CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application under 35 U.S.C. §371 of PCT Application No. PCT/GB2007/001362, filed Apr. 13, 2007, entitled “METHOD OF ION ABUNDANCE AUGMENTATION IN A MASS SPECTROMETER”, which claims the priority benefit of GB Application No. 0607542.8, filed Apr. 13, 2006, entitled “MASS SPECTROMETER WITH ION STORAGE DEVICE”, which applications are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates to a mass spectrometer and a method of mass spectrometry, in particular for performing MS n  experiments. 
     BACKGROUND TO THE INVENTION 
     Tandem mass spectrometry is a well known technique by which trace analysis and structural elucidation of samples may be carried out. In a first step, parent ions are mass analysed/filtered to select ions of a mass to change ratio of interest, and in a second step these ions are fragmented by, for example, collision with a gas such as argon. The resultant fragment ions are then mass analysed usually by producing a mass spectrum. 
     Various arrangements for carrying out multiple stage mass analysis or MS n  have been proposed or are commercially available, such as the triple quadrupole mass spectrometer and the hybrid quadrupole/time-of-flight mass spectrometer. In the triple quadrupole, a first quadrupole Q 1  acts as a first stage of mass analysis by filtering out ions outside of a chosen mass-to-charge ratio range. A second quadrupole Q 2  is typically arranged as a quadrupole ion guide arranged in a gas collision cell. The fragment ions that result from the collisions in Q 2  are then mass analysed by the third quadrupole Q 3  downstream of Q 2 . In the hybrid arrangement, the second analysing quadrupole Q 3  may be replaced by a time-of-flight (TOF) mass spectrometer. 
     In each case, separate analysers are employed before and after the collision cell. In GB-A-2,400,724, various arrangements are described wherein a single mass filter/analyser is employed to carry out filtering and analysis in both directions. In particular, an ion detector is positioned upstream of the mass filter/analyser, and ions pass through the mass filter/analyser to be stored in a downstream ion trap. The ions are then ejected from the downstream trap back through the mass filter/analyser before being detected by the upstream ion detector. Various fragmentation procedures, still employing a single mass filter/analyser, are also described, which permit MS/MS experiments to be carried out. 
     Similar arrangements are also shown in WO-A-2004/001878 (Verentchikov et al). Ions are passed from a source to a TOF analyser, which acts as an ion selector, from where ions are ejected to a fragmentation cell. From here, they pass back through the TOF analyser and are detected. For MS n , the fragment ions can be recycled through the spectrometer. US-A-2004/0245455 (Reinhold) carries out a similar procedure for MS n  but employs a high sensitivity linear trap rather than a TOF analyser to carry out the ion selection. JP-A-2001-143654 relates to an ion trap, ejecting ions on a circular orbit for mass separation followed by detection. 
     The present invention seeks against this background to provide an improved method and apparatus for MS n . 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a method of improving the detection limits of a mass spectrometer comprising: (a) generating sample ions from an ion source; (b) storing the sample ions in a first ion storage device; (c) ejecting the stored ions into an ion selection device; (d) selecting and ejecting ions of a chosen mass to charge ratio out of the ion selection device; (e) storing the ions ejected from the ion selection device in a second ion storage device without passing them back through the ion selection device; (f) repeating the preceding steps (a) to (e) so as to augment the ions of the said chosen mass to charge ratio stored in the second ion storage device; and (g) transferring the augmented ions of the said chosen mass to charge ratio back to the first ion storage device for subsequent analysis. 
     This cycle may be repeated, optionally, multiple times, so as to allow MS n . 
     The present invention thus employs a cyclical arrangement in which ions are trapped, optionally cooled, and ejected from an exit aperture. A subset of these ions are returned to the ion storage device. This cyclical arrangement provides a number of advantages over the art identified in the introduction above, which instead employs a “back and forth” procedure via the same aperture in the ion trap. Firstly, the number of devices required to store and inject ions into the ion selector is minimised (and in the preferred embodiment is just one). Modern storage and injection devices that permit very high mass resolution and dynamic range are expensive to produce and demanding to control so that the arrangement of the present invention represents a significant cost and control saving over the art. Secondly, by using the same (first) ion storage device to inject into, and receive ions back from, an external ion selection device, the number of MS stages is reduced. This in turn improves ion transport efficiency which depends upon the number of MS stages. Typically, ions ejected from an external ion selector will have very different characteristics to those of the ions ejected from the ion storage device. By loading ions into the ion storage device through a dedicated ion inlet port (a first ion transport aperture), particularly when arriving back at the ion storage device from an external fragmentation device, this process can be carried out in a well controlled manner. This minimises ion losses which in turn improves the ion transport efficiency of the apparatus. 
     This technique also allows the detection limit of the instrument to be improved, where the ions of the chosen mass to charge ratio are of low abundance in the sample. Once a sufficient quantity of these low abundance precursor ions have been built up in the second ion storage device, they can be injected back to the first ion storage device for capture there (bypassing the ion selection device) and subsequent MS n  analysis, for example. Although preferably the ions leave the first ion storage device through a first ion transport aperture and are received back into it via a second separate ion transport aperture, this is not essential in this aspect of the invention and ejection and capture through the same aperture are feasible. 
     Optionally, at the same time as the low abundance precursor ions are being moved to the second ion storage device to improve total population of these particular precursor ions, the ion selection device may continue to retain and further refine the selection of other desired precursor ions. When sufficiently narrowly selected, these precursor ions can be ejected from the ion selection device and fragmented in a fragmentation device to produce fragment ions. These fragment ions may then be transferred to the first ion storage device, and MS n  of these fragment ions may then be carried out or they may likewise be stored in the second ion storage device so that subsequent cycles may further enrich the number of ions stored in this way to again increase the detection limit of the instrument for that particular fragment ion. 
     In a second aspect, the present invention may reside in a method of improving the detection limits of a mass spectrometer comprising: (a) generating sample ions from an ion source; (b) storing the sample ions in a first ion storage device; (c) ejecting the stored ions into an ion selection device; (d) selecting and ejecting ions of analytical interest out of the ion selection device; (e) fragmenting the ions ejected from the ion selection device in a fragmentation device; (f) storing fragment ions in a second ion storage device without passing them back through the ion selection device; (g) repeating the preceding steps (a) to (f) so as to augment the fragment ions stored in the second ion storage device, and (h) transferring the augmented fragment ions back to the first ion storage device for subsequent analysis. 
