Patent Publication Number: US-7723680-B2

Title: Electron multiplier having electron filtering

Description:
BACKGROUND 
   Time of flight (TOF) mass spectrometers are used to analyze the composition of a sample. The sample is ionized, accelerated through a vacuum, and caused to impact an ion detector. Ions having a higher mass accelerate more slowly through the vacuum than ions having a lower mass. As a result, the time of flight mass spectrometer measures the time of flight of the ions, which is then used to identify the mass of the ion. This information is then used to identify the content of the sample. 
   One type of detector used in TOF mass spectrometers includes an electron multiplier. The ions enter the electron multiplier and strike a dynode. In response, the dynode releases a plurality of electrons in response to each ion that strikes it. Those ions then pass to and strike another dynode. The second dynode then releases multiple electrons in response to each electron that strikes it. This process repeats for several stages of dynodes until enough electrons are generated to induce an electrical current. The current is measured and the time at which the current is induced corresponds to the time it took the ion to pass from the ion source to the electron multiplier. 
   A difficulty arises when the sample includes constituents that are not of interest for analysis. For example, some TOF mass spectrometers are commonly used to test the composition of a discrete sample of ambient environment such as air for the presence of any undesirable constituents such as pollution, poisons, and explosives. The instrument ionizes and samples all of the constituents that happen to be present, not just undesirable constituents that are of interest for analysis. However, the ionization and sampling includes high frequency and abundant molecules such as oxygen and nitrogen even though their presence is known and not of interest. 
   A difficulty is that the dynodes degrade with use, and frequently ionizing and sampling high abundance molecules that are not of interest shortens the dynode&#39;s useful life. One technique that has been used to prevent sampling ions that are not of interest is to add an arrangement of electrodes in the mass spectrometer that deflect the undesired ions from the ion path before they reach the electron multiplier. However, these arrangements are expensive, difficult to switch, consume energy, and add bulk to the mass spectrometer. 
   SUMMARY 
   In general terms, this patent is directed to an electron multiplier having multiple dynodes. The voltage applied to at least one of the dynodes is adjusted to selectively prevent or satisfactorily reduce the flow of electrons through the electron multiplier. 
   One aspect is a detector for detecting ion impact. The detector comprises a plurality of dynodes and a power supply circuit. The plurality of dynodes are arranged in an electron cascading configuration and include at least a first dynode and a second dynode arranged to receive electrons from the first dynode and defining a path. The power supply circuit is electrically coupled to the plurality of dynodes and includes the first and second dynodes, wherein the power supply circuit is arranged to selectively adjust a potential difference between the first and second dynodes between a first state in which the second dynode has a greater voltage than the first dynode and a second state in which the second dynode has a voltage substantially similar to or less than the first dynode. 
   Another aspect is a detector for detecting ion impact. The detector comprises an ion source, ion optics, a flight tube, a plurality of dynodes, and a power supply circuit. The ion source ionizes a sample to generate ions. The ion optics receive and focus the ions from the ion source. The flight tube is positioned to define an ion path for the ions from the ion source. The plurality of dynodes are in an electron cascading configuration and are arranged to receive the ions from the ion path. The plurality of dynodes define an electron path and comprise at least a first dynode; a second dynode arranged to receive electrons from the first dynode; a third dynode arranged immediately upstream of the first dynode along the electron path to supply electrons to the first dynode; and a fourth dynode arranged immediately downstream of the second dynode along the electron path to receive electrons from the second dynode. The power supply circuit is electrically coupled to the plurality of dynodes. The power supply circuit comprises a plurality of resistive elements, each resistive element electrically connected between adjacent dynodes of the plurality of dynodes arranged in the electron cascading configuration; a pulse generator electrically coupled to the second dynode to selectively adjust a potential difference between the first and second dynodes between a first state in which the second dynode has a greater voltage than the first dynode and a second state in which the second dynode has a voltage substantially similar to or less than the first dynode; a first capacitive coupling electrically coupled between the power source and the third dynode to maintain a substantially constant third dynode voltage throughout the first state and the second state; and a second capacitive coupling electrically coupled between the fourth dynode and ground to maintain a substantially constant fourth dynode voltage throughout the first state and the second state. 
   Yet a further aspect is a method of detecting ions. The method comprises receiving ions from an ion source, the ions including an unwanted constituent; detecting impacts of the ions with a detector; and inhibiting detection of impacts of the unwanted constituent. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating an example time of flight mass spectrometer including a detector according to the present disclosure. 
