Patent Publication Number: US-2022238321-A1

Title: Ion injection to an electrostatic trap

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation under 35 U.S.C. § 120 and claims the priority benefit of co-pending U.S. patent application Ser. No. 15/600,996, filed May 22, 2017. The disclosure of the foregoing application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a method of injecting ions into an electrostatic trap from an ion storage device and a corresponding mass spectrometer. 
     BACKGROUND TO THE INVENTION 
     The use of electrostatic traps as mass analyzers, such as the orbital trapping mass analyzer (marketed under the name Orbitrap (TM)), has provided high resolution mass spectra with a high dynamic range. This type of mass spectrometry, particularly utilizing the orbital trapping mass analyzer, is increasingly used for detection of small organic molecules as well as large intact proteins and native protein complexes. 
     The intrinsic capability of this type of mass analyzer to trap molecular species at the extremes of broader mass-to-charge (m/z) ratio ranges may depend on the quality of ion injection into the electrostatic trap. To assist with understanding the injection process, it is useful to consider how an existing mass analyzer of this type is operated. 
     Referring to  FIG. 1 , there is depicted a schematic of a known mass spectrometer using an orbital trapping mass analyzer. This mass spectrometer is marketed under the name Exactive Plus (TM) by Thermo Fisher Scientific. The mass spectrometer comprises: an Atmospheric Pressure Ionization ion source  10 ; source injection optics  20 ; a bent flatapole ion guide  30 ; a transfer multipole ion optical device  40 ; a curved linear trap (CLT or C-trap)  50 ; a Z-lens  60 ; an orbital trapping mass analyzer  70 ; a Higher-Energy Collision Dissociation (HCD) collision cell  80 ; and a collector  90 . The source injection optics  20  comprises: a capillary  21 ; a S-lens  22 ; a S-lens exit lens  23 ; an injection flatapole ion optical device  24 ; and an inter-flatapole lens  25 . Also provided are: a flatapole exit lens  35 ; a split lens  36 ; a C-trap entrance lens  53 ; and a C-trap exit lens  55 . 
     As is well-known, the orbital trapping mass analyzer  70  is axially-symmetrical and comprises a spindle-shape central electrode (CE)  72  surrounded by a pair of bell-shaped outer electrodes  75 . Electric fields within the mass analyzer are used to capture and confine ions therein such that trapped ions undergo repeated oscillations in an axial direction of the analyzer whilst orbiting about the central electrode. A deflector electrode  65  is provided adjacent the entrance aperture to the orbital trapping mass analyzer  70  to deflect ions into the entrance. Ions are injected into the orbital trapping mass analyzer  70  from the CLT  50  at high energies (typically 1-2 keV per charge) to achieve dynamic trapping. If injection takes place over hundreds of microseconds at such energies, the process may last for hundreds of ion reflections. Without any collisional cooling outside of the electrostatic trap, ion stability may be compromised. To enable efficient ion trapping, a temporal spread of an ion packet in the vicinity of the injection slot should be shorter than a half period of axial ion oscillation in the electrostatic trap. Therefore, a short injection time is used, which creates tight requirements for ion capture. Although the mass analyzer in this example is of an orbital trapping type, similar considerations apply to the injection of ions into other electrostatic traps, which often have strict requirements for ion injection and capture. 
     In the example shown in  FIG. 1 , injection to the orbital trapping mass analyzer  70  involves the C-trap  50 . Ions for analysis are ejected from the C-trap  50  in a direction orthogonal to the direction in which they enter the C-trap  50  from the transfer multipole ion optical device  40 . This is effected by ramping off an RF potential applied to the rods of the C-Trap and applying extracting voltage pulses to the electrodes. The initial curvature of the C-Trap  50  and the subsequent lenses, such as Z-lens  60 , cause the ion beam to converge on the entrance to the orbital trapping mass analyzer  70 . The Z-lens  60  also provides differential pumping slots (electrostatically deflecting the ions away from the gas jet, thereby eliminating gas carryover into the analyzer) and causes spatial focusing of the ion beam into the entrance of the orbital trapping mass analyzer  70 . 
     The fast pulsing of ions from the C-Trap  50  causes ions of each mass-to-charge ratio to arrive at the entrance of the orbital trapping mass analyzer  70  as short packets only a few millimeters long. For ions of each mass-to-charge species, this corresponds to a spread of flight times of only a few hundred nanoseconds for mass-to-charge ratios of a few hundred Daltons per charge. Such durations are considerably shorter than a half-period of axial ion oscillation in the electrostatic trap  70 . When ions are injected into the orbital trapping mass analyzer  70  at a position offset from its equator, these packets start coherent axial oscillations without the need for any additional excitation cycle. 
     Injection may also rely on dynamic waveforms applied to the deflector electrode  65  and the CE  72  during an injection event. Collectively, these can be referred to as CE Injection Waveforms. The ion species entering the analyzer during an injection event experience a dynamic electric field inside the trapping region (between the CE  72  and outer electrodes  75 ) and concurrently orbit the CE  72  with a decreasing radius during several initial periods of axial oscillation. This is the process known as dynamic squeezing. Upon injection, the potential applied to the CE  72  is varied in a ramped manner, for example made more negative for the trapping of positive ions and made more positive for the trapping of negative ions. This dynamic potential at the CE reduces the ions&#39; radial position in the trapping region during an injection event and results in ion trapping and subsequent detection within the electrostatic trap. 
