Abstract:
A method and apparatus for tandem mass spectrometry is disclosed. Precursor ions are fragmented and the fragments are accumulated in parallel, by converting an incoming stream of ions from an ion source ( 10 ) into a time separated sequence of multiple precursor ions which are then assigned to their own particular channel of a multi compartment collision cell ( 40 ). In this manner, precursor ion species, being allocated to their own dedicated fragmentation cell chambers ( 41, 42 . . . 43 ) within the fragmentation cell ( 40 ), can then be captured and fragmented by that dedicated fragmentation chamber at optimum energy and/or fragmentation conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    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. 14/367,871, filed Jun. 20, 2014, which is a National Stage application under 35 U.S.C. §371 of PCT Application No. PCT/EP2012/076501, filed Dec. 20, 2012. The disclosures of each of the foregoing applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a collision cell for a tandem mass spectrometer, to a tandem mass spectrometer including a collision cell, and to a method of tandem mass spectrometry. 
       BACKGROUND OF THE INVENTION 
       [0003]    Tandem mass spectrometry (MS/MS) is an established technique for improving the throughput of mass analysis in a mass spectrometer. Traditionally, one precursor is selected at a time, subjected to fragmentation and then its fragment analysed in the same or a subsequent mass analyser. When analysing complex mixtures (such as are typical for proteomics, environmental and food analysis), so many precursors must be analysed in a limited time period that there is insufficient time to achieve a good signal-to-noise ratio for each of the precursors. In consequence, tandem mass spectrometry techniques have been developed. Here, an incident ion beam is split into packets in accordance with their mass to charge ratio (m/z) and one packet is then fragmented without the loss of another packet, or in parallel with another packet. 
         [0004]    The splitting of the ion beam into packets can be performed with a scanning device that stores ions of a broad mass range (such as a 3D ion trap: see for example WO-A-03/03010, or a linear trap with radial injection as for example in U.S. Pat. No. 7,157,698). Alternatively, ion beam splitting can be achieved through the use of a pulsed ion mobility spectrometer (eg as is disclosed in WO-A-00/70335 or U.S. Pat. No. 6,906,319), through a linear time-of-flight mass spectrometer as is shown in U.S. Pat. No. 5,206,508, or using multi-reflecting time-of-flight mass spectrometer (see, for example, WO-A-2004/008481). As yet another alternative, ion beam splitting can be achieved along a spatial coordinate as is disclosed for example in U.S. Pat. No. 7,041,968 and U.S. Pat. No. 7,947,950. 
         [0005]    In each case, this first stage of mass analysis is followed by fast fragmentation, typically in a collision cell (preferably having an axial gradient) or by a pulsed laser. The resulting fragments are analysed (preferably by employing another TOF) on a much faster time scale than the scanning duration (so called “nested times”). 
         [0006]    This approach provides throughput without compromising sensitivity. In a more traditional multi-channel MS/MS technique, by contrast, a number of parallel mass analysers (typically ion traps) are used to select one precursor each. The resultant fragments are then scanned out to an individual detector (e.g. the ion trap array shown in U.S. Pat. No. 5,206,506, or the multiple traps of U.S. Pat. No. 6,762,406). Other alternative arrangements, such as are shown in U.S. Pat. No. 6,586,727, U.S. Pat. No. 6,982,414, or U.S. Pat. No. 7,759,638, acquire all fragments from all precursors simultaneously, in one spectrum, which is then subsequently deconvoluted. However such traditional methods inherently lack dynamic range, and face challenges with reliability of identification. 
         [0007]    The very limited time which is allocated for each fragment scan (typically, 10-20 microseconds) in the “nested times” approach of the above methods presents particular challenges. In particular, the “nested times” approach, involving the splitting of ion packets in time or space, inherently cannot provide high-performance analysis of obtained fragments. Increasing the scan time would further jeopardise the analytical performance of the precursor isolation, the latter already being quite poor when compared with routine present-day MS/MS. In addition, the “nested times” approach is incompatible with increasingly popular “slow” methods of fragmentation such as electron-transfer dissociation (ETD) which require up to a few tens of milliseconds for fragmentation to take place. Finally, the low transmission of the last-stage orthogonal-acceleration TOF offsets any advantages obtained by removal of losses in the precursor selection. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention seeks to address these problems with the prior art. 
