Patent Publication Number: US-2023154741-A1

Title: Instrument for separating ions including an electrostatic linear ion trap to simultaneously trap multiple ions

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/293,852, filed May 13, 2021, which is a U.S. national stage entry of PCT Application No. PCT/US2019/013285, filed Jan. 11, 2019, which claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/774,703, filed Dec. 3, 2018, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under CHE1531823 awarded by the National Science Foundation. The United States Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to charge detection mass spectrometry instruments, and more specifically to instruments for simultaneously analyzing multiple ions with an electrostatic linear ion trap. 
     BACKGROUND 
     Mass Spectrometry provides for the identification of chemical components of a substance by separating gaseous ions of the substance according to ion mass and charge. Various instruments have been developed for determining the masses of such separated ions, and one such instrument is a charge detection mass spectrometer (CDMS). CDMS is conventionally a single-particle instrument and technique in which ion mass is determined for each ion individually as a function of measured ion mass-to-charge ratio, typically referred to as “m/z,” and measured ion charge. Some such CDMS instruments employ an electrostatic linear ion trap (ELIT) detector in which ions are made to oscillate back and forth through a charge detection cylinder. Multiple passes of ions through such a charge detection cylinder provides for multiple measurements for each ion, and such multiple measurements are then processed to determine ion m/z and charge from which the ion mass can be calculated. 
     Single particle CDMS is a time consuming process which typically requires several hours to measure and obtain a mass spectrum. It is desirable to develop CDMS instruments and techniques which decrease sample analysis durations. 
     SUMMARY 
     The present disclosure may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof. In a first aspect, a charge detection mass spectrometer (CDMS) may comprise an ion source configured to generate a beam of ions, a mass spectrometer configured to separate the generated beam of ions as a function of ion mass-to-charge ratio to produce a resulting beam of separated ions, an electrostatic linear ion trap (ELIT) including a pair of coaxially aligned ion mirrors and an elongated charge detection cylinder disposed therebetween and coaxially aligned therewith such that a longitudinal axis of the ELIT passes centrally through each, a first one of the pair of ions mirror defining an ion inlet aperture about the longitudinal axis through which the beam of separated ions enters the ELIT, at least one voltage source operatively coupled to the pair of ion mirrors and configured to produce voltages for selectively establishing electric fields therein configured to trap within the ELIT a plurality of ions in the entering beam of separated ions and to cause the plurality of trapped ions to oscillate back and forth between the pair of ion mirrors each time passing through the charge detection cylinder, and means for controlling a trajectory of the beam of separated ions entering the ion inlet aperture of the ELIT to cause the plurality of ions subsequently trapped within the ELIT to oscillate therein with a corresponding plurality of different planar ion oscillation trajectories angularly offset from one another about the longitudinal axis with each extending along the longitudinal axis and crossing the longitudinal axis in each of the pair of ion mirrors or a corresponding plurality of different cylindrical ion oscillation trajectories radially offset from one another about the longitudinal axis to form a plurality of nested cylindrical trajectories each extending along the longitudinal axis. 
     In a second aspect, an instrument for separating ions may comprise an ion source configured to generate ions from a sample, a first mass spectrometer configured to separate the generated ions as a function of mass-to-charge ratio, an ion dissociation stage positioned to receive ions exiting the first mass spectrometer and configured to dissociate ions exiting the first mass spectrometer, a second mass spectrometer configured to separate dissociated ions exiting the ion dissociation stage as a function of mass-to-charge ratio, and a charge detection mass spectrometer (CDMS) coupled in parallel with and to the ion dissociation stage such that the CDMS can receive ions exiting either of the first mass spectrometer and the ion dissociation stage, the CDMS comprising (i) at least one ion separation instrument configured to separate ions as a function of at least one molecular characteristic, (ii) an electrostatic linear ion trap (ELIT) including a pair of coaxially aligned ion mirrors and an elongated charge detection cylinder disposed therebetween and coaxially aligned therewith such that a longitudinal axis of the ELIT passes centrally through each, a first one of the pair of ions mirror defining an ion inlet aperture about the longitudinal axis through which the beam of separated ions enters the ELIT, (iii) at least one voltage source operatively coupled to the pair of ion mirrors and configured to produce voltages for selectively establishing electric fields therein configured to trap within the ELIT a plurality of ions in the entering beam of separated ions and to cause the plurality of trapped ions to oscillate back and forth between the pair of ion mirrors each time passing through the charge detection cylinder, and (iv) means for controlling a trajectory of the beam of separated ions entering the ion inlet aperture of the ELIT to cause the plurality of ions subsequently trapped within the ELIT to oscillate therein with a corresponding plurality of different planar ion oscillation trajectories angularly offset from one another about the longitudinal axis with each extending along the longitudinal axis and crossing the longitudinal axis in each of the pair of ion mirrors or a corresponding plurality of different cylindrical ion oscillation trajectories radially offset from one another about the longitudinal axis to form a plurality of nested cylindrical trajectories each extending along the longitudinal axis, wherein masses of precursor ions exiting the first mass spectrometer are measured using the CDMS, mass-to-charge ratios of dissociated ions of precursor ions having mass values below a threshold mass are measured using the second mass spectrometer, and mass-to-charge ratios and charge values of dissociated ions of precursor ions having mass values at or above the threshold mass are measured using the CDMS. 
     In a third aspect, a method is provided for simultaneously measuring at least two ions in a beam of ions supplied to an electrostatic linear ion trap (ELIT) including a pair of coaxially aligned ion mirrors and an elongated charge detection cylinder disposed therebetween and coaxially aligned therewith such that a longitudinal axis of the ELIT passes centrally through each, wherein a first one of the pair of ions mirror defines an ion inlet aperture about the longitudinal axis through which the supplied beam of ions enters the ELIT. The method may comprise controlling at least one voltage source to apply voltages to the pair of ion mirrors to establish an ion transmission electric field therein to pass the beam of ions supplied to the ion inlet aperture of the ELIT through each of the pair of ion mirrors and through the charge detection cylinder and through an ion exit defined by a second one of the pair of ion mirrors, wherein each ion transmission electric field is configured to focus ions passing therethrough toward the longitudinal axis, controlling the at least one voltage source to modify the voltages applied to the pair of ion mirrors to establish an ion reflection electric field therein to trap within the ELIT at least two of the ions in the beam of ions supplied to the ion inlet aperture of the ELIT, wherein each ion reflection electric field is configured to cause ions entering a respective one of the pair of ion mirrors from the charge detection cylinder to stop and accelerate in an opposite direction back through the charge detection cylinder and toward the other of the pair of ion mirrors while also focusing the ions toward the longitudinal axis, and controlling a trajectory of the beam of ions entering the ion inlet aperture of the ELIT to cause the at least two ions subsequently trapped within the ELIT to oscillate therein with at least two different planar ion oscillation trajectories angularly offset from one another about the longitudinal axis with each extending along the longitudinal axis and crossing the longitudinal axis in each of the pair of ion mirrors. 
     In a fourth aspect, a method is provided for simultaneously measuring at least two ions in a beam of ions supplied to an electrostatic linear ion trap (ELIT) including a pair of coaxially aligned ion mirrors and an elongated charge detection cylinder disposed therebetween and coaxially aligned therewith such that a longitudinal axis of the ELIT passes centrally through each, wherein a first one of the pair of ions mirror defines an ion inlet aperture about the longitudinal axis through which the supplied beam of ions enters the ELIT. The method may comprise controlling at least one voltage source to apply voltages to the pair of ion mirrors to establish an ion transmission electric field therein to pass the beam of ions supplied to the ion inlet aperture of the ELIT through each of the pair of ion mirrors and through the charge detection cylinder and through an ion exit defined by a second one of the pair of ion mirrors, wherein each ion transmission electric field is configured to focus ions passing therethrough toward the longitudinal axis, controlling the at least one voltage source to modify the voltages applied to the pair of ion mirrors to establish an ion reflection electric field therein to trap within the ELIT at least two of the ions in the beam of ions supplied to the ion inlet aperture of the ELIT, wherein each ion reflection electric field is configured to cause ions entering a respective one of the pair of ion mirrors from the charge detection cylinder to stop and accelerate in an opposite direction back through the charge detection cylinder and toward the other of the pair of ion mirrors while also focusing the ions toward the longitudinal axis, and controlling a trajectory of the beam of ions entering the ion inlet aperture of the ELIT to cause the at least two ions subsequently trapped within the ELIT to oscillate therein with at least two different cylindrical ion oscillation trajectories radially offset from one another about the longitudinal axis to form at least two nested cylindrical ion oscillation trajectories each extending along the longitudinal axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a simplified diagram of a CDMS system including an embodiment of an electrostatic linear ion trap (ELIT) with control and measurement components coupled thereto. 
         FIG.  2 A  is a magnified view of the ion mirror M 1  of the ELIT illustrated in  FIG.  1    in which the mirror electrodes of M 1  are controlled to produce an ion transmission electric field therein. 
         FIG.  2 B  is a magnified view of the ion mirror M 2  of the ELIT illustrated in  FIG.  1    in which the mirror electrodes of M 2  are controlled to produce an ion reflection electric field therein. 
         FIG.  3    is a simplified diagram of an embodiment of the processor illustrated in  FIG.  1   . 
         FIGS.  4 A- 4 C  are simplified diagrams of the ELIT of  FIG.  1    demonstrating sequential control and operation of the ion mirrors to capture at least one ion within the ELIT and to cause the ion(s) to oscillate back and forth between the ion mirrors and through the charge detection cylinder to measure and record multiple charge detection events. 
         FIG.  5 A  is a simplified perspective sectional view of the ELIT of  FIGS.  1 - 2 B  showing a 3-dimensional Cartesian coordinate system superimposed thereon with the origin of the coordinate system positioned at the ion inlet of the ELIT. 
         FIG.  5 B  is a magnified view of a portion of the ion inlet of the ELIT of  FIG.  5 A  as viewed along the Y-Z plane of the illustrated coordinate system. 
         FIG.  5 C  is a magnified view of a portion of the ion inlet of the ELIT of  FIG.  5 A  as viewed along the X-Y plane of the illustrated coordinate system. 
         FIG.  6    is a plot of an example planar ion oscillation trajectory within the ELIT of  FIGS.  1 - 2 B and  5 A  relative to the 3-dimensional coordinate system illustrated in  FIGS.  5 A- 5 C . 
         FIG.  7    is a plot of an example cylindrical ion oscillation trajectory within the ELIT of  FIGS.  1 - 2 B and  5 A  relative to the 3-dimensional coordinate system illustrated in  FIGS.  5 A- 5 C . 
         FIG.  8    is a plot similar to  FIG.  6    depicting example orthogonal planar oscillation trajectories of two ions simultaneously trapped within the ELIT of  FIGS.  1 - 2 B and  5 A  relative to the 3-dimensional coordinate system illustrated in  FIGS.  5 A- 5 C . 
         FIG.  9    is a plot similar to  FIG.  7    depicting example nested cylindrical oscillation trajectories of two ions simultaneously trapped within the ELIT of  FIGS.  1 - 2 B and  5 A  relative to the 3-dimensional coordinate system illustrated in  FIG.  5 A- 5 C . 
         FIG.  10    is a cross-sectional view of the two nested cylindrical oscillation trajectories plot of  FIG.  9    as viewed along section lines  10 - 10 . 
         FIG.  11    is a simplified diagram of an embodiment of a charge detection mass spectrometer including a trajectory control apparatus for selectively controlling the trajectories of ions entering the ELIT to achieve simultaneous trapping of multiple ions with a distribution of planar or cylindrical oscillation trajectories. 
         FIG.  12 A  is a simplified block diagram of an embodiment of an ion separation instrument which may include the ELIT illustrated and described herein, and which may include the charge detection mass spectrometer illustrated and described herein, and which may include any number of ion processing instruments which may form part of the ion source upstream of the ELIT and/or which may include any number of ion processing instruments which may be disposed downstream of the ELIT to further process ion(s) exiting the ELIT. 
         FIG.  12 B  is a simplified block diagram of another embodiment of an ion separation instrument in which a multi-stage mass spectrometer instrument includes the CDMS and the ELIT, and which may also include an ion trajectory control apparatus as described herein. 
     