     As above, ion ejection from the first ion storage device and ion capture back there may be through separate ion transport apertures or through the same one. 
     Ions in the first ion storage device may be mass-analysed either in a separate mass analyser, such as an Orbitrap as described in the above-referenced U.S. Pat. No. 5,886,346, or may instead be injected back into the ion selection device for mass analysis there. 
     An ion source may be provided to supply a continuous or pulsed stream of sample ions to the ion storage device. In one preferred arrangement, the optional fragmentation device may be located between such an ion source and the ion storage device instead. In either case, complicated MS n  experiments may be carried out in parallel by allowing division of (and, optionally, separate analysis of) sub populations of ions, either directly from the ion source or deriving from previous cycles of MS. This in turn results in an increase in the duty cycle of the instrument and can likewise improve the detection limits of it as well. 
     Although preferred embodiments of the invention may employ any ion selection device, it is particularly suited to and beneficial in combination with an electrostatic trap (EST). In recent years, mass spectrometers including electrostatic traps (ESTs) have started to become commercially available. Relative to quadrupole mass analysers/filters, ESTs have a much higher mass accuracy (parts per million, potentially), and relative to quadrupole-orthogonal acceleration TOF instruments, they have a much superior duty cycle and dynamic range. Within the framework of this application, an EST is considered as a general class of ion optical devices wherein moving ions change their direction of movement at least along one direction multiple times in substantially electrostatic fields. If these multiple reflections are confined within a limited volume so that ion trajectories are winding over themselves, then the resultant EST is known as a “closed” type. Examples of this “closed” type of mass spectrometer may be found in U.S. Pat. No. 3,226,543, DE-A-04408489, and U.S. Pat. No. 5,886,346. Alternatively, ions could combine multiple changes in one direction with a shift along another direction so that the ion trajectories do not wind on themselves. Such ESTs are typically referred to as of the “open” type and examples may be found in GB-A-2,080,021, SU-A-1,716,922, SU-A-1,725,289, WO-A-2005/001878, and US-A-20050103992 FIG. 2. 
     Of the electrostatic traps, some, such as those described in U.S. Pat. No. 6,300,625, US-A-2005/0,103,992 and WO-A-2005/001878 are filled from an external ion source and eject ions to an external detector downstream of the EST. Others, such as the Orbitrap as described in U.S. Pat. No. 5,886,346, employ techniques such as image current detection to detect ions within the trap without ejection. 
     Electrostatic traps may be used for precise mass selection of externally injected ions (as described, for example, in U.S. Pat. Nos. 6,872,938 and 6,013,913). Here, precursor ions are selected by applying AC voltages in resonance with ion oscillations in the EST. Moreover, fragmentation within the EST is achieved through the introduction of a collision gas, laser pulses or otherwise, and subsequent excitation steps are necessary to achieve detection of the resultant fragments (in the case of the arrangements of U.S. Pat. Nos. 6,872,938 and 6,013,913, this is done through image current detection). 
     Electrostatic traps are not, however, without difficulties. For example, ESTs typically have demanding ion injection requirements. For example, our earlier patent applications number WO-A-02/078046 and WO05124821A2 describe the use of a linear trap (LT) to achieve the combination of criteria required to ensure that highly coherent packets are injected into an EST device. The need to produce very short time duration ion packets (each of which contains large numbers of ions) for such high performance, high mass resolution devices means that the direction of optimum ion extraction in such ion injection devices is typically different from the direction of efficient ion capture. 
     Secondly, advanced ESTs tend to have stringent vacuum requirements to avoid ion losses, whereas the ion traps and fragmentors to which they may interface are typically gas filled so that there is typically at least 5 orders of magnitude pressure differential between such devices and the EST. To avoid fragmentation during ion extraction, it is necessary to minimise the product of pressure by gas thickness (typically, to keep it below 10 −3  . . . 10 −2  mm*torr), while for efficient ion trapping this product needs to be maximised (typically, to exceed 0.2 . . . 0.5 mm*torr) 
     Where the ion selection device is an EST, therefore, in a preferred embodiment of the present invention, the use of an ion storage device with different ion inlet and exit ports permits the same ion storage device to provide ions in an appropriate manner for injection into the EST, but nevertheless to allow the stream or long pulses of ions coming back from the EST via the fragmentation device to be loaded back into that first ion storage device in a well controlled manner, through the second or in certain embodiments, the third ion transport aperture. 
     Any form of electrostatic trap may be used, if this is what constitutes the ion selection device. A particularly preferred arrangement involves an EST in which the ion beam cross-section remains limited due to the focusing effect of the electrodes of the EST, as this improves efficiency of the subsequent ion ejection from the EST. Either an open or a closed type EST could be used. Multiple reflections allow for increasing separation between ions of different mass-to-charge ratios, so that a specific mass-to-charge ratio of interest may, optionally, be selected, or simply a narrower range of mass-to-charge ratios than was injected into the ion selection device. Selection could be done by deflecting unwanted ions using electric pulses applied to dedicated electrodes, preferably located in the plane of time-of-flight focus of ion mirrors. In the case of closed EST, a multitude of deflection pulses might be required to provide progressively narrowing m/z ranges of selection. 
     It is possible to use the fragmentation device in two modes: in a first mode, precursor ions can be fragmented in the fragmentation device in the usual manner, and in a second mode, by controlling the ion energy, precursor ions can pass through the fragmentation device without fragmentation. This allows both MS n  and ion abundance improvement, together or separately: once ions have been injected from the first ion storage device into the ion selection device, specific low abundance precursor ions can be ejected controllably from the ion selection device and be stored back in the first ion storage device, without having been fragmented in the fragmentation device. This may be achieved by passing these low abundance precursor ions through the fragmentation device at energies insufficient to cause fragmentation. Energy spread could be reduced for a given m/z by employing pulsed deceleration fields (e.g. formed in a gap between two flat electrodes with apertures). When ions enter a decelerating electric field on the way back from the mass selector to the first ion storage device, higher energy ions overtake lower energy ions and thus move to a greater depth in the deceleration field. After all the ions of this particular m/z enter the deceleration field, the field is switched off. Therefore ions with initially higher energy experience a higher drop in potential relatively to ground potential than the lower energy ions, thus making their energies equal. By matching the potential drop to the energy spread upon exit from the mass selector, a significant reduction of the energy spread may be achieved. Fragmentation of ions may thereby be avoided, or, alternatively, control over the fragmentation may be improved. 
