Separating ions in an ion trap

Abstract

A method is disclosed comprising: trapping ions in an ion trap (40); applying a first force on the ions within the ion trap in a first direction, said force having a magnitude that is dependent upon the value of a physicochemical property of the ions; applying a second force on these ions in the opposite direction so that the ions separate according to the physicochemical property value as a result of the first and second forces; and then pulsing or driving ions out of one or more regions of the ion trap.

Claims

1. A method of filtering ions according to at least one physicochemical property, comprising: (i) trapping ions in a linear ion trap; (ii) spatially separating the ions along a longitudinal axis of the ion trap according to said at least one physicochemical property by applying a first force on the ions within the ion trap in a first direction, wherein said physicochemical property is mass-to-charge ratio, said force having a magnitude that is dependent upon a value of said at least one physicochemical property of the ions, and applying a second force on these ions in the opposite direction, such that said first and second forces are counterbalanced at different locations within the ion trap for ions having different values of said physicochemical property and such that the axial motion of said ions is stopped such that said ions having different values of said physicochemical property are trapped within separate, different axial regions of the ion trap according to a known, determined or estimated physicochemical property distribution; (iii) performing said step of spatially separating the ions within the ion trap; (iv) selecting a desired first value, or first range of values, of said physicochemical property for first ions desired to be ejected from the ion trap; (v) determining an axial distance along the longitudinal axis of the ion trap at which said first ions are located from said known, determined or estimated physicochemical property distribution, wherein the axial distance is not at an end of the linear ion trap; and then (vi) driving said first ions out of the ion trap after performing step (v).

2. The method of claim 1, wherein ions of interest having the selected value, or range of values, of said physicochemical property are ejected from the ion trap and transmitted to an ion analyser or ion storage device and wherein unwanted ions having a value, or range of values, of said physicochemical property that are not of interest are ejected from the ion trap and discarded or neutralised.

3. The method of claim 1, wherein the ion trap has an ion trapping region having a longitudinal axis and wherein the ions are driven out of a region that is not directly adjacent an exit at a longitudinal end of the ion trapping region.

4. The method of claim 1, wherein the ion trap is an elongated ion trap and the step of driving said first ions out of the ion trap is performed by radially ejecting the first ions from the ion trap.

5. A method of mass spectrometry comprising: trapping ions in a linear ion trap; and then spatially separating the ions along a longitudinal axis of the ion trap according to mass to charge ratio so that ions having different values of mass to charge ratio are trapped in separate, different axial regions of the ion trap, wherein said step of spatially separating the ions comprises applying a first force on the ions within the ion trap in a first direction, said force having a magnitude that is dependent upon the value of said mass to charge ratio of the ions, and applying a second force on these ions in the opposite direction; wherein said first and second forces are counterbalanced at different locations within the ion trap for ions having different values of mass to charge ratio such that said ions having different values of mass to charge ratio are axially stopped and trapped within separate, different, axial regions of the ion trap; and then driving or pulsing first trapped ions having a first mass to charge ratio out of a first of said axial regions of the ion trap and into a discontinuous ion analyser at a first time, whilst retaining other ions trapped in their respective separate, different, axial regions in the ion trap; analysing said first ions in a first cycle of said discontinuous ion analyser; driving or pulsing second trapped ions having a second mass to charge ratio out of a second, different one of said axial regions of the ion trap and into the discontinuous ion analyser at a second, subsequent time, whilst retaining other ions trapped in their respective separate, different, axial regions in the ion trap, wherein said second axial region is further away from an exit of the ion trap than the first axial region from which said first ions are driven or pulsed; and analysing said second ions in a different cycle of said discontinuous ion analyser; wherein the discontinuous ion analyser is a Time of Flight mass analyser, wherein all of the ions driven or pulsed out of the ion trap at the first time are analysed in said first cycle, and wherein all of the ions driven or pulsed out of the ion trap at the second time are analysed in said second cycle.

6. The method of claim 5, wherein each of the steps of driving or pulsing trapped ions out of a region of the ion trap and into a discontinuous ion analyser, whilst retaining other ions trapped in the ion trap, comprises travelling an electric potential along at least a portion of the ion trap so as to drive the ions out of the ion trap, wherein said electric potential is traveled along a first length of the ion trap in order to drive said first ions out of the ion trap, and said electric potential is subsequently traveled along a second, different length of the ion trap in order to drive said second ions out of the ion trap.