       FIG. 2  is a cross-sectional view of an exemplary embodiment of the detector shown in  FIG. 1 . 
       FIG. 3  is an electrical schematic diagram of an exemplary embodiment of a power supply circuit of the detector shown in  FIG. 1 . 
       FIG. 4  is an electrical schematic diagram of another exemplary embodiment of a power supply circuit of the detector shown in  FIG. 1 . 
       FIG. 5  is a flow chart illustrating an example method of operating the detector shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Various embodiments will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the appended claims. 
     FIG. 1  is a block diagram illustrating example time of flight mass spectrometer  100  that is one possible application for the electron multiplier described herein. Mass spectrometer  100  includes inlet  102 , ion source  104 , ion optics  105 , flight tube  106 , ion path  107 , detector  108 , control system  110 , and output device  112 . In this exemplary embodiment, mass spectrometer  100  operates to detect the content of samples of ambient environment  90 , although other embodiments can analyze other types of samples. In various embodiments, any or all of these components can operate in a vacuum or at atmospheric pressure. Furthermore, other embodiments can include different combinations of components to form the mass spectrometer. 
   The mass spectrometer  100  is useful in analyzing the content of a sample, such as ambient environment  90 . In an exemplary embodiment, the mass spectrometer  100  performs continuous air monitoring to detect the presence (or absence) of a toxic chemical, an explosive substance, a pollutant, or various other chemicals or compositions in the ambient environment  90 . Upon detection, mass spectrometer  100  provides an output relating to the results of the analysis. Alternatively, mass spectrometer  100  generates an alarm signal, adjusts an operating mode of a machine or other device, or takes some other type of action upon detecting the presence of a predetermined compound in the ambient environment  90  or the presence above a certain threshold level within the ambient environment. Other embodiments analyze samples from sources other than an ambient environment. 
   A sample of ambient environment  90  is first received through inlet  102 . In an exemplary embodiment, inlet  102  is a pump for pumping samples of ambient environment  90  into ion source  104 . In other possible embodiments, inlet  102  is an inlet such as a vent, valve, hose, nozzle, or port through which a sample of ambient environment  90  is received into mass spectrometer  100 . In such alternative embodiments the sample can be drawn through inlet  102  by a vacuum or some other mechanism. 
   The sample flows through inlet  102  and to ion source  104 . Possible embodiments of ion source  104  include a radioactive ionization source, a plasma source, an electron ionization source, and a chemical ionization source. An example of a radioactive ionization source is nickel-65. Examples of plasma sources include resonant rings and dielectric barrier discharge devices. In an example electron ionization source, electrons are produced through thermionic emission by heating a wire filament using an electric current and passing the gas-phase sample near it. In a chemical ionization source, the sample is ionized by chemical reactions that occur between the sample and a reagent, such as methane, isobutane, or ammonia. Other possible embodiments of ion source  104  ionize the source using glow discharge, field desorption, fast atom bombardment, thermospray, desorption/ionization on silicon, Direct Analysis in Real Time, atmospheric pressure chemical ionization, secondary ion mass spectrometry, spark ionization, thermal ionization, or other ionization techniques. While certain embodiments of the ion source are described herein, the ion source  104  can include other structures and arrangements and can use yet other techniques for ionizing the sample. 
   An arrangement of ion optics  105  receives the ions from the ion source  102 , focuses them onto an ion path, and passes them into the flight tube  106 . In an exemplary embodiment, the first arrangement of ion optics  104  includes a skimmer (not shown) that collimates the ions into an ion stream flowing along an ion path that pass through the flight tube  106  and into the detector  108 . In various embodiments, the ion optics  105  also includes an electrode arrangement (not shown) that accelerates the ions along the ion path, adjusts the phase and frequency of the ions so that they enter the first mass analyzer  106  at predetermined levels, and adjusts the timing of when the ions are released into the flight tube  106  so that the exact time of flight between the ion optics  105  and the detector  108  can be determined. While certain components for the ion optics  105  are described herein, the ion optics can include other structures and arrangements 
   Sample ions flow from the ion optics and into flight tube or other drift region  106 , which defines an ion path  107  between the ion optics  105  and the detector  108 . The ion path  107  has a distance over which ions of different masses are separated due to their different velocities. In an exemplary embodiment, flight tube  106  is a conductive cylinder that is grounded to reduce or eliminate stray fields from affecting the flight of sample ions. While a certain structure for the flight tube  106  is described, the mass spectrometer  100  can include any structure that defines an ion path leading to the detector  108 . For example, other embodiments can include additional electrode arrangements for accelerating ions traveling along the ion path or as a mass filter for passing desired ions from the ion path. Quadrapole electrode arrangements are examples of such additional electrode arrangements. 