     In accordance with a further aspect of the present invention, there is provided a mass spectrometer comprising an ion storage device arranged to store ions, an ion selection device and a fragmentation/storage device. The ion selection device is arranged to receive ions stored in the first ion storage device and ejected therefrom, and to select a subset of ions from those received. The second fragmentation/storage device is arranged to receive at least some of the ions selected by the ion selection device. The second fragmentation/storage device is then configured, in use, to direct ions received from the ion selection device, or their products, back to the first ion storage device without passing them back through the ion selection device. 
     The present invention may also be found in a method of mass spectrometry comprising the steps of, in a first cycle, storing sample ions in a first ion storage device, the first ion storage device having an exit aperture and a spatially separate ion transport aperture; ejecting the stored ions out of the exit aperture into a separate ion selection device; receiving at least some of the ions ejected from the first ion storage device, or their derivatives, back through the ion transport aperture of the first ion storage device; and storing the received ions in the first ion storage device. 
     In accordance with a yet further aspect of the present invention, there is provided a method of mass spectrometry comprising storing ions in a first ion storage device; ejecting ions from the first ion storage device to an ion selection device; selecting a subset of ions within the ion selection device; ejecting the ions from the ion selection device; capturing at least some of the selected ions in one of a fragmentation device or second ion storage device; and returning at least some of the ions captured in the said one of the fragmentation device or second ion storage device respectively, or their products, to the first ion storage device along a return ion path that bypasses the ion selection device. 
     In accordance with still another aspect of the present invention there is provided a method of mass spectrometry comprising accumulating ions in an ion trap, injecting the accumulated ions into an ion selection device, selecting and ejecting a subset of the ions in the ion selection device, and storing the ejected subset of the ions directly back in the ion trap without intermediate ion storage. 
     Other preferred embodiments and advantages of the present invention will become apparent from the following description of a preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be put into practice in a number of ways and one preferred embodiment will now be described by way of example only and with reference to the accompanying drawings in which: 
         FIG. 1  shows, in block diagram form, an overview of a mass spectrometer embodying the present invention; 
         FIG. 2  shows a preferred implementation of the mass spectrometer of  FIG. 1 , including an electrostatic trap and a separate fragmentation cell; 
         FIG. 3  shows a schematic representation of one particularly suitable arrangement of an electrostatic trap for use with the mass spectrometer of  FIG. 2 ; 
         FIG. 4  shows a first alternative arrangement of a mass spectrometer embodying the present invention; 
         FIG. 5  shows a second alternative arrangement of a mass spectrometer embodying the present invention; 
         FIG. 6  shows a third alternative arrangement of a mass spectrometer embodying the present invention; 
         FIG. 7  shows a fourth alternative arrangement of a mass spectrometer embodying the present invention; 
         FIG. 8  shows a fifth alternative arrangement of a mass spectrometer embodying the present invention; 
         FIG. 9  shows an ion mirror arrangement for increasing energy dispersion of ions prior to injection into the fragmentation cell of  FIGS. 1 ,  2 , and  4 - 8 ; 
         FIG. 10  shows a first embodiment of an ion deceleration arrangement for reducing energy spread prior to injection of ions into the fragmentation cell of  FIGS. 1 ,  2 , and  4 - 8 ; 
         FIG. 11  shows a second embodiment of an ion deceleration arrangement for reducing energy spread prior to injection of ions into the fragmentation cell of  FIGS. 1 ,  2 , and  4 - 8 ; 
         FIG. 12  shows a plot of energy spread of ions as a function of the switching time of a voltage applied to the ion deceleration arrangement of  FIGS. 10 and 11 ; and 
         FIG. 13  shows a plot of spatial spread of ions as a function of the switching time of a voltage applied to the ion deceleration arrangement of  FIGS. 10 and 11 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring first to  FIG. 1 , a mass spectrometer  10  is shown in block diagram format. The mass spectrometer  10  comprises an ion source  20  for generating ions to be mass analysed. The ions from the ion source  20  are admitted into an ion trap  30  which may, for example, be a gas-filled RF multipole or a curved quadrupole as is described, for example, in WO-A-05124821. The ions are stored in the ion trap  30 , and collisional cooling of the ions may take place as is described for example in our co-pending application number GB0506287.2, the contents of which are incorporated herein by reference. 
     Ions stored in the ion trap  30  may then be pulse-ejected towards an ion selection device which is preferably an electrostatic trap  40 . Pulsed ejection produces narrow ion packets. These are captured in the electrostatic trap  40  and experience multiple reflections therein in a manner to be described in connection particularly with  FIG. 3  below. On each reflection, or after a certain number of reflections, unwanted ions are pulse-deflected out of the electrostatic trap  40 , for example to a detector  75  or to a fragmentation cell  50 . Preferably, the ion detector  75  is located close to the plane of time-of-flight focus of the ion mirrors, where the duration of the ion packets is at a minimum. Thus, only ions of analytical interest are left in the electrostatic trap  40 . Further reflections will continue to increase the separation between adjacent masses, so that further narrowing of the selection window may be achieved. Ultimately, all ions having a mass-to-charge ratio adjacent to the mass-to-charge ratio m/z of interest are eliminated. 
     After the selection process is completed, ions are transferred out of the electrostatic trap  40  into the fragmentation cell  50  which is external to the electrostatic trap  40 . Ions of analytical interest that remain in the electrostatic trap  40  at the end of the selection procedure are ejected with sufficient energy to allow them to fragment within the fragmentation cell  50 . 
     Following fragmentation in the fragmentation cell, ion fragments are transferred back into the ion trap  30 . Here they are stored, so that, in a further cycle, a next stage of MS may be carried out. In this manner, MS/MS or, indeed, MS n  may be achieved. 
     An alternative or additional feature of the arrangement of  FIG. 1  is that ions ejected from the electrostatic trap (because they are outside the selection window) may be passed through the fragmentation cell  50  without fragmentation. Typically, this could be achieved by decelerating such ions at relatively low energies so that they do not have sufficient energy to fragment in the fragmentation cell. These unfragmented ions which are outside of the selection window of immediate interest in a given cycle can be transferred onwards from the collision cell  50  to a auxiliary ion storage device  60 . In subsequent cycles (for example, when further mass spectrometric analysis of the fragment ions as described above has been completed), the ions rejected from the electrostatic trap  40  in the first instance (because they are outside of the selection window of previous interest) can be transferred from the auxiliary ion storage device  60  to the ion trap  30  for separate analysis. 