7. The method of claim 6, wherein the first length extends from a first location in the ion trap to the exit of the ion trap, whereas the second length extends from a second location in the ion trap to the exit of the ion trap, wherein the second location is further from the exit than the first location.

8. The method of claim 5, wherein said step of driving or pulsing first trapped ions out of the ion trap comprises driving or pulsing ions axially out of the ion trap; and wherein said step of driving or pulsing second trapped ions out of the ion trap comprises driving or pulsing ions axially out of the ion trap.

9. A device configured to perform the method of claim 1, comprising: an ion separator; an ion driving device; and a controller configured to: control the ion trap to trap ions therein; control the ion separator so as to spatially separate the ions within the ion trap according to said at least one physicochemical property so that ions having different values of said physicochemical property are trapped in different regions of the ion trap according a known, determined or estimated physicochemical property distribution; determine a first region of the ion trap in which ions having a preselected first value, or first range of values, of said physicochemical property are located based on said physicochemical property distribution; and then to control the ion driving device so as to drive first ions trapped in said first region out of the ion trap.

10. An ion trap for spatially separating ions according to mass to charge ratio, wherein the ion trap comprises: a plurality of electrodes; at least one AC or RF voltage supply; at least one DC voltage supply; and a controller and circuitry arranged and configured to: control the at least one AC or RF voltage supply so as to apply one or more AC or RF voltages to said electrodes so as to generate a pseudo-potential electric field that urges ions in a first direction; control the at least one DC voltage supply so as to apply one or more DC voltages to said electrodes so as to generate a DC electric field that urges ions in a second direction opposite to the first direction; and control the DC voltage supply and/or AC/RF voltage supply so as to pulse or drive ions out of one or more regions of the ion trap; wherein the ions are spatially separated within the ion trap according to mass to charge ratio so that the ions are dispersed along the ion trap according to their mass to charge ratio values without the spatially separated trapped ions being separated by potential barriers; and control the DC voltage supply and/or AC/RF voltage supply to perform a plurality of cycles of operation, wherein each cycle comprises: driving or pulsing trapped ions out of a trapping region of the ion trap, whilst retaining other ions trapped in other trapping regions of the ion trap in their respective trapping regions; wherein for each cycle of operation subsequent to a first cycle of operation, ions are driven or pulsed out of the ion trap from a trapping region that is further away from an exit of the ion trap than a trapping region from which ions were driven out in a previous cycle of operation.

11. The ion trap of claim 10, wherein said controller and circuitry are configured to control said AC or RF voltage supply and said DC voltage supply such that said pseudo-potential electric field and said DC electric field are counterbalanced at different locations within the ion trap for ions having different values of mass to charge ratio such that said ions having different values of mass to charge ratio are axially stopped and trapped within separate, different, axial regions of the ion trap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a graph of the positions at which ions of different mass to charge ratio may be trapped within the ion filter of a preferred embodiment of the present invention;

(3) FIG. 2 shows the spatial width over which any given ion will be trapped in the ion filter device;

(4) FIG. 3 shows the mass to charge ratios of the ions trapped in the filter device as a function of position in the filter;

(5) FIG. 4 shows the range of mass to charge ratios of the ions trapped in the filter device as a function of position in the filter;

(6) FIG. 5 shows an embodiment of the present invention wherein ions are axially ejected from the filter device;

(7) FIG. 6 shows an embodiment of the present invention wherein ions are orthogonally ejected from the filter device;

(8) FIG. 7A shows a schematic of part of a prior art ion mobility spectrometer comprising an ion trap and an IMS device, and FIG. 7B shows a schematic of part of an ion mobility spectrometer according to an embodiment of the present invention comprising an ion trap and an IMS device;

(9) FIG. 8 shows a graph of the DC voltage potential profile along the ion trap of FIG. 7B;

(10) FIG. 9 shows a graph of the RF voltage potential profile along the ion trap of FIG. 7B;

(11) FIG. 10 shows a graph of the position along the ion trap at which the total potential is minimum for ions of different mass to charge ratios;

(12) FIG. 11 shows the spatial width over which any given ion will be trapped in the ion trap of FIG. 7B;

(13) FIG. 12 shows the temporal spread over which an ion of any given mass to charge ratio will be ejected from the ion trap;

