Patent Publication Number: US-10784098-B2

Title: Two-and-a-half channel detection system for time-of-flight (TOF) mass spectrometer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/470,486, filed Mar. 13, 2017, the content of which is incorporated by reference herein in its entirety. 
    
    
     INTRODUCTION 
     The teachings herein relate to an ion detection system for a time-of-flight (TOF) mass analyzer or mass spectrometer. More particularly, the teachings herein relate to an ion detection system that includes a novel arrangement of electrodes or light pipes and a two-channel digitizer that allows the ion detection system to account for the non-uniform shapes of ion packets and the non-uniform shapes of microchannel plates without sacrificing dynamic range or resolution. 
     BACKGROUND 
     Currently, some conventional TOF mass analyzers use ion detection systems that include four-channel digitizers. A four-channel digitizer can include either a time-to-digital converter (TDC) or an analog-to-digital converter (ADC), for example. Multichannel ion detection systems provide two main benefits: enhanced dynamic range and improved resolution through independent calibration of channels (also known as channel alignment). The use of analog detection can in principle replace the need for multiple channels from a dynamic range aspect, which may also result in better timing resolution of an ADC. However, the channel alignment benefit would disappear. This can be partially compensated for by various means of tilting either the ion packet or detector itself, but it does not remove the adverse effect of the ion packet curvature on resolution. Therefore, expensive four-channel ADCs have conventionally been used. 
     Unfortunately, however, four-channel TDC or ADC digitizers can account for close to 10% of the cost of a TOF mass spectrometer. As a result, there is a need for multichannel ion detection systems that can provide the same dynamic range and resolution as four-channel systems but at a lower cost. 
     SUMMARY 
     Two-channel electrical, photo-electrical, and planar photo-electrical TOF ion detection systems are provided. These systems maintain the resolution and dynamic range advantages of four-channel systems but at a lower cost. 
     The electrical two-channel ion detection system includes a series of one or more microchannel plates (MCPs), two or more segmented anode electrodes plates, and two-channel digitizer. The series of one or more MCPs is impacted by ion packets in a rectangular pattern on a first side. The series converts the impacts into multiplied electrons emitted in the rectangular pattern on a second side. Ions of each ion packet impact the first side at different times along the length of the rectangular pattern following a convex shape. Due to the convex shape of the ion packets, ions of each packet impact a central inner area of the rectangular pattern before impacting two outer areas at each end of the rectangular pattern. 
     Two or more segmented anode electrode plates are arranged in a plane parallel with the series of one or more MCPs and are positioned next to the series of one or more MCPs to receive the emitted electrons from the rectangular pattern on the second side. They include one or more inner electrodes positioned to receive emitted electrons from the central inner area of the rectangular pattern and one or more outer electrodes positioned to receive emitted electrons from the two outer areas at each end of the rectangular pattern. 
     The two-channel digitizer includes a first channel electrically connected to one or more inner electrodes that converts the electrons received into a first digital value. It includes a second channel electrically connected to the one or more outer electrodes that converts the electrons received into a second digital value. The first channel and the second channel are independently calibrated to align the first digital value and the second digital value in time and account for the convex shape of the ion impacts of each ion packet and/or the curvature of the series of one or more MCPs. 
     The photo-electrical two-channel ion detection system includes series of one or more MCPs, a scintillator, two or more segmented light pipes, a first photo-multiplier tube (PMT), a second PMT, and a two-channel digitizer. 
     The series of one or more MCPs is impacted by ion packets in a rectangular pattern on a first side. They convert the impacts into multiplied electrons emitted in the rectangular pattern on a second side. Ions of each ion packet impact the first side at different times along the length of the rectangular pattern following a convex shape. Due to the convex shape of the ion packets, ions of each packet impact a central inner area of the rectangular pattern before impacting two outer areas at each end of the rectangular pattern. 
     The scintillator is positioned next to and in parallel with the series of one or more MCPs. The scintillator receives the emitted electrons in the rectangular pattern on a first side from the second side of the series of one or more MCPs. The scintillator converts the electrons into photons emitted in the rectangular pattern on its second side. 
     Two or more segmented light pipes are connected to the second side of the scintillator to receive the photons emitted. They include one or more inner light pipes positioned to receive photons from the central inner area of the rectangular pattern and one or more outer light pipes positioned to receive photons from the two outer areas at each end of the rectangular pattern. 
     The first PMT is connected to one or more inner light pipes and converts the photons received into first multiplied electrons for each packet. The second PMT is connected to one or more outer light pipes and converts the photons received into second multiplied electrons for each packet. 
     The two-channel digitizer includes a first channel electrically connected to the first PMT that converts the first multiplied electrons for each ion packet into a first digital value and a second channel electrically connected to the second PMT that converts the second multiplied electrons for each ion packet into a second digital value. The first channel and the second channel are independently calibrated to align the first digital value and the second digital value in time and account for the convex shape of the ion impacts of each ion packet and/or the curvature of the series of one or more MCPs. 
     The planar photo-electrical two-channel ion detection system includes a magnet, a plurality of conducting meshes that are transparent to ions, a planar ion-to-electron converter, a scintillator, two or more segmented light pipes, a first photo-multiplier tube (PMT), a second PMT, and a two-channel digitizer. 
     The magnet is used to produce a magnetic field. The plurality of conducting meshes are biased by a voltage source to produce an electric field. 