   The ion path has a known length, which is useful to calculate the time of flight of ions that pass from the ion optics  105  to the detector. Each type of ion has a different mass and hence a different time of travel from the ion optics  105  to the detector  108 . Accordingly, by measuring the time of travel along the ion path  107  from the ion optics to the flight tube, the system can calculate the mass of each detected ion and hence determine what type of ion it is. The relationship between the flight time and the mass can be written in the form:
 
Time= k√{square root over (m)}+c  
 
where k is a constant related to flight path and ion energy, m is the mass of the ion, and c is a small decay time, which may be introduced by the signal cable and/or detection electronics.
 
   Ions pass from flight tube  106  and enter detector  108 , which includes an electron multiplier such as the one illustrated in  FIG. 2  and described in more detail herein. Detector  106  detects the impact of the ions and provides an output to the control system  110 . Although certain types of detectors and structures of an ion detector are described herein, other embodiments can include other structures and arrangements of electrodes, dynodes, and circuits. 
   Control system  110  performs control functions for mass spectrometer  100 . In possible embodiments, control system  110  is communicatively coupled to ion source  104 , flight tube  106 , detector  108 , and output device  112 . In other embodiments, control system  110  is only communicatively coupled to some of these devices. The exemplary embodiment of control system  110  synchronizes the operation of mass spectrometer  100  and provides precise time measurements from a starting point (e.g., an electrode in the ion optics  105 ) to the detector  108 . An exemplary embodiment of control system  110  includes a high frequency clock or other timing mechanism such as a timer for determining the time of flight of ions. For example, the high frequency clock is sometimes used to set a start time that is synchronized with release of the ions from the ion optics  105  to the flight tube  106  and an end time that is set when the ions are detected by the detector  108 . 
   In one possible embodiment, control system  110  is a field-programmable gate array. Other possible embodiments of control system  110  include other types of processing devices, such a microprocessor, central processing unit (“CPUs), multiple CPU, microcontroller, programmable logic devices, digital signal processing (“DSP”) device, and the like. Some embodiments of processing devices are of any general variety such as reduced instruction set computing (RISC) devices, complex instruction set computing devices (“CISC”), or specially designed processing devices such as an application-specific integrated circuit (“ASIC”) device. 
   Output device  112  is an interface device for communicating data relating to an analysis of a sample. In the illustrated embodiment, output device  112  is communicatively connected to and under the control of control system  110 . In some possible embodiments, output device  112  is a user interface. Examples of user interfaces include output devices, such as a display, speaker, alarm, and printer, and input devices such as a keyboard, mouse, pen input device microphone, touch screen, and other input devices. In other possible embodiments, output device is a communication device. Examples of communication devices include a network communication device, a communication port, a wireless communication device, and other communication devices. In one embodiment, the output from output device  112  is a spectrum illustrating or representing the content of the sample. 
     FIG. 2  is a cross-sectional view of an exemplary embodiment of detector  108 . Detector  108  includes a plurality of dynodes, represented by even numbers from  202  to  228  (sometimes referred to herein generally as “dynodes  202 - 228 ”), and electrode  230 . 
   In some embodiments, the plurality of dynodes  202 - 228  are polished metal electrodes that are electrically coupled to a power supply circuit (such as shown in  FIGS. 3-4 ). The plurality of dynodes are arranged in an electron cascading configuration to define an electron path, represented by even numbers from  232  to  258  (sometimes referred to herein generally as “electron path  232 - 258 ”). In possible embodiments, electron cascading involves a process by which electrons ejected from one dynode (e.g., dynode  202 ) cascade downstream along the electron path  232 - 258  (e.g., to dynode  204 , and then to dynode  206 , etc.). Some embodiments of dynodes  202 - 228  include a surface treatment that increases the ability of the dynode to emit secondary electrons. 