         [0009]    According to a first aspect of the present invention, there is provided a method of tandem mass spectrometry as set out in claim  1 . 
         [0010]    The present invention thus, in a first aspect, provides for fragmentation of precursor ions and accumulation of the fragments in parallel, by converting an incoming stream of ions from an ion source into a time-separated sequence of multiple precursor ions, which are then assigned to their own particular channel of a multi compartment collision cell. In this manner, precursor ion species, being allocated to their own dedicated fragmentation cell chambers within the fragmentation cell, can then be captured and fragmented by that dedicated fragmentation chamber at optimum energy and/or fragmentation conditions. 
         [0011]    It is to be understood that the invention is equally applicable to both individual ion species (each being allocated separately to its own chosen fragmentation cell chamber), to a continuous range of masses forming a subset of the broader mass range from the ion source, and even to a selection of multiple ion species from the ion source which are not adjacent to each other in the precursor mass spectrum of the ions from the ion source. Any combination of these (i.e. a single ion species in one of the, or some of the, chambers, a continuous mass range of precursors in one of the, or some others of the, chambers, and/or a further non-continuous plurality of precursor ion species derived from the ion source) is also contemplated. Thus M i  and M j  are not to be construed narrowly in the sense of a single ion species but as a single ion species of a single m/z and/or a range of precursor ion species of different m/z. 
         [0012]    The separation in time between adjacent precursors or precursor ranges is shorter than the time of analysis of fragments subsequently in the mass analyser. Thus, high resolution analysis of fragments is possible. 
         [0013]    In order to maximise the duty cycle, ions of different precursor masses or mass ranges are preferably fragmented and stored in respective ones of the spatially separated fragmentation cell chambers, at partially overlapping times. In other words, at least two of the fragmentation cell chambers will contain precursor and/or fragment ions simultaneously, during part of the process in a first preferred embodiment. The method in one particular embodiment includes techniques for sequential emptying of the fragmentation cell by emptying an output cell chamber, then sequentially shifting the contents of the remaining chambers to a next respective cell chamber before repeating the process so as to eject ions sequentially from the output chamber in a “conveyor-type” or “shifting-type” arrangement. In an alternative embodiment, however, ions are ejected from each of the fragmentation cell chambers separately and by direct communication of each fragmentation cell chamber with the mass analyser. In other words, the different precursor ion species and their fragments in the different fragmentation cell chambers each communicate directly with a mass analyser and do not pass through other chambers between the step of ion ejection from each chamber and the mass analysis stage. 
         [0014]    The precursor ions separated in time preferably arrive at a downstream ion deflector for directing the ions to respective fragmentation cell chambers. The process preferably further comprises applying a pulsed voltage to the ion deflector to direct the ions to respective chambers. 
         [0015]    In preference, the energy of the precursor ions may be adjusted prior to entry into the fragmentation cell chambers. Furthermore, optionally, differential pumping of a channel between the ion deflector and fragmentation cell may take place. 
         [0016]    Various “traditional” and also “slow” fragmentation techniques may be employed, together or separately, within the fragmentation cell—that is, the same or different fragmentation techniques may be applied to different fragmentation cell chambers within the same fragmentation cell. Techniques such as activated ion electron transfer dissociation (ETD), multi stage ETD, and so forth may be employed. 
         [0017]    In accordance with a second aspect of the present invention, there is provided an arrangement for a tandem mass spectrometer as defined in claim  13 . 
         [0018]    The invention also extends to a tandem mass spectrometer comprising an ion source, a first stage of mass analysis, a multi-compartmental fragmentation cell and an ion deflector to populate the chambers of the fragmentation cell with precursor ions of different mass to charge ratios, together with a second stage of mass analysis downstream of that. The tandem mass spectrometer according to the present invention is defined in claim  23 . 
         [0019]    The first stage of mass analysis might be an ion trap, such as a linear ion trap with radial or axial ejection, a time of flight mass analyser such as a multi-turn or multi-reflection TOF for example; an ion mobility spectrometer; or a magnetic sector analyser or other spatially dispersing analyser. The second mass analyser may, by contrast, be a high resolution mass analyser, for instance an orbital trapping analyser such as the Orbitrap™ mass analyser or a time of flight analyser such as a multi-turn or multi-reflection TOF analyser. 