    
    
     DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of this disclosure, reference will now be made to a number of illustrative embodiments shown in the attached drawings and specific language will be used to describe the same. 
     This disclosure relates to apparatuses and techniques for simultaneously analyzing multiple ions with an electrostatic linear ion trap (ELIT) detector of a charge detection mass spectrometer (CDMS) by controlling the trajectories of ions entering the ELIT in a manner which provides for simultaneous trapping and individual measurement of multiple ions each having a different oscillation trajectory within the ELIT. In one embodiment, the ion entrance trajectories may be controlled in a manner which favors a planar oscillation trajectory geometry within the ELIT in which the trapped ions have a very low likelihood of interacting with one another. In another embodiment, the ion entrance trajectories may be controlled in a manner which favors a cylindrical oscillation trajectory geometry within the ELIT in which the trapped ions do not significantly interact with one another. In any case, such simultaneous analysis of multiple ions with an ELIT may substantially reduce sample analysis times over that achievable using conventional single-ion trapping techniques. 
     With respect to the operation of an ELIT, and for purposes of this disclosure, the phrase “charge detection event” is defined as detection of a charge associated with an ion passing a single time through a charge detector of the ELIT, and the phrase “ion measurement event” is defined as a collection of charge detection events resulting from oscillation of an ion back and forth through the charge detector a selected number of times or for a selected time period. As the oscillation of an ion back and forth through the charge detector results from controlled trapping of the ion within the ELIT as will be described in detail below, the phrase “ion measurement event” may alternatively be referred to herein as an “ion trapping event” or simply as a “trapping event,” and the phrases “ion measurement event,” “ion trapping event”, “trapping event” and variants thereof shall be understood to be synonymous with one another. 
     Referring to  FIG.  1   , a CDMS system  10  is shown including an embodiment of an electrostatic linear ion trap (ELIT)  14  with control and measurement components coupled thereto. In the illustrated embodiment, the CDMS system  10  includes an ion source  12  operatively coupled to an inlet of the ELIT  14 . The ion source  12  may illustratively be or include any conventional device, apparatus or technique for generating ions from a sample, e.g., electrospray or other conventional ion generation device, and may further include, for example, one or more devices and/or instruments for separating ions, e.g., based on ion mass, ion mass-to-charge ratio, ion mobility or other molecular characteristic, one or more devices and/or instruments for filtering ions, e.g., based on ion mass-to-charge ratio, ion mobility or other molecular characteristic, one or more devices or instruments for collecting and/or storing ions, e.g., one or more ion traps, one or more devices and/or instruments for dissociating ions, one or more devices or instruments for normalizing or shifting charge states of ions according to one or more molecular characteristics, and/or any combination thereof arranged in any order relative to the direction of ion flow. 
     In the illustrated embodiment, the ELIT  14  illustratively includes a charge detector CD surrounded by a ground chamber or cylinder GC and operatively coupled to opposing ion mirrors M 1 , M 2  respectively positioned at opposite ends thereof. The ion mirror M 1  is operatively positioned between the ion source  12  and one end of the charge detector CD, and ion mirror M 2  is operatively positioned at the opposite end of the charge detector CD. Each ion mirror M 1 , M 2  defines a respective ion mirror region or cavity R 1 , R 2  therein. The regions R 1 , R 2  of the ion mirrors M 1 , M 2 , the charge detector CD, and the spaces between the charge detector CD and the ion mirrors M 1 , M 2  together define a longitudinal axis  22  centrally therethrough which illustratively represents an ideal ion travel path through the ELIT  14  and between the ion mirrors M 1 , M 2  as will be described in greater detail below. 
     In the illustrated embodiment, voltage sources V 1 , V 2  are electrically connected to the ion mirrors M 1 , M 2  respectively. Each voltage source V 1 , V 2  illustratively includes one or more switchable DC voltage sources which may be controlled or programmed to selectively produce a number, N, of programmable or controllable voltages, wherein N may be any positive integer. Illustrative examples of such voltages will be described below with respect to  FIGS.  2 A and  2 B  to establish one of two different operating modes of each of the ion mirrors M 1 , M 2  as will be described in detail below. In any case, ions move within the ELIT  14  close to the longitudinal axis  22  extending centrally through the charge detector CD and the ion mirrors M 1 , M 2  under the influence of electric fields selectively established by the voltage sources V 1 , V 2 . 
     The voltage sources V 1 , V 2  are illustratively shown electrically connected by a number, P, of signal paths to a conventional processor  16  including a memory  18  having instructions stored therein which, when executed by the processor  16 , cause the processor  16  to control the voltage sources V 1 , V 2  to produce desired DC output voltages for selectively establishing ion transmission and ion reflection electric fields, TEF, REF respectively, within the regions R 1 , R 2  of the respective ion mirrors M 1 , M 2 . P may be any positive integer. In some alternate embodiments, either or both of the voltage sources V 1 , V 2  may be programmable to selectively produce one or more constant output voltages. In other alternative embodiments, either or both of the voltage sources V 1 , V 2  may be configured to produce one or more time-varying output voltages of any desired shape. It will be understood that more or fewer voltage sources may be electrically connected to the mirrors M 1 , M 2  in alternate embodiments. 
     The charge detector CD is illustratively provided in the form of an electrically conductive charge detection cylinder which is electrically connected to a signal input of a charge sensitive preamplifier CP, and the signal output of the charge preamplifier CP is electrically connected to the processor  16 . The voltage sources V 1 , V 2  are illustratively controlled in a manner which causes ions to be introduced into the ELIT  14  from the ion source  12 , and which selectively captures and confines an ion to oscillate therein such that the captured ion repeatedly passes through the charge detector CD. With an ion captured, i.e., trapped, within the ELIT  14  and oscillating back and forth between the ion mirrors M 1 , M 2 , the charge preamplifier CP is illustratively operable in a conventional manner to detect charges (CH) induced on the charge detection cylinder CD as the ion passes through the charge detection cylinder CD between the ion mirrors M 1 , M 2 , and to produce charge detection signals (CHD) corresponding thereto. A plurality of ion charge and oscillation period values are measured at the charge detector CD for each ion captured therein, and the results are recorded and processed to determine ion charge and mass values as will be described in greater detail below. 
     The processor  16  is further illustratively coupled to one or more peripheral devices  20  (PD) for providing peripheral device signal input(s) (PDS) to the processor  16  and/or to which the processor  16  provides signal peripheral device signal output(s) (PDS). In some embodiments, the peripheral devices  20  include at least one of a conventional display monitor, a printer and/or other output device, and in such embodiments the memory  18  has instructions stored therein which, when executed by the processor  16 , cause the processor  16  to control one or more such output peripheral devices  20  to display and/or record analyses of the stored, digitized charge detection signals. 
     Referring now to  FIGS.  2 A and  2 B , embodiments are shown of the ion mirrors M 1 , M 2  respectively of the ELIT  14  depicted in  FIG.  1   . Illustratively, the ion mirrors M 1 , M 2  are identical to one another in that each includes a cascaded arrangement of 4 spaced-apart, electrically conductive mirror electrodes. For each of the ion mirrors M 1 , M 2 , a first mirror electrode  30   1  has a thickness W 1  and defines a passageway centrally therethrough of diameter P 1 . An endcap  32  is affixed or otherwise coupled to an outer surface of the first mirror electrode  30   1  and defines an aperture A 1  centrally therethrough which serves as an ion entrance and/or exit to and/or from the corresponding ion mirror M 1 , M 2  respectively. In the case of the ion mirror M 1 , the endcap  32  is coupled to, or is part of, an ion exit of the ion source  12  illustrated in  FIG.  1   . The aperture A 1  for each endcap  32  illustratively has a diameter P 2 . 
     A second mirror electrode  30   2  of each ion mirror M 1 , M 2  is spaced apart from the first mirror electrode  30   1  by a space having width W 2 . The second mirror electrode  30   2 , like the mirror electrode  30   1 , has thickness W 1  and defines a passageway centrally therethrough of diameter P 2 . A third mirror electrode  30   3  of each ion mirror M 1 , M 2  is likewise spaced apart from the second mirror electrode  30   2  by a space of width W 2 . The third mirror electrode  30   3  has thickness W 1  and defines a passageway centrally therethrough of width P 1 . 
     A fourth mirror electrode  30   4  is spaced apart from the third mirror electrode  30   3  by a space of width W 2 . The fourth mirror electrode  30   4  illustratively has a thickness of W 1  and is formed by a respective end of the ground cylinder, GC disposed about the charge detector CD. The fourth mirror electrode  30   4  defines an aperture A 2  centrally therethrough which is illustratively conical in shape and increases linearly between the internal and external faces of the ground cylinder GC from a diameter P 3  defined at the internal face of the ground cylinder GC to the diameter P 1  at the external face of the ground cylinder GC (which is also the internal face of the respective ion mirror M 1 , M 2 ). 
     The spaces defined between the mirror electrodes  30   1 - 30   4  may be voids in some embodiments, i.e., vacuum gaps, and in other embodiments such gaps may be filled with one or more electrically non-conductive, e.g., dielectric, materials. The mirror electrodes  30   1 - 30   4  and the endcaps  32  are axially aligned, i.e., collinear, such that the longitudinal axis  22  passes centrally through each aligned passageway and also centrally through the apertures A 1 , A 2 . In embodiments in which the spaces between the mirror electrodes  30   1 - 30   4  include one or more electrically non-conductive materials, such materials will likewise define respective passageways therethrough which are axially aligned, i.e., collinear, with the passageways defined through the mirror electrodes  30   1 - 30   4  and which illustratively have diameters of P 2  or greater. Illustratively, P 1 &gt;P 3 &gt;P 2 , although in other embodiments other relative diameter arrangements are possible. 
     A region R 1  is defined between the apertures A 1 , A 2  of the ion mirror M 1 , and another region R 2  is likewise defined between the apertures A 1 , A 2  of the ion mirror M 2 . The regions R 1 , R 2  are illustratively identical to one another in shape and in volume. 
     As described above, the charge detector CD is illustratively provided in the form of an elongated, electrically conductive cylinder positioned and spaced apart between corresponding ones of the ion mirrors M 1 , M 2  by a space of width W 3 . In one embodiment, W 1 &gt;W 3 &gt;W 2 , and P 1 &gt;P 3 &gt;P 2 , although in alternate embodiments other relative width arrangements are possible. In any case, the longitudinal axis  22  illustratively extends centrally through the passageway defined through the charge detection cylinder CD, such that the longitudinal axis  22  extends centrally through the combination of the ion mirrors M 1 , M 2  and the charge detection cylinder CD. In operation, the ground cylinder GC is illustratively controlled to ground potential such that the fourth mirror electrode  30   4  of each ion mirror M 1 , M 2  is at ground potential at all times. In some alternate embodiments, the fourth mirror electrode  30   4  of either or both of the ion mirrors M 1 , M 2  may be set to any desired DC reference potential, or to a switchable DC or other time-varying voltage source. 
     In the embodiment illustrated in  FIGS.  2 A and  2 B , the voltage sources V 1 , V 2  are each configured to each produce four DC voltages D 1 -D 4 , and to supply the voltages D 1 -D 4  to a respective one of the mirror electrodes  30   1 - 30   4  of the respective ion mirror M 1 , M 2 . In some embodiments in which one or more of the mirror electrodes  30   1 - 30   4  is to be held at ground potential at all times, the one or more such mirror electrodes  30   1 - 30   4  may alternatively be electrically connected to the ground reference of the respective voltage supply V 1 , V 2  and the corresponding one or more voltage outputs D 1 -D 4  may be omitted. Alternatively or additionally, in embodiments in which any two or more of the mirror electrodes  30   1 - 30   4  are to be controlled to the same non-zero DC values, any such two or more mirror electrodes  30   1 - 30   4  may be electrically connected to a single one of the voltage outputs D 1 -D 4  and superfluous ones of the output voltages D 1 -D 4  may be omitted. 
     Each ion mirror M 1 , M 2  is illustratively controllable and switchable, by selective application of the voltages D 1 -D 4 , between an ion transmission mode ( FIG.  2 A ) in which the voltages D 1 -D 4  produced by the respective voltage source V 1 , V 2  establishes an ion transmission electric field (TEF) in the respective region R 1 , R 2  thereof, and an ion reflection mode ( FIG.  2 B ) in which the voltages D 1 -D 4  produced by the respect voltage source V 1 , V 2  establishes an ion reflection electric field (REF) in the respective region R 1 , R 2  thereof. As illustrated by example in  FIG.  2 A , once ions from the ion source  12  fly into region R 1  of the ion mirror M 1  through the inlet aperture A 1  of the ion mirror M 1 , the ions become focused towards the longitudinal axis  22  of the ion trap by an ion transmission electric field TEF established in the region R 1  of the ion mirror M 1  via selective control of the voltages D 1 -D 4  of V 1 . As a result of the focusing effect of the transmission electric field in region R 1  of the ion mirror M 1  on the ion trajectory, ions exiting the region R 1  of the ion mirror M 1  through the aperture A 2  of ion mirror M 1  attain a narrow trajectory through the charge detector CD, i.e., so as to maintain the path of ion travel through the charge detector CD close to the longitudinal axis  22 . An identical ion transmission electric field TEF may be selectively established within the region R 2  of the ion mirror M 2  via like control of the voltages D 1 -D 4  of the voltage source V 2 . In the ion transmission mode, ions entering the region R 2  from the charge detection cylinder CD via the aperture A 2  of M 2  are focused towards the longitudinal axis by the ion transmission electric field TEF within the region R 2  through the exit aperture A 1  of the ion mirror M 2 . 
     As illustrated by example in  FIG.  2 B , an ion reflection electric field REF established in the region R 2  of the ion mirror M 2  via selective control of the voltages D 1 -D 4  of V 2  acts to decelerate and stop ions entering the ion region R 2  from the charge detection cylinder CD via the ion inlet aperture A 2  of M 2 , to immediately accelerate the stopped ions in the opposite direction back through the aperture A 2  of M 2  and into the end of the charge detection cylinder CD adjacent to M 2  as depicted by the ion trajectory  38 , and to focus the ions toward the central, longitudinal axis  22  within the region R 2  of the ion mirror M 2  so as to maintain a narrow trajectory of ions through the charge detector CD. An identical ion reflection electric field REF may be selectively established within the region R 1  of the ion mirror M 1  via like control of the voltages D 1 -D 4  of the voltage source V 1 . In the ion reflection mode, ions entering the region R 1  from the charge detection cylinder CD via the aperture A 2  of M 1  are decelerated and stopped by the ion reflection electric field REF established within the region R 1 , then accelerated in the opposite direction back through the aperture A 2  of M 1  and into the end of the charge detection cylinder CD adjacent to M 1 , and focused toward the central, longitudinal axis  22  within the region R 1  of the ion mirror M 1  so as to maintain a narrow trajectory of ions through the charge detector CD. Ions that traverse the length of the ion trap and are reflected by the ion reflection electric field REF in the ion regions R 1  and R 2  in a manner that enables the ions to continue traveling back and forth along the length of the trap are considered trapped. 
     Example sets of output voltages D 1 -D 4  produced by the voltage sources V 1 , V 2  to control a respective ion mirror M 1 , M 2  to and between the ion transmission and reflection modes described above are shown in TABLE I below. It will be understood that the following values of D 1 -D 4  are provided only by way of example, and that other values of one or more of D 1 -D 4  may alternatively be used. 
     