   An example is illustrated in  FIG. 2 . An ion  200  (such as originating from ion source  104  and passing through flight tube  106 , shown in  FIG. 1 ) is supplied to detector  108 , which is interposed along the ion path. The ion is directed toward dynode  202  of the plurality of dynodes, and as a result impacts with dynode  202 . The impact of ion  200  with dynode  202  causes dynode  202  to emit electrons. This process is sometimes referred to as secondary emission. Secondary emission is the process in which surface electrons present on the dynode are emitted from the dynode upon impact with the ion or an electron. 
   A power supply circuit (e.g., shown in  FIGS. 3-4 ) is electrically coupled to the plurality of dynodes (e.g., D 1 -D 14 ), and operates to charge the plurality of dynodes  202 - 228  with a potential that increases for each dynode  202  downstream along the electron path. For example, dynode  204  has a greater potential than dynode  206 . As a result, an electric field is generated between dynode  202  and  204  that draws the electrons emitted from dynode  202  toward dynode  204 , generally along path  232 . 
   When the electrons from dynode  202  impact dynode  204 , the energy of the electrons is sufficient to cause secondary emission at dynode  204 . This secondary emission results in each electron causing dynode  204  to emit one or more electrons. Typically multiple electrons are emitted for each impact of an electron with one of the plurality of dynodes  202 - 228 . 
   The power supply circuit generates a potential on dynode  206  that is greater than dynode  204 , generating an electric field between dynode  204  and  206 . The electric field causes he electrons emitted from dynode  204  to be drawn toward dynode  206 , generally along path  234 . The electrons impact with dynode  206 , themselves causing a secondary emission of one or more electrons. Typically multiple electrons are emitted. This process continues along electron path  232 - 258 , and acts to increasingly multiply the number of electrons moving along electron path  232 - 258  with each impact with a dynode. As a result, a single impact of ion  200  with dynode  202  can result in a large number of electrons moving along electron path  232 - 258 . 
   In the exemplary embodiment, detector  108  includes fourteen dynode plates  202 - 228 . Other embodiments of detector  108  include various numbers of dynode plates. The number of dynode plates is generally a trade off between the gain of the detector and peak width of the resulting output. For example, the greater the number of dynode plates, the greater the gain of the detector, because each dynode plate increases the number of electrons moving along the electron path. On the other hand, the electrons do not all move in precisely the same path. Some electrons will follow a shorter distance path, while other electrons will follow a longer distance path, resulting in a range of distances of electron travel. With each added dynode, this range of distances increases accordingly. Those electrons that follow a shorter-distance path will pass through the detector in a shorter period of time than electrons that follow a longer-distance path. Therefore, the peak width of the resulting output signal is increased with each added dynode plate. If too many dynode plates are included, the output signal generated from one type of ion may become indistinguishable from the adjacent output signal generated from another type of ion. 
   Some embodiments of detector  108  include a number of dynode plates in a range from about five dynode plates to about twenty-four dynode plates. This range has been found to be sufficient to generate sufficient gain, while maintaining adequate separation between output pulses of adjacent ions. Another possible embodiment includes about fifteen dynode plates. Fifteen dynode plates provides increased gain over embodiments with fewer dynode plates, but maintains a good separation between output pulses from adjacent ions. Other embodiments include various other numbers and arrangements of dynode plates. 
   In some embodiments, after the electrons have traveled downstream along the electron path  232 - 258  they are collected by electrode  230 . In some possible embodiments, electrode  230  is connected to a load, such as a 50 ohm load, and acts like a current source to the load. The current is then detected in any desired manner, such as by a voltage meter across the 50 ohm load. Other possible embodiments do not include a separate electrode  230 , but rather the final dynode (e.g., dynode  228 ) performs this function. 
   Some possible embodiments described herein are operated to prevent or satisfactorily reduce the flow of electrons and thereby blank out the detection of known high abundance ions to suppress the electron flow caused by these ions. In this way the effective life of the detector is increased in some embodiments. Because the high abundance ions are already known to be present, there is not a need to continually analyze the sample for the presence of these ions. Other embodiments are operated to blank out undesired ions that are not in high abundance. 
   Another advantage is that blanking out detection can eliminate or reduce the need for mass ion filters such as quadrapole electron arrangements or an ion deflecting pulse in the flight path. Eliminating such filters simplifies the structure of the mass spectrometer and may permit a smaller and more compact mass spectrometer. However, other embodiments can use the electron multiplier described herein with ion filters or in a mass spectrometer that uses ion filters. 