         [0020]    Embodiments of the present invention thus provide for a method and apparatus which permits sufficient time to fragment ions including more recent “slow” techniques such as electron transfer dissociation. The multi channel arrangement of the fragmentation cell allows sufficient time for high performance analysis of fragment ions. 
         [0021]    Various other preferred features of the present invention will be apparent from the appended claims and from the following specific description of some preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention may be put into practice in a number of ways and some embodiments will now be described by way of example only and with reference to the accompanying figures in which: 
           [0023]      FIG. 1  shows a highly schematic arrangement of a first embodiment of a tandem mass spectrometer with a multi compartmental fragmentation cell in accordance with the present invention; 
           [0024]      FIG. 2   a  and  FIG. 2   b  show, respectively, front and side sectional views of the fragmentation cell arrangement of  FIG. 1  in further detail; 
           [0025]      FIG. 3  shows a highly schematic layout of a tandem mass spectrometer in accordance with a second embodiment of the present invention, again with a multi compartmental fragmentation cell; 
           [0026]      FIG. 4  shows a side sectional view of the multi compartmental fragmentation cell of  FIG. 3  in further detail; and 
           [0027]      FIG. 5  shows a particular preferred arrangement of multi compartmental fragmentation cell suitable for use with the arrangement of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    Referring first to  FIG. 1 , a highly schematic block diagram of the components for a tandem mass spectrometer embodying the present invention is shown. The embodiment of  FIG. 1  may be referred to herein as being of a “conveyor-type”. In the arrangement of  FIG. 1 , ions are introduced from an ion source  10  into a first stage of mass analysis  20 . The ion source  10  may be continuous, quasi continuous (such as, for example, an electrospray ionisation source) or pulsed such as a MALDI source. In  FIG. 1 , ion optics and various other components necessary for transporting ions between various stages of the tandem mass spectrometer are not shown, for clarity, though these will in any event be familiar to the skilled person. 
         [0029]    The first stage of mass analysis  20  may be one of an ion trap, such as a linear ion trap with radial or axial ejection, a time of flight (TOF) analyser of any known type, including but not limited to multi-turn and multi-reflection TOFs, an ion mobility spectrometer of any known type, or a spatially dispersing analyser such as a magnetic sector or distance-of-flight analyser. 
         [0030]    The first stage of mass analysis  20  ejects precursor ions. Ions of different mass to charge ratios, m/z, emerge from the first stage of mass analysis at different moments in time, or separate in time of flight downstream of the first stage of mass analysis. In either case, precursor ions of different mass to charge ratios arrive at a rastering device  30  such as an ion deflector at different times. The rastering device  30  deflects precursor ions with mass to charge ratios m 1 , m 2  . . . m N  into corresponding chambers  1 ,  2  . . . N of a fragmentation cell  40 . Each mass to charge ratio m 1 , m 2  . . . m N  represents a single ion species having a single mass to charge ratio, or alternatively a range of precursor ions having a commensurate range of mass to charge ratios. Techniques for parallel analysis of multiple mass ranges using the arrangement of  FIG. 1  will be summarised below; a particularly preferred approach to the analysis of a relatively broad mass range of precursors by segmentation into a plurality of narrower precursor mass ranges, and targeted fragmentation of different segments in multiple scan cycles, is described in our co-pending application entitled “Method of Tandem mass spectrometry”, filed at the UKIPO on the same day as the present application, and incorporated by reference in its entirety. 
         [0031]    Each collision cell chamber  1 ,  2  . . . N is denoted as  41 ,  42  . . .  43  in  FIG. 1 . For specially dispersing analysers, the rastering device  30  is inherently integrated with the mass analyser  20  in a single unit. 
         [0032]    Ions enter each fragmentation cell chamber and are fragmented there. The resulting fragments, and any remaining precursor ions, are stored within the respective chamber. 
         [0033]    The particular, optimal fragmentation conditions (energy collision gas, collision technique, slow, such as ETD, or fast as collision-induced dissociation)—can be selected for each collision cell chamber in accordance with the anticipated precursor ion. The rastering device  30  is under the control of a controller  60  and may use information from calibration or ion optical modelling, or previous mass spectra, to control the distribution of the different ion species arriving at the rastering device  30 . 
         [0034]    Once ions have been stored in the fragmentation cell chambers sufficient for the required degree of fragmentation, ions are ejected from the fragmentation cell  40  to a second stage of mass analysis  50 . 
