with the interior surface. A variable power supply has a voltage output coupled to the cathode end and the emitter end of the electron multiplier. The voltage output changes dynamically to adjust the electron output from the electron multiplier. An anode is located in proximity to the electron multiplier. An electrometer is coupled to the anode in proximity to the electron multiplier to measure the current generated by the electron output. A low pass filter circuit is coupled to the emitter end to the ground of the electrometer to attenuate emitter voltage changes. A bias circuit is coupled to the emitter end to stabilize emitter to anode voltage difference. 
     Another disclosed example is a method of stabilizing voltage output from an ion detector having an electron multiplier with a cathode, an anode and an emitter between the cathode and the anode. An ion stream is received via the electron multiplier. An electron stream is produced by applying a voltage between the electron multiplier cathode and anode. Electron multiplication is dynamically adjusted in the electron multiplier by changing the voltage. The voltage drop between the electron multiplier anode and a common ground of an electrometer is stabilized via a zener diode. Noise generated by the zender diode is attenuated via a low pass filter. The emitter voltage is filtered to eliminate changes to the emitter voltage caused by the dynamic gain controlling voltage input via a bias circuit. 
     The foregoing and additional aspects and embodiments of the present invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
         FIG. 1  is a view of an electron multiplier used in an ion detector for mass spectrometry applications; 
         FIG. 2  is a circuit diagram of the electrical circuit equivalent for the electron multiplier in  FIG. 1 ; and 
         FIG. 3  is a circuit diagram of the example filter and bias circuit in the electron detector in  FIG. 1 . 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
       FIG. 1  is a cross section view of an ion detector  100  that may be used in the electron detector module of a mass spectrometer. The ion detector  100  has a flat mounting board  102  that sits a vacuum feed through ring  104 . The mounting board  102  also mounts an annular electrometer shield  106 . An electron multiplier  107  has a cylindrical channel body  108  with an interior surface, a cathode end  110  and an opposite emitter end  112 . The opposite emitter end  112  has a hole  114  for the emission of electrons. The cathode end  110  has a conically shaped section  116  with an open ion entrance  118  that receives an ion stream  120 . A cathode  122  is created at the cathode end  110  with an electrical connection to a voltage source as will be explained below. An emitter  124  is coupled to the cylindrical channel body  108  near the opposite emitter end  112 . The electron multiplier  107  produces an electron stream output from ions colliding with the interior surface of the cylindrical channel body  108 . An anode  126  is placed in proximity to the hole  114  to receive emitted electrons from the hole  114 . Electron current collected by the anode  126  is converted to ion signal voltage by an electrometer as will be explained below. The system that includes the electron multiplier  107  stabilizes anode bias under a dynamic voltage input. A cathode cap  128  fits over the ion entrance  118 . The cylindrical channel body  108  has a glass or ceramic body. The interior of the cylindrical channel body  108  and the conically shaped section  116  are coated with a partially conducting glass which is chemically treated to allow high electron emission. Metallic coatings are applied to the exterior of each end  110  and  112  to provide connection to the interior coating of the cylindrical channel body  108 . 
     A bias circuit board  130  has a circular opening for the insertion of the cylindrical channel body  108 . The electron multiplier  107  is housed in an assembly that includes the threaded anti-corona cathode cap  128 , an insulating plastic body  132 , a helical cathode spring  134 , a bias board contact spring  136 , and a threaded metal end cap  138 . In this example, the emitter contact coating is extended up the outside of the glass stem to make contact to the bias board contact spring  138  and therefore to the bias circuit board  130 . The helical cathode spring  134  compresses the cathode cap  128  into the cylindrical channel body  108 . The end cap  138  is connected to the electrometer shield  106  via a retaining set screw so the electrometer, emitter and collector are well shielded by the enclosure formed by the electrometer shield  106  and the end cap  138 . The anode  126  is inserted through a series of washers  140  seated in the electrometer shield  106  to isolate the enclosure. 