       
         
           
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Ion Mirror Operating Mode 
                 Output Voltages (volts DC) 
               
               
                   
               
             
            
               
                 Transmission 
                 V1: D1 = 0, D2 = 95, D3 = 135, D4 = 0 
               
               
                   
                 V2: D1 = 0, D2 = 95, D3 = 135, D4 = 0 
               
               
                 Reflection 
                 V1: D1 = 190, D2 = 125, D3 = 135, D4 = 0 
               
               
                   
                 V2: D1 = 190, D2 = 125, D3 = 135, D4 = 0 
               
               
                   
               
            
           
         
       
     
     While the ion mirrors M 1 , M 2  and the charge detection cylinder CD are illustrated in  FIGS.  1 - 2 B  as defining cylindrical passageways therethrough, it will be understood that in alternate embodiments either or both of the ion mirrors M 1 , M 2  and/or the charge detection cylinder CD may define non-cylindrical passageways therethrough such that one or more of the passageway(s) through which the longitudinal axis  22  centrally passes represents a cross-sectional area and profile that is not circular. In still other embodiments, regardless of the shape of the cross-sectional profiles, the cross-sectional areas of the passageway defined through the ion mirror M 1  may be different from the passageway defined through the ion mirror M 2 . 
     Referring now to  FIG.  3   , an embodiment is shown of the processor  16  illustrated in  FIG.  1   . In the illustrated embodiment, the processor  16  includes a conventional amplifier circuit  40  having an input receiving the charge detection signal CHD produced by the charge preamplifier CP and an output electrically connected to an input of a conventional Analog-to-Digital (A/D) converter  42 . An output of the A/D converter  42  is electrically connected to a first computing device or circuit  50  (P 1 ). The amplifier  40  is operable in a conventional manner to amplify the charge detection signal CHD produced by the charge preamplifier CP, and the A/D converter is, in turn, operable in a conventional manner to convert the amplified charge detection signal to a digital charge detection signal CDS. The computing device  50  illustratively includes or is coupled to a one or more conventional memory units, and the computing device  50  is illustratively operable to store therein the charge detection signals CDS for each charge detection event in an ion measurement event such that an ion measurement event record stored in the memory of the processor circuit  50  includes multiple charge detection event measurements. 
     The processor  16  illustrated in  FIG.  3    further includes a conventional comparator  44  having a first input receiving the charge detection signal CHD produced by the charge preamplifier CP, a second input receiving a threshold voltage CTH produced by a threshold voltage generator (TG)  46  and an output electrically connected to the computing device  50 . The comparator  44  is illustratively operable in a conventional manner to produce a trigger signal TR at the output thereof which is dependent upon the magnitude of the charge detection signal CDH relative to the magnitude of the threshold voltage CTH. In one embodiment, for example, the comparator  44  is operable to produce an “inactive” trigger signal TR at or near a reference voltage, e.g., ground potential, as long as CHD is less than CTH, and is operable to produce an “active” TR signal at or near a supply voltage of the circuitry  40 ,  42 ,  44 ,  46 ,  50  when CHD is at or exceeds CTH. In alternate embodiments, the comparator  44  may be operable to produce an “inactive” trigger signal TR at or near the supply voltage as long as CHD is less than CTH, and is operable to produce an “active” trigger signal TR at or near the reference potential when CHD is at or exceeds CTH. Those skilled in the art will recognize other differing trigger signal magnitudes and/or differing trigger signal polarities that may be used to establish the “inactive” and “active” states of the trigger signal TR so long as such differing trigger signal magnitudes and/or different trigger signal polarities are distinguishable by the computing device  50 , and it will be understood that any such other different trigger signal magnitudes and/or differing trigger signal polarities are intended to fall within the scope of this disclosure. In any case, the comparator  44  may additionally be designed in a conventional manner to include a desired amount of hysteresis to prevent rapid switching of the output between the reference and supply voltages. 
     In the illustrated embodiment, the computing device  50  is operable, i.e., programmed, to control the threshold voltage generator  46  to produce the threshold voltage CTH. In one embodiment, the threshold voltage generator  46  is illustratively implemented in the form of a conventional controllable DC voltage source configured to be responsive to a digital threshold control signal THC, e.g., in the form of a single serial digital signal or multiple parallel digital signals, to produce an analog threshold voltage CTH having a polarity and a magnitude defined by the digital threshold control signal THC. In alternate embodiments, the threshold voltage generator  46  may be provided in the form of a conventional digital-to-analog (D/A) converter responsive to a serial or parallel digital threshold voltage TCH to produce an analog threshold voltage CTH having a magnitude, and in some embodiments a polarity, defined by the digital threshold control signals THC. In some such embodiments, the D/A converter may form part of the processor circuit  50 . Those skilled in the art will recognize other conventional circuits and techniques for selectively producing the threshold voltage CTH of desired magnitude and/or polarity, and it will be understood that any such other conventional circuits and/or techniques are intended to fall within the scope of this disclosure. 
     The computing device  50  is operable to control the voltage sources V 1 , V 2  as described above with respect to  FIGS.  2 A,  2 B  to selectively establish ion transmission and reflection fields within the regions R 1 , R 2  of the ion mirrors M 1 , M 2  respectively. In one embodiment, the computing device  50  is illustratively provided in the form of a field programmable gate array (FPGA) programmed as just described to collect and store charge detection signals CDS for charge detection events and for ion measurement events, to produce the threshold control signal(s) THC from which the magnitude and/or polarity of the threshold voltage CTH is determined or derived, and to control the voltage sources V 1 , V 2  based on the charge detection signals CHD relative to the threshold voltage CTH as determined by monitoring the trigger output signal TR produced by the comparator  44 . In this embodiment, the memory  18  described with respect to  FIG.  1    is integrated into, and forms part of, the programming of the FPGA. In alternate embodiments, the computing device  50  may include and/or be provided in the form of one or more conventional microprocessors or controllers and one or more accompanying memory units incorporated therein or coupled thereto and having instructions stored therein which, when executed by the one or more microprocessors or controllers, cause the one or more microprocessors or controllers to operate as just described. In other alternate embodiments, the computing device  50  may be implemented purely in the form of one or more conventional or application-specific hardware circuits designed to operate as described above, or as a combination of one or more such hardware circuits and at least one microprocessor or controller operable to execute instructions stored in memory to operate as described above. 
     In any case, the embodiment of the processor  16  depicted in  FIG.  3    further illustratively includes a second computing device  52  coupled to the first computing device  50  and also to the one or more peripheral devices  20  illustrated in  FIG.  1   . In some alternate embodiments, the computing device  52  may include at least one of the one or more peripheral devices  20 . In any case, the computing device  52  is illustratively operable to process the ion measurement event information stored by the first computing device  50  to determine ion mass information. The computing device  52  may be or include one or more conventional microprocessors and/or controllers, one or more programmable circuits, e.g., one or more field-programmable gate arrays, and/or one or more application-specific integrated circuits (ASICs). In some embodiments, the computing device  52  may be provided in the form of any conventional computer or computing device capable of processing the ion measurement event information, i.e., having sufficient computing power, to determine, display, store and conduct at least some amount of analysis of ion mass information. In one embodiment, the computing device  52  may be provided or included in the form of a conventional personal computer (PC), although in other embodiments the computing device  52  may be or be included in one or more computers or computing devices with greater or lesser computing power. 
     The voltage sources V 1 , V 2  are illustratively controlled by the computing device  50  in a manner which selectively establishes ion transmission and ion reflection electric fields in the region R 1  of the ion mirror M 1  and in the region R 2  of the ion mirror M 2  to cause an ion to be introduced into the ELIT  14  from the ion source  12 , and to then cause the introduced ion to be selectively captured and confined to oscillate within the ELIT  14  such that the captured ion repeatedly passes through the charge detector CD between M 1  and M 2 . Referring to  FIGS.  4 A- 4 C , simplified diagrams of the ELIT  14  of  FIG.  1    are shown depicting an example of such sequential control and operation of the ion mirrors M 1 , M 2  of the ELIT  14 . In the following example, the computing device  50  will be described as controlling the operation of the voltage sources V 1 , V 2  in accordance with its programming, although it will be understood that in alternate embodiments the operation of the voltage source V 1  and/or the operation of the voltage source V 1  may be controlled, at least in part, by the computing device  52  in accordance with its programming. 
     As illustrated in  FIG.  4 A , the ELIT control sequence begins with the computing device  50  controlling the voltage source V 1  to control the ion mirror M 1  to the ion transmission mode of operation (T) by establishing an ion transmission field within the region R 1  of the ion mirror M 1 , and also controlling the voltage source V 2  to control the ion mirror M 2  to the ion transmission mode of operation (T) by likewise establishing an ion transmission field within the region R 2  of the ion mirror M 2 . As a result, an ion generated by the ion source  12  is drawn into the ion mirror M 1  and transmitted, i.e., accelerated, through M 1  into the charge detection cylinder CD by the ion transmission field established in the region R 1 . The ion then passes through the charge detection cylinder CD and into the ion mirror M 2  where the ion transmission field established within the region R 2  of M 2  transmits, i.e., accelerates, the ion through the exit aperture A 1  of M 2  as illustrated by the ion trajectory  60  depicted in  FIG.  4 A . 
     Referring now to  FIG.  4 B , after both of the ion mirrors M 1 , M 2  have been operating in ion transmission operating mode for a selected time period and/or until successful ion transmission therethrough has been achieved, e.g., by monitoring the charge detection signals CDS captured by the computing device  50 , the computing device  50  is illustratively operable to control the voltage source V 2  to control the ion mirror M 2  to the ion reflection mode (R) of operation by establishing an ion reflection field within the region R 2  of the ion mirror M 2 , while maintaining the ion mirror M 1  in the ion transmission mode (T) of operation as shown. As a result, an ion generated by the ion source  12  flies into the ion mirror M 1  and is transmitted through M 1  into the charge detection cylinder CD by the ion transmission field established in the region R 1  as just described with respect to  FIG.  4 A . The ion then passes through the charge detection cylinder CD and into the ion mirror M 2  where the ion reflection field established within the region R 2  of M 2  reflects ions to cause them to travel in the opposite direction and back into the charge detection cylinder CD, as illustrated by the ion trajectory  62  in  FIG.  4 B . 
     Referring now to  FIG.  4 C , after the ion reflection electric field has been established in the region R 2  of the ion mirror M 2  and the ion is moving within the ELIT  14 , the processor circuit  50  is operable to control the voltage source V 1  to control the ion mirror M 1  to the ion reflection mode (R) of operation by establishing an ion reflection field within the region R 1  of the ion mirror M 1 , while maintaining the ion mirror M 2  in the ion reflection mode (R) of operation in order to trap the ion within the ELIT  14 . In some embodiments, the computing device  50  is illustratively operable, i.e., programmed, to control the ELIT  14  in a “random trapping mode” or “continuous trapping mode” in which the computing device  50  is operable to control the ion mirror M 1  to the reflection mode (R) of operation after the ELIT  14  has been operating in the state illustrated in  FIG.  4 B , i.e., with M 1  in ion transmission mode and M 2  in ion reflection mode, for a selected time period. Until the selected time period has elapsed, the ELIT  14  is controlled to operate in the state illustrated in  FIG.  4 B . 
     The probability of trapping an ion in the ELIT  14  is relatively low using the random trapping mode of operation due to the timed control of M 1  to the ion reflection mode of operation without any confirmation that an ion is contained within the ELIT  14 . The number of trapped ions within the ELIT  14  during the random trapping mode of operation follows a Poisson distribution and, with the ion inlet signal intensity adjusted to maximize the number of single ion trapping events, it can be shown that only about 37% of trapping events in the random trapping mode can contain a single ion. If the ion inlet signal intensity is too small, most of the trapping events will be empty, and if it is too large most will contain multiple ions. 