     The planar ion-to-electron converter is impacted by ion packets in a rectangular pattern on a first side. The planar ion-to-electron converter converts the impacts into multiplied electrons emitted in the rectangular pattern on the same first side. Ions of each ion packet impact the first side at different times along the length of the rectangular pattern following a convex shape. Due to the convex shape of the ion packets, ions of each packet impact a central inner area of the rectangular pattern before impacting two outer areas at each end of the rectangular pattern. 
     The scintillator is positioned side by side with the planar ion-to-electron converter. The scintillator receives the emitted electrons in the rectangular pattern on a first side from the first side of the planar ion-to-electron converter. The scintillator converts the electrons into photons emitted in the rectangular pattern on its second side. The magnet and the plurality of conducting meshes are positioned to create the magnetic field and the electric field in front of the first side of the planar ion-to-electron converter and the first side of the scintillator so that the magnetic field and the electric field send the emitted electrons in a semi-circular path from the planar ion-to-electron converter to the scintillator. 
     Two or more segmented light pipes are connected to the second side of the scintillator to receive the photons emitted. They include one or more inner light pipes positioned to receive photons from the central inner area of the rectangular pattern and one or more outer light pipes positioned to receive photons from the two outer areas at each end of the rectangular pattern. 
     The first PMT is connected to one or more inner light pipes and converts the photons received into first multiplied electrons for each packet. The second PMT is connected to one or more outer light pipes and converts the photons received into second multiplied electrons for each packet. 
     The two-channel digitizer includes a first channel electrically connected to the first PMT that converts the first multiplied electrons for each ion packet into a first digital value and a second channel electrically connected to the second PMT that converts the second multiplied electrons for each ion packet into a second digital value. The first channel and the second channel are independently calibrated to align the first digital value and the second digital value in time and account for the convex shape of the ion impacts of each ion packet and/or the curvature of the series of one or more MCPs. 
     These and other features of the applicant&#39;s teachings are set forth herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The skilled artisan will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way. 
         FIG. 1  is a side view of a time-of-flight (TOF) ion detection system showing exemplary ion packets that each has an ideal shape and an ideal orientation just before they impact a microchannel plate (MCP) of the TOF ion detection system. 
         FIG. 2  is a side view of a TOF ion detection system showing exemplary ion packets that each has an ideal shape and a non-ideal orientation just before they impact an MCP of a TOF ion detection system. 
         FIG. 3  is a side view of a TOF ion detection system showing exemplary ion packets that each has a non-ideal shape and an ideal orientation just before they impact an MCP of a TOF ion detection system. 
         FIG. 4  is a side view of a TOF ion detection system showing exemplary ion packets that each has a non-ideal shape and an ideal orientation just before they impact an MCP of a TOF ion detection system that also has a non-ideal shape. 
         FIG. 5  is a side view of a TOF ion detection system showing how the digitized signals of exemplary ion packets that each has a non-ideal shape are obtained using four electrodes and a four-channel digitizer to improve resolution. 
         FIG. 6  is a front view of the impact side of the MCPs of  FIG. 5  showing that ion packets impact the MCPs in a rectangular pattern. 
         FIG. 7  is a front view of the four electrodes of  FIG. 5 . 
         FIG. 8  is an exemplary series of timing diagrams showing how the measurements from the four channels of the four-channel digitizer in  FIG. 5  are aligned or combined to compensate for the non-ideal shape of ion packets and improve the overall resolution of an ion detection system. 
         FIG. 9  is a side view of the same TOF ion detection system as shown in  FIG. 5  with exemplary ion packets that overlap. 
         FIG. 10  is an exemplary series of timing diagrams showing how the measurements from the four channels of the four-channel digitizer in  FIG. 9  are aligned or combined to compensate for the non-ideal shape of ion packets and improve the overall resolution of an ion detection system even when ion packets overlap. 
         FIG. 11  is a front view of three segmented anode electrode plates showing how three electrodes can be electrically connected producing just two sets of electrodes and used to detect the inner and outer portions of a rectangular pattern of emitted electrons, in accordance with various embodiments. 
         FIG. 12  is a front view of two segmented anode electrode plates showing how just two electrodes can be configured to detect the inner and outer portions of a rectangular pattern of emitted electrons, in accordance with various embodiments. 
         FIG. 13  is a side view of an electrical two-channel ion detection system for a TOF mass analyzer, in accordance with various embodiments. 
         FIG. 14  is a side view of a photo-electrical two-channel ion detection system for a TOF mass analyzer, in accordance with various embodiments. 
         FIG. 15  is a side view of a planar ion-to-electron photo-electrical two-channel ion detection system for a TOF mass analyzer, in accordance with various embodiments. 
     
    
    
     Before one or more embodiments of the present teachings are described in detail, one skilled in the art will appreciate that the present teachings are not limited in their application to the details of construction, the arrangements of components, and the arrangement of steps set forth in the following detailed description or illustrated in the drawings. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DESCRIPTION OF VARIOUS EMBODIMENTS 
     Two-Channel Ion Detection System 
     As described above, some conventional time-of-flight (TOF) mass analyzers use ion detection systems that include four-channel digitizers. A four-channel digitizer can include either a time-to-digital converter (TDC) or an analog-to-digital converter (ADC), for example. Multichannel ion detection systems provide two main benefits: enhanced dynamic range and improved resolution through independent calibration of channels (also known as channel alignment). 
     Unfortunately, however, four-channel TDC or ADC digitizers are expensive and can account for close to 10% of the cost of a TOF mass spectrometer. As a result, there is a need for multichannel ion detection systems that can provide the same or similar dynamic range and resolution as four-channel systems but at a lower cost. 