     The bias circuit board  130  is mounted near the opposite emitter end  112  of the cylindrical channel body  108 . The bias board contact spring  136  is a conductive toroidal spring that ensures connection between the circular opening of the bias circuit board  130  and the conductive coating of the emitter end  112 . The electrometer shield  106  encloses the opposite emitter end  112  of the cylindrical channel body  108  and creates an electron emission chamber  142  that includes a gap between the hole  114  and the anode  126 . The anode  126  is mounted on an electrometer circuit board  150 . The interior of the vacuum feed through ring  104  includes a groove  152  that interfaces with the mounting board  102 . An O-ring  154  contains the vacuum seal potting epoxy within the vacuum feed through ring  104 . The epoxy fills the annular space between the vacuum feed through ring  104  and the electrometer shield  106 . The epoxy also fills the space above the electrometer circuit board  150 . The washers  140  stop the epoxy from entering the chamber  142 . An electrical connector  156  is located on the opposite end of the mounting board  102  to provide inputs and outputs to the electrometer circuit board  150 . 
     In operation, the ion stream  120  produced from a sample compound is directed through the cylindrical channel body  108 . A voltage is applied between the cathode  122  and the anode  126  to determine the electrons produced by the electron multiplier  107  from the ion stream  120 . The applied voltage between the anode  126  and the emitter  124  (termed the channel voltage) determines the gain value for the electron multiplier  107 . The electron output exiting the emitter end  112  generate a current on the anode  126  which is measured by electronic components on the electrometer circuit board  150 . 
       FIG. 2  is a circuit diagram of an equivalent circuit  200  of the electron multiplier  107  combined with the bias circuit on the bias circuit board  130  in  FIG. 1 . The equivalent circuit  200  includes a cathode node  202 , an emitter node  204  and an anode node  206 . The cylindrical channel body  108  in  FIG. 1  is represented by a channel circuit  210  between the cathode  122  and the emitter  124  in  FIG. 1 . The channel circuit  210  has the electrical impedance equivalent of a channel resistance  212  and a channel capacitance  214  representing the capacitance between the cathode node  202  and the emitter node  204 . 
     A capacitance represented by the capacitor  220  is created between the emitter node  204  and the anode node  206  by the close proximity of these two conductors. An emitter current  222  is the gain amplified electron current measured by the electrometer as will be explained below. Any bias circuit connected to the emitter node  204  must be able to maintain a constant potential on the emitter node  204  regardless of currents entering the emitter node  204  from the channel circuit  210  or the capacitance  220  in addition to emitter current  222 . 
       FIG. 3  is a circuit diagram of the compensation circuit  300  that is connected to the electron multiplier  107  from  FIG. 1 . As explained above, the components of the compensation circuit  300  are mounted on the bias circuit board  130  in  FIG. 1 . Like elements are labeled with identical element numbers in  FIG. 1 . The compensation circuit  300  includes a low pass filter  302  and a bias circuit  304 . An electrometer  306  is coupled to the compensation circuit  300  and the anode  126  of the electron multiplier  107 . The components of the electrometer  306  are mounted on the electrometer circuit board  150  in  FIG. 1 . The low pass filter  302  is connected from the multiplier emitter  124  to the ground of the electrometer  306 . The bias circuit  304  is coupled in parallel to the low pass filter  302  and the electrometer  306 . The electron multiplier  107  is powered by a voltage circuit  308  that produces a voltage value between the cathode  122  and the anode  126 . As explained above, the voltage circuit  308  is a variable power supply producing a dynamic range of voltages to the electron multiplier  107  in order to provide measurements for different compounds with different ion streams. The voltage output of the voltage circuit  308  changes dynamically to adjust the electron output or multiplication from the electron multiplier  107 . The compensation circuit  300  therefore corrects from distortions from the components of the equivalent circuit  200  shown in  FIG. 2  to allow the dynamic ranging of the voltage to the electron multiplier  107 . Specifically, distortions may be generated from the currents from the channel resistance  212  and from the capacitive currents through the capacitance  214  from rapid changes in voltage to the electron multiplier  107 . 
