Title of Invention: Mass spectrometer Abstract: A mass spectrometer is disclosed comprising a first ion trap or ion guide, a single ion mobility spectrometer or separator stage and a second ion trap or ion guide arranged downstream of the ion mobility spectrometer or separator. A mode of operation includes passing ions from said first ion trap or ion guide to said device and onwards to said second ion trap or ion guide and then passing at least some of said ions or at least some fragment, daughter, product or adduct ions derived from said ions from said second ion trap or ion guide onwards to said first ion trap or ion guide. Detailed Description: Part general_description_paragraphs: Part CROSS-REFERENCE TO RELATED APPLICATION: This application is a continuation of U.S. patent application Ser. No. 12/092,601 filed on Aug. 20, 2008 which represents a National Stage of International Application No. PCT/GB2006/004202 filed on Nov. 10, 2006 and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/737,150 filed on Nov. 16, 2005 and United Kingdom Application No. 0522933.1 filed on Nov. 10, 2005. The entire contents of these applications are incorporated by reference. Part BACKGROUND OF THE INVENTION: The present invention relates to a mass spectrometer and a method of mass spectrometry. Mass spectrometry is an established technique for identifying and quantifying molecules, including molecules of biological interest. It is a primary technique for identifying proteins due to its unparalleled speed, sensitivity and specificity. Strategies for the analysis of proteins may involve either analysis of the intact protein or more commonly digestion of the protein using a specific protease that cleaves at predictable residues along the peptide backbone. This provides smaller stretches of peptide sequence that are more amenable to analysis via mass spectrometry. It is known to perform experiments which involve the separation of a complex digest mixture by liquid chromatography which is directly interfaced to a tandem mass spectrometer using Electrospray Ionisation (ESI). MS and MS/MS spectra may be collected throughout the chromatographic separation and this information may be used to search databases directly for matching sequences leading to identification of the parent protein. The known approach can be used to identify proteins that are present at low endogenous concentrations. However, such digest mixtures may contain many hundreds if not thousands of components many of which will co-elute from the chromatography column. Methods designed for analysis of digest mixtures aim to identify as many of the peaks as possible within the complex mixture. However, as sample complexity increases it becomes increasingly difficult to select each individual precursor or parent ion for subsequent fragmentation. One method of increasing the peak capacity is to fragment a large number of parent or precursor ions simultaneously and then to record their product or fragment ions. Product or fragment ions may be associated with parent or precursor ions according to the closeness of alignment of their LC elution times. Eventually, however, as the sample complexity increases this method may also fail. Another approach to the problem of highly complex mixtures is to improve the separation capability. Addition of a further orthogonal separation stage can be particularly effective, especially if the time requirements for each separation process and for the mass spectrometer do not overlap. One known method which may be used to separate ions prior to analysis by mass spectrometry is that of ion mobility spectrometry or gas phase electrophoresis. One form of an ion mobility spectrometer or separator comprises a drift tube or cell wherein an axial electric field is maintained in the presence of a buffer gas. Higher mobility ions pass more quickly along the length of the ion mobility spectrometer or separator than lower mobility ions. As a result ions are separated according to their ion mobility. A known ion mobility spectrometer or separator may operate at or around atmospheric pressure or under a partial vacuum at a pressure down to as low as about 0.01 mbar. The known ion mobility spectrometer or separator operating under a partial vacuum comprises a plurality of electrodes having apertures. A DC voltage gradient is maintained along the length of the ion mobility spectrometer or separator and the electrodes are connected to an AC or RF voltage supply. This form of ion mobility spectrometer or separator is advantageous in that the AC or RF voltage which is applied to the electrodes results in radial confinement of the ions passing through the ion mobility spectrometer or separator. Radial confinement of the ions results in higher ion transmission compared with an ion mobility spectrometer or separator which does not confine ions radially. An ion mobility spectrometer or separator is known wherein ions are confined radially by an inhomogeneous RF field in an ion guide and ions are propelled forward by a potential hill or barrier that is progressively applied along the axis of the ion guide in the presence of a buffer gas. Appropriate selection of the amplitude and velocity of the potential hill or barrier which is translated along the length of the ion guide and the type and pressure of gas allows ions to slip selectively over the potential hill or barrier according to their ion mobility. This in turn allows ions having different ion mobilities to be transported at different velocities along the ion guide and thereby to become temporally separated. The additional separation gained by the use of ion mobility separation (IMS) or gas phase electrophoresis increases the peak capacity of a mass spectrometer. This benefit is gained irrespective of whether or not other separation techniques such as Liquid Chromatography (LC) are also used. Furthermore, the benefit gained by the use of ion mobility separation is equally relevant to tandem mass spectrometers (MS/MS) in which parent ions may be mass analysed and then selected parent ions may be induced to fragment by Collision Induced Decomposition and wherein the resulting fragment or daughter ions are then mass analysed. It is desired to provide a mass spectrometer having an improved ability to separate ions according to their ion mobility. Part SUMMARY OF THE INVENTION: According to an aspect of the present invention there is provided a mass spectrometer comprising: a first ion trap or ion guide comprising a plurality of electrodes; a device for separating ions according to their ion mobility or rate of change of ion mobility with electric field strength, the device being arranged downstream of the first ion trap or ion guide; and a second ion trap or ion guide comprising a plurality of electrodes arranged downstream of the device, wherein the second ion trap or ion guide is arranged and adapted in a mode of operation to pass or transmit ions from the second ion trap or ion guide to the device. According to the preferred embodiment the first ion trap or ion guide is preferably arranged and adapted in a mode of operation to receive ions which emerge from the device. The first ion trap or ion guide is preferably arranged and adapted in a mode of operation to receive ions which emerge from the device and to pass or transmit at least some of the ions, or at least some fragment, daughter, product or adduct ions derived from the ions, from the first ion trap or ion guide to the device. The first ion trap or ion guide may comprise a multipole rod set or a segmented multipole rod set ion trap or ion guide comprising a quadrupole rod set, a hexapole rod set, an octapole rod set or a rod set comprising more than eight rods. Alternatively, the first ion trap or ion guide may comprise an ion tunnel or ion funnel ion trap or ion guide comprising a plurality of electrodes or at least 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 electrodes having apertures through which ions are transmitted in use, wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the electrodes have apertures which are of substantially the same size or area or which have apertures which become progressively larger and/or smaller in size or in area. According to another embodiment the first ion trap or ion guide may comprise a stack or array of planar, plate or mesh electrodes, wherein the stack or array of planar, plate or mesh electrodes comprises a plurality or at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 planar, plate or mesh electrodes and wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the planar, plate or mesh electrodes are arranged generally in the plane in which ions travel in use. According to another embodiment the first ion trap or ion guide may comprise an ion trap or ion guide comprising a plurality of groups of electrodes arranged axially along the length of the ion trap or ion guide, wherein each group of electrodes comprises: (a) a first and a second electrode and means for applying a DC voltage or potential to the first and second electrodes in order to confine ions in a first radial direction within the ion guide; and (b) a third and a fourth electrode and means for applying an AC or RF voltage to the third and fourth electrodes in order to confine ions in a second radial direction within the ion guide. The second radial direction is preferably orthogonal to the first radial direction. According to the preferred embodiment the first ion trap or ion guide comprises an ion tunnel or ion funnel ion trap or ion guide wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the electrodes have internal diameters or dimensions selected from the group consisting of: (i) ≦1.0 mm; (ii) ≦2.0 mm; (iii) ≦3.0 mm; (iv) ≦4.0 mm; (v) ≦5.0 mm; (vi) ≦6.0 mm; (vii) ≦7.0 mm; (viii) ≦8.0 mm; (ix) ≦9.0 mm; (x) ≦10.0 mm; and (xi) >10.0 mm. The first ion trap or ion guide preferably further comprises first AC or RF voltage means arranged and adapted to apply an AC or RF voltage to at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the plurality of electrodes of the first ion trap or ion guide in order to confine ions radially within the first ion trap or ion guide. The first AC or RF voltage means is preferably arranged and adapted to apply an AC or RF voltage having an amplitude selected from the group consisting of: (i) <50 V peak to peak; (ii) 50-100 V peak to peak; (iii) 100-150 V peak to peak; (iv) 150-200 V peak to peak; (v) 200-250 V peak to peak; (vi) 250-300 V peak to peak; (vii) 300-350 V peak to peak; (viii) 350-400 V peak to peak; (ix) 400-450 V peak to peak; (x) 450-500 V peak to peak; and (xi) >500 V peak to peak. The first AC or RF voltage means is preferably arranged and adapted to apply an AC or RF voltage having a frequency selected from the group consisting of: (i) <100 kHz; (ii) 100-200 kHz; (iii) 200-300 kHz; (iv) 300-400 kHz; (v) 400-500 kHz; (vi) 0.5-1.0 MHz; (vii) 1.0-1.5 MHz; (viii) 1.5-2.0 MHz; (ix) 2.0-2.5 MHz; (x) 2.5-3.0 MHz; (xi) 3.0-3.5 MHz; (xii) 3.5-4.0 MHz; (xiii) 4.0-4.5 MHz; (xiv) 4.5-5.0 MHz; (xv) 5.0-5.5 MHz; (xvi) 5.5-6.0 MHz; (xvii) 6.0-6.5 MHz; (xviii) 6.5-7.0 MHz; (xix) 7.0-7.5 MHz; (xx) 7.5-8.0 MHz; (xxi) 8.0-8.5 MHz; (xxii) 8.5-9.0 MHz; (xxiii) 9.0-9.5 MHz; (xxiv) 9.5-10.0 MHz; and (xxv) >10.0 MHz. The first ion trap or ion guide is preferably arranged and adapted to receive a beam or group of ions and to convert or partition the beam or group of ions such that a plurality or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 separate packets of ions are confined and/or isolated in the first ion trap or ion guide at any particular time. Each packet of ions is preferably separately confined and/or isolated in a separate real