         [0035]    In the embodiment of  FIG. 1 , fragment ions and any remaining precursor ions from each of the fragmentation cell chambers are ejected sequentially to the mass analyser  50  via a single exit aperture  45  for the fragmentation cell  40 . Specifically, fragment and any remaining precursor ions from the fragmentation cell chamber  41  which is closest to the mass analyser  50  are injected into that mass analyser for mass analysis. Chamber  41  may thus be termed the output chamber. There is then a short delay (preferably less than 1-5 ms), whilst fragment and any remaining precursor ions from the second closest fragmentation cell chamber  42  are shifted into the fragmentation cell chamber  41 , which is closest to the mass analyser  50 . This is achieved by applying displacing DC voltages to the electrodes of the second closest fragmentation cell chamber  42 . 
         [0036]    Similar displacing DC voltages are sequentially applied to each of the remaining fragmentation cell chambers, so that the ion populations shift by 1 fragmentation cell chamber at a time towards the mass analyser  50 , once the previous population has been ejected from the fragmentation cell chamber closest to the mass analyser  50 . 
         [0037]    After the first shift of the different fragment ions from the fragmentation cell chambers  41 ,  42  . . .  43 , the n-th fragmentation cell chamber  43 , which is furthest from the mass analyser  50 , is empty. Interleaving may then be carried out, whereby that n-th fragmentation cell chamber  43  is filled with either the same precursor species as was previously injected into that fragmentation cell chamber  43 , or alternatively, a different precursor ion species. Thus, the embodiment of  FIG. 1  preferably employs a one dimensional array of shifting cells. In other embodiments two dimensional arrays can be arranged. 
         [0038]    Turning now to  FIG. 2 , the rastering device  30  and fragmentation cell  40  of  FIG. 1  is shown in further detail. The rastering device  30  is preferably a pair of deflector plates with pulsed voltages applied to them. Optionally, the rastering device  30  may be complemented by an energy lift  31 , which is pulsed in synchronisation (under the control of the controller  60 ) with the rastering device  30 , and adjusts the ion energy of precursor ions so that each precursor ion species enters its respective fragmentation cell chamber at an energy optimum for the required degree of fragmentation. The energy lift  31  may be located before or after the rastering device  30 . However, if the first stage of mass analysis  20  is a time of flight analyser, then it is desirable that both the rastering device  30  and the energy lift  31  are located close to the plane of TOF focusing. 
         [0039]    Each of the fragmentation cell chambers  41  . . .  43  is preferably formed of an RF-only multipole filled with collision gas. The chambers function not only to fragment ions, but also to ensure collisional cooling of the fragments. 
         [0040]    The ions are deflected to a particular fragmentation cell chamber and traverse a differentially pumped volume labelled generally at  35  in  FIG. 2  before entering entrance deflectors  81  . . .  83  of the fragmentation cell. Each cell chamber  41  . . .  43  has its own entrance deflector in this embodiment. The entrance deflectors  81  . . .  83  align the ion trajectory of incident ions of a particular mass to charge ratio with the axis of the fragmentation cell chamber into which these ions will be injected, and ensures the maximum acceptance of the ion beam. Although not shown in  FIG. 2 , it will also be understood that deceleration optics might also be included, as the ion energy is advantageously reduced from typically 1-3 keV/charge, down to 5-150 eV/charge. 
         [0041]    Upon entering the fragmentation cell chambers  41  . . .  43 , ions experience multiple collisions with collision gas, and fragment. A decelerating voltage between the entrance deflector  81  . . .  83  and the entrance aperture  41   a  . . .  43   a  of each fragmentation cell chamber may provide for an optimum collision energy alternatively or in addition to the optional energy lift  31 . If non-collisional fragmentation techniques are used, then ions should enter the cell chambers at energies below fragmentation level. To simplify deceleration of ions by allowing higher energies at the entry and still avoiding fragmentation, light collision gases such as helium or hydrogen could be used. Fragments and remaining precursor ions are reflected at the far end of each fragmentation cell chamber by an appropriate DC voltage, and those ions subsequently lose energy through collisions so that they concentrate near the axis of each fragmentation cell chamber. 