     One of ordinary skill in the art can appreciate that the terms “mass analyzer” and “mass spectrometer” can be used interchangeably. Generally, a mass analyzer refers to a device at one or more stages of a mass spectrometer. In other words, the mass analyzer is typically just one component of a mass spectrometer. However, it is common in industry practice to refer to an entire mass spectrometer in terms of its mass analyzer. For example, a mass spectrometer that includes a TOF mass analyzer is often referred to as a TOF mass spectrometer even though the TOF mass analyzer is just one component. 
     Resolution and Channel Alignment 
     One of the benefits of a multichannel ion detection system is improved resolution through independent calibration of channels, called channel alignment. Channel alignment is needed due to the non-ideal way in which ion packets are shaped when they impact the detector. 
       FIG. 1  is a side view  100  of a TOF ion detection system showing exemplary ion packets that each has an ideal shape and an ideal orientation just before they impact a microchannel plate (MCP) of the TOF ion detection system. An MCP is a device that converts ion impacts on one side of the MCP to electron emissions on the corresponding other side of the MCP. Typically, an MCP produces many electrons for each ion impact. As a result, an MCP acts as a multiplier or amplifier of ion impacts. Due to this amplification effect, multiple MCPs can also be used in series to increase the amplification of ion impacts. 
     The shapes of ion packets  101  and  102  are ideal with respect to MCP  110  of  FIG. 1  because they are essentially the same flat shape as MCP  110 . In other words, due to this shape, all of the ions of ion packet  101  will strike MCP  110  at the same time and all of the ions of ion packet  102  will also strike MCP  110  at the same time. 
     The orientations of ion packets  101  and  102  are ideal with respect to MCP  110  because they are essentially parallel to MCP  110 . Again, this orientation allows all of the ions of ion packet  101  to strike MCP  110  at the same time and all of the ions of ion packet  102  to strike MCP  110  at the same time. 
     The shape and orientation of ion packets are important because they affect the resolution of a TOF ion detection system. In a TOF ion detection system, resolution essentially refers to how well the distance between ion packets can be measured. In other words, the highest resolution would be the minimum distance between two ion packets where those two different ion packets could still be resolved. 
     The ideal shape and ideal orientation of ion packets  101  and  102  in  FIG. 1  allows for a very high resolution. Ion packets with this shape and orientation can be resolved even if they are placed much closer than ion packets  101  and  102 . Ion packets, however, with non-ideal shapes and non-ideal orientations can degrade resolution by increasing the minimum distance between two ion packets where those two different ion packets can still be resolved. 
       FIG. 2  is a side view  200  of a TOF ion detection system showing exemplary ion packets that each has an ideal shape and a non-ideal orientation just before they impact an MCP of a TOF ion detection system. In  FIG. 2 , ion packets  201  and  202  are oriented at an angle, or are tilted, with respect to MCP  110 . This tilting of ion packets  201  and  202  within the ion beam causes a decrease in resolution. 
     This decrease in resolution can be seen by determining if ion packets  201  and  202  can be placed closer together and still be distinguished at MCP  110 . If ion packet  201  is placed closer to ion packet  202  its leading edge immediately starts to overlap the trailing edge of ion packet  202 . If these edges overlap, the ion packets cannot be distinguished at MCP  110 . This means that ion packets  201  and  202  cannot be placed much closer together. Therefore, a comparison of  FIGS. 1 and 2  show how a non-ideal orientation can degrade resolution. 
     In practice, it is common for TOF mass analyzers to produce ion packets with tilted or non-ideal orientations. Fortunately, however, there is a conventional remedy to this problem. In order to compensate for the tilted packets, the MCP can be correspondingly tilted in a calibration step to account for ion packets with tilted or non-ideal orientations. Non-ideal ion packet shape can also degrade resolution. 
       FIG. 3  is a side view  300  of a TOF ion detection system showing exemplary ion packets that each has a non-ideal shape and an ideal orientation just before they impact an MCP of a TOF ion detection system. In  FIG. 3 , ion packets  301  and  302  have an arched sausage or convex shape with respect to MCP  110 . The length  311  of ion packet  301  is about 40 mm, and the depth of convexity  312  of ion packet  301  is much less than 1 mm, for example. The convex shape of ion packets  301  and  302  in TOF mass analyzers is common. 
     This convex shape reduces the resolution of the ion detection system. Like ion packets  201  and  202  of  FIG. 2 , ion packets  301  and  302  of  FIG. 3  cannot be resolved at MCP  110  if they are much closer than is shown in  FIG. 3 . This is because, for example, the two trailing edges of ion packet  302  would overlap with the leading edge of ion packet  301  if ion packets  301  and  302  are placed any closer together. Like ion packets, MCPs can also have non-ideal shapes. In practice, MCPs often have a convex shape. 
       FIG. 4  is a side view  400  of a TOF ion detection system showing exemplary ion packets that each has a non-ideal shape and an ideal orientation just before they impact an MCP of a TOF ion detection system that also has a non-ideal shape. In  FIG. 4 , MCP  210  has a convex shape that is often seen in practice. Any convexity of the MCP will also affect the resolution of the ion detection system. 
     Although, ion packets  310  and  320  also have convex shapes the amount of convexity seen in MCPs and ion packets typically do not correspond. As a result, both often have to be taken into account in order to improve resolution. 