     A detailed discussion of this injection is also provided in International Patent Publication No. WO-02/078046 and the contents of this document are incorporated herein by reference. For the mass spectrometer shown in  FIG. 1 , detection of ions having a m/z ratio between 50 Thomsons (Th, equivalent to Daltons per elementary electrical charge) and 6000 Th is routinely possible. Improving (and where possible, optimizing) the range of m/z ratios that can be readily detected is desirable. Achieving such improvements remains a challenge, however. 
     SUMMARY OF THE INVENTION 
     Against this background, there is provided a method of injecting ions into an electrostatic trap in line with claim  1  and a mass spectrometer as defined in claim  22 . Further features of the invention are detailed in the dependent claims. The mass spectrometer is operable to perform mass analysis of ions that have been captured in the electrostatic trap by the method of injecting ions. An injection event comprises two main parts: (a) applying an ejection potential to an ion storage device; and (b) applying one or more injection potentials to an electrode, which may be associated with the electrostatic trap (preferably, the electrostatic trap is of an orbital trapping type). The ejection potential causes ions stored in the ion storage device to be ejected towards the electrostatic trap. The one or more injection potentials cause the ions ejected from the ion storage device to be captured by the electrostatic trap. In particular, synchronous injection potentials of different amplitudes may be applied concurrently to the multiple electrodes associated with the electrostatic trap (such as a deflector and central electrode). The ion storage device is beneficially a linear ion trap and preferably a curved linear trap (termed CLT or C-trap), especially when an orbital trapping type electrostatic trap is used. 
     Conventionally, (a) and (b) have been started at the same time. Advantageously, the present invention starts (a) and (b) at different times. The start times (or at least, the difference between the start times, in terms of direction and/or magnitude) are beneficially selected based on desired values of mass-to-charge ratios of ions to be captured by the electrostatic trap (which may be covered by one or multiple ranges of mass-to-charge ratios). In other words, to capture ions that include those having a specific range of mass-to-charge ratios, either: (a) may be started before (b); or (b) may be started before (a), and the selection from these two options depends on the specific range of mass-to-charge ratios. In another sense, the length of time between the start of (a) and the start of (b) may depend on the specific range of mass-to-charge ratios. 
     By the use of this technique detection of ions having m/z ratios as low as 35 Th or as high as 20000 Th (or higher) is possible, which is a significantly wider range than for the existing mode of operation, with improvements at both ends of the range. Moreover, the m/z range of the mass spectrometer can be advantageously tuned for optimized ion detection. In this way, the ratio of highest and lower m/z ratios in a spectrum can be as high as 40:1 and possibly higher. For example, a mass spectrum may be generated based on multiple “micro-scans” in the electrostatic trap, that is from respective multiple ion injections into the electrostatic trap, taken at different delay times between the ejection and injection potentials, in order to achieve a higher range of m/z ratios. In other words, each scan is based on a different delay time and provides a mass spectrum of ions of a different range of m/z ratios. A sum of such spectra thereby provides a “composite” mass spectrum of a higher range of m/z ratios than each individual scan. 
     It has been discovered that, where the desired range of mass-to-charge ratios of ions to be captured by the electrostatic trap covers a range lower than a threshold mass-to-charge ratio (for instance, around 100 Thomsons), (b) should beneficially start before (a). The duration (magnitude) of this time difference may be at least that of an induction (settling) time period associated with the one or more injection potentials. The induction period may be around 1 μs, so (b) may start around 3 μs before (a). Preferably, (b) may start before (a) with a time difference of between 1 μs to 5 μs, 2 μs to 4 μs or about 3 μs. 
     In contrast, if the desired range of mass-to-charge ratios of ions to be captured by the electrostatic trap covers a range higher than a limit mass-to-charge ratio (about 8000 Thomsons, for example), (a) should advantageously start before (b). That is, the start of applying the one or more injection potentials is delayed with respect to the start of the ejection potential being applied. The duration of this time difference may be based on one or more of: a time period associated with the ejection potential; a time period associated with the one or more injection potentials; and a time period associated with a flight time for ions between the ion storage device and the electrostatic trap, especially a flight time for ions having a mass-to-charge ratio of at least the limit mass-to-charge ratio. In particular, the time difference may be greater than the flight time for ions between the ion storage device and the electrostatic trap but less than the sum of the flight time for ions between the ion storage device and the electrostatic trap (typically, at least 15 μs for ions of about m/z 8,000 and higher) and the discharge time constant associated with the one or more injection potentials (around 10 μs, for instance). Therefore, a time difference of between 15-25 μs, for example about 20 μs, may be used in practice. However, longer delays of (b) after (a) might be employed for trapping the highest m/z ions, for example time differences between 25 and 50 μs. 
     For example, where the electrostatic trap is of an orbital trapping type, it comprises a central electrode and a co-axial outer electrode. The co-axial outer electrode usually comprises a pair of bell-shaped outer electrodes. Then, the step of applying one or more injection potentials may comprise applying a trapping injection potential to the central electrode and/or the deflector. This may be a ramping potential from a first injection potential level to a second, lower injection potential level. The second potential level may be a zero potential. For trapping positive ions, the trapping injection potential to the central electrode is preferably a ramping potential that changes from a first negative potential level to a lower (that is, more negative) potential level. For example, the first potential level may be in the range from −3.2 kV to −3.7 kV and the second lower potential may be about −5 kV. For trapping negative ions, these polarities would be reversed (that is, applying positive potentials to the central electrode). The second potential level is preferably the final potential applied to the central electrode: that is, the potential applied to the electrode during detection of the ions in the electrostatic trap following the injection process. The duration of the potential ramp on the central electrode from the first to the second potential level can be in the range 5 μs to 200 μs, such as 5 μs to 100 μs, but preferably 5 μs to 50 μs. 