axial potential well formed within the first ion trap or ion guide. The mass spectrometer preferably further comprises means arranged and adapted to urge at least some ions upstream and/or downstream through or along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the first ion trap or ion guide in a mode of operation. According to the preferred embodiment first transient DC voltage means are provided which are arranged and adapted to apply one or more transient DC voltages or potentials or one or more transient DC voltage or potential waveforms to the electrodes forming the first ion trap or ion guide in order to urge at least some ions upstream and/or downstream along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the first ion trap or ion guide. According to a less preferred embodiment AC or RF voltage means may be provided and may be arranged and adapted to apply two or more phase-shifted AC or RF voltages to electrodes forming the first ion trap or ion guide in order to urge at least some ions upstream and/or downstream along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the first ion trap or ion guide. The mass spectrometer preferably comprises means arranged and adapted in a mode of operation to maintain at least a portion of the first ion trap or ion guide at a pressure selected from the group consisting of: (i) >0.0001 mbar; (ii) >0.001 mbar; (iii) >0.01 mbar; (iv) >0.1 mbar; (v) >1 mbar; (vi) >10 mbar; (vii) >1 mbar; (viii) 0.0001-100 mbar; and (ix) 0.001-10 mbar. According to an embodiment first acceleration means are preferably provided which are arranged and adapted to accelerate ions into the first ion trap or ion guide wherein in a mode of operation at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the ions are caused to fragment or react upon entering the first ion trap or ion guide. The first acceleration means preferably accelerates ions from the ion mobility spectrometer or separator into the first ion trap or ion guide. A control system is preferably arranged and adapted to switch or repeatedly switch the potential difference through which ions pass prior to entering the first ion trap or ion guide between a relatively high fragmentation or reaction mode of operation wherein ions are substantially fragmented or reacted upon entering the first ion trap or ion guide and a relatively low fragmentation or reaction mode of operation wherein substantially fewer ions are fragmented or reacted or wherein substantially no ions are fragmented or reacted upon entering the first ion trap or ion guide. In the relatively high fragmentation or reaction mode of operation ions entering the first ion trap or ion guide are preferably accelerated through a potential difference selected from the group consisting of: (i) ≧10 V; (ii) ≧20 V; (iii) ≧30 V; (iv) ≧40 V; (v) ≧50 V; (vi) ≧60 V; (vii) ≧70 V; (viii) ≧80 V; (ix) ≧90 V; (x) ≧100 V; (xi) ≧110 V; (xii) ≧120 V; (xiii) ≧130 V; (xiv) ≧140 V; (xv) ≧150 V; (xvi) ≧160 V; (xvii) ≧170 V; (xviii) ≧180 V; (xix) ≧190 V; and (xx) ≧200 V. In the relatively low fragmentation or reaction mode of operation ions entering the first ion trap or ion guide are preferably accelerated through a potential difference selected from the group consisting of: (i) ≦20 V; (ii) ≦15 V; (iii) ≦10 V; (iv) ≦5V; and (v) ≦1V. The device which is preferably arranged downstream of the first ion trap or ion guide preferably comprises an ion mobility spectrometer or separator which is preferably arranged to separate ions according to their ion mobility. The device preferably comprises a gas phase electrophoresis device. The ion mobility spectrometer or separator is preferably arranged to temporally separate ions according to their ion mobility which emerge from or which have been transmitted or received from the first ion trap or ion guide and/or the second ion trap or guide. The ion mobility spectrometer or separator may comprise either: (i) a drift tube comprising one or more electrodes and means for maintaining an axial DC voltage gradient or a substantially constant or linear axial DC voltage gradient along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the drift tube; (ii) a multipole rod set or a segmented multipole rod set comprising a quadrupole rod set, a hexapole rod set, an octapole rod set or a rod set comprising more than eight rods; (iii) an ion tunnel or ion funnel comprising a plurality of electrodes or at least 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 electrodes having apertures through which ions are transmitted in use, wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the electrodes have apertures which are of substantially the same size or area or which have apertures which become progressively larger and/or smaller in size or in area; (iv) a stack or array of planar, plate or mesh electrodes, wherein the stack or array of planar, plate or mesh electrodes comprises a plurality or at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 planar, plate or mesh electrodes wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the planar, plate or mesh electrodes are arranged generally in the plane in which ions travel in use; or (v) a plurality of groups of electrodes arranged axially along the length of the ion trap or ion guide, wherein each group of electrodes comprises: (a) a first and a second electrode and means for applying a DC voltage or potential to the first and second electrodes in order to confine ions in a first radial direction within the device; and (b) a third and a fourth electrode and means for applying an AC or RF voltage to the third and fourth electrodes in order to confine ions in a second radial direction (which is preferably orthogonal to the first radial direction) within the device. According to the preferred embodiment the ion mobility spectrometer or separator preferably comprises an