     The low pass filter  302  includes a bypass capacitor  310  that is coupled between the emitter  124  and the ground of the electrometer  306 . This filter bandpass is primarily determined by the multiplier channel resistance  212  in  FIG. 2 , a bias resistor from the bias circuit  304  and the bypass capacitor  310 . The capacitor value of the bypass capacitor  310  and the bias resistor value in the bias circuit  304  are chosen to shunt capacitive currents from the cathode  122  to the ground of the electrometer  306  thus attenuating the emitter voltage changes and subsequent coupling to the electrometer  306 . Thus voltage generated as a result of dynamic voltage changes applied to the cathode node  202  in  FIG. 2  is eliminated. 
     The bias circuit  304  includes a zener diode  320 , an input resistor  322  and a capacitor  324 . The zener diode  320  ensures a stable low frequency bias required for efficient electrometer collection of electrons from the emitter  124  on the cylindrical channel body  108  of the multiplier  107 . The reverse current through the zener diode  320  is on the average equal to the channel current which is the channel voltage divided by the channel resistance  212  in  FIG. 2 . The value of the input resistor  322  is chosen to give both a minimum loading of a low pass filter created by the capacitor  324  and a minimum voltage drop generated by the multiplier channel current  222  in  FIG. 2 . The capacitor  324  therefore attenuates the intrinsic noise generated by the zener diode  320 . 
     The electrometer  306  in this example includes an operational amplifier  330  having a first input  332  coupled to the anode  126  and a second input  334  coupled to a ground that serves as the ground for the electrometer  306 . The anode  126  is coupled via a resistor  336  to the first input  332 . A resistor  338  is coupled in a feedback configuration between one end of the resistor  336  and an output  340  of the operational amplifier  330 . A capacitor  352  is coupled to the opposite end of the resistor  336  and the output  340  of the operational amplifier  330 . The voltage pins of the operational amplifier  330  are coupled to the positive and negative voltage inputs of the electrical connector  156 . A capacitor  354  and a capacitor  356  connects the voltage pins of the operational amplifier  330  to ground to smooth out the voltage inputs. The electrometer  306  therefore outputs voltage proportional to the current sensed on the anode  126  which is a function of the electrons produced by the electron multiplier  107 . 
     The voltage circuit  308  includes a high voltage power supply  360  and a power regulator  362 . The power regulator  362  may be adjusted to provide different output voltages to the cathode  122  of the electron multiplier  107 . Other controls (not shown) in the mass spectrometer system set the output voltage of the power regulator  362  according to the range of ion emission from the compounds being analyzed. 
     In this example, the components of the compensation circuit  300  may be mounted or affixed on the bias circuit board  130  at the opposite emitter end  112  of the electron multiplier  107  and within the electrometer shield  106  as shown in  FIG. 1 . Use of surface mount components allows reduced size, reduces stray coupling and minimizes shielding requirements for the compensation circuit  300 . 
     The ion detector  100  when operated with the high voltage power supply  360  is sensitive to ripple and the voltage change created by rapid slewing of channel voltage during gain changes that occur during dynamic ranges of ion fluxes created by the regulator  362 . Capacitive coupling of changes in multiplier emitter voltage to the electrometer input  332  in  FIG. 3  introduces an error in the output of the electrometer  306  absent the compensation circuit  300 . Ripple on a high voltage supply such as the voltage circuit  308  for the electron multiplier  107  results in noise on the signal baseline. Rapid changes in a set point for the high voltage produce output transients may create errors in the measured ion signal. The compensation circuit  300  connected between the bottom of the channel resistance  212  in  FIG. 2  and the electrometer ground reference eliminates both the feed-through of channel voltage noise and baseline shifts resulting from intentional modulation of the multiplier gain from the power regulator  362 . This is particularly useful in mass spectrometer detectors where rapid changes of gain settings are required to maintain wide dynamic range of ion fluxes. 
     The compensation circuit  300  eliminates these perturbations without resorting to a separate anode bias supply. The small size of the compensation circuit  300  allows it to be incorporated into the electron multiplier  107 , reducing shielding requirements since it is contained within the electrometer shield  106 . The compensation circuit  300  therefore may be included in the vacuum created by the vacuum feed through ring  104 . 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.