(14) FIG. 13 shows a plot of drift time through the IMS device of FIG. 7B as a function of mass to charge ratio;

(15) FIG. 14 shows two plots of resolution R against mass to charge ratio of the ions for the instruments of FIGS. 7A and 7B;

(16) FIG. 15 shows a schematic of part of a ToF mass spectrometer;

(17) FIG. 16 shows the mass of ions analysed by the ToF mass analyser as a function of the pulse number in the pusher sequence;

(18) FIG. 17 shows the position along the ion trap from which ions are swept out of, as a function of the pulse number in the pusher sequence;

(19) FIG. 18 shows the length of the region of the ion trap from which ions are extracted, as a function of the pulse number in the pusher sequence; and

(20) FIG. 19 shows a schematic of an ion trap according to an embodiment of the invention.

DETAILED DESCRIPTION

(21) Embodiments of the present invention provide an ion trap that separates ions within the ion trap according to a physicochemical property, e.g., mass to charge ratio.

(22) A first set of embodiments provide a method and apparatus for mass filtering ions by trapping the ions in an ion trap of an ion filtering device, spatially separating the ions within the ion trap according to their mass to charge ratios, and then selectively ejecting ions from one or more sections of the ion trap such that ions having only the desired mass to charge ratios are onwardly transmitted.

(23) A second set of embodiments provide a method and apparatus for improving the resolution of ion mobility measurements by trapping ions in an ion trap, spatially separating the ions within the ion trap according to their mass to charge ratios, and then pulsing the separated ions into an ion mobility separator (IMS) in a single pulse.

(24) A third set of embodiments provide a method and apparatus for improving the analysis of ions in a discontinuous ion analyser by trapping ions in an ion trap, spatially separating the ions within the ion trap according to their mass to charge ratios, and then driving or pulsing ions out of different regions of the ion trap into the discontinuous ion analyser at different times.

(25) The embodiments described herein separate the ions within an ion trap according to mass to charge ratios. The ions may be spatially separated within the ion trap using a combination of a pseudo-potential electric field and a DC electric field. For example, the ion trap may comprise a stacked ring ion guide (or other ion guide) and a controller and related electronic circuitry may control a voltage supply to apply an axial DC voltage V.sub.dc(z) along the length of the ion trap having the following profile:
V.sub.dc(z)=d.sub.1z+d.sub.2z.sup.2(equation 1)
where z is the distance from the end of the ion trap, and d.sub.1 and d.sub.2 are the coefficients of the linear and quadratic terms.

(26) Similarly, the controller and related electronic circuitry may control a voltage supply to apply RF voltages to the electrodes so that the RF voltage function along the ion trap, V.sub.x(z), is as follows:
V.sub.x(z)=p.sub.1z+p.sub.2z.sup.2(equation 2)
Where p.sub.1 and p.sub.2 are the coefficients of the linear and quadratic terms.

(27) This results in an axial pseudo-potential profile, V.sub.ps(z,m), given by:

(28) $\begin{array}{cc}{V}_{ps}\left(z,m\right)=\frac{{q\left(\frac{d}{\mathrm{dz}}\left[V_x\left(z\right)\right]\right)}^2}{4\mathrm{Mm}{}^2}& \left(\mathrm{equation}3\right)\end{array}$
where M is the atomic mass unit, m is the mass to charge ratio of a given ion, and is the applied RF voltage frequency.

(29) Let

(30) $\begin{array}{cc}\left(\right)=\frac{q}{4M{}^2}& \left(\mathrm{equation}4\right)\end{array}$
Therefore, the pseudo-potential generated along the ion trap is given by:

(31) $\begin{array}{cc}{V}_{ps}\left(z,m\right)=\frac{{\left(\right)\left[p_1+2p_{2}z\right]}^2}{m}& \left(\mathrm{equation}5\right)\end{array}$

(32) The total potential for a given mass to charge ratio m and distance z from the end of the ion trap, V.sub.tot, is simply the sum of V.sub.DC and V.sub.ps, and is therefore given by:

(33) $\begin{array}{cc}{V}_{\mathrm{tot}}=d_{1}z+d_2{z}^2+\frac{{\left(\right)\left[p_1+2p_{2}z\right]}^2}{m}& \left(\mathrm{equation}6\right)\end{array}$

(34) Ions of any given mass to charge ratio m will be located at a distance z from the end of the ion trap, at a distance where V.sub.tot is a minimum for that mass. Differentiating equation 6 and solving for the minima gives equation 7 below:

(35) $\begin{array}{cc}z=-\frac{1}{2}\frac{d_1}{d_2}\left[\frac{4\left(\right)\frac{p_1{p}_2}{d_1}+m}{4\left(\right)\frac{p_2^2}{d_2}+m}\right]& \left(\mathrm{equation}7\right)\end{array}$
This equation can be used to calculate the position z at which the pseudo-potential minimum for any given ion is located, and hence the position at which that ion will remain trapped.