ion tunnel or ion funnel wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the electrodes have internal diameters or dimensions selected from the group consisting of: (i) ≦1.0 mm; (ii) ≦2.0 mm; (iii) ≦3.0 mm; (iv) ≦4.0 mm; (v) ≦5.0 mm; (vi) ≦6.0 mm; (vii) ≦7.0 mm; (viii) ≦8.0 mm; (ix) ≦9.0 mm; (x) ≦10.0 mm; and (xi) >10.0 mm. The ion mobility spectrometer or separator preferably further comprises a second AC or RF voltage means arranged and adapted to apply an AC or RF voltage to at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the plurality of electrodes of the ion mobility spectrometer or separator in order to confine ions radially within the ion mobility spectrometer or separator. The second AC or RF voltage means is preferably arranged and adapted to apply an AC or RF voltage having an amplitude selected from the group consisting of: (i) <50 V peak to peak; (ii) 50-100 V peak to peak; (iii) 100-150 V peak to peak; (iv) 150-200 V peak to peak; (v) 200-250 V peak to peak; (vi) 250-300 V peak to peak; (vii) 300-350 V peak to peak; (viii) 350-400 V peak to peak; (ix) 400-450 V peak to peak; (x) 450-500 V peak to peak; and (xi) >500 V peak to peak. The second AC or RF voltage means is preferably arranged and adapted to apply an AC or RF voltage having a frequency selected from the group consisting of: (i) <100 kHz; (ii) 100-200 kHz; (iii) 200-300 kHz; (iv) 300-400 kHz; (v) 400-500 kHz; (vi) 0.5-1.0 MHz; (vii) 1.0-1.5 MHz; (viii) 1.5-2.0 MHz; (ix) 2.0-2.5 MHz; (x) 2.5-3.0 MHz; (xi) 3.0-3.5 MHz; (xii) 3.5-4.0 MHz; (xiii) 4.0-4.5 MHz; (xiv) 4.5-5.0 MHz; (xv) 5.0-5.5 MHz; (xvi) 5.5-6.0 MHz; (xvii) 6.0-6.5 MHz; (xviii) 6.5-7.0 MHz; (xix) 7.0-7.5 MHz; (xx) 7.5-8.0 MHz; (xxi) 8.0-8.5 MHz; (xxii) 8.5-9.0 MHz; (xxiii) 9.0-9.5 MHz; (xxiv) 9.5-10.0 MHz; and (xxv) >10.0 MHz. According to the preferred embodiment the ion mobility spectrometer or separator preferably comprises second transient DC voltage means arranged and adapted to apply one or more transient DC voltages or potentials or one or more transient DC voltage or potential waveforms to electrodes forming the ion mobility spectrometer or separator in order to urge at least some ions upstream and/or downstream along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the ion mobility spectrometer or separator. According to a less preferred embodiment the ion mobility spectrometer or separator may comprise AC or RF voltage means arranged and adapted to apply two or more phase-shifted AC or RF voltages to electrodes forming the ion mobility spectrometer or separator in order to urge at least some ions upstream and/or downstream along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the ion mobility spectrometer or separator. The mass spectrometer preferably comprises means arranged and adapted in a mode of operation to maintain at least a portion or substantially the whole of the ion mobility spectrometer or separator at a pressure selected from the group consisting of: (i) >0.001 mbar; (ii) >0.01 mbar; (iii) >0.1 mbar; (iv) >1 mbar; (v) >10 mbar; (vi) >100 mbar; (vii) 0.001-100 mbar; (viii) 0.01-10 mbar; and (ix) 0.1-1 mbar. According to a less preferred embodiment the device arranged downstream of the first ion trap or ion guide may comprise a Field Asymmetric Ion Mobility Spectrometer (“FAIMS”) device which is arranged to separate ions according to their rate of change of ion mobility with electric field strength. The Field Asymmetric Ion Mobility Spectrometer device may comprise at least a first electrode and a second electrode. Ions may be arranged to be received, in use, between the first and second electrodes. According to an embodiment the FAIMS device may further comprise means for applying: (i) an asymmetric periodic voltage waveform to the first and/or second electrodes, wherein the asymmetric periodic voltage waveform has a peak positive voltage and a peak negative voltage; and (ii) a DC compensation voltage to the first and/or second electrodes, wherein the DC compensation voltage preferably acts to counterbalance or counteract a force which would otherwise cause desired ions to drift towards the first and/or second electrodes. According to an embodiment the second ion trap or ion guide may comprise either: (i) a multipole rod set or a segmented multipole rod set comprising a quadrupole rod set, a hexapole rod set, an octapole rod set or a rod set comprising more than eight rods; (ii) an ion tunnel or ion funnel comprises a plurality of electrodes or at least 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 electrodes having apertures through which ions are transmitted in use, wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the electrodes have apertures which are of substantially the same size or area or which have apertures which become progressively larger and/or smaller in size or in area; (iii) a stack or array of planar, plate or mesh electrodes, wherein the stack or array of planar, plate or mesh electrodes comprises a plurality or at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 planar, plate or mesh electrodes arranged generally in the plane in which ions travel in use, wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the planar, plate or mesh electrodes are arranged generally in the plane in which ions travel in use; or (iv) an ion trap or ion guide comprising a plurality of groups of electrodes arranged axially along the length of the ion trap or ion guide, wherein each group of electrodes comprises: (a) a first and a second electrode and means for applying a DC voltage or potential to the first and second electrodes in order to confine ions in a first radial direction within the ion guide; and (b) a third and a fourth electrode and means for applying an AC or RF voltage to the third and fourth electrodes in order to confine ions in a second radial direction (which is preferably