     FIG. 3  is an electrical schematic diagram of an exemplary embodiment of a power supply circuit  300  of detector  108 . Power supply circuit  300  includes a plurality of resistive elements R 1 -R 15 , a voltage source (Vcc), ground connections (G), and a pulse generator (Vpulse), and capacitive couplings C 1  and C 2 . Power supply circuit  300  is electrically coupled to the plurality of dynodes  202 - 228  at D 1 -D 14  (shown in  FIG. 2 ). 
   In some embodiments, power supply circuit  300  supplies power to the plurality of dynodes  202 - 228 . When operating in a detection state, power supply circuit  300  operates to apply a voltage gradient between the entry dynode (e.g.,  202  at D 1 ) and the exit dynode (e.g.,  228  at D 14 ). 
   In the illustrated embodiment, power supply circuit  300  includes a plurality of resistive elements R 1 -R 15  connected in a series orientation from R 1  to R 15 , forming a voltage dividing circuit. The first resistive element R 1  of the plurality of resistive elements R 1 -R 15  is connected to voltage source Vcc. The last resistive element R 15  of the plurality of resistive elements R 1 -R 15  is connected to ground. 
   In this example, the dynode plates are electrically coupled to the intersections of adjacent resistive elements R 1 -R 15 . For example, the first dynode plate  202  is electrically coupled between resistive element R 1  and R 2  (at D 1 ). The second dynode  204  is electrically coupled between resistive element R 2  and R 3  (at D 2 ), and so on. As a result, when the resistances of R 1 -R 15  are substantially matched, the voltage drop is divided evenly across each of the resistive elements, such that a substantially equal potential difference exists between each dynode plate. In other embodiments, the resistances are not substantially matched and the voltage drop is not evenly divided across each of the resistive elements. In addition, other embodiments use other power supply circuits to energize the dynodes, rather than the plurality of resistive elements. 
   In the illustrated embodiment, power supply circuit  300  is operating in a negative mode, such that Vcc is negative compared to ground. In some embodiments Vcc is in a range from about −1000 volts to about −20,000 volts. This range of voltages generates a voltage difference between adjacent dynode plates that is sufficient to draw electrons downstream along electron path  232 - 258  (shown in  FIG. 2 ). In another possible embodiment, Vcc is calculated by multiplying the voltage difference that is desired between adjacent dynodes by the number of dynodes. For example, if it is desired that the voltage difference between dynodes be about 100 volts, and there are fourteen dynodes, then Vcc is set at −1400 volts to achieve the desired 100 volt potential difference between each dynode. 
   In another possible embodiment, power supply circuit  300  is operated in a positive mode, such that Vcc is connected to the last resistive element (e.g., R 15 ) in place of the ground connection, and the ground connection is made to resistive element R 1  in place of Vcc. In this embodiment, Vcc is set to a positive voltage, such as in a range from about 1000 volts to about 20,000 volts. Other possible embodiments do not have a ground connection at either end of resistive elements R 1 -R 15 , but rather have two sources that generate a desired potential difference between R 1  and R 15 . 
   During the detection mode, power supply circuit  300  operates to maintain a substantially equal potential difference between each dynode. Power supply circuit  300  also operates in a blanking mode. During the blanking mode, some embodiments of power supply circuit  300  operate to adjust the potential difference between two or more adjacent dynodes, such that the voltage is substantially similar to each other. In other embodiments, the voltage at one dynode is adjusted such that it is less than the voltage at an adjacent upstream dynode. When the voltage across two adjacent dynodes is substantially similar, the electric field between the two dynodes is reduced, eliminated, or reversed, such that electrons are not drawn to the downstream dynode. Similarly, if the voltage at a dynode is less than an adjacent upstream dynode, the electric field between the two dynodes resists the electron movement from the upstream dynode to the downstream dynode. In this way, electron flow is blanked when power supply circuit  300  is operated in the blanking mode. However, even when operating in the blanking mode, some embodiments will still have some electrons that pass by the blanked dynodes. Nonetheless, the amount of electron flow in these embodiments will still be greatly reduced. 
   In some possible embodiments, the potential difference between adjacent dynodes is adjusted during the blanking mode so that it is in a range from about 50 percent to about negative 100 percent of the potential difference during the detection mode. For example, if the potential difference from dynode D 4  to D 5  is 100 volts during the detection mode, the potential difference from dynode D 4  to D 5  during the blanking mode is in a range from about 50 volts to about −100 volts. In another embodiment, the voltages at the adjacent dynodes are adjusted so that they are substantially similar. In one embodiment, the voltages at adjacent dynodes are substantially similar when they are within 10 percent of each other. 