     Four-Channel Digitizer 
     Conventional TOF ion detection systems have compensated for the loss of resolution caused by the convex shape of ion packets and the convex shape of an MCP by using four electrodes and a four-channel digitizer. 
       FIG. 5  is a side view  500  of a TOF ion detection system showing how the digitized signals of exemplary ion packets that each has a non-ideal shape are obtained using four electrodes and a four-channel digitizer to improve resolution. In  FIG. 5 , two MCPs  510  positioned in series are impacted by ion packets  301  and  302 , which have convex shapes. Multiplied electrons produced by MCPs  510  are collected by four segmented anode electrode plates  521 ,  522 ,  523 , and  524 . Each of anode electrode plates  521 ,  522 ,  523 , and  524  is electrically connected to a separate channel of four-channel digitizer  530 . 
     Four-channel digitizer  530  is, for example, an ADC or a TDC. Each of anode electrode plates  521 ,  522 ,  523 , and  524  can also be electrically connected to four-channel digitizer  530  through a four-channel preamplifier (not shown), for example. A four-channel preamplifier amplifies the electrical signal received from the electrode plates. 
     MCPs  510  essentially translate an ion impact image on one side to a corresponding electron emission image on the other side. Although ion packets  301  and  302  have convex shapes, their images on either side of MCPs  510  have a rectangular pattern or shape. 
       FIG. 6  is a front view  600  of the impact side of the MCPs of  FIG. 5  showing that ion packets impact the MCPs in a rectangular pattern. In  FIG. 6 , side  511  of MCPs  510  of  FIG. 5  are impacted by ion packets  301  and  302  of  FIG. 5  in a rectangular pattern or image  305 . Because ion packets  301  and  302  of  FIG. 5  have a convex shape, ions of each packet impact the central or inner portion of rectangular pattern  305  of  FIG. 6  first. Later in time, ions of each packet impact the outer two edges of rectangular pattern  305 . Typically, rectangular pattern  305  has a width  307  of about 10 mm and a length  309  of about 40 mm. Electrons are emitted from the other side of MCPs  510  of  FIG. 5  in the same rectangular pattern as rectangular pattern  305 . 
       FIG. 7  is a front view  700  of the four electrodes of  FIG. 5 .  FIG. 7  shows how four segmented anode electrode plates  521 ,  522 ,  523 , and  524  are positioned to detect ions from a circular MCP, for example. Electrons are emitted onto electrode  521 ,  522 ,  523 , and  524  using an MCP producing corresponding rectangular pattern  305  of electrons. 
     Each of anode electrode plates  521 ,  522 ,  523 , and  524  is able to detect a different part of the rectangular pattern  305  over time. Note that the rectangular pattern is most convex along the length of the rectangular pattern, because the rectangular pattern is much longer than it is wide. By detecting different parts of rectangular pattern  305  over time the convex shape of each ion packet is detected. 
     Returning to  FIG. 5 , the four channels  531 ,  532 ,  533 , and  534  of four-channel digitizer  530  are calibrated to combine or align the measurements from the different channels at different times to account for the lengthwise convexity of the ion packets. 
       FIG. 8  is an exemplary series of timing diagrams  800  showing how the measurements from the four channels of the four-channel digitizer in  FIG. 5  are aligned or combined to compensate for the non-ideal shape of ion packets and improve the overall resolution of an ion detection system. Each of the timing diagrams is a plot of the intensity of the electron flux as a function of time. 
     In  FIG. 8 , timing diagram  831  shows intensities  812  and  811  for ion packets  302  and  301 , respectively, of  FIG. 5  measured in channel  531  of four-channel digitizer  530  of  FIG. 5 . Timing diagram  832  of  FIG. 8  shows intensities  822  and  821  for ion packets  302  and  301 , respectively, of  FIG. 5  measured in channel  532  of four-channel digitizer  530  of  FIG. 5 . Timing diagram  833  of  FIG. 8  shows intensities  832  and  831  for ion packets  302  and  301 , respectively, of  FIG. 5  measured in channel  533  of four-channel digitizer  530  of  FIG. 5 . Finally, timing diagram  834  of  FIG. 8  shows intensities  842  and  841  for ion packets  302  and  301 , respectively, of  FIG. 5  measured in channel  534  of four-channel digitizer  530  of  FIG. 5 . 
     In timing diagram  850  of  FIG. 8 , the intensities measured in timing diagrams  831 ,  832 ,  833 , and  834  are combined. For example, these values are summed in diagram  850 . This results in two intensity peaks for each of ion packets  302  and  301  of  FIG. 5 , one that is a combination of measurements from the two inner electrode plates  522  and  523  of  FIG. 5  and one that is a combination of measurements from the two outer electrode plates  521  and  524  of  FIG. 5 . For example, in timing diagram  850  of  FIG. 8 , peaks  851  and  852  are the two intensity peaks measured from ion packet  302  of  FIG. 5  and peaks  853  and  854  are the two intensity peaks measured from ion packet  301  of  FIG. 5 . 
     Note that in  FIG. 5 , due to the convex shape of the ion packets, the time difference between the detection of the central or inner ions of an ion packet at electrodes  522  and  523  and the detection of the outer ions of an ion packet at electrodes  521  and  524  is Δt  501 . In  FIG. 8 , this Δt  501  as the difference between the centers of peaks  851  and  852  and Δt  502  as the difference between the centers of peaks  853  and  854 . This time difference Δt  501  or Δt  502  produced by the convex shapes of the ion packets decreases the detection resolution. It decreases the detection resolution by decreasing the space between the intensities that can be measured for two different packets. In other words, as shown in timing diagram  850 , because the intensities of the single ion packet are spread out over time due to the convex shape of the ion packet, the resolution is reduced. 