orthogonal to the first radial direction) within the ion guide. According to an embodiment the second ion trap or ion guide preferably comprises an ion tunnel or ion funnel wherein at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the electrodes have internal diameters or dimensions selected from the group consisting of: (i) ≦1.0 mm; (ii) ≦2.0 mm; (iii) ≦3.0 mm; (iv) ≦4.0 mm; (v) ≦5.0 mm; (vi) ≦6.0 mm; (vii) ≦7.0 mm; (viii) ≦8.0 mm; (ix) ≦9.0 mm; (x) ≦10.0 mm; and (xi) >10.0 mm. The second ion trap or ion guide may further comprise third AC or RF voltage means arranged and adapted to apply an AC or RF voltage to at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the plurality of electrodes of the second ion trap or ion guide in order to confine ions radially within the second ion trap or ion guide. The third AC or RF voltage means is preferably arranged and adapted to apply an AC or RF voltage having an amplitude selected from the group consisting of: (i) <50 V peak to peak; (ii) 50-100 V peak to peak; (iii) 100-150 V peak to peak; (iv) 150-200 V peak to peak; (v) 200-250 V peak to peak; (vi) 250-300 V peak to peak; (vii) 300-350 V peak to peak; (viii) 350-400 V peak to peak; (ix) 400-450 V peak to peak; (x) 450-500 V peak to peak; and (xi) >500 V peak to peak. The third AC or RF voltage means is preferably arranged and adapted to apply an AC or RF voltage having a frequency selected from the group consisting of: (i) <100 kHz; (ii) 100-200 kHz; (iii) 200-300 kHz; (iv) 300-400 kHz; (v) 400-500 kHz; (vi) 0.5-1.0 MHz; (vii) 1.0-1.5 MHz; (viii) 1.5-2.0 MHz; (ix) 2.0-2.5 MHz; (x) 2.5-3.0 MHz; (xi) 3.0-3.5 MHz; (xii) 3.5-4.0 MHz; (xiii) 4.0-4.5 MHz; (xiv) 4.5-5.0 MHz; (xv) 5.0-5.5 MHz; (xvi) 5.5-6.0 MHz; (xvii) 6.0-6.5 MHz; (xviii) 6.5-7.0 MHz; (xix) 7.0-7.5 MHz; (xx) 7.5-8.0 MHz; (xxi) 8.0-8.5 MHz; (xxii) 8.5-9.0 MHz; (xxiii) 9.0-9.5 MHz; (xxiv) 9.5-10.0 MHz; and (xxv) >10.0 MHz. The second ion trap or ion guide is preferably arranged and adapted to receive a beam or group of ions and to convert or partition the beam or group of ions such that a plurality or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 separate packets of ions are confined and/or isolated in the second ion trap or ion guide at any particular time. Each packet of ions is preferably separately confined and/or isolated in a separate real axial potential well formed within the second ion trap or ion guide. According to an embodiment the mass spectrometer preferably comprises means arranged and adapted to urge at least some ions upstream and/or downstream through or along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the second ion trap or ion guide in a mode of operation. According to an embodiment the mass spectrometer further comprises third transient DC voltage means arranged and adapted to apply one or more transient DC voltages or potentials or one or more transient DC voltage or potential waveforms to the electrodes forming the second ion trap or ion guide in order to urge at least some ions upstream and/or downstream along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the second ion trap or ion guide. According to a less preferred embodiment the mass spectrometer may further comprise AC or RF voltage means arranged and adapted to apply two or more phase-shifted AC or RF voltages to electrodes forming the second ion trap or ion guide in order to urge at least some ions upstream and/or downstream along at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the axial length of the second ion trap or ion guide. The mass spectrometer preferably further comprises means arranged and adapted in a mode of operation to maintain at least a portion of the second ion trap or ion guide at a pressure selected from the group consisting of: (i) >0.0001 mbar; (ii) >0.001 mbar; (iii) >0.01 mbar; (iv) >0.1 mbar; (v) >1 mbar; (vi) >10 mbar; (vii) >1 mbar; (viii) 0.0001-100 mbar; and (ix) 0.001-10 mbar. According to an embodiment the mass spectrometer preferably further comprises second acceleration means arranged and adapted to accelerate ions into the second ion trap or ion guide wherein in a mode of operation at least 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 100% of the ions are caused to fragment or react upon entering the second ion trap or ion guide. The second acceleration means preferably accelerates ions from the ion mobility spectrometer or separator into the second ion trap or ion guide. The mass spectrometer preferably comprises a control system arranged and adapted to switch or repeatedly switch the potential difference through which ions pass prior to entering the second ion trap or ion guide between a relatively high fragmentation or reaction mode of operation wherein ions are substantially fragmented or reacted upon entering the second ion trap or ion guide and a relatively low fragmentation or reaction mode of operation wherein substantially fewer ions are fragmented or reacted or wherein substantially no ions are fragmented or reacted upon entering the second ion trap or ion guide. In the relatively high fragmentation or reaction mode of operation ions entering the second ion trap or ion guide are preferably accelerated through a potential difference selected from the group consisting of: (i) ≧10 V; (ii) ≧20 V; (iii) ≧30 V; (iv) ≧40 V; (v) ≧50 V; (vi) ≧60 V; (vii) ≧70 V; (viii) ≧80 V; (ix) ≧90 V; (x) ≧100 V; (xi) ≧110 V; (xii) ≧120 V; (xiii) ≧130 V; (xiv) ≧140 V; (xv) ≧150 V; (xvi) ≧160 V; (xvii) ≧170 V; (xviii) ≧180 V; (xix) ≧190 V; and (xx) ≧200 V. In the relatively low fragmentation or reaction mode of operation ions entering the second ion trap or ion guide are preferably accelerated through a potential difference selected from the group consisting of: (i) ≦20 V; (ii) ≦15 V; (iii) ≦10 V; (iv) ≦5V; and (v) ≦1V. According to an embodiment the mass spectrometer preferably