     Moreover the auxiliary ion storage device  60  can be used to increase the number of ions of a particular mass to charge ratio, particularly when these ions have a relatively low abundance in the sample to be analysed. This is achieved by using the fragmentation device in non-fragmentation mode and setting the electrostatic trap to pass only ions of particular mass to charge ratio that is of interest but which is of limited abundance. These ions are stored in the auxiliary ion storage device  60  but are augmented by additional ions of that same chosen mass to charge ratio selected and ejected from the electrostatic trap  40  using similar criteria in subsequent cycles. Ions of multiple m/z ratios could be stored together as well, e.g. by using several ejections from the trap  40  with different m/z. 
     Of course, either the previously unwanted precursor ions, or the precursor ions that are of interest but which have a low abundance in the sample and thus first need to be increased in number, can be the subject of subsequent fragmentation for MS n . In that case, the auxiliary ion storage device  60  could first eject its contents into the fragmentation cell  50 , rather than transferring its contents directly back to the ion trap  30 . 
     Mass analysis of ions can take place at various locations and in various ways. For example, ions stored in the ion trap may be mass-analysed in the electrostatic trap  40  (more details of which are set out below in connection with  FIG. 2 ). Additionally or alternatively, a separate mass analyser  70  may be provided in communication with the ion trap  30 . 
     Turning now to  FIG. 2 , a preferred embodiment of a mass spectrometer  10  is shown in more detail. The ion source  20  shown in  FIG. 2  is a pulsed laser source (preferably a matrix-assisted laser desorption ionization (MALDI) source in which ions are generated through irradiation from a pulsed laser source  22 ). Nevertheless, a continuous ion source, such as an atmospheric pressure electrospray source, could equally be employed. 
     Between the ion trap  30  and the ion source  20  is a pre-trap  24  which may, for example, be a segmented RF-only gas-filled multipole. Once the pre-trap is filled, ions in it are transferred into the ion trap  30 , which in the preferred embodiment is a gas-filled RF-only linear quadrupole, via a lens arrangement  26 . The ions are stored in the ion trap  30  until the RF is switched off and a DC voltage is applied across the rods. This technique is set out in detail in our co-pending applications, published as GB-A-2,415,541 and WO-A-2005/124821, the details of which are incorporated herein in their entirety. 
     The applied voltage gradient accelerates ions through ion optics  32  which may, optionally, include a grid or electrode  34  arranged to sense charge. The charge-sensing grid  34  permits estimation of the number of ions. It is desirable to have an estimate of the number of ions since, if there are too many ions, the resulting mass shifts become difficult to compensate. Thus, if the ion number exceeds a predefined limit (as estimated using the grid  34 ), all ions may be discarded and an accumulation of ions in the pre-trap  24  may be repeated, with a proportionally lowered number of pulses from the pulsed laser  22 , and/or a proportionally shorter duration of accumulation. Other techniques for controlling the number of trapped ions could be employed, such as are described in U.S. Pat. No. 5,572,022, for example. 
     After acceleration through the ion optics  32  the ions are focused into short packets between 10 and 100 ns long for each m/z and enter the mass selector  40 . Various forms of ion selection device may be employed, as will become apparent from the following. If the ion selection device is an electrostatic trap, for example, the specific details of that are not critical to the invention. For example, the electrostatic trap, if employed, may be open or closed, with two or more ion mirrors or electric sectors, and with or without orbiting. At present, a simple and preferred arrangement of an electrostatic trap embodying the ion selection device  40  is shown in  FIG. 3 . This simple arrangement comprises two electrostatic mirrors  42 ,  44  and two modulators  46 ,  48  that either keep ions on a recurring path or deflect them outside of this path. The mirrors may be formed of either a circular or a parallel plate. As the voltages on the mirrors are static, they may be sustained with very high accuracy, which is favourable for stability and mass accuracy within the electrostatic trap  40 . 
     The modulators  46 ,  48  are typically a compact pair of openings with pulsed or static voltages applied across them, normally with guard plates on both sides to control fringing fields. Voltage pulses with rise and fall times of less than 10-100 ns (measured between 10% and 90% of peak) and amplitudes up to a few hundred volts are preferable for high-resolution selection of precursor ions. Preferably, both modulators  46  and  48  are located in the planes of time-of-flight focusing of the corresponding mirrors  42 ,  44  which, in turn, may preferably but do not necessarily coincide with the centre of the electrostatic trap  40 . Typically, ions are detected through image current detection (which is in itself a well known technique and is not therefore described further). 
     Returning again to  FIG. 2 , after a sufficient number of reflections and voltage pulses within the electrostatic trap  40 , only a narrow mass range of interest is left in the electrostatic trap  40 , thus completing precursor ion selection. Selected ions in the EST  40  are then deflected on a path that is different from their input path and which leads to the fragmentation cell  50 , or alternatively the ions may pass to detector  75 . Preferably, this diversion to the fragmentation cell is performed through a deceleration lens  80  which is described in further detail in connection with  FIGS. 9 to 13  below. The ultimate energy of the collisions within the fragmentation cell  50  may be adjusted by appropriate biasing of the DC offset on the fragmentation cell  50 . 
     Preferably, the fragmentation cell  50  is a segmented RF-only multipole with axial DC field created along its segments. With appropriate gas density in the fragmentation cell (detailed below) and energy (which is typically between 30 and 50 V/kDa), ion fragments are transported through the cell towards the ion trap  30  again. Alternatively or concurrently, ions could be trapped within the fragmentation cell  50  and then be fragmented using other types of fragmentation such as electron transfer dissociation (ETD), electron capture dissociation (ECD), surface-induced dissociation (SID), photo-induced dissociation (PID), and so forth. 
     Once the ions have been stored in the ion trap  30  again, they are ready for onward transmission towards the electrostatic trap  40  for a further stage of MS n , or towards the electrostatic trap  40  for mass analysis there, or alternatively towards the mass analyser  70  which may be a time-of-flight (TOF) mass spectrometer or an RF ion trap or FT ICR or, as shown in  FIG. 2 , an Orbitrap mass spectrometer. Preferably, the mass analyser  70  has its own automatic gain control (AGC) facilities, to limit or regulate space charge. In the embodiment of  FIG. 2 , this is carried out through an electrometer grid  90  on the entrance to the Orbitrap  70 . 