   During the blanking mode, most of the electrons are not drawn toward the downstream dynode because the voltage is less than or substantially similar to the upstream dynode. As a result, the electrons will typically be absorbed by another structure within the detector. If the structure is not a dynode, the electron will typically not be detected by the detector. If the structure is another dynode, the velocity of the electron will typically not be great enough to liberate more electrons, such that the electron does not result in secondary emission at the detector. 
   As described in more detail herein, power supply circuit  300  is operated in the blanking mode, for example, at a time when the detector is expected to receive a known high abundance ion. In this way the detection of the high abundance ion is suppressed. 
   The illustrated embodiment of power supply circuit  300  operates in the blanking mode by utilizing pulse generator (Vpulse) and capacitive coupling C 1  and C 2 . The pulse generator is electrically coupled to one of the dynode plates, such as dynode plate  210  (at D 5 ). In the illustrated embodiment, the pulse generator includes isolating capacitor C 3  and a resistive element R 16 . 
   In some embodiments, power supply circuit  300  begins to operate in the blanking mode by generating a voltage pulse with pulse generator Vpulse. In some embodiments, the voltage pulse is a negative voltage pulse that is supplied to one of the dynode plates (e.g., dynode plate  210  at D 5 ). The negative voltage pulse is substantially similar to or greater than the potential difference between the dynode plate (e.g.,  210  at D 5 ) and the adjacent upstream dynode plate (e.g., dynode plate  210  at D 4 ). As a result, the voltage at the dynode plate (e.g.,  212  at D 5 ) is reduced, such that the adjacent dynode plates have a substantially similar voltage, such that the electric field is eliminated, or at least reduced to such a level that most electrons will not flow to the downstream dynode (e.g.,  212  at D 5 ) from the upstream dynode (e.g.,  210  at D 4 ). In another embodiment, the voltage at the dynode plate (e.g.,  212  at D 5 ) is reduced such that the voltage is less than the upstream dynode (e.g.,  210  at D 4 ). 
   Capacitive coupling is electrically coupled to each adjacent dynode plate, including the upstream dynode plate (e.g.,  208  at D 4 ) and the downstream dynode plate (e.g.,  212  at D 6 ). The first capacitive coupling C 1  is electrically coupled between Vcc and the upstream dynode plate (e.g.,  208  at D 4 ). The second capacitive coupling is electrically coupled between the downstream dynode plate (e.g.,  212  at D 6 ) and ground. 
   When operating in the detection mode, capacitive coupling C 1  and C 2  stores up energy. This energy is then used by capacitive coupling C 1  and C 2  when operating in the blanking mode, to maintain the voltage potential at the dynode plates to which they are electrically coupled. In this way, for example, the voltage at dynode plate  208  (D 4 ) is maintained constant or relatively constant during the blanking mode. Without capacitive coupling C 1 , the voltage at dynode plate  208  (D 4 ) would tend to adjust away from the voltage at dynode plate  210  (D 5 ), resulting in an undesired electric field between the adjacent dynode plates during blanking. Although certain types, arrangements, and structures of capacitive coupling are described herein as used in a power supply circuit, other embodiments include other types, structures, structures, and arrangements for storing charge and energizing the dynodes. 
   In the illustrated embodiment the fourth and fifth dynodes ( 208  at D 4  and  210  at D 5 ) are used for blanking, such that the potential at the fifth dynode  210  is adjusted to be substantially similar to the voltage at the fourth dynode  208 . In other embodiments, any two or more adjacent dynodes can be used for blanking. 
   One problem, however, of using the first and second dynodes, for example, is that ion  200  will sometimes travel through detector  108  and impact with one of the downstream dynodes (e.g., dynode  206 ) despite detector  108  being operated in the blanking mode. However, it has been found that ion  200  is less likely to bypass the blanking dynodes the further downstream they are. On the other hand, the further downstream the blanking occurs, the higher the current that will be generated in the upstream dynodes due to the impact of ion  200 . If the blanking occurs too far downstream in the detector, such as at the thirteenth and fourteenth dynodes (e.g.,  226  and  228 ), the upstream dynodes (e.g., dynode  212 ) will suffer from degradation from the excess current flow. As a result, it has been found to be beneficial in some embodiments to configure the power supply circuit  300  to supply the blanking pulse to a dynode located in a range from the fourth dynode to the seventh dynode. Other embodiments have other detector geometries that will benefit from having the blanking pulse delivered to a dynode outside of this range. 