     However, because multiple channels are used to measure different parts of the convex shape of an ion packet, it is possible to compensate for the spreading out of intensities. This is shown in timing diagram  860 . Essentially, peaks  851  and  852  for ion packet  302  of  FIG. 5  are combined into peak  861 , and peaks  853  and  854  for ion packet  302  of  FIG. 5  are combined into peak  862  in timing diagram  860  of  FIG. 8 . In other words, digitizer  530  of  FIG. 5  is calibrated to align the intensities of channels  531  and  534  with the intensities of channels  532  and  533 . This calibration is done, for example, using the calibration equation m=a×(t−t 0 ) 2 , where m is mass, a is slope, t is time, and to is the time offset. Once calibrated, the intensities of all four channels are combined. 
     Timing diagram  860  of  FIG. 8  shows that the resolution has been restored. In other words, the spacing between the peaks ( 861  and  862 ) of different packets has been increased. This can be shown more clearly if the ion packets of  FIG. 5  are overlapping. 
       FIG. 9  is a side view  900  of the same TOF ion detection system as shown in  FIG. 5  with exemplary ion packets that overlap. In  FIG. 9 , the leading of ion packet  901  overlaps with the trailing edge of ion packet  902 . If only one electrode and one digitizing channel were used, ion packets  901  and  902  could not be distinguished. However, by using separated electrodes and a four-channel digitizer, packets  901  and  902  can be distinguished. 
       FIG. 10  is an exemplary series of timing diagrams  1000  showing how the measurements from the four channels of the four-channel digitizer in  FIG. 9  are aligned or combined to compensate for the non-ideal shape of ion packets and improve the overall resolution of an ion detection system even when ion packets overlap. In  FIG. 10 , timing diagram  1031  shows intensities  1012  and  1011  for ion packets  902  and  901 , respectively, of  FIG. 9  measured in channel  531  of four-channel digitizer  530  of  FIG. 9 . Timing diagram  1032  of  FIG. 10  shows intensities  1022  and  1021  for ion packets  902  and  901 , respectively, of  FIG. 9  measured in channel  532  of four-channel digitizer  530  of  FIG. 9 . Timing diagram  1033  of  FIG. 10  shows intensities  1032  and  1031  for ion packets  902  and  901 , respectively, of  FIG. 9  measured in channel  533  of four-channel digitizer  530  of  FIG. 9 . Finally, timing diagram  1034  of  FIG. 10  shows intensities  1042  and  1041  for ion packets  902  and  901 , respectively, of  FIG. 9  measured in channel  534  of four-channel digitizer  530  of  FIG. 9 . 
     In timing diagram  1050  of  FIG. 10 , the intensities measured in timing diagrams  1031 ,  1032 ,  1033 , and  1034  are combined. This results in two intensity peaks for each of ion packets  902  and  901  of  FIG. 9 , one that is a combination of measurements from the two inner electrode plates  522  and  523  of  FIG. 9  and one that is a combination of measurements from the two outer electrode plates  521  and  524  of  FIG. 9 . For example, in timing diagram  1050  of  FIG. 10 , peaks  1051  and  1052  are the two intensity peaks measured from ion packet  902  of  FIG. 9  and peaks  1053  and  1054  are the two intensity peaks measured from ion packet  901  of  FIG. 9 . 
     Note in  FIG. 10  that peak  1052  of ion packet  902  of  FIG. 9  overlaps with peak  1053  of ion packet  901  of  FIG. 9 . This shows that the overlap caused by the convex shapes of the ion packets in  FIG. 9  reduces the resolution. 
     However, because multiple channels are used to measure different parts of the convex shape of an ion packet, it is possible to compensate for this overlap. This is shown in timing diagram  1060 . Essentially, peaks  1051  and  1052  for ion packet  902  of  FIG. 9  are combined into peak  1061 , and peaks  1053  and  1054  for ion packet  902  of  FIG. 9  are combined into peak  1062  in timing diagram  1060  of  FIG. 10 . This is done, for example, by recalibrating channels  531  and  534  to match peak position on channels  532  and  533 . Once recalibrated, the intensities of all four channels are combined and the overlap is eliminated. 
     Two-Channel Digitizer 
     As described above, however, four-channel TDC or ADC digitizers are expensive and can account for close to 10% of the cost of a TOF mass spectrometer. In various embodiments, a two-channel digitizer is used instead to reduce the cost of a TOF mass spectrometer. 
     It is possible to use a two-channel digitizer due to the symmetric nature of the convexity of ion packets and MCPs. As shown in  FIG. 8 , timing diagrams  831  and  834  for the outer channels have measured intensities at similar times. Likewise, timing diagrams  832  and  833  for the inner channels also have measured intensities at similar times. 
     As a result, in various embodiments, it is possible to measure intensities from just two sets of electrodes using a two-channel digitizer. For example, in  FIG. 7 , outer electrodes  521  and  524  can be electrically connected and inner electrodes  521  and  524  can be electrically connected producing just two sets of electrodes. Each of these sets of electrodes can then be connected to a channel of a two-channel digitizer. Similarly, inner electrodes  521  and  524  of  FIG. 7  can be combined into a single inner electrode. 