     An optional detector  75  may be placed on one of the exit paths from the electrostatic trap  40 . This may be used for a multitude of purposes. For example, the detector may be employed for accurate control of the number of ions during a pre-scan (that is, automatic gain control), with ions arriving directly from the ion trap  30 . Additionally or alternatively, those ions outside of the mass window of interest (in other words, unwanted ions from the ion source, at least in that cycle of the mass analysis) may be detected using the detector. As a further alternative, the selected mass range in the electrostatic  40  may be detected with high resolution, following multiple reflections in the EST as described above. Still a further modification may involve the detection of heavy singly-charged molecules, such as proteins, polymers and DNAs with appropriate post-acceleration stages. By way of example only, the detector may be an electron multiplier or a microchannel/microsphere plate which has single ion sensitivity and can be used for detection of weak signals. Alternatively, the detector may be a collector and can thus measure very strong signals (potentially more than 10 4  ions in a peak). More than one detector could be employed, with modulators directing ion packets towards one or another according to spectral information obtained, for example, from the previous acquisition cycle. 
       FIG. 4  illustrates an arrangement which is essentially similar to the arrangement of  FIG. 2  though with some specific differences. As such, like reference numerals denote parts common to the arrangements of  FIGS. 2 and 4 . 
     The arrangement of  FIG. 4  again comprises an ion source  20  which supplies ions to a pre-trap which in the embodiment of  FIG. 4  is a auxiliary ion storage device  60 . Downstream of that pre-trap/auxiliary ion storage device  60  is a ion trap  30  (which in the preferred embodiment is a curved trap) and a fragmentation cell  50 . In contrast to the arrangement of  FIG. 2 , however, the arrangement of  FIG. 4  locates the fragmentation cell between the ion trap  30  and the auxiliary ion storage device  60 , that is, on the “source” side of the ion trap, rather than between the ion trap and the electrostatic trap as it is located in  FIG. 2 . 
     In use, ions are built up in the ion trap  30  and then orthogonally ejected from it through ion optics  32  to an electrostatic trap  40 . A first modulator/deflector  100  downstream of the ion optics  32  directs the ions from the ion trap  30  into the EST  40 . Ions are reflected along the axis of the EST  40  and, following ion selection there, they are ejected back to the ion trap  30 . To assist with ion guiding in that process, an optional electric sector (such as a toroidal or cylindrical capacitor)  110  may be employed. A deceleration lens is located between the electric sector  110  and the return path into the ion trap  30 . Deceleration may involve pulsed electric fields as described above. 
     Due to the low pressure in the ion trap  30 , ions arriving back at that trap  30  fly through it and fragment in the fragmentation cell  50  which is located between that ion trap  30  and the auxiliary ion storage device  60  (i.e. on the ion source side of the ion trap  30 ). The fragments are then trapped in the ion trap  30 . 
     As with  FIG. 2 , an Orbitrap mass analyser  70  is employed to allow accurate mass analysis of ions ejected from the ion trap  30  at any chosen stage of MS n . The mass analyser  70  is located downstream of the ion trap (i.e. on the same side of the ion trap as the EST  40 ) and a second deflector  120  “gates” ions either to the EST  40  via the first deflector  100  or into the mass analyser  70 . 
     Other components shown in  FIG. 4  are RF only transport multipoles that act as interfaces between the various stages of the arrangement as will be well understood by those skilled in the art. Between the ion trap  30  and the fragmentation cell  50  may also be located an ion deceleration arrangement (see  FIGS. 9-13  below). 
       FIG. 5  shows a further alternative arrangement to that shown in  FIG. 2  and  FIG. 4  and like components are once again labelled with like reference numerals. The arrangement of  FIG. 5  is similar to that of  FIG. 2  in that ions are generated by an ion source  20  and then pass through (or bypass) a pre-trap and auxiliary ion storage device  60  before being stored in a ion trap  30 . Ions are orthogonally ejected from the ion trap  30 , through ion optics  32 , and are deflected by a first modulator/deflector  100  onto the axis of an EST  40 , as with  FIG. 4 . 
     In contrast to  FIG. 4 , however, as an alternative to ion selection in the EST  40 , ions may instead be deflected by modulator/deflector  100  into an electric sector  110  and from there into a fragmentation cell  50  via an ion deceleration arrangement  80 . Thus (in contrast to  FIG. 4 ) the fragmentation cell  50  is not on the source side of the ion trap  30 . Following ejection from the fragmentation cell  50 , ions pass through a curved transport multipole  130  and then a linear RF only transport multipole  140  back into the ion trap  30 . An Orbitrap or other mass analyser  70  is again provided to permit accurate mass analysis at any stage of MS n . 
       FIG. 6  shows still a further alternative arrangement which is essentially identical in concept to the arrangement of  FIG. 2 , except that the EST  40  is not of the “closed” type trap illustrated in  FIG. 3 , but is instead of the open type as is described in the documents set out in the introduction above. 
     More specifically, the mass spectrometer of  FIG. 6  comprises an ion source  20  which provides a supply of ions to a pre-trap/auxiliary ion store  60  (further ion optics is also shown but is not labelled in  FIG. 6 ). Downstream of the pre-trap/auxiliary ion storage device  60  is a further ion storage device which in the arrangement of  FIG. 6  is once again a curved ion trap  30 . Ions are ejected from the curved trap  30  in an orthogonal direction, through ion optics  32 , towards an EST  40 ′ where the ions undergo multiple reflections. A modulator/deflector  100 ′ is located towards the “exit” of the EST  40 ′ and this permits ions to be deflected either into a detector  150  or to a fragmentation cell  50  via an electric sector  110  and an ion decelerator arrangement  80 . From here, ions may be injected back into the ion trap  30  once more, again through an entrance aperture which is distinct from the exit aperture through which ions pass on their way to the EST  40 ′. The arrangement of  FIG. 6  also includes associated ion optics but this is not shown for the sake of clarity in that Figure. 
     In one alternative, the EST  40 ′ of  FIG. 6  may employ parallel mirrors (see, for example, WO-A-2005/001878) or elongate electric sectors (see, for example, US-A-2005/0103992). More complex shapes of trajectories or EST ion optics could be used. 