   Power supply circuit  300  is operated in the blanking mode for a time period sufficient to suppress detection of one or more ions, such as a known high abundance ion. The time period varies in different embodiments based on factors such as the length of the flight tube, the acceleration of the ions, the ions to be blanked, and other factors. In one embodiment, the duration of the blanking pulse is in a range from about 100 picoseconds to about five nanoseconds. This range is typically sufficient to blank the detection of one or more undesired ions, but short enough so as to not inhibit detection of all subsequent ions. 
   After operating in the blanking mode, power supply circuit is operable to return the detector to the detection mode. Some embodiments will have a period of time that it takes for the power supply circuit to transition from the blanking mode back to the detection mode. For example, it will take dynode plate  210  some time to return to the appropriate detection mode voltage. In some embodiments, this restart time is in a range from about three nanoseconds to about ten nanoseconds. Therefore, the total time that power supply circuit  300  is blanked from detection of ions is the time of the blanking pulse plus the restart time. After the restart time has passed, the detector is operable to detect ion impacts with the detector. 
   A benefit of some embodiments according to the present disclosure is that the restart time is relatively short. One reason for this relatively short restart time is that some embodiments operate to adjust a dynode plate potential only tens or hundreds of volts during blanking, rather than a thousand or more volts. Switching of tens to hundreds of volts can be accomplished more rapidly than switching of a thousand or more volts, for example. Other embodiments operate to adjust the potential between the first dynode plate and the last dynode plate (and accordingly all intermediate dynode plates) to substantially the same voltage. Although these embodiments are also effective in inhibiting ion detection, the restart time will be relatively long because the entire voltage (e.g., 1400 volts) has to be reestablished. In contrast, by adjusting the potential between only two adjacent dynodes, only that portion of the voltage (e.g., 100 volts) has to be reestablished. As a result, the restart time after blanking is much faster 
   Other embodiments are possible with various modifications to the illustrated embodiment. For example, some embodiments will adjust more than two dynode plates to have a substantially similar voltage when operating in the blanking mode. Other embodiments will not use a pulse generator (Vpulse) but will instead use a charge dumping, a switch (such as illustrated in  FIG. 4 ), or other methods of voltage adjustment. In another embodiment, the power supply circuit operates to adjust the voltage at an upstream dynode to be substantially similar to or greater than the voltage at a downstream dynode, thereby blanking the detector. 
     FIG. 4  is an electrical schematic diagram of another exemplary embodiment of a power supply circuit  350  of detector  108 . Power supply circuit  350  is very similar to power supply circuit  300 , shown in  FIG. 3 , except that rather than using pulse generator (Vpulse, shown in  FIG. 3 ) to adjust the voltage at a blanking dynode (e.g.,  210  at D 5 ), power supply circuit  350  utilizes a switch S 1 . When operating in the detection mode, switch S 1  is maintained open, such that it does not influence the dynode voltages. The switch S 1  is then closed by power supply circuit  350  to operate in the blanking mode. When switch S 1  is closed, the blanking dynode (e.g.,  210  at D 5 ) is electrically coupled to the upstream dynode (e.g.,  208  at D 5 ). As a result, the voltage at the blanking dynode (e.g.,  210  at D 5 ) is adjusted to be substantially similar to the upstream dynode (e.g.,  208  at D 5 ), thereby blanking the detector. Capacitive coupling C 1 , C 2 , C 3 , and switch S 2  operate to maintain the voltage at the upstream and downstream dynodes (e.g.,  208  at D 4  and  212  at D 6 ) substantially constant during the blanking mode, and to quickly return the voltage to the appropriate levels when transitioning back to the detection mode. Switch S 2  is operated opposite switch S 1 . For example, when switch S 1  is open, switch S 2  is closed, and when switch S 1  is closed, switch S 2  is open. 
   When power supply circuit  350  is operating in the detection mode, switch S 1  is open and switch S 2  is closed. At this time, capacitor C 3  is charged due to the potential difference across resistor R 6  (and between D 5  and D 6 ). To adjust to the blanking mode, switch S 1  is closed and switch S 2  is opened. Closing of switch S 1  shorts the dynodes at D 5  and D 6  together, causing the voltage at each dynode to become substantially similar. At the same time, switch S 2  is opened, such that capacitor C 3  stores its charge during the blanking mode. When the blanking mode is complete, switch S 1  is opened and switch S 2  is closed. Upon closing of switch S 2 , the charge from capacitor C 3  is discharged to the dynode at D 5  to quickly restore the potential difference between the dynodes at D 4  and D 5  to return to the proper detection mode. Without switch S 2  and capacitor C 3 , a recovery current must flow through resistor R 6 , possibly resulting in the time required to restore the voltage of the dynode at D 5  to the proper potential being significantly longer. The delayed recovery could possibly result in a failure of the detector to detect a desired ion. 