       FIG. 11  is a front view  1100  of three segmented anode electrode plates showing how three electrodes can be electrically connected producing just two sets of electrodes and used to detect the inner and outer portions of a rectangular pattern of emitted electrons, in accordance with various embodiments. In FIG.  11 , outer electrode plates  1121  and  1123  are electrically connected, producing a first set of electrodes. Single electrode plate  1122  provides the second set of electrodes. As a result, electrode plates  1121  and  1123  are electrically connected to a first channel of a two-channel digitizer, and electrode plate  1122  is electrically connected to a second channel of the two-channel digitizer. 
     In this configuration, outer electrode plates  1121  and  1123  detect the outer portions of the rectangular pattern of emitted electrons  305  from an MCP. Similarly, single electrode plate  1122  detects the inner portion of the rectangular pattern of emitted electrons  305  from the MCP. 
     It is also possible to have two sets of electrodes where each set only includes one electrode. Essentially, an inner or central electrode can be surrounded by an outer ring electrode. Such a configuration can be circular. However, almost any other shape is possible. For example, a central rectangle can be surrounded by a rectangular ring. 
       FIG. 12  is a front view  1200  of two segmented anode electrode plates showing how just two electrodes can be configured to detect the inner and outer portions of a rectangular pattern of emitted electrons, in accordance with various embodiments. In  FIG. 12 , outer ring electrode plate  1221  is electrically connected to a first channel of a two-channel digitizer. Inner disk electrode plate  1222  is electrically connected to a second channel of the two-channel digitizer. 
     In this configuration, outer ring electrode plate  1221  detects the outer portions of the rectangular pattern of emitted electrons  305  from an MCP. Similarly, inner disk electrode plate  1222  detects the inner portion of the rectangular pattern of emitted electrons  305  from the MCP. 
     Two-Channel Digitizer Dynamic Range 
     As described above, one advantage of a four-channel digitizer is a high dynamic range. Dynamic range is the ratio of the largest and smallest values a digitizer can measure. For example, a four-channel 10-bit ADC has a dynamic range of 4×2 10  or 4,096. 
     Replacing a four-channel digitizer with a two-channel digitizer would reduce the dynamic range by a factor of two. However, the dynamic range can be recaptured by increasing the number of bits of the two-channel digitizer. For example, a 14-bit two-channel ADC can be used. In this case, the dynamic range is 2×2 14  or 32,768 values. As a result, the dynamic range lost by decreasing the number of channels can not only be recovered but it can also be increased by increasing the number of bits used. 
     Two-Channel Electrical Ion Detection System 
       FIG. 13  is a side view  1300  of an electrical two-channel ion detection system for a time-of-flight (TOF) mass analyzer, in accordance with various embodiments. The electrical two-channel ion detection system includes series of one or more microchannel plates  1310 , two or more segmented anode electrodes plates  1321 ,  1322 , and  1323 , and two-channel digitizer  1330 . 
     In various embodiments, two-channel digitizer  1330  is a two-channel analog-to-digital converter (ADC). In various embodiments, two-channel digitizer  1330  is a two-channel time-to-digital converter (TDC). Further, in various embodiments, two-channel digitizer  1330  can include a pre-amplifier for each of its two channels. 
     The first plate of series of one or more microchannel plates  1310  is impacted by ion packets  1301  and  1302  in a rectangular pattern on a first side  1311  of series of one or more microchannel plates  1310 . Series of one or more microchannel plates  1310  converts the impacts into multiplied electrons emitted in the rectangular pattern on a second side  1312  of series of one or more microchannel plates  1310 . The longer side of the rectangular pattern is the length and a shorter side of the rectangular pattern is the width. Ions of each ion packet impact first side  1311  at different times along the length of the rectangular pattern following a convex shape. Due to the convex shape of ion packets  1301  and  1302 , ions of each packet impact a central inner area of the rectangular pattern before impacting two outer areas at each end of the rectangular pattern. 
     Two or more segmented anode electrode plates  1321 ,  1322 , and  1323  are arranged in a plane parallel with series of one or more microchannel plates  1310 . Two or more electrodes  1321 ,  1322 , and  1323  are positioned next to series of one or more microchannel plates  1310  to receive the emitted electrons from the rectangular pattern on second side  1312  of series of one or more microchannel plates  1310 . Two or more electrodes  1321 ,  1322 , and  1323  together have an area large enough to receive electrons from the rectangular pattern. 
     Two or more electrodes  1321 ,  1322 , and  1323  include one or more inner electrodes  1322  positioned to receive emitted electrons from the central inner area of the rectangular pattern. Two or more electrodes  1321 ,  1322 , and  1323  include one or more outer electrodes  1321  and  1323  positioned to receive emitted electrons from the two outer areas at each end of the rectangular pattern. 
     In various embodiments, the one or more inner electrodes include one inner electrode, as shown in  FIG. 11 . In various embodiments, the one or more inner electrodes include two inner electrodes that are electrically connected (not shown). 
     In various embodiments, the one or more outer electrodes include two electrodes that are electrically connected and each of the two electrodes receives electrons from different areas of the two outer areas at each end of the rectangular pattern as shown in  FIG. 11 . 
     In various embodiments, the one or more inner electrodes include a single disk electrode, the one or more outer electrodes include a single ring electrode, and the disk electrode and the ring electrode are concentric, as shown in  FIG. 12 . 