     The ejection potential may be applied by reducing a magnitude of a potential applied to an electrode of the ion storage device, such that the ions stored in the ion storage device are ejected towards the electrostatic trap. Reducing a magnitude of a potential applied to an electrode of the ion storage device beneficially comprises switching off the potential, such as an RF potential applied to one or more electrodes of the ion storage device, for example an RF potential applied to multipole rod electrodes. The ejection potential may be alternatively, or preferably additionally, applied by applying an extraction potential to one or electrodes of the ion storage device, preferably in the form of one or more DC potentials applied to one or more electrodes. In one embodiment, opposite polarity DC potentials can be applied to at least two electrodes of the ion storage device providing a push and pull of the ions in the ion storage device to eject them from the device. The duration of the ejection potential applied to the ion storage device may be in the range 5 μs to 40 μs, preferably 10 μs to 20 μs. 
     The one or more injection potentials may comprise a deflecting injection potential, applied to an ion deflector between the ion storage device and the electrostatic trap. This may cause the ions to travel towards (and/or be focused on an entrance aperture of) the electrostatic trap. Additionally or alternatively, the one or more injection potentials may comprise a trapping injection potential applied to an electrode of the electrostatic trap. 
     In embodiments where the electrostatic trap is an orbital trapping electrostatic trap, the trapping injection potential can be applied to a central electrode of the electrostatic trap about which the captured ions orbit. Application of the trapping injection potential and deflecting injection potential may be started at the same time. This is beneficial from the perspective of simplicity. Where they are not started at the same time, the time difference with respect to applying the ejection potential refers to the first to start of the trapping injection potential and deflecting injection potential. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be put into practice in a number of ways, and a preferred embodiment will now be described by way of example only and with reference to the accompanying drawings, in which: 
         FIG. 1  depicts a schematic of a known mass spectrometer using an orbital trapping mass analyzer; 
         FIG. 2 a    illustrates signal waveforms for injection and ejection potentials applied to parts of the mass spectrometer of  FIG. 1 , in accordance with one embodiment; 
         FIG. 2 b    illustrates signal waveforms for injection and ejection potentials applied to parts of the mass spectrometer of  FIG. 1 , in accordance with another embodiment; 
         FIG. 3  depicts a schematic block diagram of a control system, in accordance with an embodiment; 
         FIG. 4 a    shows example mass spectra for ion species having a low mass-to-charge ratio range, where an existing approach is used; 
         FIG. 4 b    shows example mass spectra for ion species having a low mass-to-charge ratio range, where an embodiment is used. 
         FIG. 5 a    shows first example mass spectra for ion species having a high mass-to-charge ratio range, where an existing approach is used; 
         FIG. 5 b    shows first example mass spectra for ion species having a high mass-to-charge ratio range, where an embodiment is used in accordance with a first approach; 
         FIG. 6 a    shows second example mass spectra for ion species having a high mass-to-charge ratio range, where an existing approach is used; and 
         FIG. 6 b    shows second example mass spectra for ion species having a high mass-to-charge ratio range, where an embodiment is used in accordance with a second approach. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     The discussion below references the known mass spectrometer depicted in  FIG. 1 . Nevertheless, it will be understood that the techniques described herein are applicable to a wide range of other mass spectrometers, which may use different types of mass analyzer and different ways to inject ions into the mass analyzer. The approaches described herein are especially applicable to electrostatic traps with upstream ion storage, such that injection from the ion storage device to the electrostatic trap involves ejection from the ion storage device. The invention may find application in embodiments where there is a difference in the time of arrival of ions at the electrostatic trap after ejection from the ion storage device that depends on the m/z of the ions. The invention may additionally (or alternatively) find application in embodiments where there is an induction (settling) time period associated with the one or more injection potentials. 
     It has been discovered that the conventional parameters of ion ejection from the C-Trap  50  to the orbital trapping mass analyzer  70  may cause loss of ions of low mass-to-charge (m/z) ratio and/or high m/z ratio. This may occur for different reasons, as will now be explained. 
     One reason why ions of high m/z ratio may be lost is as follows. Modelling has allowed determination of the flight times of ions of a given m/z ratio from the C-Trap  50  to the entrance port of the orbital trapping mass analyzer  70 . As explained above, ions are ejected from the C-Trap  50  by reducing the RF potential applied to its rod electrodes and applying an extraction voltage pulse (typically push and pull voltages applied to respective electrodes of the C-Trap  50 ). The modelling has shown that following such ejection (a purge event), ions of higher m/z ratio, such as those with an m/z ratio of 8,000 or greater, arrive at the electrostatic trap entrance in approximately 15 μs. 