     In other embodiments, the computing device  50  is operable, i.e., programmed, to control the ELIT  14  in a “trigger trapping mode” which illustratively carries a substantially greater probability of trapping a single ion therein. In a first version of the trigger trapping mode, the computing device  50  is operable to monitor the trigger signal TR produced by the comparator  44  and to control the voltage source V 1  to control the ion mirror M 1  to the reflection mode (R) of operation to trap an ion within the ELIT  14  if/when the trigger signal TR changes the “inactive” to the “active” state thereof. In some embodiments, the processor circuit  50  may be operable to control the voltage source V 1  to control the ion mirror M 1  to the reflection mode (R) immediately upon detection of the change of state of the trigger signal TR, and in other embodiments the processor circuit  50  may be operable to control the voltage source V 1  to control the ion mirror M 1  to the reflection mode (R) upon expiration of a predefined or selectable delay period following detection of the change of state of the trigger signal TR. In any case, the change of state of the trigger signal TR from the “inactive” state to the “active” state thereof results from the charge detection signal CHD produced by the charge preamplifier CP reaching or exceeding the threshold voltage CTH, and therefore corresponds to detection of a charge induced on the charge detection cylinder CD by an ion contained therein. With an ion thus contained within the charge detection cylinder CD, control by the computing device  50  of the voltage source V 1  to control the ion mirror M 1  to the reflection mode (R) of operation results in a substantially improved probability, relative to the random trapping mode, of trapping a single ion within the ELIT  14 . Thus, when an ion has entered the ELIT  14  via the ion mirror M 1  and is detected as either passing the first time through the charge detection cylinder CD toward the ion mirror M 2  or as passing back through the charge detection cylinder CD after having been reflected by the ion reflection field established within the region R 2  of the ion mirror M 2  as illustrated in  FIG.  4 B , the ion mirror M 1  is controlled to the reflection mode (R) as illustrated in  FIG.  4 C  to trap the ion within the ELIT  14 . It is also desirable to optimize the signal intensity with trigger trapping as briefly described above with respect to the random trapping mode of operation. In trigger trapping mode with optimized ion inlet signal intensity, for example, it has been shown that trapping efficiency, defined as the ratio between single-ion trapping events and all acquired trapping events, can approach 90% as compared to 37% with random trapping. However, if the ion inlet signal intensity is too large the trapping efficiency will be less than 90% and it will be necessary to reduce the ion inlet signal intensity. 
     In a second version of the trigger trapping mode, the process or step illustrated in  FIG.  4 B  is omitted or bypassed, and with the ELIT  14  operating as illustrated in  FIG.  4 A  the computing device  50  is operable to monitor the trigger signal TR produced by the comparator  44  and to control both voltage sources V 1 , V 2  to control the respective ion mirrors M 1 , M 2  to the reflection mode (R) of operation to trap or capture an ion within the ELIT  14  if/when the trigger signal TR changes the “inactive” to the “active” state thereof. Thus, when an ion has entered the ELIT  14  via the ion mirror M 1  and is detected as passing the first time through the charge detection cylinder CD toward the ion mirror M 2  as illustrated in  FIG.  4 A , the ion mirrors M 1  and M 2  are both controlled to the reflection mode (R) as illustrated in  FIG.  4 C  to trap the ion within the ELIT  14 . 
     In any case, with both of the ion mirrors M 1 , M 2  controlled to the ion reflection operating mode (R) to trap an ion within the ELIT  14 , the ion is caused by the opposing ion reflection fields established in the regions R 1  and R 2  of the ion mirrors M 1  and M 2  respectively to oscillate back and forth between the ion mirrors M 1  and M 2 , each time passing through the charge detection cylinder CD as illustrated by the ion trajectory  64  depicted in  FIG.  4 C . In one embodiment, the computing device  50  is operable to maintain the operating state illustrated in  FIG.  4 C  until the trapped ion passes through the charge detection cylinder CD a selected number of times. In an alternate embodiment, the computing device  50  is operable to maintain the operating state illustrated in  FIG.  4 C  for a selected time period after controlling M 1  (and M 2  in some embodiments) to the ion reflection mode (R) of operation. In either embodiment, the ion detection event information resulting from each pass by the ion through the charge detection cylinder CD is temporarily stored in or by the computing device  50 . When the ion has passed through the charge detection cylinder CD a selected number of times or has oscillated back-and-forth between the ion mirrors M 1 , M 2  for a selected period of time, the total number of ion detection events stored in or by the computing device  50  defines an ion measurement event and, upon completion, the ion measurement event is passed to, or retrieved by, the computing device  52 . The sequence illustrated in  FIGS.  4 A- 4 C  then returns to that illustrated in  FIG.  4 A  where the voltage sources V 1 , V 2  are controlled by the computing device  50  as described above to control the ion mirrors M 1 , M 2  respectively to the ion transmission mode (T) of operation by establishing an ion transmission fields within the regions R 1 , R 2  of the ion mirrors M 1 , M 2  respectively. The illustrated sequence then repeats for as many times as desired. 
     In one embodiment, the ion measurement event data are processed by computing, e.g., with the computing device  52  or with the computing device  50 , a Fourier Transform of the recorded collection of charge detection events, i.e., of the recorded ion measurement event data. Illustratively, the computing device  52  is operable to compute such a Fourier Transform using any conventional digital Fourier Transform (DFT) technique such as for example, but not limited to, a conventional Fast Fourier Transform (FFT) algorithm. In any case, the computing device  52  is then illustratively operable to compute an ion mass-to-charge ratio value (m/z), an ion charge value (z) and ion mass values (m), each as a function of the computed Fourier Transform. The computing device  52  is illustratively operable to store the computed results in the memory  18  and/or to control one or more of the peripheral devices  20  to display the results for observation and/or further analysis. 
     It is generally understood that the mass-to-charge ratio (m/z) of ion(s) oscillating back and forth between opposing ion mirrors M 1 , M 2  of an ELIT  14  is inversely proportional to the square of the fundamental frequency ff of the oscillating ion(s) according to the equation: 
         m/z=C/ff 2, 
     where C is a constant that is a function of the ion energy and also a function of the dimensions of the respective ELIT  14 , and the fundamental frequency ff is determined directly from the computed Fourier Transform in a conventional manner. The value of the ion charge, z, is proportional to the magnitude FTMAG of the FT fundamental frequency, taking into account the number of ion oscillation cycles. In some cases, the magnitude(s) of one or more of the harmonic frequencies of the FFT may be added to the magnitude of the fundamental frequency for purposes of determining the ion charge, z. In any case, ion mass, m, is then calculated as a product of m/z and z. The processor circuit  52  is thus operable to compute m/z=C/ff2, z=F(FTMAG) and m=(m/z)(z). 
     Multiple, e.g., hundreds or thousands or more, ion trapping events are typically carried out for any particular sample from which the ions are generated by the ion source  12 , and ion mass-to-charge, ion charge and ion mass values are determined/computed for each such ion trapping event. The ion mass-to-charge, ion charge and ion mass values for such multiple ion trapping events are, in turn, combined to form spectral information relating to the sample. Such spectral information may illustratively take different forms, examples of which include, but are not limited to, ion count vs. mass-to-charge ratio, ion charge vs. ion mass (e.g., in the form of an ion charge/mass scatter plot), ion count vs. ion mass, ion count vs. ion charge, or the like. 
     Charge detection mass spectrometry (CDMS) is conventionally a single-ion analysis technique in which an ion is directed into an ion detection or measurement stage for measurement of the ion&#39;s charge and mass-to-charge ratio (m/z) from which the ion&#39;s mass is then determined. This process is repeated multiple times, e.g., hundreds or thousands of times, to produce a mass spectrum of the sample being analyzed. The ion detection or measurement stage may take any of several different forms including, for example, but not limited to an orbitrap mass analyzer, an electrostatic linear ion trap (ELIT) or other single ion measurement stage or instrument. In the case of an ELIT of any design, including that illustrated in  FIGS.  1 - 4 C  and described in detail above, ions entering the ELIT are typically tightly focused toward the center of the ion inlet aperture such that their entrance trajectories are generally collinear with the longitudinal axis of the ELIT. In conventional ELIT operation, only single ion trapping events are analyzed because two or more trapped ions oscillating back and forth through the ELIT typically have an unacceptably high probability of interacting with one another in a manner which adversely influences the stabilities of their oscillation trajectories within the ELIT, thereby leading to inaccurate m/z and charge measurements. 
     In order to accurately measure the m/z and charge of an ion in an ELIT, its longitudinal oscillation frequency must be as stable as possible. When multiple ions enter into and are trapped in an ELIT, the trapped ions exert a repulsive force on one another that is proportional to the distance between them. This repulsive force deflects the ion oscillation trajectories within the ELIT, and as ions exchange momentums as a result of such interactions the energies of the oscillating ions also change. Ion oscillation trajectory and energy fluctuations during trapping events are undesirable because they decrease the certainty with which the ion oscillation frequencies can be determined, thereby decreasing the accuracy of the m/z measurements. Ion oscillation trajectory fluctuations also decrease the certainty in ion charge determinations as such fluctuations can affect the distance of ion penetration into the regions R 1 , R 2  of the ion mirrors M 1 , M 2 , thereby changing the duty cycle of the charge detection signal CH (see, e.g.,  FIGS.  4 A- 4 C ) and decreasing the certainty in the distributions of the signal harmonics. 
     Referring now to  FIGS.  5 A and  6 - 7   , different ion oscillation trajectories within the ELIT  14  of  FIGS.  1 - 2 B and  4 A- 4 C  are considered in which Coulombic repulsion between multiple trapped ions is minimized. Referring specifically to  FIG.  5 A , a perspective cross-sectional view is shown of the ELIT  14  of  FIGS.  1 - 2 B and  4 A- 40    with a three-dimensional Cartesian coordinate system superimposed thereon. In the illustrated example, the z-axis extends centrally through the charge detection cylinder CD and the regions R 1  and R 2  of the ion mirrors M 1  and M 2  respectively, and is thus collinear with the central longitudinal axis  22  of the ELIT  14  as illustrated in  FIGS.  1  and  2 A- 2 B . For purposes of this description, the regions R 1 , R 2  and the charge detection cylinder CD of the ELIT  14  are assumed to be cylindrically symmetric such that the x-axis of the coordinate system, running normal to the z-axis, defines a lateral or transverse plane bisecting the regions R 1 , R 2  and the charge detection cylinder CD as illustrated by example in  FIG.  5 A . The y-axis of the coordinate system, likewise running normal to the z-axis, defines a medial, (or vertical or longitudinal) plane bisecting the regions R 1 , R 2  and the charge detection cylinder CD. The zero intersection of the x, y and z axes is arbitrarily located at the ion inlet A 1  of the ELIT  14  flush with the inner wall of the endcap  32  as best illustrated in  FIG.  5 B . 
     Two limiting forms of single ion oscillation trajectories within the ELIT  14  have been identified in which Coulombic repulsion between multiple trapped ions is minimized. One such single ion oscillation trajectory is illustrated by example in  FIG.  6    in the form of a planar ion oscillation trajectory  80 , and the other is illustrated by example in  FIG.  7    in the form of a cylindrical ion oscillation trajectory  90 . 
     The planar ion oscillation trajectory  80  illustratively represents a planar trajectory of ion travel back and forth through the regions R 1 , R 2  and CD of the ELIT  14 . In the example illustrated in  FIG.  6   , the planar ion oscillation trajectory  80  includes a planar frustum  82  with a flared base, an inverted but otherwise identical planar frustum  84  with a flared base and a generally rectangular plane  86  joining the frusta  82 ,  84 . The opposed planar frusta  82 ,  84  illustratively represent the flared conical ion trajectories within the regions R 1 , R 2  of the ion mirrors M 1 , M 2  respectively, and the rectangular plane  86  illustratively represents the planar ion trajectory through the charge detection cylinder CD. An ion in the planar ion oscillation trajectory  80  illustrated in  FIG.  6    thus oscillates back and forth through the ELIT  14  with a planar oscillation trajectory extending along the longitudinal (z) axis  22  such that its oscillation trajectory is largely constrained to a single line in the x-y plane as it moves along the z-axis  22 . As also illustrated in  FIG.  6   , the planar ion oscillation trajectory  80  passes through the z-axis  22  at least once during each oscillation; once in the region R 1  (although not necessarily through the longitudinal center of the charge detection cylinder CD) and once in the region R 2 . 