further comprises an ion gate or deflection system arranged upstream and/or downstream of the device arranged downstream of the first ion trap or ion guide. The ion gate or deflection system preferably attenuates ions exiting the device which have an undesired transit time through the device or an undesired ion mobility, mass to charge ratio or rate of change of ion mobility with electric field strength. A first mass filter or mass analyser may be arranged upstream and/or downstream of the first ion trap or ion guide. The first mass filter or mass analyser is preferably selected from the group consisting of: (i) a quadrupole rod set mass filter or mass analyser; (ii) a Time of Flight mass filter or mass analyser; (iii) a Wein filter; and (iv) a magnetic sector mass filter or mass analyser. In a mode of operation the first mass filter or mass analyser may be operated in a substantially non-resolving or ion guiding mode of operation. In another mode of operation the first mass filter or mass analyser may be scanned or a mass to charge ratio transmission window of the first mass filter or mass analyser may be varied with time. A second mass filter or mass analyser may be arranged upstream and/or downstream of the second ion trap or ion guide. The second mass filter or mass analyser is preferably selected from the group consisting of: (i) a quadrupole rod set mass filter or mass analyser; (ii) a Time of Flight mass filter or mass analyser; (iii) a Wein filter; and (iv) a magnetic sector mass filter or analyser. In a mode of operation the second mass filter or mass analyser may be operated in a substantially non-resolving or ion guiding mode of operation. In another mode of operation the second mass filter or mass analyser is preferably scanned or a mass to charge ratio transmission window of the second mass filter or mass analyser is preferably varied with time. According to an embodiment in a mode of operation the first mass filter or mass analyser and/or the second mass filter or mass analyser may be scanned or a mass to charge ratio transmission window of the first mass filter or mass analyser and/or the second mass filter or mass analyser may be varied with time preferably in synchronism with the operation of the device or the ion mobility or rate of change of ion mobility with electric field strength of ions emerging from and/or being transmitted to the device. According to an embodiment in a mode of operation the first mass filter or mass analyser is preferably scanned or a mass to charge ratio transmission window of the first mass filter or mass analyser is preferably varied with time in synchronism with the operation of the second mass filter or mass analyser. The mass spectrometer may comprise a collision, fragmentation or reaction device arranged and adapted to fragment ions by Collision Induced Dissociation (“CID”). According to a less preferred embodiment the mass spectrometer may comprise a collision, fragmentation or reaction device selected from the group consisting of: (i) a Surface Induced Dissociation (“SID”) fragmentation device; (ii) an Electron Transfer Dissociation fragmentation device; (iii) an Electron Capture Dissociation fragmentation device; (iv) an Electron Collision or Impact Dissociation fragmentation device; (v) a Photo Induced Dissociation (“PID”) fragmentation device; (vi) a Laser Induced Dissociation fragmentation device; (vii) an infrared radiation induced dissociation device; (viii) an ultraviolet radiation induced dissociation device; (ix) a nozzle-skimmer interface fragmentation device; (x) an in-source fragmentation device; (xi) an ion-source Collision Induced Dissociation fragmentation device; (xii) a thermal or temperature source fragmentation device; (xiii) an electric field induced fragmentation device; (xiv) a magnetic field induced fragmentation device; (xv) an enzyme digestion or enzyme degradation fragmentation device; (xvi) an ion-ion reaction fragmentation device; (xvii) an ion-molecule reaction fragmentation device; (xviii) an ion-atom reaction fragmentation device; (xix) an ion-metastable ion reaction fragmentation device; (xx) an ion-metastable molecule reaction fragmentation device; (xxi) an ion-metastable atom reaction fragmentation device; (xxii) an ion-ion reaction device for reacting ions to form adduct or product ions; (xxiii) an ion-molecule reaction device for reacting ions to form adduct or product ions; (xxiv) an ion-atom reaction device for reacting ions to form adduct or product ions; (xxv) an ion-metastable ion reaction device for reacting ions to form adduct or product ions; (xxvi) an ion-metastable molecule reaction device for reacting ions to form adduct or product ions; and (xxvii) an ion-metastable atom reaction device for reacting ions to form adduct or product ions. According to an embodiment the collision, fragmentation or reaction device preferably forms at least part of the first ion trap or ion guide and/or the device and/or the second ion trap or ion guide. According to an alternative embodiment the collision, fragmentation or reaction device may be arranged upstream and/or downstream of the first ion trap or ion guide and/or the device and/or the second ion trap or ion guide. The mass spectrometer preferably comprises an ion source. The ion source is preferably selected from the group consisting of: (i) an Electrospray ionisation (“ESI”) ion source; (ii) an Atmospheric Pressure Photo Ionisation (“APPI”) ion source; (iii) an Atmospheric Pressure Chemical Ionisation (“APCI”) ion source; (iv) a Matrix Assisted Laser Desorption Ionisation (“MALDI”) ion source; (v) a Laser Desorption Ionisation (“LDI”) ion source; (vi) an Atmospheric Pressure Ionisation (“API”) ion source; (vii) a Desorption Ionisation On Silicon (“DIOS”) ion source; (viii) an Electron Impact (“EI”) ion source; (ix) a Chemical Ionisation (“CI”) ion