     Returning to  FIG. 13 , two-channel digitizer  1330  includes a first channel  1331  electrically connected to one or more inner electrodes  1322 . Two-channel digitizer  1330  converts the electrons received by one or more inner electrodes  1322  for each ion packet into a first digital value. 
     Two-channel digitizer  1330  includes a second channel  1332  electrically connected to one or more outer electrodes  1321  and  1323 . Two-channel digitizer  1330  converts the electrons received by one or more outer electrodes  1321  and  1323  for each packet into a second digital value. 
     First channel  1331  and second channel  1332  are independently calibrated to align the first digital value and the second digital value in time and account for the convex shape of the ion impacts of each ion packet. 
     In various embodiments, first channel  1331  and second channel  1332  are further independently calibrated to align the first digital value and the second value digital in time and account for curvature of series of one or more microchannel plates  1310 . 
     Two-Channel Photo-Electrical Ion Detection System 
     In various embodiments, a two-channel ion detection system for a TOF mass analyzer can include optical components to detect the electrons produced by series of one or more MCPs. Essentially, these optical components replace the segmented anode electrode plates of the electrical systems described above. As a result, all of the configurations of electrodes described above also apply to the optical components or light pipes of a photo-electrical system. 
       FIG. 14  is a side view  1400  of a photo-electrical two-channel ion detection system for a TOF mass analyzer, in accordance with various embodiments. The photo-electrical two-channel ion detection system includes series of one or more microchannel plates  1410 , scintillator  1420 , two or more segmented light pipes  1431 ,  1432 ,  1433  and  1434 , first photo-multiplier tube (PMT)  1441 , second PMT  1442 , and two-channel digitizer  1450 . 
     In various embodiments, two-channel digitizer  1450  is a two-channel analog-to-digital converter (ADC). In various embodiments, two-channel digitizer  1450  is a two-channel time-to-digital converter (TDC). 
     The first one of series of one or more microchannel plates  1410  is impacted by ion packets  1401  in a rectangular pattern on a first side  1411  of series of one or more microchannel plates  1410 . Series of one or more microchannel plates  1410  converts the impacts into multiplied electrons emitted in the rectangular pattern on a second side  1412  of series of one or more microchannel plates  1410 . A longer side of the rectangular pattern is the length and a shorter side of the rectangular pattern is the width. Due to the convex shape of ion packet  1401 , for example, ions of each packet impact a central inner area of the rectangular pattern before impacting two outer areas at each end of the rectangular pattern. 
     Scintillator  1420  is positioned in parallel with series of one or more microchannel plates  1410  and next to series of one or more microchannel plates  1410 . Scintillator  1420  receives the emitted electrons in the rectangular pattern on a first side  1421  of scintillator  1420  from second side  1412  of series of one or more microchannel plates  1410 . Scintillator  1420  converts the electrons into photons emitted in the rectangular pattern on a second side  1422  of scintillator  1420 . 
     Two or more segmented light pipes  1431 ,  1432 ,  1433 , and  1434  are connected to second side  1422  of scintillator  1420  to receive the photons from second side  1422  of scintillator  1420 . Two or more segmented light pipes  1431 ,  1432 ,  1433 , and  1434  together have an area large enough to receive photons from the rectangular pattern. Two or more light pipes  1431 ,  1432 ,  1433 , and  1434  include one or more inner light pipes  1432  and  1433  positioned to receive photons from the central inner area of the rectangular pattern. Two or more light pipes  1431 ,  1432 ,  1433 , and  1434  include one or more outer light pipes  1431  and  1434  positioned to receive photons from the two outer areas at each end of the rectangular pattern. 
     In various embodiments, the one or more inner light pipes include one light pipe, similar to the one electrode of  FIG. 11 . In various embodiments, the one or more light pipes include two inner light pipes that are connected, as shown in  FIG. 14 . 
     In various embodiments, the one or more outer light pipes include two light pipes that are connected and each of the two light pipes receives photons from different areas of the two outer areas at each end of the rectangular pattern as shown in  FIG. 14 . 
     In various embodiments, the one or more inner light pipes include a single disk light pipe, the one or more outer light pipes include a single ring light pipe, and the disk light pipe and the ring light pipe are concentric, similar to the electrodes shown in  FIG. 12 . 
     Returning to  FIG. 14 , first photo-multiplier tube  1441  is connected to one or more inner light pipes  1432  and  1433  and converts the photons received by one or more inner light pipes  1432  and  1433  into first multiplied electrons for each packet. Second photo-multiplier tube  1442  is connected to one or more outer light pipes  1431  and  1434  and converts the photons received by one or more outer light pipes  1431  and  1434  into second multiplied electrons for each packet. 
     Two-channel digitizer  1450  includes a first channel  1451  electrically connected to first photo-multiplier tube  1441  that converts the first multiplied electrons for each ion packet into a first digital value. Two-channel digitizer  1450  includes a second channel  1452  electrically connected to second photo-multiplier tube  1442  that converts the second multiplied electrons for each ion packet into a second digital value. 
     First channel  1451  and second channel  1452  are independently calibrated to align the first digital value and the second digital value in time and account for the convex shape of the ion impacts of each ion packet. 
     In various embodiments, first channel  1451  and second channel  1452  are further independently calibrated to align the first digital value and the second digital value in time and account for the curvature of series of one or more microchannel plates  1410 . 
     Two-Channel Planar Photo-Electrical Ion Detection System 
     In various embodiments, a two-channel ion detection system for a TOF mass analyzer can include a planar ion-to-electron converter, a magnetic field, and optical components to detect ions. Essentially, the planar ion-to-electron converter, a combination of electric and magnetic fields, and optical components replace the MCPs and the segmented anode electrode plates of the electrical systems described above. 