         [0042]    Shifting of ions between the various fragmentation cell chambers  41  . . .  43  precedes as follows, with reference particularly to  FIG. 2B . The multipole rods  61  and  62  define the first fragmentation cell chamber  41 , the rods  62  also define the second fragmentation cell chamber  42 , along with multipole rods  63 . Rods  63  and  64  define the third fragmentation cell chamber  43 , and so forth. 
         [0043]    The DC offset on the rods  62 ,  63  . . . is raised relative to the DC offset on the rods  61 . Suitably, the potential difference is 20-30 volts. The offset on the rods  61  is, in its turn, raised relative to a DC offset on electrodes  71 , such as 5 volts. The electrodes  71  form a part of a curved linear trap, to be described below, which acts to permit orthogonal ejection of ions from the fragmentation cell  40 . 
         [0044]    Each of the electrodes  61 ,  62 ,  63  . . . and  71  have RF voltages applied to them during the process of trapping and transfer. As a result, ions in the fragmentation cell chamber  41  are forced to move between electrodes  61  and  71  and into a curved linear trap  70  which is best seen in  FIG. 2A . Such a curved linear trap, also termed a C-trap, is described for example in WO 2008/081334. Once ions from the fragmentation cell chamber  41  have entered the curved linear trap  70 , they are stored along a curved axis and pulsed out into the mass analyser  50 . The process is described in WO-A-05/124,821. After that, the DC offset on the rods  61  is raised, for example, to 10 volts, and the DC offset on the rods  62  is lowered, for example, to ground potential. The DC offset on the rods  63  . . . is kept high (for example, 20-30 volts), so that ions from the fragmentation cell chamber  42  are then forced into the fragmentation cell chamber  41  by the resulting transverse electric field created by the potential difference. This sequence is repeated across the entire parallel array of ion traps constituted by the N fragmentation cell chambers  41  . . .  43 . In other words, the DC offset on the rods  62  is raised whilst the offset on rod  63  is lowered, resulting in a transfer of content of the fragmentation cell chamber  43  into the fragmentation cell chamber  42 , and so forth. Whilst ions are transferred from one fragmentation cell chamber to another, the fragmentation cell chamber itself is preferably not filled by the corresponding precursor ion species. 
         [0045]    The mass analyser  50  may, in preference, be of the orbital trapping or time of flight type. For example, the Orbitrap mass analyser, or a multi-turn or multi-reflection time of flight mass analyser might be employed. Furthermore, each of the fragmentation cell chambers might be employed to store fragments from several precursors (preferably from considerably different mass to charge ratios), to increase throughput (“multiplexing”). Also, the transfer of ions from one fragmentation cell chamber to another might be accompanied by crude mass selection, as a consequence of the applied DC fields, and also further fragmentation, to yield further generation of fragments (MS N , N=3, 4 . . . ). This also allows activated-ion ETD and multi-stage ETD to be accomplished. 
         [0046]      FIG. 3  shows an alternative embodiment of a tandem mass spectrometer with a fragmentation cell having parallel fragmentation cell chambers. As with  FIG. 1 ,  FIG. 3  shows the spectrometer in highly schematic block form for simpler explanation of the operation of it.  FIG. 4  shows the novel fragmentation cell arrangement of  FIG. 3  in more detail. 
         [0047]    In  FIG. 3 , as may be seen, the tandem mass spectrometer comprises an ion source  10  of pulsed, quasi continuous or continuous type, such as an electrospray or MALDI ion source, in a similar manner to that of the  FIG. 1  embodiment. Ions from the ion source enter the first stage of mass analysis  20  which, again, may be an ion trap, such as, preferably a linear ion trap with radial or axial ejection, a time of flight analyser of any known type, including a multi-turn and/or multi-reflection TOF device, an ion mobility spectrometer of any known type, or a spatially dispersing analyser, such as a magnetic sector analyser. 
         [0048]    Ions within the first mass analyser are ejected so that they arrive at a rastering device  30  such that ions of different mass to charge ratio arrive at different times. 