       FIG. 7  shows still a further embodiment of a mass spectrometer in accordance with aspects of the present invention. As with  FIG. 4 , the spectrometer comprises an ion source  20  which supplies ions to a pre-trap which, as in the embodiment of  FIG. 4 , is a auxiliary ion storage device  60 . Downstream of that pre-trap/auxiliary ion storage device  60  is a ion trap  30  (which in the preferred embodiment is a curved trap) and a fragmentation cell  50 . The fragmentation cell  50  could be located on either side of the ion trap  30  though in the embodiment of  FIG. 7  the fragmentation cell  50  is shown between the ion source  20  and the ion trap  30 . As with the previous embodiments, an ion deceleration arrangement  80  is located in preference between the ion trap  30  and the fragmentation cell  50 . 
     In use, ions enter the ion trap  30  via an ion entrance aperture  28  and are accumulated in the ion trap  30 . They are then orthogonally ejected through an exit aperture  29  which is separate from the entrance aperture  28 , to an electrostatic trap  40 . In the arrangement shown in  FIG. 7 , the exit aperture is elongate in a direction generally perpendicular to the direction of ion ejection (i.e., the exit aperture  29  is slot-like). The ion position within the trap  30  is controlled so that the ions exit through one side (the left hand side as shown in  FIG. 7 ) of the exit aperture  29 . Control of the position of the ions within the ion trap may be achieved in a number of ways, such as by applying differing voltages to electrodes (not shown) on the ends of the ion trap  30 . In one particular embodiment, ions may be ejected in a compact cylindrical distribution from the middle of the ion trap  30  whilst being recaptured as a much longer cylindrical distribution (as a result of divergence and aberrations within the system) of a much greater angular size. 
     Modified ion optics  32 ′ are sited downstream of the exit from the ion trap  30 , and, downstream of that, a first modulator/deflector  100 ″ directs the ions into the EST  40 . Ions are reflected along the axis of the EST  40 . As an alternative to the directing of the ions from the ion trap  30  into the EST  40 , the ions may instead be deflected by a deflector  100 ″ downstream of the ion optics  32 ′ into an Orbitrap mass analyser  70  or the like. 
     In the embodiment of  FIG. 7 , the ion trap  30  operates both as a decelerator and as an ion selector. The extraction (dc) potential across the ion trap  30  is switched off and the trapping (rf) potential is switched on at the exact point at which ions of interest come to rest in the ion trap  30  following their return from the EST  40 . To inject into and eject from the EST  40 , the voltages on the mirror within the EST  40  ( FIG. 3 ) which is closest to the lenses is switched off in a pulsed manner. After ions of interest are captured in the ion trap  30 , they are accelerated towards the fragmentation cell  50  on either side of the ion trap  30 , where fragment ions are generated and then trapped. After that, the fragment ions can be transferred to the ion trap  30  once more. 
     By ejecting ions from a first side of an elongate slot and capturing them back at or towards a second side of such a slot, the path of ejection from the ion trap  30  is not parallel to the path of recapture into that trap  30 . This in turn may allow injection of the ions into the EST  40  at an angle relative to the longitudinal axis of that EST  40 , as is shown in the embodiments of  FIGS. 4 and 5 . 
     Of course, although a single slot-like exit aperture  29  is shown in  FIG. 7 , with ions exiting it towards a first side of that slot but being received back from the EST  40  via the other side of that slot, two (or more) separate but generally adjacent transport apertures (which may or may not then be elongate in the direction orthogonal to the direction of travel of ions through them) could instead be employed, with ions exiting via a first one of these transport apertures but returning into the ion trap  30  via an adjacent transport aperture. 
     Indeed, not only could the slot like exit aperture  29  of  FIG. 7  be subdivided into separate transport apertures spaced in an generally orthogonal direction to the direction of travel of the ions during ejection and injection, but the curved ion trap  30  of  FIG. 7  could itself be subdivided into separate segments. Such an arrangement is shown in  FIG. 8 . 
     The arrangement of  FIG. 8  is very similar to that of  FIG. 7 , in that the spectrometer comprises an ion source  20  which supplies ions to a pre-trap which is a auxiliary ion storage device  60 . Downstream of that pre-trap/auxiliary ion storage device  60  is a ion trap  30 ′ (to be described further below) and a fragmentation cell  50 . As with the arrangement of  FIG. 7 , the fragmentation cell  50  in  FIG. 8  could be located on either side of the ion trap  30 ′ though in the embodiment of  FIG. 8  the fragmentation cell  50  is shown between the ion source  20  and the ion trap  30 ′, the ion trap  30 ′ and the fragmentation cell  50  being separated by an optional ion deceleration arrangement  80 . 
     Downstream of the ion trap  30  is a first modulator/deflector  100 ′″ which directs the ions into the EST  40  from an off axis direction. Ions are reflected along the axis of the EST  40 . To eject the ions from the EST  40  back to the ion trap  30 , a second modulator/deflector  100 ″ in the EST  40  is employed. As an alternative to the directing of the ions from the ion trap  30  into the EST  40 , the ions may instead be deflected by the deflector  100 ′″ into an Orbitrap mass analyser  70  or the like. 
     The curved ion trap  30 ′ comprises in the embodiment of  FIG. 8 , three adjoining segments  36 , 37 , 38 . The first and third segments  36 , 38  each have an ion transport aperture so that ions are ejected from the ion trap  30 ′ via the first transport aperture in the first segment  36 , into the EST  40 , but are received back into the ion trap  30 ′ via a second, spatially separate transport aperture in the third segment  38 . To achieve this, the same RF voltage may be applied to each segment of the ion trap  30 ′ (so that in that sense the ion trap  30 ′ acts as a single trap despite the several trap sections  36 , 37 , 38 ) but with different DC offsets applied to each section so that the ions are not distributed centrally in the axial direction of the curved ion trap  30 ′. In use, ions are stored in the ion trap  30 ′. By suitable adjustment of the DC voltage applied to the ion trap segments  36 ,  37 ,  38 , ions are caused to leave the ion trap  30 ′ via the first segment  36  for off axis injection into the EST  40 . The ions return to the ion trap  30 ′ and enter via the aperture in the third segment  38 . 
     By maintaining the DC voltage on first and second segments  36  and  37  at a lower amplitude than the DC voltage applied to the third segment  38  when the ions are re-trapped from the EST  40 , the ions can be accelerated (eg by 30-50 ev/kDa) along the curved axis of the ion trap  30 ′ so that they undergo fragmentation. In this manner the ion trap  30 ′ is operable both as a trap and as a fragmentation device. 
     The resultant fragment ions are then cooled and squeezed into the first segment  36  by increasing the DC offset voltage on the second and third segments  37 ,  38  relative to the voltage on the first segment  36 . 