     The cylindrical ion oscillation trajectory  90  illustrated by example in  FIG.  7    represents a generally cylindrical trajectory of ion travel back and forth through the regions R 1 , R 2  and CD of the ELIT  14 . In the illustrated example, the cylindrical ion oscillation trajectory  90  includes a frustum  92  with a flared base, an inverted but otherwise identical frustum  94  with a flared base and a central cylinder  96  joining the frusta  92 ,  94 . As with the planar trajectory  80  illustrated in  FIG.  6   , the opposed frusta  92 ,  94  illustratively represent the flared conical ion trajectories within the regions R 1 , R 2  of the ion mirrors M 1 , M 2  respectively, and the central cylinder  96  illustratively represents the cylindrical ion trajectory through the charge detection cylinder CD. An ion in the cylindrical ion oscillation trajectory  90  illustrated in  FIG.  7    illustratively undergoes an orbital motion in the x-y plane as it oscillates back and forth through the ELIT  14  along the z-axis  22  such that the cylindrical oscillation trajectory  90  extends along and about the z-axis  22 . As a result of such orbital motion, the cylindrical ion oscillation trajectory  90  does not pass though the z-axis  22  in either region R 1 , R 2  or in any other region of the ELIT  14  as also illustrated in  FIG.  7   . 
     It has been determined that the planar and cylindrical ion oscillation trajectories  80 ,  90  respectively illustrated in  FIGS.  6  and  7    each depend in large part upon ion entrance conditions; specifically, upon ion entrance trajectories. As such, the trajectory of an ion entering the aperture A 1  of the region R 1  of the ELIT  14  can be controlled in a manner which favors a planar ion oscillation trajectory of the type illustrated in  FIG.  6   , or which favors a cylindrical ion oscillation trajectory of the type illustrated in  FIG.  7   . In particular, such control of ion entrance trajectory may take the form of one or a combination of controlling an amount or magnitude of radial offset of the entering ion relative to the z-axis  22  and controlling an angle of ion entrance relative to the z-axis  22 . The “angle of ion entrance relative to the z-axis” may be alternatively referred to herein as an “angular divergence,” and it will be understood that these two terms are to be considered interchangeable. 
     The radial offset of an ion entering the ELIT  14  is generally the distance between the z-axis  22  and a line parallel with the z-axis  22 . Referring to  FIG.  5 B  for example, the dashed line oz is parallel to but offset from the z-axis  22 , and oz thus represents one example radial offset condition. As illustrated in  FIG.  5 B , the ion  70  traveling into the aperture A 1  of the region R 1  of the ELIT  14  along the radial offset line oz thus represents an ion entrance trajectory T 1  having a radial offset only of “oz”, i.e., having substantially no or negligible angular divergence (conversely, having an angular divergence of substantially 0°). The angular divergence of an ion entering the ELIT  14 , on the other hand, is generally an angle relative to the z-axis  22  or relative to a radial offset, if any, at which an ion enters the ELIT  14 . As also illustrated in  FIG.  5 B , the ion  72  traveling into the aperture A 1  of the region R 1  of the ELIT  14  at an angle DA 1  relative to the z-axis  22  thus represents an ion entrance trajectory T 1  having a divergence angle only of DA 1 , i.e., having substantially no or negligible radial offset relative to the z-axis  22 . Finally, the ion  74  depicted in  FIG.  5 B  as traveling into the aperture A 1  of the region R 1  of the ELIT  14  at an angle DA 2  relative to the radial offset oz represents an ion entrance trajectory T 3  having both a radial offset relative to the z-axis  22  of “oz” and a divergence angle of DA 2  relative to the radial offset oz. It should be noted that in cases in which the ion entering the ELIT  14  has a radial offset and an angular divergence, the two may, but need not, be along the same direction in the x-y plane. 
     The ion entrance trajectory, e.g., in terms of a radial offset and/or an angular divergence, determines whether an ion entering the ELIT  14  follows a planar or a cylindrical ion oscillation trajectory within the ELIT  14 . For example, an ion entering the aperture A 1  of the ion mirror M 1  at the z-axis  22  with or without a divergence angle will adopt a planar ion oscillation trajectory of the type illustrated in  FIG.  6   . An ion entering the aperture A 1  of the ion mirror M 1  with a radial offset relative to the z-axis  22  but with no (or negligible) divergence angle, e.g., a collimated entrance trajectory, will likewise adopt a planar ion oscillation trajectory. On the other hand, an ion entering the aperture A 1  of the ion mirror M 1  with both a radial offset relative to the z-axis  22  and a divergence angle pointing in any direction other than the offset axis will adopt a cylindrical ion oscillation trajectory. 
     Because the ELIT  14  is assumed to be cylindrically symmetric as described above, the three-dimensional ion reflection electric field (REF) that is induced within the regions R 1 , R 2  during the ion reflection mode of operation of the ion mirrors M 1 , M 2  can be described with respect to a two-dimensional radial slice at an arbitrary location through the ion mirror M 1  of the ELIT  14  along the x-y plane as illustrated by example in  FIG.  5 C . Referring to  FIG.  5 C , an ion  78  within the region R 1  of the ion mirror M 1  is shown radially offset from the z-axis by a radial distance r. Within the region R 1  (and also within the region R 2  of the ion mirror M 2 ), the ion reflection electric field REF operates to reflect the ion  78  entering R 1  from the charge detection cylinder CD back toward and into the charge detection cylinder CD as described above with respect to  FIG.  2 B . In addition to reflecting the ion  78  back toward the charge detection cylinder CD, the ion reflection electric field REF also forces the ion toward the z-axis. This force is illustratively represented in the x-y plane of  FIG.  5 C  by the vector F, and the direction of the vector F, as just described, always points toward the z-axis. 
     The velocity of the ion  78  positioned within the region R 1  of the ion mirror M 1  is represented in the x-y plane of  FIG.  5 C  by the vector v. The ion velocity vector v forms an angle α v  with the x-y coordinate system, and the electric field force vector F likewise forms an angle α F  with the x-y coordinate system. The difference between the force vector angle α F  and the velocity vector angle α v  is the angle β illustrated in  FIG.  5 C , wherein β represents the deviation of the ion  78  from collinearity of the two vectors F and v. When the velocity vector v is aligned with the force vector F such that the angle β is 0° or 180°, the ion  78  will assume a planar ion oscillation trajectory of the type illustrated in  FIG.  6   . When, on the other hand, the angle β between the force and velocity vectors v and F respectively is significantly greater or less than 0° or 180°, the ion  78  will assume a cylindrical ion oscillation trajectory of the type illustrated in  FIG.  7   . If either the angular divergence or the radial offset of an entering ion is zero or sufficiently small when the radial offset and the angular divergence of the entering ion are in the same direction in the x-y plane, then velocity vector v will point toward the z-axis and the ion oscillation trajectory will be planar. However, if the angular divergence and the radial offset are at different angles in the x-y plane, the ion oscillation trajectory will be cylindrical. 
     By suitably controlling the entrance trajectories of multiple ions entering the ELIT  14 , it is possible to favor a distribution of planar ion oscillation trajectories in which the likelihood of interactions, and thus Coulombic repulsion, between the multiple trapped ions simultaneously oscillating back and forth through the ELIT  14  is acceptably low. Referring to  FIG.  8   , for example, a plot is shown which represents a planar distribution of multiple ions  100  simultaneously trapped and oscillating back and forth within the ELIT  14 . In the illustrated example, the planar distribution of ions  100  includes two planar ion oscillation trajectories  80 ,  80 ′ defining an angle A R  therebetween in the x-y plane. Each planar ion oscillation trajectory  80 ,  80 ′ represents a single ion trapped and oscillating within the ELIT  14 . In the example illustrated in  FIG.  8   , the ELIT  14  thus has two ions trapped and oscillating back and forth therein, with each ion following one of two different planar ion oscillation trajectories  80 ,  80 ′ each with a unique α F  wherein the difference between α F  of one ion and α F  for the other ion is the angle A R . In the illustrated example, A R  is approximately 90° such that the two planes  80 ,  80 ′ are orthogonal, although it will be understood that A R  may alternatively be greater or less than 90°. As also illustrated in  FIG.  8   , the two ions can potentially interact with one another only along the z-axis where the two planes  80 ,  80 ′ intersect with one another, and in some embodiments the likelihood of interaction between the two ions may therefore be acceptably low. It will be understood that the planar ion distribution of two ions illustrated in  FIG.  8    is provided only by way of example, and that in other implementations the ion entrance trajectories may be controlled to trap two or more ions each having a different planar ion oscillation trajectory that is offset from adjacent planar ion oscillation trajectories by angle A R  of less than 90°. Two or more ions having different planar ion oscillation trajectories will thus be angularly offset from one another about the z-axis  22  by their respective planar angles A R . 
     Given the ion entrance conditions discussed above with respect to  FIGS.  5 B and  5 C , ion entrance trajectories which favor a planar distribution of ions within the ELIT  14  can illustratively be controlled in several different ways. Examples include, but are not limited to, injecting a collimated beam of ions with a large radial distribution of ions into the aperture A 1  of the ion mirror M 1  while keeping voltages D 1 -D 4  of power supply V 1  grounded and the central, longitudinal axis of the beam centered on the z-axis  22  so as to produce a distribution of radial offsets centered at the z-axis  22 , and injecting a collimated beam of ions into the aperture A 1  of the ion mirror M 1  and then varying the focusing power of the ion transmission electric field of the ion region R 1  in the ion mirror M 1  by manipulating voltages D 1 -D 4  of V 1  to impart an angular convergence on the ion beam towards a focal point that lies on the z-axis  22 . Any such control of the ion entrance trajectories will allow for the trapping of two or more ions within the ELIT  14  which will favor two or more corresponding planar ion oscillation trajectories within the ELIT  14  each forming an angle A R  in the x-y plane relative to adjacent trajectories which extends along the z-axis  22 . Various single or multiple stage instruments may be implemented as part of the ion source  12  illustrated in  FIG.  1   , or disposed between the ion source  12  and the ELIT  14 , for suitably controlling ion entrance trajectories in a manner which favors a planar distribution of ions oscillating back and forth within the ELIT  14 . An example embodiment of one such instrument is illustrated in  FIG.  11    and will be described in detail below. 
     By suitably controlling the entrance trajectories of multiple ions entering the ELIT  14 , it is also possible to favor a distribution of cylindrical ion oscillation trajectories in which the likelihood of close interactions, and thus Coulombic repulsion, between the multiple trapped ions simultaneously oscillating back and forth through the ELIT  14  is minimized. Examples include, but are not limited to, focusing a collimated beam of ions into a point along the z-axis  22  and sweeping the point along a line of radial offsets relative to the z-axis  22 , focusing a collimated beam of ions into a plane at the aperture A 1  of the ion mirror M 1  and offsetting from the z-axis  22 , and injecting an uncollimated beam of ions into the ELIT  14 . Referring to  FIG.  9   , for example, a plot is shown which represents a cylindrical distribution of multiple ions  110  simultaneously trapped and oscillating back and forth within the ELIT  14 . In the illustrated example, the cylindrical distribution of ions  110  includes two cylindrical ion oscillation trajectories  90 ,  90 ′ with the cylindrical ion oscillation trajectory  90 ′ completely nested within the cylindrical ion oscillation trajectory  90 . Each cylindrical ion oscillation trajectory  90 ,  90 ′ represents a single ion trapped and oscillating within the ELIT  14 . In the example illustrated in  FIG.  9   , the ELIT  14  thus has two ions trapped and oscillating back and forth therein, with each ion following one of two different cylindrical ion oscillation trajectories  90 ,  90 ′ and with one trajectory  90 ′ completely nested within the other trajectory  90 . With this configuration, the ion following the trajectory  90  thus has no opportunity to significantly interact with the ion following the trajectory  90 ′ and vice versa. It will be understood that the cylindrical ion distribution of two ions illustrated in  FIG.  9    is provided only by way of example, and that in other implementations the ion entrance trajectories may be controlled to trap three or more ions with successively nested cylindrical ion oscillation trajectories. 
     Referring now to  FIG.  10   , the outer cylindrical ion oscillation trajectory  90  is illustrated as having an inner radius IR 1  and an outer radius OR 1  along the section line  10 - 10  of  FIG.  9   , wherein the radial distance between IR 1  and OR 1  defines the thickness of the cylindrical trajectory  90 . The inner cylindrical ion oscillation trajectory  90 ′ similarly has an inner radius IR 2  and an outer radius OR 2  along the section line  10 - 10  of  FIG.  9   , wherein the radial distance between IR 2  and OR 2  defines the thickness of the cylindrical trajectory  90 ′. The radial distance between the inner radius IR 1  of the outer cylindrical ion oscillation trajectory  90  and the outer radius OR 2  of the inner cylindrical ion oscillation trajectory  90 ′ is DR. 