         [0049]    A system controller  60  controls the rastering device  30  to direct incident ions to a chosen one of multiple fragmentation cell chambers  41 ,  42  . . .  43  within in a fragmentation cell  40 . The fragmentation cell chambers  41 ,  42  . . .  43  are arranged in parallel as can be seen in  FIGS. 3 and 4 . Thus, for example, ions with a first mass to charge ratio m 1  may be directed by the rastering device  30 , under the control of the controller  60 , to a first of the fragmentation cell chambers  41 . Ions of a second mass to charge ratio m 2 , arriving at the rastering device  30  at different time to the ions of mass to charge ratio m 1 , may be directed to the second fragmentation cell chamber  42 , and so forth. It will of course be understood that the order of arrival of precursor ions at the rastering device  30  need not be related to the physical order of the fragmentation cell chambers. Whilst it may be, in practical terms, easiest to scan incident ions arriving at the rastering device  30  in sequence, into successive adjacent ones of the fragmentation cell chambers, in other words, this is by no means essential as with the arrangement of  FIG. 1  and  FIG. 2 , either calibration or ion optical modelling or previous mass spectra may be employed to allow the controller  60  suitably to control the rastering device  30  to direct appropriate precursor ions into appropriate fragmentation cell chambers. 
         [0050]    Once ions have been injected by the rastering device  30  into a particular fragmentation cell chamber  41 ,  42  . . .  43 , appropriate fragmentation conditions can be applied data dependently (that is, for example, as a result of pre scans, calibration and so forth), so that fragmentation of ions in a particular fragmentation cell chamber takes place under conditions that are optimised for the particular precursor ion species. For example, the collision energy for the particular ion species may be tuned to that ion species under the control of the controller  60 . Energy lift means as described above in respect of  FIG. 1  may optionally be employed in the  FIG. 3  embodiment as well. 
         [0051]    Unlike the arrangement of  FIGS. 1 and 2 , however, the output of each fragmentation cell chamber  41 ,  42  . . .  43 , is in direct communication with an output exit of the fragmentation cell  40 . By this means, ions in any one of the fragmentation cell chambers can be ejected, independently of the others and without the need to pass ions through any other fragmentation cell chambers, via the fragmentation cell ion exit, to a second stage mass analyser  50 . The second stage (external) mass analyser  50  may, as with the arrangement of  FIGS. 1 and 2 , be a high resolution mass analyser such as an orbital electrostatic trap, a time of flight mass spectrometer and so forth. The second stage mass analyser  50  collects and detects the fragment ions and any remaining precursor ions which are ejected to it from the individual fragmentation cell chambers within the fragmentation cell  40 . The results of the detection of the ejected ions by the second stage mass analysis  50  can be sent to the controller  60  for post processing or onward transmission to a pc (not shown in  FIG. 3 ). 
         [0052]    The arrangement of  FIG. 3 , in contrast to the arrangement of  FIG. 1 , allows for direct and independent transfer of ions from each fragmentation cell chamber to the second stage of mass analysis  50 , without first passing through other fragmentation cell chambers. This allows greater freedom of operation and a larger variation in fill times for precursors of different intensities. 
         [0053]    Turning now more particularly to  FIG. 4 , a part of the tandem mass spectrometer  FIG. 3  is shown, between the rastering device  30  and the second stage mass analysis  50 , in further detail. Ions are scanned by the rastering device  30  into a chosen one of the fragmentation cell chambers  41 ,  42  . . .  43  through respective input deflectors  81 ,  82  . . .  83  adjacent input apertures  41   a ,  42   a  . . .  43   a . The volume between the rastering device  30  and the multiple input deflectors  81 ,  82  . . .  83  is differentially pumped and this is shown generally at reference numeral  35 . 
         [0054]    In the arrangement of  FIGS. 3 and 4 , ions exit each fragmentation cell chamber in the reverse sequence to their entry. This procedure may be seen best with reference to  FIG. 4 . Ions are firstly released by dropping the voltage on the exit aperture  41   b ,  42   b  . . .  43   b  on a particular fragmentation cell chamber  41 ,  42  . . .  43 . After that, the ions are accelerated by applying a voltage between the exit aperture of a particular fragmentation cell chamber  41 ,  42  . . .  43  and its exit deflector  91 ,  92  . . .  93 . Ions leave the exit deflector of a particular fragmentation cell chamber where they pass across a second differentially pumped volume  95  ( FIG. 4 ) as they are directed by the exit deflector to arrive at an exit deflector  90  arranged within or adjacent to the exit aperture of the fragmentation cell  40 . 