(36) For example, the curvature C(m) of the minima may be calculated from the second differential with respect to z of V.sub.tot in equation 6, giving:

(37) $\begin{array}{cc}C\left(m\right)=2d_2+\frac{8p_2^2\left(\right)}{m}& \left(\mathrm{equation}8\right)\end{array}$

(38) As long as the value of C(m) is greater than zero then equation 7 gives the position of the minimum at which ions of a given mass to charge ratio will be trapped.

(39) By optimisation of the DC potential parameters, d.sub.1 and d.sub.2, and the RF potential parameters, p.sub.1 and p.sub.2, it is possible to obtain reasonable voltage levels and good spatial separation of the ions within the ion trap.

(40) As described above, this ion trap may be used to separate ion according to a physicochemical property (e.g., mass to charge ratio) in the first, second and third sets of embodiments described herein.

(41) Various embodiments of said first set of embodiments will now be described, in which the ion trap is used in an ion filter which selectively ejects ions from one or more sections of the ion trap so that ions having only the desired values of a physicochemical property (e.g., mass to charge ratio) are onwardly transmitted.

(42) As described above, with optimisation of the DC potential parameters, d.sub.1 and d.sub.2, and the RF potential parameters, p.sub.1 and p.sub.2, it is possible to obtain reasonable voltage levels and good spatial separation of the ions within the ion trap. In embodiments described below the optimisation of the parameters assumes an ion trap length of 0.2 m and a distribution of ions in the trap for ions having a range of mass to charge ratios of 100 to 2000 Da.

(43) FIG. 1 shows a graph of the position z at which the total potential V.sub.tot is minimum as a function of ion mass to charge ratio, for ions of mass to charge ratio m between 100 and 2000 Da and for optimised values of d.sub.1=1254 and d.sub.2=2280 which give a maximum V.sub.dc=150 V; and p.sub.1=560.96 and p.sub.2=43519.8 which give a maximum V.sub.x=400 V, =100 kHz.

(44) The ions are cooled by a buffer or background gas in the ion trap and they reach thermal energy. A worst case assumption would be that the residual axial energy V of the ions was approximately ten times this value, say V=0.25 eV. This assumption then allows the spatial width z(m) along the z-direction of the ion trap that any given ion will reside within to be determined, i.e. +/0.25 V from the central potential position.

(45) The spatial width z(m) along the ion trap in the z-direction within which ions of any given mass to charge ratio are trapped can be determined from the curvature C(m) given in equation 8 above, such that:

(46) $\begin{array}{cc}z\left(m\right)=\sqrt{\frac{8V}{C\left(m\right)}}& \left(\mathrm{equation}9\right)\end{array}$

(47) FIG. 2 shows this spatial width z(m) as a function of the mass to charge ratio of the ions.

(48) FIG. 3 shows the mass to charge ratios of the ions in the ion guide as a function of the electrode number in the ion guide forming the ion trap, i.e. effectively as a function of the distance z along the ion trap. It can be seen that the mass to charge ratios of the ions trapped in the ion trap increase along the length of the ion trap. It can therefore be seen that ions of different mass to charge ratios can be ejected from the ion trap by ejecting ions from different regions of the length of the ion trap.

(49) FIG. 4 shows the range of mass to charge ratios that are nominally trapped adjacent each electrode in the ion trap, as a function of the electrode number in the ion trap, i.e. effectively as a function of the distance z along the ion trap. This shows a decreasing mass range with increasing distance z from the end of ion trap, with a maximum range of approximately 17 Da.