     The dynamic Central Electrode (CE) injection waveforms, conventionally starting at the same time as the ejection potentials for the C-trap ejection event, result in a reduced potential on the CE  72  and therefore a reducing field strength is applied to the CE  72  during injection to provide for the trapping of ions (concurrently an increasing dynamic potential is applied to the deflector electrode  65 ). For positive ions, an increasing deflector voltage as a function of time may steer ions into the injection slot and a lower voltage (more negative voltage) is applied to the CE  72  to reduce the ions&#39; orbital radius during injection. The increasing voltage on the deflector may compensate the effect of negative field sagging into the deflector region, so that the deflection field at the injection point remains nearly constant and independent of the time-varying negative potential applied to the CE  72 . The reducing field strength means that the ions with high m/z ratio, arriving into the electrostatic trap later than the low m/z ions, experience a field from the potential on the CE  72  that is already significantly reduced in amplitude. Hence, the remaining dynamic field that can be used for trapping these higher m/z ions is reduced. The efficiency of trapping such ions is therefore reduced, since a dynamic field is required for trapping ions in the electrostatic trap. 
     In the case of an orbital trapping electrostatic trap of the type shown in  FIG. 1 , The CE injection waveforms are generated using: coupling resistors R CE =1 MΩ for the CE  72  and R DEFL =2.5 MΩ for the deflector electrode  65 ; and CE  72  and deflector electrode  65  intrinsic capacitances to ground, C CE ≈10 pF and C DEFL ≈5 pF, respectively. As a consequence, time constants of the exponentially varying electric fields (resulting from the CE injection waveforms), R CE C CE  and R DEFL C DEFL , are about 10 μs and about 12.5 μs, respectively. In view of these time constants, the initial amplitude of the varying field is reduced 5-fold, and only 20% of the remaining dynamic field could be used for trapping these higher m/z species by the time that these ions enter the region between the outer detection electrodes  75  and CE  72 . Since the CE injection waveforms and resultant fields exponentially decrease in magnitude, the efficiency of trapping is further reduced proportional to the rate of change in voltage (or field strength) over time. 
     An explanation for why ions of low m/z ratio may be lost is now considered. The rapidly changing injection waveform applied to the CE  72  can have an induction period. This may be around 1 μs for CE  72  in a more recent design of orbital trapping mass analyzer  70 , depending on the electronics used for application of this waveform. Such a long induction period may mean that ions having a low m/z ratio (less than or no greater than 100 Th) would experience low, if any, dynamic trapping field. These ions would then escape the electrostatic trap during an injection event. 
     It has therefore been established that, in principle, the loss of both ions with low m/z ratios and ions with high m/z ratios is due to the timing mismatch between the arrival of ions into the electrostatic trap that have been ejected from the upstream ion storage device (due to a change in the field confining the ions within that storage device), such as C-trap  50 , and the dynamic capture field generated by one or more electrodes associated with the electrostatic trap, such as the deflection field and/or the injection field. This timing mismatch results from the existing approach, which starts applying the potentials to generate or adjust these ejection and capture fields at the same time. Adjustment of the time at which those fields are changed or applied can affect the ability to capture ions of a specific m/z ratio range within the electrostatic trap. 
     In general terms, there may be considered a method of injecting ions into an electrostatic trap, comprising: applying an ejection potential to an ion storage device, to cause ions stored in the ion storage device to be ejected towards the electrostatic trap; and applying one or more injection potentials to one or more electrodes, to cause the ions ejected from the ion storage device to be captured by the electrostatic trap. Then, the steps of applying the ejection potential and applying the one or more injection potentials are advantageously each started at respective different times. The times are beneficially selected based on desired values of mass-to-charge ratios of ions to be captured by the electrostatic trap. 
     In other words, the difference between the time at which the step of applying the ejection potential is started; and the time at which the step of applying the one or more injection potentials is started is preferably controlled. Specifically, the magnitude, direction or both of this difference may be selected based on the desired range of mass-to-charge ratios of ions to be captured by the electrostatic trap. The difference (effectively a delay) can be programmed on the basis of the desired m/z range, which may be user-defined and provided as an input. 
     This general approach can be implemented as a computer program or programmable or programed logic, configured to perform any method described herein when operated by a processor. The computer program may be stored on a computer readable medium. Also considered may be a mass spectrometer, comprising: an ion storage device, configured to receive ions for analysis (for example when a receiving potential is applied to the device), store the received ions (for example when a storing potential is applied to the device) and eject the stored ions (for example when an ejection potential, such as described above, is applied to the device); an electrostatic trap, arranged to receive the ions ejected from the ion storage device; and a controller, configured to apply potentials to parts of the mass spectrometer. The electrostatic trap is preferably of the orbital trapping type as described herein. The controller may be configured to operate in accordance with any method steps (alone or in combination) described herein. It may have structural features (one or more of: one or more inputs; one or more outputs; one or more processors; logic; and circuitry) configured to perform any one or more of these method steps. The controller may comprise a computer or processor for executing a computer program or programmable or programmed logic configured to perform any of the methods described herein. The controller may comprise trigger circuitry to start the ejection potential and one or more injection potentials. The controller may comprise a programmable delay generator and/or a clock for implementing a time difference between respective start times of applying the ejection potential to the ion storage device and applying the one or more injection potentials to the electrodes of the electrostatic trap. Information relating to values of the mass-to-charge ratios of the ions to be captured by the electrostatic trap can be input to the controller. Such input information can be utilized with the programmable delay generator and/or clock for implementing the time difference between the start times of the potentials. 