     For optimal operation, fragmentation devices in particular require that the spread of energies of the ions injected into them is well controlled and held within a range of about 10-20 eV, since higher energies result in only low-mass fragments whereas lower energies provide little fragmentation. Many existing mass spectrometer arrangements, as well as the novel arrangements described in the embodiments of  FIGS. 1 to 7  here, on the other hand, result in an energy spread of ions arriving at a fragmentation cell far in excess of that desirable narrow range. For example, in the arrangement of  FIGS. 1 to 7 , the ions may spread in energy in the ion trap  30 ,  30 ′ due to spatial spread in that trap; due to space charge effects (e.g. Coulomb expansion during multiple reflections) in the EST  40 , and due to the accumulated effect of aberrations in the system. 
     In consequence some form of energy compensation is desirable.  FIGS. 9 to 11  show some specific but schematic examples of parts of an ion deceleration arrangement  80  for achieving that goal, and  FIGS. 12 and 13  show energy spread reduction and spatial spread for a variety of different parameters applied to such ion deceleration arrangements. 
     In order to achieve a suitable level of energy compensation, employing some of the embodiments described above, it is desirable to increase the ion energy dispersion. In other words, the beam thickness for a hypothetical monoenergetic ion beam is preferably smaller than the separation of two such hypothetical monoenergetic ion beams by the desired energy difference of 10-20 eV as explained above. Although a degree of energy dispersion could of course be achieved by physically separating the fragmentation cell  50  from the ion trap  30  or EST  40  by a significant distance (so that the ions can disperse in time), such an arrangement is not preferred as it increases the overall size of the mass spectrometer, requires additional pumping, and so forth. 
     Instead it is preferable to include a specific arrangement to allow deliberate energy dispersion without unduly increasing the distance between the fragmentation cell  50  and the component of the mass spectrometer upstream from it (ion trap  30  or EST  40 ).  FIG. 9  shows one suitable device. In  FIG. 9 , an ion mirror arrangement  200  forming an optional part of the highly schematically represented ion deceleration arrangement  80  of  FIGS. 2-7  is shown. The ion mirror arrangement  200  comprises an array of electrodes  210  terminating in a flat mirror electrode  220 . Ions are injected into the ion mirror arrangement from the EST  40  and are reflected by the flat mirror electrode  220  resulting in increased dispersion of the ions by the time they exit back out of the ion mirror arrangement and arrive at the fragmentation cell  50 . An alternative approach to the introduction of energy dispersion is shown in  FIG. 11  and described further below. 
     Once the degree of energy dispersion has been increased for example with the ion mirror arrangement  200  of  FIG. 9 , ions are next decelerated. In general terms this may be achieved by applying a pulsed DC voltage to a decelerating electrode arrangement such as that illustrated in  FIG. 10  and labelled  250 . The decelerating electrode arrangement  250  of  FIG. 10  comprises an array of electrodes with an entrance electrode  260  and an exit electrode  270  between which is sandwiched a ground electrode  280 . Preferably the entrance and exit electrodes are combined with differential pumping sections so as to reduce the pressure gradually between the (upstream) ion mirror arrangement  200  at a relatively low pressure, the decelerating electrode arrangement  250  at an intermediate pressure, and the relatively higher pressure required by the (downstream) fragmentation cell  50 . By way of example only, the ion mirror arrangement  200  may be at a pressure of around 10 −8  mBar, the decelerating electrode arrangement  250  may have a lower pressure limit of around 10 −5  mBar rising to around 10 −4  mBar via differential pumping, with a pressure in the range of 10 −3  to 10 −2  mBar or so in the fragmentation cell  50 . To provide pumping between the exit of the decelerating electrode arrangement  250  and the fragmentation cell  50 , an additional RF only multipole such as, most preferably, an octapole RF device, could be employed. This is shown in  FIG. 11  to be described below. 
     To achieve deceleration, DC voltages on one or both of the lenses  260 ,  270  are switched. The time at which this occurs depends upon the specific mass to charge ratio of ions of interest. In particular, when ions enter a decelerating electric field, higher energy ions overtake lower energy ions and thus move to a greater depth in the deceleration field. After all the ions of this particular m/z enter the deceleration field, the field is switched off. Therefore ions with initially higher energy experience a higher drop in potential relatively to ground potential than the lower energy ions, thus making their energies equal. By matching the potential drop to the energy spread upon exit from the mass selector, a significant reduction of the energy spread may be achieved. 
     It will be understood that this technique permits energy compensation for ions of a certain range of mass to charge ratios, and not for an indefinitely wide range of different mass to charge ratios. This is because in a finite decelerating lens arrangement, only ions of a certain range of mass to charge ratios will be caused to undergo an amount of deceleration that can be matched to their energy spread. Any ions of widely differing mass to charge ratios to that selected will of course either be outside of the decelerating lens when it is switched, or likewise undergo a degree of deceleration but, having a largely different mass to charge ratio, the amount of deceleration will not then be balanced by the initial energy spread, i.e. the deceleration and penetration distance of higher energy ions will not then be matched to the deceleration and penetration distance of lower energy ions. Having said that, however, the skilled person will readily understand that this does not prohibit the introduction of ions of widely differing mass to charge ratios into the ion deceleration arrangement  80 , only that only ions of one particular range of mass to charge ratios of interest will undergo the appropriate degree of energy compensation to prepare them properly for the fragmentation cell  50 . Thus, the ions can either be filtered upstream of the ion deceleration arrangement  80  (so that only ions of a single mass to charge ratio of interest enter it in a given cycle of the mass spectrometer) or alternatively a mass filter can be employed downstream of the ion deceleration arrangement  80 . Indeed, it is even possible to use the fragmentation cell  50  itself to discard ions not of the mass to charge ratio of interest and which have been suitably energy compensated. 
       FIG. 11  shows an alternative arrangement for decelerating ions and also optionally defocusing them as well. Here, the defocusing is achieved within the EST  40  (only a part of which is shown in  FIG. 11 ) by pulsing the DC voltage on one of the electrostatic mirrors  42 ,  44  ( FIG. 3 ) at a time when ions of a mass to charge ratio of interest are in the vicinity of that electrostatic mirror  42 ,  44  (because of the manner in which the EST  40  operates, the time at which ions of a particular m/z arrive at the electrostatic mirrors  42 ,  44  is known). Applying a suitable pulse to that electrostatic mirror  42  or  44  results in that mirror  42 ,  44  having a defocusing rather than a focusing effect on those ions. 