     FIG. 5  is a flow chart illustrating an example method  400  of detecting ions. Method  400  includes operations  402 ,  404 ,  406 ,  408 ,  410 ,  412 ,  414 , and  416 . Method  400  begins with operation  402  during which a sample is ionized having a plurality of constituents. In some embodiments, the sample includes an analyte of interest and a known unwanted constituent. It is desirable to detect the analyte of interest and to blank the detection of the unwanted constituent. In some embodiments, operation  402  involves ionizing the sample with ion source  104 , such as shown in  FIG. 1 . 
   At the time that the sample is ionized, operation  404  is then performed to set a start time. In one embodiment, operation  404  involves checking a clock to determine a current time. In another embodiment, operation  404  involves resetting a clock to zero. 
   The sample is then accelerated into or along a flight tube in operation  406 . In one embodiment acceleration of the ionized sample involves applying an electric field to the ionized sample to propel the ion into or along the flight tube, such as flight tube  106  shown in  FIG. 1 . Once in the flight tube, the ion separates from other ions having different masses. 
   Operation  408  is then performed to determine whether the current time minus the start time of operation  404  is equal to the flight time of the unwanted constituent. In some embodiments the flight time of the unwanted constituent is read from memory, such as measured and stored during a prior sampling. In another embodiment, the flight time is determined by looking up the flight time from a look up table according to the type of unwanted constituent. The flight time is compared to the difference between the current time and the start time. If they are equal, then operation  410  is performed. Otherwise, operation  414  is performed. 
   During operation  410  the detector is blanked to inhibit the detection of the unwanted constituent, which is predicted to be impacting the detector at approximately the present time. In some embodiments, operation  410  begins a short time period prior to the time determined in operation  408 . Operation  410  continues for a predetermined time period. At operation  410 , the detector begins to operate in the blanking mode. In some embodiments of operation  410 , the potentials at two or more adjacent dynodes are adjusted such that they are substantially similar. 
   After operation  410 , the detector is restarted in operation  412 . In some embodiments, restarting of the detector involves a process of returning one or more dynodes to the appropriate detection voltages, such as by removing a supplied pulse, opening a switch, and other methods of voltage adjustment. In some embodiments, the termination of operation  412  marks the end of the blanking mode. 
   Operation  414  is performed to detect ion impacts, such as with detector  108 , shown in  FIG. 1 . In some embodiments, operation  414  involves measuring a current or a voltage from an output electrode (e.g., electrode  230 , shown in  FIG. 2 ). Some embodiments also determine the time of detection of the ion, and store the data in memory. Some embodiments store both the time data and the current or voltage data, and associate the data together. In this way, data relating to both the mass and the abundance of the detected ion is stored. In some embodiments, the stored data is analyzed to generate an output spectrum indicative of the content of the sample. 
   Operation  414  is then performed to determine whether detection is complete. In some embodiments, operation  414  determines whether a current time minus the start time is equal to or greater than the maximum flight time of any analyte of interest in the sample. If so, operation  414  determines that detection is complete and returns to operation  402  to perform the next sample. If not, operation  414  returns to operation  408 . 
   Some possible embodiments of method  400  include a process of automatic or manual calibration. For example, the detector can be operated without blanking for one or more samples to determine the time of flight of the unwanted constituent. If the unwanted constituent is a high abundance ion, the time of flight is easily determined by evaluating the resulting spectrum. The time of flight is then stored in memory and used for the decision of operation  408 . In some embodiments, the calibration process is repeated on a regular basis to ensure that the appropriate time of flight is being used. 
   The exemplary embodiment of the electron multiplier described herein is used as part of an ion detector for a mass spectrometer. However, various embodiments of the electron multiplier can be used in different types of mass spectrometers other than the one described herein. It also may be used in applications other than mass spectrometry. 
   The various embodiments described above are provided by way of illustration only and should not be construed to limit the claims attached hereto. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the following claims.