     The inner and outer radii of a cylindrical ion oscillation trajectory can be controlled by controlling the magnitude of the radial offset of the ion entrance trajectory relative to the z-axis  22 . Thus, if multiple ions enter the ELIT  14  via the aperture A 1  of the ion mirror M 1  with a radial distribution, the resulting multiple cylindrical ion oscillation trajectories within the ELIT  14  will each have different, independent radii, which will contribute to minimizing the likelihood of close interactions between the multiple trapped ions. The thickness of a cylindrical ion oscillation trajectory in relation to the average radius of the trajectory can similarly be controlled by controlling the magnitude of the angular divergence of the ion entrance trajectory relative to the radial offset line parallel with the z-axis  22 . For example, the thinnest cylindrical ion oscillation trajectories are produced when β approaches 90°. Thin cylindrical ion oscillation trajectories are preferable in embodiments in which it is desirable to nest or stack many cylindrical ion oscillation trajectories within the ELIT  14 . As compared with planar ion oscillation trajectories, the ELIT  14  can accommodate substantially more ions simultaneously oscillating back and forth therein with nested cylindrical ion oscillation trajectories because each such nested cylindrical ion oscillation trajectory occupies a unique region within the ELIT  14 , i.e., a region that is separate and distinct from those occupied by all other cylindrical ion oscillation trajectories. 
     Given the ion entrance conditions discussed above with respect to  FIGS.  5 B and  5 C , ion entrance trajectories which favor a cylindrical distribution of ions within the ELIT  14  can illustratively be controlled in several different ways. Examples include, but are not limited to, one or more of the example techniques described above with respect to the planar distribution of ions but doing so with an uncollimated beam of ions such that the ions not only have a distribution of radial offsets but also a distribution of divergence angles. In any case, as the radial offset of an entering ion increases in radial distance from the z-axis  22  of the ELIT  14 , so too does the magnitude of the force vector F (see  FIG.  5 C ) pointing toward the z-axis  22 . For a particular velocity vector v, the inner radius of a cylindrical ion oscillation trajectory resulting from an ion entering the ELIT  14  with a relatively greater radial offset is thus less than that of a cylindrical ion oscillation trajectory resulting from an ion entering the ELIT  14  with a relatively lower radial offset because the magnitude of the force vector F acting on the former is less than that acting on the latter. 
     Moreover, as the radial offset of an entering ion increases, the angle of divergence, represented by the magnitude of the velocity vector v that points away from the force vector F, of the entering ion must also increase in order to cause the entering ion to adopt a cylindrical ion oscillation trajectory. This is so because if the velocity vector v is pointing along the same plane as the force vector F, i.e., where β is 0 or 180 degrees, the ion motion will be influenced only by the force vector, thereby causing the entering ion to adopt an oscillation trajectory that lies in the same plane as the force vector F as described above with respect to  FIG.  5 C . Any ion velocity vector v component that is not coplanar with the force vector F, i.e., where β is anything larger or smaller than 0 or 180, causes the ion to rotate in the x-y plane while oscillating back and forth along the z-axis  22  because no force acts on the ion in the direction of its rotation. The magnitude of the force vector F towards the z-axis  22  experienced by an ion trapped in the ELIT  14  is directly proportional to the radial offset of the ion. Since the force vector F acting on an ion increases with the radial offset, an ion oscillating farther away from the z-axis  22  is subjected to a larger force vector F towards the z-axis  22  which causes its oscillation trajectory to become dominated by the force vector F and becomes more planar. To compensate for this effect and induce the formation of cylindrical ion oscillation trajectories at all radial offsets in the ELIT  14 , as the magnitude of the force vector F increases, so too must the magnitude of the velocity vector v in a direction that is perpendicular to the force vector F by a commensurate amount which ensures that the entering ion will adopt a cylindrical oscillation trajectory. Various single or multiple stage instruments may be implemented as part of the ion source  12  illustrated in  FIG.  1   , or disposed between the ion source  12  and the ELIT  14 , for suitably controlling ion entrance trajectories in a manner which favors a cylindrical distribution of ions oscillating back and forth within the ELIT  14 . An example embodiment of one such instrument is illustrated in  FIG.  11    and will be described in detail below. 
     Based on the foregoing, the nested cylindrical ion oscillation trajectories illustrated by example in  FIG.  9    are superior to the angularly distributed planar ion oscillation trajectories illustrated by example in  FIG.  8    in terms minimizing interactions between multiple ions trapped within the ELIT  14 . However, while the angularly distributed planar ion oscillation trajectories do not completely eliminate the potential for ion interaction within the ELIT  14 , the probability of such ion interaction is substantially reduced as compared with conventional ion entrance control techniques. Moreover, based on the design of the ELIT  14  illustrated in  FIGS.  1 - 2 B  and described above, the oscillation frequency stability of nested cylindrical ion oscillation trajectories is superior to that of the angularly distributed planar ion oscillation trajectories. In other words, fluctuations in ion oscillation frequency during a trapping event within the ELIT  14  are greater for planar ion oscillation trajectories than for cylindrical ion oscillation trajectories. Since the oscillation frequency within the ELIT  14  is used to determine ion mass-to-charge ratio (m/z), m/z determination uncertainty is therefore expected to be smaller for cylindrical ion oscillation trajectories than for planar ion oscillation trajectories. This may not be the case for other ELIT designs, and indeed it is to be understood that the concepts illustrated in the attached figures and described herein may be implemented with ELIT designs and configurations different in one or more aspects from the ELIT  14  illustrated in  FIGS.  1 - 2 B  and described herein. Moreover, it is possible that the design of the ELIT  14  may be modified and/or that an ELIT or other ion trap may be designed, in a manner which reduces such fluctuations in oscillation frequency. 
     It is also possible to split the charge detection cylinder CD of the ELIT  14  into two halves along the longitudinal axis and either connect a separate detection circuit as shown in  FIG.  3    to each charge detection cylinder half and independently analyze the signal coming from each half, or perform a differential measurement between the two charge detection cylinder halves using a differential amplifier. In the former case where a separate circuit is used for each half, the digitized signal for each half may be analyzed by fast Fourier transform and the magnitude of the fundamental frequency peak related to the average proximity of an ion to each charge detection cylinder half over the course of a trapping event. In other words, a cylindrical ion oscillation trajectory with an outer radius such as OR 1  of  90  in  FIG.  10    will oscillate at a particular average distance from the two charge detection cylinder halves. The magnitude of the fundamental frequency peak in the Fourier transform depends on how close the ion was to the charge detection cylinder halves. From this, the outer radius of the cylindrical ion oscillation trajectory can be deduced and used to correct the measured ion m/z to account for deviations from the actual ion m/z that result from ion oscillation trajectory distributions. In the latter case, a differential amplifier can be used to monitor the difference in signal between the two charge detection cylinder halves. Ion oscillation frequencies that are very close to the z-axis  22 , i.e., its outer radius is small, would produce a small difference in signal between the halves because the ion is a similar distance from each half. However, an ion oscillation trajectory that has a large outer radius is much closer to one charge detection cylinder half than the other which will result in a large difference in signal between the two halves. A fast Fourier transform can be employed to measure the fundamental frequency magnitude from the digitized differential amplifier signal and related to the ion oscillation trajectory outer radius, lending this as a method of ascertaining the three-dimensional ion oscillation trajectory to correct for deviations in the measured m/z of an ion that arise from trajectory distributions. Alternatively, the charge detection cylinder CD can be left whole and additional charge detection cylinders can be located in any other region of the trap where the oscillating ion would produce an induced image charge on the additional cylinders that is representative of the ion oscillation trajectory. 
     As described above, it is possible to tune the ion entrance trajectories, i.e., the trajectories of ions entering the ELIT  14 , in a manner which favors a distribution of planar or cylindrical ion oscillation trajectories within the ELIT  14 , and some example techniques for controlling ion entrance trajectories to favor each trajectory are briefly described above. Such examples of controlling ion entrance trajectories to favor a distribution of planar ion oscillation trajectories illustratively include, but are not limited to, injecting a collimated beam of ions with a large radial distribution of ions into the aperture A 1  of the ion mirror M 1  while keeping the voltages D 1 -D 4  of power supply V 1  grounded and the central, longitudinal axis of the beam centered on the z-axis  22  so as to produce a distribution of radial offsets centered at the z-axis  22 , and injecting a collimated beam of ions into the aperture A 1  of the ion mirror M 1  and then varying the focusing power of the ion transmission electric field of the ion region R 1  in the ion mirror M 1  by manipulating voltages D 1 -D 4  of V 1  to impart an angular convergence on the ion beam towards a focal point that lies on the z-axis  22 . Alternatively, focusing a collimated beam of ions into a point along the z-axis  22  and sweeping the point along a line of radial offsets relative to the z-axis  22 , focusing a collimated beam of ions into a plane at the aperture A 1  of the ion mirror M 1  and offsetting the plane from the z-axis  22 , and injecting an uncollimated, i.e., convergent or divergent, beam of ions that includes not only a distribution of radial offsets but also a distribution of angular divergence into the aperture A 1  of the ion mirror M 1  are example techniques for controlling ion entrance trajectories to favor a distribution of cylindrical ion oscillation trajectories. Any such control of the ion entrance trajectories will allow for the trapping of two or more ions within the ELIT  14  which will favor a distribution of planar or cylindrical ion oscillation trajectories respectively. In this regard, an embodiment is shown in  FIG.  11    of a charge detection mass spectrometer (CDMS)  100  which includes the ion source  12  illustrated in  FIG.  1    and described above, which includes the ELIT  14  illustrated in  FIGS.  1 - 2 B  and described above and which includes an example embodiment of an ion trajectory control apparatus  101  for selectively controlling the trajectories of ions exiting the ion source  12  and entering the ELIT  14  in a manner which achieves simultaneous trapping of multiple ions in and by the ELIT  14  and which favors a distribution within the ELIT  14  of planar or cylindrical ion oscillation trajectories. 
     Referring now to  FIG.  11   , the ion trajectory control apparatus  101  illustratively includes a multi-stage ion trajectory control instrument  105  disposed between the ion source  12  and the ELIT  14  and operatively coupled to one or more voltage sources  108  and to signal detection circuitry  110 . The one or more voltage sources  108  may illustratively include any number of conventional voltage sources configured to produce one or more constant or switching DC voltages of selectable polarity and/or magnitude, and any number of conventional voltage sources configured to produce one or more time-varying, i.e., AC, voltages of selectable frequency and/or peak magnitude. One or any combination of the one or more voltage sources  108  may be manually controllable and/or may be operatively coupled to a conventional processor  112  for processor control thereof. One or more of the voltage sources  108  may also be used to control one or more operational features of the ion source  12 , and in some embodiments the one or more voltage sources  108  may include the voltage sources V 1  and V 2  illustrated in  FIG.  1    and operable to control operation of the ELIT  14  as described above. 
     The signal detection circuitry  110  illustratively includes one or more conventional signal sensors and conventional signal detection circuitry for detecting one or more operating conditions of the ion trajectory control instrument  105 . In some embodiments, the signal detection circuitry  110  may include the charge preamplifier CP operatively coupled to the ELIT  14  as illustrated in  FIG.  1    and described above. In any case, the signal detection circuitry  110  is operatively coupled to the processor  112 , and signals detected by the circuitry  110  are thus provided to the processor  112  for processing thereof. 
     The processor  112  illustratively includes, or is operatively coupled to, at least one conventional memory unit  114  for storing operating instructions for the processor  114  and to store data collected and/or processed by the processor  112 . As it relates to the operation and control of the ion trajectory control instrument  105 , the memory unit(s)  114  illustratively has one or more sets of instructions stored therein which, when executed by the processor  112 , cause the processor  112  to control one or more of the voltage sources  108  based, at least in part, on one or more signals produced by the signal detection circuitry  110 , in a manner which selectively controls the trajectories of ions exiting the ion source  12  and entering the ELIT  14  so as to achieve simultaneous trapping of multiple ions in and by the ELIT  14  and which causes the ions entering the ELIT  14  to adopt a distribution therein of planar or cylindrical ion oscillation trajectories. The processor  112  may include one or more conventional computing devices in the form of any one or combination of one or more conventional microprocessors and/or controllers, one or more field programmable gate arrays (FPGAs), one or more application specific integrated circuits (ASICs), one or more conventional personal, lap top, desk top, tablet or other computers, or the like. 