(50) The above example uses the DC and RF voltage profiles set out in equations 1 and 2 in order to generate the illustrated mass to charge ratio profile along the ion trap. However, different, choices of DC and/or RF voltage profiles may be used to tailor the mass to charge ratio distribution along the ion trap. Also, the ions may be separated within the ion trap such that they are arranged in order of increasing or decreasing mass to charge ratio by altering the arrangement of the DC and RF voltages applied the ion trap.

(51) FIG. 5 shows a schematic of an embodiment that may be used as a high pass mass to charge ratio filter, mimicking a low mass to charge ratio cut-off device. The system comprises a source of ions 1 (e.g. electrospray, REIMS, DESI, etc.), an ion trap or ion accumulation device 2 (e.g. SRIG trap), an ion guide filter 3 comprising an ion trap for separating the ions according to mass to charge ratio (e.g., as described above), an ion neutralising/destroying device 4 (e.g. pDRE lens), and onward transmission ion optics or a mass analyser 5. Device 5 may be a mass to charge ratio analyser, such as Time of Flight (ToF) mass analyser or Orbitrap etc. Alternatively, device 5 may be a gas filled ion guide that converts the pulsed beam received therein into a pseudo-continuous ion beam.

(52) In use, ions from ion source 1 are accumulated in the ion trap 2 and are then pulsed into the ion filter 3. The ion filter 3 comprises an ion trap of the type described above for separating the ions. Opposing forces are applied to the ions in the ion trap so as to cause them to separate within the filter 3 according to mass to charge ratio, as described above. The ions are allowed a short period of time after being pulsed into the filter 3 (typically a few milliseconds to a few tens of milliseconds) to cool, spatially separate and take up their equilibrium spatial positions. Ions having different mass to charge ratios become arranged at different positions within the filter 3. For example, the ion distribution may be the same as that shown in FIG. 3. In this embodiment the ions are arranged in order of mass to charge ratio within the filter 3, with the lower mass ions arranged towards the exit of the filter 3. The instrument in this embodiment acts as a high pass device and it is therefore desired to discard ions below a threshold mass to charge ratio. In order to do this, a controller and associated circuitry controls a voltage supply so as to apply a DC voltage to the ion trap of the filter 3 that travels along the portion of the filter 3 in which the unwanted ions are stored so as to force these ions out of the filter 3.

(53) The unwanted ions are swept out of the filter 3 and into the ion neutralising/destroying device 4, which at this point is activate and neutralises or destroys the unwanted ions. The ion neutralising/destroying device 4 may be adapted and configured to electronically neutralise the ions. For example, the ion neutralising/destroying device 4 may comprise a controller and associated electronic circuitry that control a voltage supply to apply a voltage to an electrode that causes the unwanted ions to be deflected onto a surface that electrically neutralises them, e.g., so that they cannot be analysed by a downstream ion analyser. For example, the ion neutralising/destroying device 4 may deflect the ion onto an electrode that electrically neutralises the ions. Alternatively, the ion neutralising/destroying device 4 may react the ions with ions of opposite polarity so as to neutralise them. The neutralising device 4 is then deactivated and the remaining, desired ions are swept out of the filter 3 by travelling a DC voltage along the filter 3. As the ion neutralising/destroying device 4 has been deactivated, the desired ions are transmitted therethrough to the device 5.

(54) The ion neutralising/destroying device 4 may alternatively be replaced with a device that simply discards the ions. For example, a device comprising a controller and associated electronic circuitry that control a voltage supply to apply a voltage to an electrode that causes the unwanted ions to ejected from the instrument, e.g., so that they cannot be analysed by a downstream ion analyser, may be used.

(55) As an alternative to separating and arranging the ions in the filter 3 in the above manner, the ions could be separated in the filter 3 such that the higher mass to charge ratio ions are arranged towards the exit of the filter 3. As such, a DC voltage may be travelled along the portion of the filter 3 in which the desired ions are stored so as to force these ions out of the filter 3. These desired ions may then be onwardly transmitted to device 5. Ions in the filter 3 having masses below the threshold value may be discarded, e.g. by subsequently transmitting them to the ion neutralising/destroying device 4 or ejecting them from the instrument in the manners described above. For example, the controller may switch off the trapping voltages in the filter 3 such that these unwanted ions are no longer trapped.