         [0055]      FIG. 5  shows a preferred embodiment of a fragmentation cell arrangement, in cross-sectional view. The fragmentation cell arrangement of  FIG. 5  includes the rastering device  30  of  FIGS. 1 to 4 , a differentially pumped volume  35  between the rastering device  30  and the fragmentation cell  40 ′ indicated by the broken line, various stages of differential pumping to be further described below, an exit aperture deflector  90  and a second stage of mass analysis  50 . The embodiment of  FIG. 5  addresses several issues, firstly to reduce complexity of construction taking into account the difference in ion energies, the multiplicity of channels, and so forth, secondly to reduce ion losses when decelerating the precursor ions to low energies prior to injection into the individual fragmentation cell chambers and thirdly to provide a suitable arrangement for differential pumping of the cell. 
         [0056]    In further detail, still referring to  FIG. 5 , precursor ions arrive at the rastering device  30  and are deflected by that towards one or other of the multiple fragmentation cell chambers  41 ,  42  . . .  43 . Each of these fragmentation cell chambers has entrance aperture deflectors  81 ,  82 ,  83  to adjust the direction of travel of the incident ions from the rastering device and guide them into the respective fragmentation cell chamber. Each fragmentation cell chamber itself is of integrated construction. This integrated fragmentation cell chamber construction addresses the first of the above noted issues, namely how to construct the fragmentation cell chambers so as to address the differences in ion energies, the multiplicity of channels and so forth. As may be seen In  FIG. 5 , each fragmentation cell chamber is comprised of RF electrodes implemented as parts of a plate having multiple apertures. In other words, the multiple fragmentation cell chambers are formed from horizontally stacked plates with multiple apertures, each horizontally stacked plate having an aperture which aligns with the others to form the longitudinal axes of the various fragmentation cell chambers. The deflectors at the entrance apertures,  81 ,  82 ,  83  and also the end electrodes, are provided with different DC voltages for the different channels (fragmentation cell chambers) and these are implemented as printed circuit boards (PCBs) with individual conductors provided to each of the channels. The parts of the fragmentation cell arrangement of  FIG. 5  constituting the entrance deflectors and end electrodes are labelled  120  and  130  respectively 
         [0057]    To address the problem of losses during deceleration of precursor ions to low energies, an Einzel lens  100  is integrated into each of the fragmentation cell chambers. A suitable lens is described, for example, for O&#39;Connor et al, J. Am. Soc. Mass Spectrom.; 1991, 2, pages 322-335. 
         [0058]    The problems of differential pumping of the fragmentation cell can be addressed by the creation of elongated areas of pressure gradient having aspect ratios of channel length to inscribed diameter in excess of about 10-50. In the case the cell consists of a sequence of N apertures with gaps between them, the aspect ratio (AR) is around N. 
         [0059]    For example, for a system of 50 fragmentation cell chambers, each having an inner diameter (ID) of 4 mm, the pressure could be reduced from P c =3.10 −3  mbar in the nitrogen filled fragmentation cell  40 ′, to a pressure P p =6.10 −4  mbar in the volumes labelled  101  and  102  in  FIG. 5 , with AR=20 (the sections labelled  111  and  112  in  FIG. 5 ) and a pumping speed in the volumes  101  and  102  of  FIG. 5 , of 40 litres per second in total. The pressure can then be reduced to P f =5.10 −5  mbar in the volumes labelled  35  and  94  in  FIG. 5 , with a further AR=20 (sections  113  and  114  of  FIG. 5 ) at a pumping speed of 100 litres per second in total in these volumes, 
         [0060]    In addition to the conventional molecular flow, there is also jetting of ions over the direct line of sight from one pressure region to another, resulting in additional increase of pressure, to consider. However, for AR &gt;10 and a pressure drop less than ten fold, this effect is negligible. However, regions  111  to  114  of  FIG. 5  could also be implemented as curved rather than straight sections, so that the line of sight from the high pressure region is then blocked. 
         [0061]    It is desirable that ions are already decelerated at the start of the pressure gradient described above, and it is also preferable that the DC gradient is applied along the entire length of the fragmentation cell. On the output side of it, ions are already collisionally cooled so that they concentrate upon the axis of the fragmentation cell chamber, and might pass through a much smaller hole (for example, a hole having a 2 mm inner diameter). This allows the length of the region  114  to be reduced. 