     In the illustrated embodiment, the ion trajectory control instrument  105  includes a number of cascaded ion trajectory control stages. It will be understood that such stages are illustrated only by way of example, and that alternate embodiments of the instrument  105  may include more or fewer ion trajectory control stages. In any case, the instrument  105  depicted in  FIG.  11    illustratively includes an image charge detection array stage  102  having an ion inlet at one end configured to receive ions generated by the ion source  12  and an ion outlet at an opposite end that is operatively coupled to an ion inlet of an ion deflector/offset stage  104  having an ion outlet operatively coupled to an ion inlet of an ion focusing stage  106 . An ion outlet of the ion focusing stage  106  is operatively coupled to the ion inlet aperture A 1  of the ion mirror M 1  of the ELIT  14  such that ions exiting the ion focusing stage  106  enter the ELIT  14  via the aperture A 1  of the ion mirror M 1 . 
     The image charge detection array stage  102  illustratively includes at least two spaced-apart arrays  102 A,  102 B of conventional image charge detectors. As ions exit the ion source  12  in the form of a beam and pass sequentially through the image charge detector arrays  102 A,  102 B, conventional image charge detection circuitry included as part of the signal detection circuitry  110  provides respective image charge detection signals to the processor  112  from which the processor  112  is operable to determine the positions of the ions passing sequentially through each array  102 A,  102 B. From this information, the trajectory of the ion beam exiting the stage  102  can be determined. It will be understood that although the image charge detection array stage  102  is illustrated in  FIG.  11    and described herein as including only two spaced-apart image charge detector arrays, alternate embodiments of the stage  102  may include more or fewer spaced-apart image charge detector arrays. 
     The ion deflector/offset stage  104  illustratively includes one or more conventional ion deflectors and/or one or more conventional ion offset apparatuses. Based on the computed trajectory of the ion beam exiting the stage  102 , the processor  112  is illustratively operable to compute, e.g., in real-time, adjustments to the ion beam trajectory required to achieve an ion entrance trajectory which will favor a selected planar or cylindrical distribution of ion oscillation trajectories within the ELIT  14  as described in detail hereinabove. Such computed adjustments are illustratively fed to the one or more ion deflectors and/or one or more ion offset apparatuses in the stage  104  in the form of control signals, and the one or more ion deflectors and/or one or more ion offset apparatuses are responsive to such control signals to selectively alter the trajectory of the ion beam passing therethrough, e.g., by controlling either or both of a radial offset of the ion beam relative to the z-axis  22  and an angle of the ion beam relative to the z-axis  22  and/or relative to an axis that passes through the ELIT  14  and that is parallel with the z-axis  22 . 
     The ion focusing stage  106  illustratively includes one or more conventional ion focusing elements. The adjusted ion beam trajectory exiting the ion deflector/offset stage  104  is suitably focused as it passes through the one or more ion focusing elements, and the ion beam emerging from the ion focusing stage  106  is passed into the ELIT  14  via the ion inlet aperture A 1  of the ion mirror M 1  as described above. 
     As illustrated by dashed-line representation in  FIG.  11   , one example ion entrance trajectory produced by the ion trajectory control instrument  105  may be a collimated ion beam  120  which is radially offset from the z-axis  22  of the ELIT  14  and which is suitably manipulated using any of the techniques described above so as to favor a distribution of planar ion oscillation trajectories within the ELIT  14 . As also illustrated by dashed-line representation in  FIG.  11   , another example ion entrance trajectory produced by the ion trajectory control instrument  105  may be an uncollimated ion beam  130  which is radially offset from the z-axis  22  of the ELIT  14  and includes a distribution of divergence angles, and which is suitably manipulated using any of the techniques described above so as to favor a distribution of cylindrical ion oscillation trajectories within the ELIT  14 . 
     In some alternate embodiments, the ion trajectory control instrument  105  may be or include at least one conventional ion trap that is controlled by the processor  112  in a conventional manner to collect ions therein, to focus the collected ion toward the z-axis  22  passing through the ion trap, and to then selectively release the collected ions. Upon release, the exiting ions will expand radially about the z-axis  22  and may thereafter be focused by one or more focusing elements into the ELIT  14 . In this embodiment, the ion beam exiting the ion trap will include an angular distribution of ions distributed radially about the z-axis  22 , and such an ion entrance trajectory will thus favor a distribution of cylindrical ion oscillation trajectories. 
     In addition to or in place of the ion trajectory control instrument  105 , one or more magnetic and electric field generators may suitably positioned relative to the ELIT  14  and selectively controlled in a manner which controls or guides the ion oscillation trajectories within the ELIT  14 . If, for example, the generated magnetic field lines extend along the z-axis  22 , ions trapped within the ELIT  14  will undergo a cyclotron motion as they oscillate back and forth through the ELIT  14 . Also, a collimated ion beam can be injected into a magnetic lens positioned between the ion source  12  and ELIT  14  aligned with the ELIT  14  z-axis  22 . The lens would impart a radial Lorentz force on the ions as they travel through the lens that can give them a radial velocity with a magnitude that is proportional to the ion distance from the z-axis  22  and in a direction that may give rise to cylindrical ion oscillation trajectories. The magnetic field strength of the lens can be adjusted by varying the electric current in the lens coil so as to cause ions to enter the ELIT  14  with trajectories that favor the formation of planar or cylindrical ion oscillation trajectories. Such control may induce or enhance a desired ion oscillation trajectory or distribution of ion oscillation trajectories within the ELIT  14 . 
     Those skilled in the art will recognize other conventional instruments and combinations of conventional instruments that may be used to guide and control ion inlet trajectories according to ion inlet conditions described herein which result in planar or cylindrical distributions of ion oscillation trajectories with an electrostatic linear ion trap such as the ELIT  14  illustrated in the attached figures and described herein. It will be understood that any such other conventional instruments and combinations thereof are contemplated by, and are intended to fall within the scope of, this disclosure. 
     In any case, with multiple ions oscillating back and forth through the ELIT  14  with either a planar or cylindrical distribution of ion oscillation trajectories, charges induced on the charge detection cylinder CD of the ELIT  14  by the multiple ions passing therethrough are detected by the charge preamplifier CP, and corresponding charge detection signals CHD are passed to the processor  16  for the duration of a trapping event as described above with respect to  FIGS.  1 - 4 C . When the stored collection of charge detection signals for a trapping event are processed using a conventional Fourier transform algorithm as described above, multiple fundamental frequency peaks will emerge, each corresponding to a respective one of the multiple trapped ions. The harmonic peaks associated with each such fundamental peak may then be easily identified, and ion charge, mass-to-charge and mass may then be determined as described above for each of the multiple trapped ions. 
     Referring now to  FIG.  12 A , a simplified block diagram is shown of an embodiment of an ion separation instrument  200  which may include the ELIT  14  illustrated and described herein, and which may include the charge detection mass spectrometer  100  illustrated and described herein, and which may include any number of ion processing instruments which may form part of the ion source  12  upstream of the ELIT  14  and/or which may include any number of ion processing instruments which may be disposed downstream of the ELIT  14  to further process ion(s) exiting the ELIT  14 . In this regard, the ion source  12  is illustrated in  FIG.  12 A  as including a number, Q, of ion source stages IS 1 -IS Q  which may be or form part of the ion source  12 , where Q may be any positive integer. Alternatively or additionally, an ion processing instrument  202  is illustrated in  FIG.  12 A  as being coupled to the ion outlet of the ELIT  14 , wherein the ion processing instrument  210  may include any number, R, of ion processing stages OS 1 -OS R , where R may be any positive integer. 
     Turning now to the ion source  12 , it will be understood that the source  12  of ions entering the ELIT may be or include, in the form of one or more of the ion source stages IS 1 -IS Q , one or more conventional sources of ions as described above, and may further include one or more conventional instruments for separating ions according to one or more molecular characteristics (e.g., according to ion mass, ion mass-to-charge, ion mobility, ion retention time, or the like) and/or one or more conventional ion processing instruments for collecting and/or storing ions (e.g., one or more quadrupole, hexapole and/or other ion traps), for filtering ions (e.g., according to one or more molecular characteristics such as ion mass, ion mass-to-charge, ion mobility, ion retention time and the like), for fragmenting or otherwise dissociating ions, for normalizing or shifting ion charge states, and the like. It will be understood that the ion source  12  may include one or any combination, in any order, of any such conventional ion sources, ion separation instruments and/or ion processing instruments, and that some embodiments may include multiple adjacent or spaced-apart ones of any such conventional ion sources, ion separation instruments and/or ion processing instruments. 
     Turning now to the ion processing instrument  202 , it will be understood that the instrument  202  may be or include, in the form of one or more of the ion processing stages OS 1 -OS R , one or more conventional instruments for separating ions according to one or more molecular characteristics (e.g., according to ion mass, ion mass-to-charge, ion mobility, ion retention time, or the like), one or more conventional instruments for collecting and/or storing ions (e.g., one or more quadrupole, hexapole and/or other ion traps), one or more conventional instruments for filtering ions (e.g., according to one or more molecular characteristics such as ion mass, ion mass-to-charge, ion mobility, ion retention time and the like), one or more conventional instruments for fragmenting or otherwise dissociating ions, one or more conventional instruments for normalizing or shifting ion charge states, and the like. It will be understood that the ion processing instrument  202  may include one or any combination, in any order, of any such conventional ion separation instruments and/or ion processing instruments, and that some embodiments may include multiple adjacent or spaced-apart ones of any such conventional ion separation instruments and/or ion processing instruments. In any implementation which the ion source  12  and/or the ion processing instruments  202  includes one or more mass spectrometers, any one or more such mass spectrometers may be of any conventional design including, for example, but not limited to a time-of-flight (TOF) mass spectrometer, a reflectron mass spectrometer, a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer, a quadrupole mass spectrometer, a triple quadrupole mass spectrometer, a magnetic sector mass spectrometer, or the like. 
     As one specific implementation of the ion separation instrument  200  illustrated in  FIG.  12 A , which should not be considered to be limiting in any way, the ion source  12  illustratively includes 3 stages, and the ion processing instrument  202  is omitted. In this example implementation, the ion source stage IS 1  is a conventional source of ions, e.g., electrospray, MALDI or the like, the ion source stage IS 2  is a conventional ion filter, e.g., a quadrupole or hexapole ion guide, and the ion source stage IS 3  is a mass spectrometer of any of the types described above. In this embodiment, the ion source stage IS 2  is controlled in a conventional manner to preselect ions having desired molecular characteristics for analysis by the downstream mass spectrometer, and to pass only such preselected ions to the mass spectrometer, wherein the multiple ions simultaneously analyzed by the ELIT  14  will be the preselected ions separated by the mass spectrometer according to mass-to-charge ratio. The preselected ions exiting the ion filter may, for example, be ions having a specified ion mass or mass-to-charge ratio, ions having ion masses or ion mass-to-charge ratios above and/or below a specified ion mass or ion mass-to-charge ratio, ions having ion masses or ion mass-to-charge ratios within a specified range of ion mass or ion mass-to-charge ratio, or the like. In some alternate implementations of this example, the ion source stage IS 2  may be the mass spectrometer and the ion source stage IS 3  may be the ion filter, and the ion filter may be otherwise operable as just described to preselect ions exiting the mass spectrometer which have desired molecular characteristics for analysis by the downstream ELIT  14 . In other alternate implementations of this example, the ion source stage IS 2  may be the ion filter, and the ion source stage IS 3  may include a mass spectrometer followed by another ion filter, wherein the ion filters each operate as just described. 
     As another specific implementation of the ion separation instrument  200  illustrated in  FIG.  12 A , which should not be considered to be limiting in any way, the ion source  12  illustratively includes 2 stages, and the ion processing instrument  202  is again omitted. In this example implementation, the ion source stage IS 1  is a conventional source of ions, e.g., electrospray, MALDI or the like, the ion source stage IS 2  is a conventional mass spectrometer of any of the types described above. In this implementation, the instrument  200  takes the form of the charge detection mass spectrometer (CDMS)  100  in which the ELIT  14  is operable to simultaneously analyze multiple ions exiting the mass spectrometer. 