(56) The instrument may alternatively be configured as a low pass filter. For example, the ions could be separated so as to be arranged in order of mass to charge ratio within the ion filter, with the lower mass ions arranged towards the exit of the filter 3. However, this embodiment is a low pass filter and so only ions below a threshold mass to charge ratio are desired. As such, a DC voltage may be travelled along the portion of the mass filter in which the desired ions are stored so as to force these ions out of the filter 3. These desired ions may then be onwardly transmitted to device 5. Ions in the filter having masses above the threshold value may be neutralised or discarded, for example, in the manners described above.

(57) Alternatively, the ions could be separated so as to be arranged in order of mass to charge ratio within the ion filter, with the higher mass ions arranged towards the exit of the filter 3. In this embodiment, the a DC voltage is travelled along the portion of the mass filter 3 in which the unwanted ions are stored so as to force these ions out of the filter 3. These unwanted ions are neutralised or discarded, e.g. in the manners described above. The desired ions may subsequently be swept out of the filter 3 by a travelling DC potential and onwardly transmitted to device 5.

(58) Alternatively, the device may be operated as a band pass filter. In this embodiment the ions are separated in the filter 3 in the same manner as described above. It is desired to neutralise or discard ions below a first threshold mass to charge ratio and to neutralise or discard ions above a second threshold value. If the ions are arranged in order of mass such that the low mass to charge ratios are arranged toward the exit of the filter 3, then a DC voltage is travelled along the portion of the filter 3 in which the ions having masses below the first threshold are stored so as to force these ions out of the filter 3. These ions are then neutralised or discarded, e.g. in the manners described above. A DC voltage is then travelled along the portion of the filter 3 in which the desired ions are stored, i.e. the ions having masses between the first and second threshold values. These desired ions are swept out of the filter 3 and are onwardly transmitted to device 5. Ions in the filter 3 having masses above the second threshold value may then be neutralised or discarded, e.g. in the manners described above.

(59) Alternatively, the ions could be arranged in order of mass to charge ratio within the ion filter, with the higher mass ions arranged towards the exit of the filter 3. In this embodiment, a DC voltage is travelled along the portion of the filter 3 in which the ions having masses above the second threshold are stored so as to force these ions out of the filter 3. These unwanted ions are neutralised or discarded, e.g. in the manners described above. A DC voltage is then travelled along the portion of the filter 3 in which the desired ions are stored, i.e. the ions having masses between the first and second threshold values. These desired ions are swept out of the filter 3 and are onwardly transmitted to device 5. Ions in the filter 3 having mass to charge ratios below the first threshold value may then be neutralised or discarded, e.g. in the manners described above.

(60) FIG. 6 shows a schematic of an instrument that is the same as that shown in FIG. 5, except wherein ions are orthogonally transferred from the ion filter 3 into an adjacent device 6, rather than being further transmitted along the longitudinal axis of the instrument. As such, the ion neutralising/destroying device 4 may be omitted and replaced by a device 6 that has a controller and associated circuitry for controlling a voltage supply to apply voltages to the ion trap so as to orthogonally eject ions from the filter 3. The device 6 may be, for example, an ion guide (such as a Stepwave ion guide) that is conjoined with filter 3 so that ions travelling along the longitudinal axis of filter 3 may be selectively radially ejected into device 6 so as to travel along the longitudinal axis of the device 6.

(61) The instrument may be operated in any of the modes described above in relation to FIG. 5, except that when the ions have been separated along the length of the filter 3 the desired ions may be orthogonally ejected from the filter 3 into the ion guide 6. This is in contrast to the arrangement of FIG. 5, wherein the desired ions are swept out of the filter 3 along its longitudinal axis. According to the instrument illustrated in FIG. 6, the ions received in device 6 may be onwardly transmitted to device 5.

(62) Alternatively, rather than transferring desired ions into device 6, only unwanted ions may be transferred into the adjacent device 6. The desired ions may then be transferred along the longitudinal axis of filter 3 to an ion analyser.

(63) The instrument may be operated both in simple filter modes in which the instrument may be operated in either low pass, high pass or band pass modes. Alternatively, the instrument may be operated in complex filter modes. For example, the instrument may be operated multi-pass filter modes in which ions are first trapped, separated and filtered according to a low pass, high pass or band pass mode in filter 3; and at least some of the desired ions transmitted by the filter 3 are subsequently reintroduced into the filter 3 and are trapped, separated and filtered again according to a low pass, high pass or band pass mode.