     The details of the selection of delays for ion injection are now considered in more depth. Referring now to  FIG. 2 a   , there are illustrated signal waveforms for injection and ejection potentials applied to parts of the mass spectrometer of  FIG. 1 , in accordance with an embodiment. These waveforms are intended to illustrate the principle of “delayed” ion injection into the orbital trapping mass analyzer  70 . The rising edge of a pre-trigger signal  101  triggers a reduction of the voltage waveform  105  applied to the CE  72  to a start voltage of, say, −3.7 kV. This takes place prior to application of a CLT pulse trigger signal  102  to the CLT  50  to start a voltage pulse  103  applied to the CLT (that is, an ejection potential applied to the CLT  50  to eject ions from the CLT  50 ). Next, the rising edge of an injection pulser trigger signal  104  causes the CE injection waveform  105  to ramp down further to −5 kV (from −3.7 kV), during the ion injection. Synchronously with the CE injection waveform  105 , a deflector injection waveform  106  is applied to the deflector electrode  65 . Note that the deflector injection waveform  106  is a positive going pulse, used to mitigate field sagging effect in the injection slot due to the negative going pulse applied to the CE  72  during injection. 
     As shown on the figure, the injection waveform  105  applied to the CE  72  and an injection waveform  106  applied to the deflector electrode  65 , both started from the injection pulse trigger signal  104 , are shifted in time by an injection delay period  110 , relative to a synchronization pulse  102 , which triggers application of the ejection potential  103  to the C-trap  50 . The waveforms are shown as repeating, since multiple spectra are normally acquired per single experiment. The left and right-hand side waveforms of the drawing correspond to two different spectra taken at the same delay time  110  between CLT trigger  102  and CE trigger  104 . The term “delayed” in this context simply refers to shifting in time, as the CE injection waveform  105  and deflector injection waveform  106  may start after the CLT ejection pulse  103  or vice versa. The waveforms  105  and  106  may be collectively referred to herein as injection waveforms. If the injection waveforms  105 ,  106  start after the CLT ejection pulse  103 , this is referred to as a positive delay. 
     If the injection waveforms start before the CLT ejection pulse  103 , this is termed a negative delay. Referring next to  FIG. 2 b   , there are illustrated signal waveforms for injection potentials being applied to the mass spectrometer of  FIG. 1  before ejection potentials, in accordance with another embodiment. Where the waveforms of  FIG. 2 b    are the same as those of  FIG. 2 a   , the same reference numerals are used. For this embodiment, the injection delay period  120  is negative, because the CE trigger waveform  114  precedes the CLT trigger pulse  102 . As a result, the CE injection waveform  115  and deflector injection waveform  116  start before the CLT ejection pulse  103 . The magnitude of the negative injection delay period  120  shown in  FIG. 2 b    is smaller than the magnitude of the positive injection delay period  110  shown in  FIG. 2   a.    
     It should be noted that the distance (and thus, the time-of-flight, TOF, separation) between the deflector electrode  65  and the CE  72  is much smaller than the distance (and hence TOF separation) between the CLT  50  and the deflector electrode  65 . In view of this, it is simplest to trigger the deflector injection waveforms  106 ,  116  and CE injection waveform  105 ,  115  at the same time, although some shifting between these two signals may be considered in alternative approaches. For example, the CE injection waveform  105 ,  115  could start shortly after the deflector injection waveform  106 ,  116 . 
     A controller is therefore used to manage and synchronize signal timing appropriately. Referring next to  FIG. 3 , there is depicted a schematic block diagram of a control system, in accordance with an embodiment. This comprises a Field Gate Programmable Array (FPGA) controller  200 , which provides outputs to: a CLT RF board  240  that applies potentials to the CLT  250 ; and a CE pulser board  220 , supplying potentials to the central electrode and deflector  230 . The CLT  250  of this drawing is equivalent to the CLT  50  of  FIG. 1  and the central electrode and deflector  230  of  FIG. 3  are equivalent to the CE  72  and deflector electrode  65  of the  FIG. 1 . The FPGA controller  200  employs a high-precision clock to generate a CLT trigger  205  and a delayed CE inject trigger  210  on separate channels. The delay of the CE inject trigger  210  is programmable at the controller  200 . The CLT trigger  205  handles the logic on the CLT RF board  240  and is synchronous with ion ejection from the CLT  250 , while the CE inject trigger  210  starts the injection waveforms applied to the central electrode and the deflector  230  and provides for ion injection into the electrostatic ion trap. 
     In this way, synchronization of the CLT trigger signal  102  and injection waveforms  105  and/or  106  is achieved using the on-board high-precision clock of FPGA controller  200 . The time-shifting of the waveforms relative to one another can enable ion injection into the electrostatic field region to be triggered such that the CE injection waveform  105  is at the optimum level and the rate of change of field strength in the electrostatic trap is high for ions of the desired mass-to-charge ratio. In view of the considerations discussed above regarding the reasons for the loss of injected ions, the magnitude and/or direction of the delay (or time shift) can be selected based on the range of m/z ratios for the ions desired for capture. In the case of ions with low m/z ratios (no more or less than 100 Th), the CE injection waveform  105  (and deflector injection waveform  106 ) is enabled approximately 3 μs prior to switching off the RF waveform applied to the CLT  50  and applying the extraction voltage (ion purging), as counted by periods of the RF waveform applied to the CLT  50 . Typically, the RF applied to the CLT  50  is at a frequency of 3 MHz, so counting 10 RF periods provides a delay of 3 μs. As above, this delay is referred to as “negative”, as the CE injection potential  105  is applied before the CLT ejection pulse  103 . This time shift is related to the induction period for the injection waveform applied to the CE  72 , as discussed above. 