source; (x) a Field Ionisation (“FI”) ion source; (xi) a Field Desorption (“FD”) ion source; (xii) an Inductively Coupled Plasma (“ICP”) ion source; (xiii) a Fast Atom Bombardment (“FAB”) ion source; (xiv) a Liquid Secondary Ion Mass Spectrometry (“LSIMS”) ion source; (xv) a Desorption Electrospray Ionisation (“DESI”) ion source; (xvi) a Nickel-63 radioactive ion source; (xvii) an Atmospheric Pressure Matrix Assisted Laser Desorption Ionisation (“AP-MALDI”) ion source; and (xviii) a Thermospray ion source. The ion source may comprise a pulsed or continuous ion source. The mass spectrometer preferably further comprises separation means for separating molecules from a mixture of other molecules prior to being ionised. The separation means is preferably selected from the group consisting of: (i) High Performance Liquid Chromatography (“HPLC”); (ii) anion exchange; (iii) anion exchange chromatography; (iv) cation exchange; (v) cation exchange chromatography; (vi) ion pair reversed-phase chromatography; (vii) chromatography; (vii) single dimensional electrophoresis; (ix) multi-dimensional electrophoresis; (x) size exclusion; (xi) affinity; (xii) reverse phase chromatography; (xiii) Capillary Electrophoresis Chromatography (“CEC”); (xiv) electrophoresis; (xv) ion mobility separation; (xvi) Field Asymmetric Ion Mobility Separation (“FAIMS”); and (xvi) capillary electrophoresis. According to an embodiment the ion source is provided with an eluent over a period of time. The eluent is preferably separated from a mixture by means of liquid chromatography or capillary electrophoresis. According to a less preferred embodiment the ion source may be provided with an eluent over a period of time wherein the eluent has been separated from a mixture by means of gas chromatography. The mass spectrometer preferably further comprises a mass analyser. The mass analyser is preferably arranged downstream of the second ion trap or ion guide. The mass analyser is preferably selected from the group consisting of: (i) a quadrupole mass analyser; (ii) a 2D or linear quadrupole mass analyser; (iii) a Paul or 3D quadrupole mass analyser; (iv) a Penning trap mass analyser; (v) an ion trap mass analyser; (vi) a magnetic sector mass analyser; (vii) Ion Cyclotron Resonance (“ICR”) mass analyser; (viii) a Fourier Transform Ion Cyclotron Resonance (“FTICR”) mass analyser; (ix) an electrostatic or orbitrap mass analyser; (x) a Fourier Transform electrostatic or orbitrap mass analyser; (xi) a Fourier Transform mass analyser; (xii) a Time of Flight mass analyser; (xiii) an orthogonal acceleration Time of Flight mass analyser; (xiv) an axial acceleration Time of Flight mass analyser; and (xv) a quadrupole rod set mass filter or mass analyser. According to an aspect of the present invention there is provided a method of mass spectrometry comprising: providing a first ion trap or ion guide; separating ions according to their ion mobility or rate of change of ion mobility with electric field strength in a device, the device being arranged downstream of the first ion trap or ion guide; providing a second ion trap or ion guide arranged downstream of the device; and passing or transmitting ions from the second ion trap or ion guide to the device. According to an aspect of the present invention there is provided a mass spectrometer comprising: a device for separating ions according to their ion mobility or rate of change of ion mobility with electric field strength; wherein in a mode of operation at a first time ions are passed in a first direction through the device and wherein at a second later time ions are passed in second direction through the device, wherein the second direction is different or opposed to the first direction. According to an aspect of the present invention there is provided a method of mass spectrometry comprising: separating ions according to their ion mobility or rate of change of ion mobility with electric field strength in a device; and passing ions at a first time in a first direction through the device and passing ions at a second later time in a second direction through the device, wherein the second direction is different or opposed to the first direction. According to an aspect of the present invention there is provided a mass spectrometer comprising: a device for separating ions according to their ion mobility or rate of change of ion mobility with electric field strength; an ion trap or ion guide which is arranged to receive ions emerging from the device and which are being transmitted in a first direction; and acceleration means arranged and adapted to cause ions emerging from the device to be fragmented or to react upon entering the ion trap or ion guide so that a plurality of fragment, daughter, product or adduct ions are formed; wherein in a mode of operation at least some of the plurality of fragment, daughter, product or adduct ions are transmitted or passed from the ion trap or ion guide to the device in a second direction which is different from or opposed to the first direction. According to an aspect of the present invention there is provided a method of mass spectrometry comprising: separating ions according to their ion mobility or rate of change of ion mobility with electric field strength in a device; and providing an ion trap or ion guide which is arranged to receive ions emerging from the device and which are being transmitted in a first direction; accelerating ions emerging from the device so that the ions are fragmented or react upon entering the ion trap or ion guide so that a plurality of fragment, daughter, product or adduct ions are formed; and transmitting or passing at least some of the plurality of fragment, daughter, product or adduct ions from the ion trap or ion guide to the device in a second direction which is different from or opposed to the first direction. According