         [0062]    It will be appreciated that various modifications to the foregoing preferred embodiments can be contemplated. For example, in the embodiment of  FIG. 3 , each of the fragmentation cell chambers might form an individual mass analyser, such as a linear ion trap with axial or radial ejection (preferable with rectilinear type). In this case, ions are ejected with the help of an additional resonant excitation, preferably applied perpendicularly to the plane of the drawings. 
         [0063]    Furthermore, in each of the embodiments described above, during trapping in the fragmentation cell chambers, ions might be subjected to electron transfer dissociation (ETD), electron capture dissociation (ECD), electron ionisation dissociation (EID) or other ion-ion, ion-molecule, ion-photon (e.g. irradiation by laser) reactions, metastable-atom dissociation, and so forth. Anions for ETD could be introduced either from the other end of the fragmentation cell, or via the same first stage of mass analysis  20  and rastering device  30 . 
         [0064]    Moreover, it is to be understood that many different schemes for ion capture and fragmentation within the multiple parallel fragmentation cell chambers are envisaged. In one embodiment, for example, the controller  60  may control the rastering device  30  to direct precursor ions of only a single ion species/mass to charge ratio into a respective separate one of the multiple fragmentation cell chambers. Within each chamber, as discussed, each ion can be fragmented, or not, under conditions optimal for the particular ion species and charge state in the particular fragmentation cell chamber. In particular, whilst it may be that each (single) ion species in each fragmentation cell chamber  41  . . .  43  is fragmented (though optimally under different fragmentation conditions), in other embodiments, some but not all of the ion species in the fragmentation cell  40  are fragmented. Thus what is ejected from the chambers (either using the conveyor ejection scheme of  FIGS. 1 and 2  or the individual ejection technique employed with the arrangements of  FIGS. 3-5 ) may be a mixture of both unfragmented precursor ions from some of the chambers and the fragments of precursor ions from other chambers. 
         [0065]    In that case, the process can be repeated for multiple scan cycles, for the same or at least overlapping mass ranges from the ion source, but with different fragmentation schemes applied to the different scan cycles. For example, in cycle  1 , with 50 fragmentation cell chambers, chamber numbers  1 ,  2 ,  5 ,  9  and  32  might receive specific precursor ions m 1  m 2  m 5  m 9  and m 32  respectively (under the control of the controller  60  and the rastering device  30 ) but then store those precursor ions of masses m 1  m 2  m 5  m 9  and m 32  in the respective chambers and subsequently eject them to the mass analyser  50  without fragmentation. The remaining chambers may fragment the ions of masses m 3  m 4  m 6-8  m 10-31  and m 33-50 . In a second cycle of the arrangement, for example, a different subset of chambers can fragment the same or a different set of precursor ions (for example, in scan cycle  2 , precursor ions of masses m 19-24  and m 36  might instead be allowed to pass through the fragmentation cell  40  without fragmentation). As well or instead, different fragmentation conditions can be applied in different cycles. 
         [0066]    By taking this multicycle approach, and using different fragmentation parameters in each cycle, it is possible to deconvolve and decode mixtures of fragment and precursor ions in the mass analyser, and hence arrive at separate fragment and precursor spectra without the need to obtain these separately. That said, a single cycle is sufficient, particularly where the analyte is of known or suspected identity, and/or by judiciously selecting the chambers and their content precursor masses. 
         [0067]    Still further, whilst the invention has been described above, for the sake of simplicity and clarity of explanation, in the context of only a single precursor species having a single mass to charge ratio within each fragmentation cell chamber, the invention is by no means so limited. For example, the controller  60  and the rastering device  30  may together be configured to subdivide the precursor ions from the ion source and having a relatively broad mass range, into a plurality of segments some or all of which contains multiple precursor ions across a relatively narrower mass range forming a subset of the broad mass range (with some containing only a single ion species). Thus it is to be understood that reference to a “mass”, or a “mass to charge ratio” is intended to mean both a single ion species having a single mass/mass to charge ratio, and also a mass range containing two or more different ion species and/or two or more different mass to charge ratios (whether or not those different mass to charge ratios are discriminated during analysis, should they have a very similar m/z). 
         [0068]    The techniques for parallel processing of such segments containing multiple precursor species—and indeed a more detailed explanation of some exemplary decoding strategies, where multiple cycles with differing fragmentation cell chamber fragmentation schemes are employed, are set out in our above mentioned co-pending application entitled “Method of tandem mass spectrometry”, filed at the UKIPO on the same date as the present application.