     As yet another specific implementation of the ion separation instrument  200  illustrated in  FIG.  12 A , which should not be considered to be limiting in any way, the ion source  12  illustratively includes 2 stages, and the ion processing instrument  202  is omitted. In this example implementation, the ion source stage IS 1  is a conventional source of ions, e.g., electrospray, MALDI or the like, and the ion source stage IS 2  is a conventional single or multiple-stage ion mobility spectrometer. In this implementation, the ion mobility spectrometer is operable to separate ions, generated by the ion source stage IS 1 , over time according to one or more functions of ion mobility, and the ELIT  14  is operable to simultaneously analyze multiple ions exiting the ion mobility spectrometer. In an alternate implementation of this example, the ion processing instrument  202  may include a conventional single or multiple-stage ion mobility spectrometer as a sole stage OS 1  (or as stage OS 1  of a multiple-stage instrument). In this alternate implementation, the ELIT  14  is operable to simultaneously analyze multiple ions generated by the ion source stage IS 1 , and the ion mobility spectrometer OS 1  is operable to separate ions exiting the ELIT  14  over time according to one or more functions of ion mobility. As another alternate implementation of this example, single or multiple-stage ion mobility spectrometers may follow both the ion source stage IS 1  and the ELIT  14 . In this alternate implementation, the ion mobility spectrometer following the ion source stage IS 1  is operable to separate ions, generated by the ion source stage IS 1 , over time according to one or more functions of ion mobility, the ELIT  14  is operable to simultaneously analyze multiple ions exiting the ion source stage ion mobility spectrometer, and the ion mobility spectrometer of the ion processing stage OS 1  following the ELIT  14  is operable to separate ions exiting the ELIT  14  over time according to one or more functions of ion mobility. In any implementations of the embodiment described in this paragraph, additional variants may include a mass spectrometer operatively positioned upstream and/or downstream of the single or multiple-stage ion mobility spectrometer in the ion source  12  and/or in the ion processing instrument  202 . 
     As still another specific implementation of the ion separation instrument  200  illustrated in  FIG.  12 A , which should not be considered to be limiting in any way, the ion source  12  illustratively includes 2 stages, and the ion processing instrument  202  is omitted. In this example implementation, the ion source stage IS 1  is a conventional liquid chromatograph, e.g., HPLC or the like configured to separate molecules in solution according to molecule retention time, and the ion source stage IS 2  is a conventional source of ions, e.g., electrospray or the like. In this implementation, the liquid chromatograph is operable to separate molecular components in solution, the ion source stage IS 2  is operable to generate ions from the solution flow exiting the liquid chromatograph, and the ELIT  14  is operable to simultaneously analyze multiple ions generated by the ion source stage IS 2 . In an alternate implementation of this example, the ion source stage IS 1  may instead be a conventional size-exclusion chromatograph (SEC) operable to separate molecules in solution by size. In another alternate implementation, the ion source stage IS 1  may include a conventional liquid chromatograph followed by a conventional SEC or vice versa. In this implementation, ions are generated by the ion source stage IS 2  from a twice separated solution; once according to molecule retention time followed by a second according to molecule size, or vice versa. The ability to analyze trapping events containing multiple simultaneously trapped ions is highly valuable in experiments where the mass spectrometer is coupled to a chromatographic technique. When molecules are sufficiently separated in the chromatograph, they elute from the chromatograph in bursts, each on the order of seconds to minutes in duration, where each burst occurs only once for each molecule per sample injection into the chromatograph. The abundance of ions exiting the chromatograph as a function of the elution time can be considered the elution profile. When these bursts are introduced into the mass spectrometer, the ion beam intensity becomes a function of the chromatographic elution profile. Unlike conventional CDMS implementation which only analyzes single-ion trapping events and requires over an hour to collect sufficient data for the generation of a mass spectrum, simultaneous trapping of multiple ions and the subsequent analysis of multiple-ion trapping events significantly decrease the time necessary to collect a mass spectrum, making it possible to acquire a mass spectrum in several minutes. This also means that bursts in the ion beam intensity that coincide with the elution of separated molecules from the chromatograph can be characterized by CDMS in the same timeframe as the elution profile. In any implementations of the embodiment described in this paragraph, additional variants may include a mass spectrometer operatively positioned between the ion source stage IS 2  and the ELIT  14 . 
     Referring now to  FIG.  12 B , a simplified block diagram is shown of another embodiment of an ion separation instrument  210  which illustratively includes a multi-stage mass spectrometer instrument  220  and which also includes the CDMS  100  including the ELIT  14  and, in some embodiments, the ion trajectory control apparatus  105  as described above, implemented as a high-mass ion analysis component. In the illustrated embodiment, the multi-stage mass spectrometer instrument  220  includes an ion source (IS)  12 , as illustrated and described herein, followed by and coupled to a first conventional mass spectrometer (MS 1 )  204 , followed by and coupled to a conventional ion dissociation stage (ID)  206  operable to dissociate ions exiting the mass spectrometer  204 , e.g., by one or more of collision-induced dissociation (CID), surface-induced dissociation (SID), electron capture dissociation (ECD) and/or photo-induced dissociation (PID) or the like, followed by and coupled to a second conventional mass spectrometer (MS 2 )  208 , followed by a conventional ion detector (D)  212 , e.g., such as a microchannel plate detector or other conventional ion detector. The CDMS  100 , is coupled in parallel with and to the ion dissociation stage  206  such that the CDMS  100  may selectively receive ions from the mass spectrometer  204  and/or from the ion dissociation stage  206 . 
     MS/MS, e.g., using only the ion separation instrument  220 , is a well-established approach where precursor ions of a particular molecular weight are selected by the first mass spectrometer  204  (MS 1 ) based on their m/z value. The mass selected precursor ions are fragmented, e.g., by collision-induced dissociation, surface-induced dissociation, electron capture dissociation or photo-induced dissociation, in the ion dissociation stage  206 . The fragment ions are then analyzed by the second mass spectrometer  208  (MS 2 ). Only the m/z values of the precursor and fragment ions are measured in both MS 1  and MS 2 . For high mass ions, the charge states are not resolved and so it is not possible to select precursor ions with a specific molecular weight based on the m/z value alone. However, by coupling the instrument  220  to the CDMS  100  as illustrated in  FIG.  12 B , it is possible to select a narrow range of m/z values and then use the CDMS  100  to determine the masses of the m/z selected precursor ions. The mass spectrometers  204 ,  208  may be, for example, one or any combination of a magnetic sector mass spectrometer, time-of-flight mass spectrometer or quadrupole mass spectrometer, although in alternate embodiments other mass spectrometer types may be used. In any case, the m/z selected precursor ions with known masses exiting MS 1  can be fragmented in the ion dissociation stage  206 , and the resulting fragment ions can then be analyzed by MS 2  (where only the m/z ratio is measured) and/or by the CDMS instrument  100  (where the m/z ratios and charges of multiple ions are measured simultaneously). Low mass fragments, i.e., dissociated ions of precursor ions having mass values below a threshold mass value, e.g., 10,000 Da (or other mass value), can thus be analyzed by conventional MS, using MS 2 , while high mass fragments (where the charge states are not resolved), i.e., dissociated ions of precursor ions having mass values at or above the threshold mass value, can be analyzed by the CDMS  100 . 
     It will be understood that the dimensions of the various components of the ELIT  14  and the magnitudes of the electric fields established therein, as implemented in any of the systems  10 ,  100 ,  200 ,  210  illustrated in the attached figures and described above, may illustratively be selected as to establish a desired duty cycle of ion oscillation within the ELIT  14 , corresponding to a ratio of time spent by the ion(s) in the charge detection cylinder CD and a total time spent by the ion(s) traversing the combination of the ion mirrors M 1 , M 2  and the charge detection cylinder CD during one complete oscillation cycle. For example, a duty cycle of approximately 50% may be desirable for the purpose of reducing noise in fundamental frequency magnitude determinations resulting from harmonic frequency components of the measured signals. Details relating to such dimensional and operational considerations for achieving a desired duty cycle, e.g., such as 50%, are illustrated and described in co-pending U.S. Patent Application Ser. No. 62/616,860, filed Jan. 12, 2018, co-pending U.S. Patent Application Ser. No. 62/680,343, filed Jun. 4, 2018 and co-pending International Patent Application No. PCT/US2019/013251, filed Jan. 11, 2019, all entitled ELECTROSTATIC LINEAR ION TRAP DESIGN FOR CHARGE DETECTION MASS SPECTROMETRY, the disclosures of which are all expressly incorporated herein by reference in their entireties. 
     It will be further understood that one or more charge detection optimization techniques may be used with the ELIT  14  in any of the systems  10 ,  100 ,  200 ,  210  illustrated in the attached figures and described herein e.g., for trigger trapping or other charge detection events. Examples of some such charge detection optimization techniques are illustrated and described in co-pending U.S. Patent Application Ser. No. 62/680,296, filed Jun. 4, 2018 and in co-pending International Patent Application No. PCT/US2019/013280, filed Jan. 11, 2019, both entitled APPARATUS AND METHOD FOR CAPTURING IONS IN AN ELECTROSTATIC LINEAR ION TRAP, the disclosures of which are both expressly incorporated herein by reference in their entireties. 
     It will be further understood that one or more charge calibration or resetting apparatuses may be used with the charge detection cylinder CD of the ELIT  14  in any of the systems  10 ,  100 ,  200 ,  210  illustrated in the attached figures and described herein. An example of one such charge calibration or resetting apparatus is illustrated and described in co-pending U.S. Patent Application Ser. No. 62/680,272, filed Jun. 4, 2018 and in co-pending International Patent Application No. PCT/US2019/013284, filed Jan. 11, 2019, both entitled APPARATUS AND METHOD FOR CALIBRATING OR RESETTING A CHARGE DETECTOR, the disclosures of which are both expressly incorporated herein by reference in their entireties. 
     It will be still further understood that the ELIT  14  illustrated in the attached figures and described herein, as part of any of the systems  10 ,  100 ,  200 ,  210  also illustrated in the attached figures and described herein, may alternatively be provided in the form of at least one ELIT array having two or more ELITs or ELIT regions and/or in any single ELIT including two or more ELIT regions, and that the concepts described herein are directly applicable to systems including one or more such ELITs and/or ELIT arrays. Examples of some such ELITs and/or ELIT arrays are illustrated and described in co-pending U.S. Patent Application Ser. No. 62/680,315, filed Jun. 4, 2018 and in co-pending International Patent Application No. PCT/US2019/013283, filed Jan. 11, 2019, both entitled ION TRAP ARRAY FOR HIGH THROUGHPUT CHARGE DETECTION MASS SPECTROMETRY, the disclosures of which are both expressly incorporated herein by reference in their entireties. 
     It will be further understood that one or more ion source optimization apparatuses and/or techniques may be used with one or more embodiments of the ion source  12  illustrated and described herein as part of or in combination with any of the systems  10 ,  150 ,  180 ,  200 ,  220  illustrated in the attached figures and described herein, some examples of which are illustrated and described in co-pending U.S. Patent Application Ser. No. 62/680,223, filed Jun. 4, 2018 and entitled HYBRID ION FUNNEL-ION CARPET (FUNPET) ATMOSPHERIC PRESSURE INTERFACE FOR CHARGE DETECTION MASS SPECTROMETRY, and in co-pending International Patent Application No. PCT/US2019/013274, filed Jan. 11, 2019 and entitled INTERFACE FOR TRANSPORTING IONS FROM AN ATMOSPHERIC PRESSURE ENVIRONMENT TO A LOW PRESSURE ENVIRONMENT, the disclosures of which are both expressly incorporated herein by reference in their entireties. 
     It will be further understood that any of the systems  10 ,  100 ,  200 ,  210  illustrated in the attached figures and described herein may be implemented in or as part of systems configured to operate in accordance with real-time analysis and/or real-time control techniques, some examples of which are illustrated and described in co-pending U.S. Patent Application Ser. No. 62/680,245, filed Jun. 4, 2018 and co-pending International Patent Application No. PCT/US2019/013277, filed Jan. 11, 2019, both entitled CHARGE DETECTION MASS SPECTROMETRY WITH REAL TIME ANALYSIS AND SIGNAL OPTIMIZATION, the disclosures of which are both expressly incorporated herein by reference in their entireties. 
     While this disclosure has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described and that all changes and modifications that come within the spirit of this disclosure are desired to be protected. For example, it will be understood that the ELIT  14  illustrated in the attached figures and described herein is provided only by way of example, and that the concepts, structures and techniques described above may be implemented directly in ELITs of various alternate designs. Any such alternate ELIT design may, for example, include any one or combination of two or more ELIT regions, more, fewer and/or differently-shaped ion mirror electrodes, more or fewer voltage sources, more or fewer DC or time-varying signals produced by one or more of the voltage sources, one or more ion mirrors defining additional electric field regions, or the like.