     Once defocused, the ions can then be ejected out of the EST by applying a suitable deflecting field to the deflector  100 / 100 ′/ 100 ″. The defocused ions then travel towards a decelerating electrode arrangement  300  which decelerates ions of the selected m/z as explained above in connection with  FIG. 10 , by matching the initial energy spread to the drop in potential across the electric field defined by the decelerating electrode arrangement  300 . 
     Finally, ions exit the decelerating electrode arrangement  300  through termination electrodes  310  and pass through an exit aperture  320  into an octapole RF only device  330  to provide the desirable pumping described above. 
       FIGS. 12 and 13  show plots of energy spread and spatial spread of ions of a specific mass to charge ratio, respectively, as a function of switching time of the DC voltage applied to the ion decelerating electrodes. 
     It can be seen from  FIG. 12  that the reduction in energy spread achieved by an embodiment of the present invention can be as much as a factor of 20, reducing a beam with +/−50 eV spread to one of +/−2.4 eV. A longer switching time produces a smaller spatial spot size but a larger final energy spread with the particular decelerator system described here. The example is given here to show that beam characteristics other than energy spread must be considered, not to suggest that deceleration for optimal final energy spread always produces an increase in spatial spread of the final beam. 
     Other designs of decelerating lens used with other energy defocused beams could produce a still greater reduction in energy spread. Those skilled in the art will realise that there are many potential uses for the invention as a result. The use for which the invention was particularly addressed was that of improving the yield and type of fragment ions produced in a fragmentation process. As was noted earlier, for efficient fragmentation of parent ions, 10-20 eV ion energies are required, and clearly a great many ions in a beam having +/−50 eV energy spreads will be well outside that range. Ions having too high an energy predominantly fragment to low mass fragments which can make identification of the parent ion difficult, whilst a higher proportion of ions of low energy do not fragment at all. Without energy compensation, a parent ion beam having +/−50 eV energy spread directed towards a fragmentation cell would either produce a high abundance of low mass fragments, if all the beam were allowed to enter the fragmentation cell, or if only ions having the highest 20 eV of energy were allowed to enter (by use of a potential barrier prior to entry, for example) a great many ions would have been lost, and the process would be highly inefficient. The inefficiency would depend upon the energy distribution of the ions in the beam, with perhaps 90% of the beam being lost or unable to fragment due to insufficient ion energy. 
     By using the foregoing techniques, fragmentation of ions in the fragmentation cell may thereby be avoided if it is desired to pass ions through the fragmentation cell  50  (or store them there) in a given cycle of the mass spectrometer intact. Alternatively, control over the fragmentation may be improved when it is desired to carry out MS/MS or MS^n experiments. 
     Other uses for the ion deceleration technique described may be found in other ion processing techniques. Many ion optical devices can only function well with ions having energies within a limited energy range. Examples include electrostatic lenses, in which chromatic aberrations cause defocusing, RF multipoles or quadrupole mass filters in which the number of RF cycles experienced by the ions as they travel the finite length of the device is a function of the ion energy, and magnetic optics which disperse in both mass and energy. Reflectors are typically designed to provide energy focusing so as to compensate for a range of ion beam energies, but higher order energy aberrations usually exist and an energy compensated beam such as is provided by the present invention will reduce the defocusing effect of those aberrations. Again, those skilled in the art will realise that these are only a selection of possible uses for the described technique. 
     Returning now to the arrangements of FIGS.  2  and  4 - 8 , in general terms, effective operation of each of the gas-filled units shown in these Figures depends upon the optimum choice of collision conditions and is characterised by collision thickness P·D, where P is the gas pressure and D is the gas thickness traversed by ions (typically, D is the length of the unit). Nitrogen, helium or argon are examples of collision gases. In the presently preferred embodiment, it is desirable that the following conditions are approximately achieved: 
     In the pre-trap  24 , it is desirable that P·D&gt;0.05 mm·torr, but is preferably &lt;0.2 mm·torr. Multiple passes may be used to trap ions, as described in our co-pending Patent Application No. GB0506287.2. 
     The ion trap  30  preferably has a P·D range of between 0.02 and 0.1 mm·torr, and this device could also extensively use multiple passes. 
     The fragmentation cell  50  (using collision-induced dissociation, CID) has a collision thickness P·D&gt;0.5 mm·torr and preferably above 1 mm·torr. 
     For any auxiliary ion storage device  60  employed, the collision thickness P·D is preferably between 0.02 and 0.2 mm torr. On the contrary, it is desirable that the electrostatic trap  40  is sustained at high vacuum, preferably at or better than 10 −8  torr. 
     The typical analysis times in the arrangement of  FIG. 2  are as follows:
         Storage in the pre-trap  24 : typically 1-100 ms; Transfer into the curved trap  30 : typically 3-10 ms;   Analysis in the EST  40 : typically 1-10 ms, in order to provide selection mass resolution in excess of 10,000;   Fragmentation in the fragmentation cell  50 , followed by ion transfer back into the curved trap  30 : typically 5-20 ms;   Transfer through the fragmentation cell  50  into a second ion storage device  60 , if employed, without fragmentation: typically 5-10 ms; and   Analysis in a mass analyser  70  of the Orbitrap type: typically 50-2,000 ms.       

     Generally, the duration of a pulse for ions of the same m/z should be well below 1 ms, preferably below 10 microseconds, while a most preferable regime corresponds to ion pulses shorter than 0.5 microseconds (for m/z between about 400 and 2000). In alternative terms and for other m/z, the spatial length of the emitted pulse should be well below 10 m, and preferably below 50 mm, while a most preferable regime corresponds to ion pulses shorter than 5-10 mm. It is particularly desirable to employ pulses shorter than 5-10 mm when employing Orbitrap and multi-reflection TOF analysers. 
     Although one specific embodiment has been described, the skilled reader will readily appreciate that various modifications could be contemplated.
A method of improving the detection limits of a mass spectrometer by: generating sample ions from an ion source; storing the sample ions in a first ion storage device; ejecting the stored ions into an ion selection device; selecting and ejecting ions of a chosen mass to charge ratio out of the ion selection device; storing the ions ejected from the ion selection device in a second ion storage device without passing them back through the ion selection device; repeating the preceding steps so as to augment the ions of the said chosen mass to charge ratio stored in the second ion storage device; and transferring the augmented ions of the said chosen mass to charge ratio back to the first ion storage device for subsequent analysis.