       FIG. 15  is a side view  1500  of a planar ion-to-electron photo-electrical two-channel ion detection system for a TOF mass analyzer, in accordance with various embodiments. The planar ion-to-electron photo-electrical two-channel ion detection system includes planar ion-to-electron converter  1510 , normal magnetic field  1513 , scintillator  1420 , two or more segmented light pipes  1431 ,  1432 ,  1433  and  1434 , first photo-multiplier tube (PMT)  1441 , second PMT  1442 , and two-channel digitizer  1450 . 
     In various embodiments, two-channel digitizer  1450  is a two-channel analog-to-digital converter (ADC). In various embodiments, two-channel digitizer  1450  is a two-channel time-to-digital converter (TDC). 
     Planar ion-to-electron converter  1510  is impacted by ion packets  1401  in a rectangular pattern on a first side  1511  of planar ion-to-electron converter  1510 . Planar ion-to-electron converter  1510  includes a material that has high electron emission probability per impinging ion such as CVD diamond or oxides or other materials known for their high secondary emission coefficients. 
     The two-channel ion detection system further includes a DC homogeneous magnetic field  1513  from a permanent or electromagnet (not shown). Magnetic field  1513  is established in front of planar ion-to-electron converter  1510  and scintillator  1420 . Electric field  1414  is also established in front of planar ion-to-electron converter  1510  and scintillator  1420 . Electric field  1514  is established by appropriately biasing highly transparent meshes  1515  using a voltage source (not shown), for example. Electric field  1514  and magnetic field  1513  are designed to cause electrons also emitted from first side  1511  of planar ion-to-electron converter  1510  to move in a semi-circular path to scintillator  1420 . U.S. Pat. No. 7,180,060, which is herein incorporated by reference, describes use of a planar ion-to-electron converter, a magnetic field, and an electric field to move electrons emitted from ions in a semi-circular path to a detector member. 
     Planar ion-to-electron converter  1510  converts the impacts into electrons emitted in the same rectangular pattern on the same first side  1511  of planar ion-to-electron converter  1510 . A longer side of the rectangular pattern is the length and a shorter side of the rectangular pattern is the width. Due to the convex shape of ion packet  1401 , for example, ions of each packet impact a central inner area of the rectangular pattern before impacting two outer areas at each end of the rectangular pattern. 
     Scintillator  1420  is positioned side by side with planar ion-to-electron converter  1510 . Scintillator  1420  receives the emitted electrons in the rectangular pattern on a first side  1421  of scintillator  1420  from first side  1511  of planar ion-to-electron converter  1510 . Scintillator  1420  converts the electrons into photons emitted in the rectangular pattern on a second side  1422  of scintillator  1420 . 
     Two or more segmented light pipes  1431 ,  1432 ,  1433 , and  1434  are connected to second side  1422  of scintillator  1420  to receive the photons from second side  1422  of scintillator  1420 . Two or more segmented light pipes  1431 ,  1432 ,  1433 , and  1434  together have an area large enough to receive photons from the rectangular pattern. Two or more light pipes  1431 ,  1432 ,  1433 , and  1434  include one or more inner light pipes  1432  and  1433  positioned to receive photons from the central inner area of the rectangular pattern. Two or more light pipes  1431 ,  1432 ,  1433 , and  1434  include one or more outer light pipes  1431  and  1434  positioned to receive photons from the two outer areas at each end of the rectangular pattern. 
     In various embodiments, the one or more inner light pipes include one light pipe, similar to the one electrode of  FIG. 11 . In various embodiments, the one or more light pipes include two inner light pipes that are connected, as shown in  FIG. 15 . 
     In various embodiments, the one or more outer light pipes include two light pipes that are connected and each of the two light pipes receives photons from different areas of the two outer areas at each end of the rectangular pattern as shown in  FIG. 15 . 
     In various embodiments, the one or more inner light pipes include a single disk light pipe, the one or more outer light pipes include a single ring light pipe, and the disk light pipe and the ring light pipe are concentric, similar to the electrodes shown in  FIG. 12 . 
     Returning to  FIG. 15 , first photo-multiplier tube  1441  is connected to one or more inner light pipes  1432  and  1433  and converts the photons received by one or more inner light pipes  1432  and  1433  into first multiplied electrons for each packet. Second photo-multiplier tube  1442  is connected to one or more outer light pipes  1431  and  1434  and converts the photons received by one or more outer light pipes  1431  and  1434  into second multiplied electrons for each packet. 
     Two-channel digitizer  1450  includes a first channel  1451  electrically connected to first photo-multiplier tube  1441  that converts the first multiplied electrons for each ion packet into a first digital value. Two-channel digitizer  1450  includes a second channel  1452  electrically connected to second photo-multiplier tube  1442  that converts the second multiplied electrons for each ion packet into a second digital value. 
     First channel  1451  and second channel  1452  are independently calibrated to align the first digital value and the second digital value in time and account for the convex shape of the ion impacts of each ion packet. 
     In various embodiments, first channel  1451  and second channel  1452  are further independently calibrated to align the first digital value and the second digital value in time and account for the curvature of series of one or more microchannel plates  1410 . 
     While the present teachings are described in conjunction with various embodiments, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art. 
     Further, in describing various embodiments, the specification may have presented a method and/or process as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the various embodiments.