to another aspect of the present invention there is provided a mass spectrometer comprising: a first ion trap or ion guide; a device for separating ions according to their ion mobility or rate of change of ion mobility with electric field strength, the device being arranged downstream of the first ion trap or ion guide; a second ion trap or ion guide arranged downstream of the first ion trap or guide; wherein in a mode of operation ions are passed from the first ion trap or ion guide to the device and onwards to the second ion trap or ion guide whereupon at least some of the ions or at least some fragment, daughter, product or adduct ions derived from the ions are then passed from the second ion trap or ion guide to the device and onwards to the first ion trap or ion guide. According to another aspect of the present invention there is provided a method of mass spectrometry comprising: providing a first ion trap or ion guide; providing a device for separating ions according to their ion mobility or rate of change of ion mobility with electric field strength, the device being arranged downstream of the first ion trap or ion guide; providing a second ion trap or ion guide arranged downstream of the first ion trap or guide; and passing ions from the first ion trap or ion guide to the device and onwards to the second ion trap or ion guide; and then passing at least some of the ions or at least some fragment, daughter, product or adduct ions derived from the ions from the second ion trap or ion guide to the device and onwards to the first ion trap or ion guide. According to the preferred embodiment a mass spectrometer is provided which comprises only a relatively few stages. However, the mass spectrometer is preferably nonetheless capable of performing relatively complex experiments which conventionally would require a mass spectrometer comprising a greater number of stages to perform. For example, a mass spectrometer according to an embodiment of the present invention comprising LC-IMS-MS stages is preferably able to perform similar experiments to those which may be performed using a conventional mass spectrometer comprising LC-IMS-IMS-MS or LC-IMS-CID-IMS-MS stages. A mass spectrometer according to the preferred embodiment preferably comprising a single ion mobility separation stage is preferably also able to perform particularly complex experiments which would conventionally require a mass spectrometer comprising, for example, LC-IMS-CID-IMS-CID-MS stages to perform wherein second generation fragment or daughter ions are produced and are subsequently mass analysed. The conventional approach to designing a mass spectrometer has been to provide multiple ion mobility separation and ion fragmentation stages in series. Ions pass sequentially through multiple stages from one stage of the mass spectrometer to the next. The number of ion mobility separation and ion fragmentation stages which need to be provided is determined by the desired capability of the mass spectrometer. The conventional approach leads to a mass spectrometer comprising a large number of discrete stages and which is relatively lengthy and complex. The mass spectrometer design is relatively inflexible and the range of experiments that can be performed by such a conventional mass spectrometer is limited by the number and arrangement of the various stages. A conventional mass spectrometer comprising a single ion fragmentation stage is not, for example, able to produce second generation fragment or daughter ions. In addition when only part of the capability of a mass spectrometer is utilised (e.g. an LC-IMS-MS analysis is required using a mass spectrometer comprising LC-IMS-CID-IMS-MS stages) then the extra stages are unnecessary and can compromise performance. For example, the elution profile of an ion species leaving a first ion mobility separation region may not be able to be measured directly since the ions may have to pass through an additional ion mobility separation region before reaching an ion detector. Conventional mass spectrometers comprising numerous multiple stages in series also require a more complex calibration procedure and this can lead to less certainty in the results. A mass spectrometer according to the preferred embodiment preferably enables a more compact and substantially more flexible mass spectrometer to be provided. A particularly preferred aspect of the present invention is that ions in a mode of operation are passed back upstream at least once through a single ion mobility separation stage or section. According to an embodiment a mass spectrometer is preferably provided comprising an ionisation source and an ion mobility spectrometer or separator comprising an RF ion guide wherein ions are confined near to the central axis. Ions are preferably propelled along the length of the ion mobility spectrometer or separator from one end to the other end in either direction. The mass spectrometer preferably further comprises an ion detector. In a preferred embodiment ions are preferably propelled along the axis of the ion mobility spectrometer or separator first in one direction (e.g. downstream) and then preferably in the opposite or reverse direction (e.g. upstream). The ions are preferably separated according to their ion mobility in at least one pass through the length of the ion mobility spectrometer or separator. Each time ions are passed along or through the length of the ion mobility spectrometer or separator the ions may or may not be separated according to their ion mobility.
{'bibliographic_information': "{'country': 'US', 'doc-number': '14617050', 'kind': 'B2', 'date': '20150209', 'extension': '19', 'disclaimer_text': 'This patent is subject to a terminal disclaimer.', 'invention_title': 'Mass spectrometer'}", 'source_file': './grant/2016/ipg161213.xml', 'source_data': 'USPTO-Grant'}