     In the case of ions having higher m/z ratios (at least or greater than 8000 Th), the CE injection waveform  105  (and deflector injection waveform  106 ) is enabled about 20 μs after switching off the RF waveform applied to the CLT  50  (ion purging) and this delay is referred to as “positive”. The RF applied to the CLT  50  is switched off by the time the waveforms  105  and  106  are applied, so the positive delay is implemented by a delay generator on the FPGA controller  200 . The magnitude of the time shift relates to the time of flight of ions of these m/z ratios from the CLT  50  to the entrance of the electrostatic trap  70  and the time constants of the exponentially varying potentials (or electric fields generated) at the deflector electrode  65  and/or CE  72 . 
     Phase correction of ion signals injected into the orbital trapping mass analyzer  70  may be achieved to enable enhanced Fourier Transform and further advanced signal processing approaches, such as discussed in “Enhanced Fourier transform for Orbitrap mass spectrometry”, Lange et al, International Journal of Mass Spectrometry, Volume 377, 1 Feb. 2015, Pages 338-344. 
     Referring to the general terms discussed above, one approach that may be considered is when the desired range of mass-to-charge ratios of ions to be captured by the electrostatic trap covers a range lower than (or no greater than) a threshold mass-to-charge ratio. In that case, the times are selected such that the step of applying the one or more injection potentials precedes the step of applying the ejection potential. Preferably, the threshold mass-to-charge ratio is 100 Th, although it may be 70, 75, 80, 90, 110, 120, 130, 140 or 150, for example. 
     Another approach that may be considered in addition (or alternatively) is when the desired range of mass-to-charge ratios of ions to be captured by the electrostatic trap covers a range higher than a limit mass-to-charge ratio. Then, the times may be selected such that the step of applying the ejection potential precedes the step of applying the one or more injection potentials. The limit mass-to-charge ratio is preferably 8000 Th, but may be 7000 Th, 9000 Th or 10000 Th, for instance. 
     The magnitude of the difference between the time at which the step of applying the ejection potential is started (the duration of the delay) and the time at which the step of applying the one or more injection potentials is started is at least 1, 2, 3, 4, 5, 10, 15, 20 or 25 μs. Additionally or alternatively, the magnitude of the difference may be no more than 1, 2, 3, 4, 5, 10, 15, 20 or 25 μs. For example, applying the one or more injection potentials may precede the step of applying the ejection potential by at least and/or no more than one of: 1, 2, 3, 4 or 5 μs, for example by a time difference in one of the ranges: 1 to 5 μs, 1 to 4 μs or 2 to 4 μs. Applying the ejection potential may precede the step of applying the one or more injection potentials by at least and/or no more than one of: 10, 15, 20 or 25 μs. 
     The magnitude of the difference between the time at which the step of applying the ejection potential is started and the time at which the step of applying the one or more injection potentials is started is advantageously based on one or more of: a time period associated with the ejection potential; a time period associated with the one or more injection potentials; and a time period associated with a flight time for ions between the ion storage device and the electrostatic trap. For example, the time period associated with the one or more injection potentials may be an induction period associated with an electrode to which one of the injection potentials is applied. Then, the magnitude of the difference may be at least and/or no more than 1, 2, 3, 4, 5 or 10 times an induction period associated with the one or more injection potentials (especially for ions having a mass-to-charge ratio below the threshold). 
     Additionally or alternatively, the magnitude of the difference may be based on (at least or greater than) one or more of: a discharge time constant associated with the one or more injection potentials; and a flight time for ions between the ion storage device and the electrostatic trap (especially for ions having a mass-to-charge ratio above the limit mass-to-charge ratio). In particular, the magnitude of the difference may be greater than (or at least) the flight time for ions between the ion storage device and the electrostatic trap but less than (or no more than) the sum of the flight time for ions between the ion storage device and the electrostatic trap and the discharge time constant associated with the one or more injection potentials. The discharge time constant associated with the one or more injection potentials may be dependent on at least one resistance and at least one capacitance associated with the electrode to which the one or more injection potentials is applied (for example, the product of the resistance and the capacitance). Additionally or alternatively, the discharge time constant may be programmable or adjustable, for instance using digital circuitry. The digital circuitry may comprise field-programmable gate array (FPGA) circuitry. The discharge time constant may be adjustable based on one or more of: a user-defined mass-to-charge range; and lowest and/or highest mass-to-charge limits. In this way, trapping and detection of higher m/z ions (for instance, at least or greater than 8000 Th) in the orbital trapping mass analyzer  70  can be performed using an injection waveform with a greater discharge time constant. 
     This aspect (variation of the discharge time constant) can, in some embodiments, be used alternatively to applying the ejection potential and the one or more injection potentials at different times. Thus, in another aspect, the invention provides a method of injecting ions into an electrostatic trap, comprising: applying an ejection potential to an ion storage device, to cause ions stored in the ion storage device to be ejected towards the electrostatic trap; and applying one or more injection potentials to one or more electrodes, to cause the ions ejected from the ion storage device to be captured by the electrostatic trap; and wherein a discharge time constant associated with the one or more injection potentials is adjustable based on desired values of mass-to-charge ratios of ions to be captured by the electrostatic trap, such as one or more of: a user-defined mass-to-charge range; and lowest and/or highest mass-to-charge limits. 
     In this way, trapping and detection of higher m/z ions (for instance, at least or greater than a first threshold level, say around 8000 Th) in the mass analyzer can be performed using an injection waveform with a relatively greater discharge time constant compared to trapping and detection of lower m/z ions (for instance, no more than or less than a second threshold, say around 100 Th) in the mass analyzer. The trapping and detection of such lower m/z ions can be performed using an injection waveform with a relatively smaller discharge time constant. The first and second thresholds are preferably different (as above), but they may be the same. Where the first and second thresholds are different, ions of m/z between the first and second thresholds may be performed using an injection waveform with the relatively greater discharge time constant, the relatively smaller discharge time constant or a discharge time constant between the relatively greater discharge time constant and the relatively smaller discharge time constant (for instance, around 10 μs). 
     The discharge time constant for an injection waveform applied to one or more trapping electrodes (such as applied to a central electrode of an orbital trapping electrostatic trap) is typically the same as the discharge time constant for an injection waveform applied to one or more deflection electrodes associated with the electrostatic trap (for deflecting the ions into the trap during the injection process). Alternatively, the discharge time constants may be different. The discharge time constant (or plurality of discharge time constants) may be as low as 5 μs, 10 μs, 15 μs and 25 μs. The discharge time constant (or plurality of discharge time constants) may be no greater than (or less than) 10 μs, 15 μs and 25 μs or 40 μs. For example, for higher m/z ions (greater than or at least the first threshold), the discharge time constant may be around 15 μs, 25 μs or 40 μs (or in a range between any two of these values, for example in the range 15 to 40 μs, or 15 to 25 μs, or 25 to 40 μs, or at least or greater than any of these values, for example greater than 15 μs, greater than 25 μs, or greater than 40 μs). For lower m/z ions (less or no more than the second threshold), the discharge time constant may be around 5 μs or 10 μs (or in a range between these values, that is in a range 5 to 10 μs, or less than or no more than these values, for example less than 10 μs, or less than 5 μs). Any of the features described herein with respect to this aspect, relating to the discharge time constant, may also be combined with any other aspect of this disclosure. 
     In the preferred embodiment, the electrostatic trap comprises a central electrode and a co-axial outer electrode, for example where the electrostatic trap is of an orbital trapping type. Then, the step of applying one or more injection potentials preferably comprises applying a trapping injection potential to the central electrode. In this case for trapping positive ions, the trapping injection potential may be a ramping potential from a first (negative) injection potential level to a second, lower (more negative) injection potential level. In the case of trapping negative ions, the trapping injection potential may be a ramping potential from a first (positive) injection potential level to a second, higher (more positive) injection potential level. Additionally or alternatively, an ion deflector may be provided between the ion storage device and the electrostatic trap. Then, the step of applying one or more injection potentials may comprise applying a deflecting injection potential to the ion deflector, to cause the ions to travel towards (optionally, focused on an entrance aperture of) the electrostatic trap. The step of applying one or more injection potentials preferably comprises applying a trapping injection potential to an electrode of the electrostatic trap. Where the electrostatic trap is an orbital trapping electrostatic trap, the trapping injection potential may be applied to a central electrode of the electrostatic trap about which the captured ions orbit. In preferred cases, both the deflecting injection potential and the trapping injection potential are applied. Then, the steps of applying the trapping injection potential and applying the deflecting injection potential are optionally started at the same time. 
     The step of applying the ejection potential optionally comprises reducing a magnitude of, preferably switching off, a potential applied to one or more electrodes of the ion storage device, such as an RF potential used to store ions in the device, in particular such that the ions stored in the ion storage device are ejected towards the electrostatic trap. Preferably, applying the ejection potential comprises simultaneously with reducing or switching off the potential used to store ions in the ion storage device, applying an extraction potential (preferably DC potential) to one or more electrodes of the ion storage device to extract ions from the device towards the electrostatic trap. The magnitude of the potential applied to the electrode of the ion storage device may be reduced to zero. In the preferred embodiment, the ion storage device is a curved linear trap. 
     In some embodiments, the step of applying an ejection potential is started by applying an ejection trigger signal to an ejection switch controlling application of the ejection potential. Additionally or alternatively, the step of applying one or more injection potentials is started by applying one or more injection trigger signals to at least one injection switch controlling application of the one of more injection potentials. In some embodiments, an RF potential with a predetermined frequency is generated, for instance as a potential for confining ions within the ion storage device. Then, the difference between respective start times of the steps of applying the ejection potential and applying the one or more injection potentials is optionally measured using the predetermined frequency of the RF potential, for example by counting periods of the RF potential. Since the RF potential is a high and stable frequency (at least 2 or 3 MHz) potential, periods of at least 1 μs can be accurately measured in this way. Additionally or alternatively, the difference between respective start times of the steps of applying the ejection potential and applying the one or more injection potentials may be measured by a clock. 
     The electrostatic trap is preferably operable to perform mass analysis of ions that have been captured in the electrostatic trap, for example by image current detection of ion oscillations in the trap (the frequencies of which depend mass-to-charge ratios of the ions) and signal processing (for example Fourier transformation) of the detected signal to provide a mass spectrum of the ions. In embodiments where the electrostatic trap comprises a central electrode and a co-axial outer electrode, such as in an orbital trapping mass analyzer, the co-axial outer electrode is preferably split into at least two parts that are used to detect the image current of the oscillating ions as known in the art, for example as implemented in Orbitrap (RTM) mass analyzers. 
     The advantages of the described approach will now be discussed by way of some example. Referring next to  FIG. 4 , there are shown example mass spectra for ion species having a low mass-to-charge ratio range, where (a) an existing approach is used and (b) an embodiment is used. These mass spectra are intended to show the efficiency of trapping ions with lower m/z ratios, using (a) a standard approach (no delay between the injection waveforms  105  and  106  and the synchronization pulse  102  applied to the C-trap  50 ) and (b) when a 3 μs negative delay is applied (that is, injection potentials were applied before the ejection potential is applied to the storage device). A mass spectrometer in accordance with  FIG. 1  was used for these tests. A comparison of these two mass spectra shows that the use of a negative delay between the CLT synchronization pulse  102  and CE injection waveforms  105  and  106  results in a significant signal-to-noise improvement for the lower mass part of the spectrum and in particular, a signal-to-noise improvement by a factor of 5 for immonium ions at m/z 74.10. 
     Referring next to  FIGS. 5 and 6 , there are shown example mass spectra for ion species having a high mass-to-charge ratio range, where (a) an existing approach is used and (b) an embodiment is used. These figures are intended to show signal-to-noise improvement for ions with higher m/z ratios, due to introduction of a programmable delay between the CLT synchronization pulse  102  and injection waveforms  105  and  106 . These experiments were performed in native MS mode of a mass spectrometer in accordance with  FIG. 1 , using GroEL protein complex (molecular weight 801 kDa), which encompasses two non-covalently bound heptameric rings, resulting in formation of a 14-mer complex. This protein complex was further collisionally activated in the HCD cell  80  to produce counter complexes of both 13-mer and 12-mer species. A direct voltage bias of −200 V was applied in the region of the HCD cell  80 . In  FIG. 5 , a pressure of 1.4×10 −4  mbar (1.4×10 −2  Pa) was used in the C-trap  50  and in  FIG. 6  a pressure of 7.7×10 −5  mbar (7.7×10 −3  Pa) was used in the C-trap  50 . In both  FIG. 5  and  FIG. 6 , the first mass spectrum (a) was generated using an existing, standard approach (no delay between the injection waveforms  103  and  104  and the synchronization pulse  105  applied to the C-trap  50 ). In  FIG. 5 , the second mass spectrum (b) was generated using a 25 μs positive delay between the CLT synchronization pulse  102  and the injection waveforms  105  and  106 . In  FIG. 6 , the second mass spectrum (b) was generated using a 20 μs positive delay between the CLT synchronization pulse  102  and the injection waveforms  105  and  106 . 
     In  FIG. 5 , the precursor signal is observed at an m/z ratio of 12K. Charge state envelopes of 13-mer and 12-mer counter complexes are observed at m/z ratios of 18K and 34K, respectively. In  FIG. 6 , the ejected subunit signal is detected at an m/z ratio of 2200, with a lower signal-to-noise ratio. Charge state envelopes of 13-mer and 12-mer counter complexes are again observed at m/z ratios of 18K and 34K, respectively. In both cases, the signal-to-noise ratio of the charge state envelopes of the 13-mer counter complex is significantly improved, as evidenced by comparing against 13-mer signals in the mass spectra in  FIGS. 5 ( a ) and 6( a )  respectively. Moreover, using the “delayed” ion injection approach, the signals of the charge state envelopes of 12-mer counter complex were acquired at a signal-to-noise ratio exceeding 50. This is again observable in  FIGS. 5( b ) and 6( b ) . These high m/z species could not be detected under standard conditions, as shown in  FIGS. 5( a ) and 6( a ) . 
     It can be seen from the above description that the invention advantageously can enable highly efficient detection of both lower m/z (for example less than or no more than 100 Th or 80 Th) and higher m/z (for example at least or greater than 8,000, 12,000, 16,000 or 20,000 Th) ions using an electrostatic trap. Thus, an electrostatic trap, such as an Orbitrap (RTM) mass analyzer for example, can be employed efficiently for mass spectrometry of small molecules and large macromolecular assemblies. Higher signal-to-noise ratios of detection can be achieved than with prior art methods. The ion injection can be tuned and optimized for the mass range of ions that it is desired to captured and/or analyze. For example, a programmable delay between starting the ejection potential applied to the ion storage device and the one or more injection potentials applied to the electrostatic trap can be used, which can be responsive to a user-defined m/z range. The ratio of highest and lower m/z in a spectrum can be in the range of 40:1. 
     Although a specific embodiment has been described, the skilled person will appreciate that various modifications and alternations are possible. In particular, different configurations of mass spectrometer, with different types of electrostatic trap and/or ion storage device may be used. The threshold or limit for what constitutes a low and/or high m/z range may be varied depending on the types of electrostatic trap and/or ion storage device. Also, the specific signals used to effect ejection from the ion storage device and/or injection to the electrostatic trap may change. The magnitude of the delay between the ejection and injection waveforms being applied may be varied depending on a range of factors, including the values of m/z ratios of ions desired to be captured in the electrostatic trap. The electrostatic trap is preferably operated as a mass analyzer, but this need not be so and it may be used for other purposes in addition or as an alternative. 
     It will therefore be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as “a” or “an” (such as an analogue to digital convertor) means “one or more” (for instance, one or more analogue to digital convertor). Throughout the description and claims of this disclosure, the words “comprise”, “including”, “having” and “contain” and variations of the words, for example “comprising” and “comprises” or similar, mean “including but not limited to”, and are not intended to (and do not) exclude other components. 
     The use of any and all examples, or exemplary language (“for instance”, “such as”, “for example” and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Any steps described in this specification may be performed in any order or simultaneously unless stated or the context requires otherwise. 
     All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).