(64) It is contemplated that the desired ions may be transmitted in a mass to charge ratio dependent manner to a downstream device whose operation is scanned with time. For example, the scanned device may be a resolving quadrupole or other multipole in which the mass to charge ratios transmitted by the quadrupole or multipole is scanned with time. This coupling serves to increase the duty cycle of the scanned device.

(65) Alternatively the separated ions may be fragmented or reacted in an active collision cell and, for example, a SWATH type experiment may be conducted. For example, ions having selected mass to charge ratio ranges may be transmitted as separate pulses from the filter 3 into an active collision, fragmentation or reaction cell, where the pulse shape or separation is maintained. The collision energy or the fragmentation or reaction rate may be alternated or switched between a high collision, fragmentation or reaction value in which substantial fragmentation or reaction of the ions is performed and a low collision, fragmentation or reaction value in which substantial fragmentation or reaction of the ions is not performed. The precursor ions from the first of the modes may be mass and/or ion mobility analysed and the fragment or product ions from the second of the modes may be mass and/or ion mobility analysed. The fragment or product ions may be assigned to their respective precursor ions, e.g., based on the times that they are mass and/or ion mobility analysed or based on their ion signal intensity profile shapes.

(66) Various embodiments of said second set of embodiments will now be described, in which the ion trap is used to separate ions and then eject the separated ions into an ion mobility separator (IMS) in a single pulse.

(67) FIG. 7A shows a schematic arrangement of part of a prior art ion mobility spectrometer. The spectrometer comprises an ion trap 20 arranged upstream of an IMS device 24. Ions are trapped in the ion trap 20 prior to being pulsed into the IMS device 24. Prior to injection into the IMS device 24, the ions are distributed substantially throughout the entire length of the ion trapping device 20 so as to maximise the ion storage capacity.

(68) Rokushika et al (Rokushika S, Hatano H, Bairn M A, Hill H H, Anal Chem 1985 (57) pp 1902-1907) showed that the resolution of an IMS device is dependent upon the temporal width of the ion injection pulse into the device and the diffusion broadening that occurs along the drift path within the IMS device. The resolution of an IMS device can be described the following equation:

(69) $\begin{array}{cc}R=\frac{t}{\sqrt{\left[W_0^2+W_d^2\right]}}& \left(\mathrm{equation}10\right)\end{array}$
where t is the drift time of the ion along the drift path, W.sub.0 is the initial ion pulse width, and W.sub.d is the diffusion broadened peak width.

(70) The diffusion broadened peak width W.sub.d is given by:

(71) $\begin{array}{cc}{W}_d=\sqrt{\frac{16\ln \left(2\right){\mathrm{kTt}}^2}{\mathrm{qEL}}}& \left(\mathrm{equation}11\right)\end{array}$
where k is the Boltzmann constant, T is temperature, t is the drift time of the ion along the drift path, q is the electronic charge, E is the electric field in the drift region, and L is the length of the drift region.

(72) The prior art arrangement of providing an ion trap device 20 upstream of the IMS device 24 is advantageous in that it increases the ion population that can be injected into the IMS device 24 at any one time. However, prior to injection, ions of any given mass to charge ratio are distributed throughout the ion trap 20 and so the initial ion pulse width W.sub.0 for ions of any given mass is relatively wide, resulting in a relatively low resolution for ion mobility measurement when the ions are pulsed into the IMS device.

(73) The second set of embodiments of the present invention provide an improvement in measured ion mobility resolution through a reduction of the magnitude of W.sub.0 for any given type of ion.

(74) FIG. 7B shows a schematic arrangement of part of an ion mobility spectrometer according to an embodiment of the present invention. The instrument is the same as that in FIG. 7A, except that it is configured to spatially separate the ions in the ion trap 20 according to their mass to charge ratio. In the illustrated example, ions of a first mass are separated to one end of the ion trap 20 and ions of another mass are separated towards the other end of the ion trap 20. Although only two groups of ions 26,28 are shown, other groups of ions of different mass to charge ratios may be separated and stored in groups that are arranged between the two illustrated groups 26,28.

(75) As the ions are separated within the ion trap 20, ions of any given mass to charge ratio become confined within a relatively small region within the ion trap 20. As such, when the ions are injected into the IMS device 24, the initial ion pulse width W.sub.0 for ions of any given mass to chare ratio is relatively narrow, even though a relatively large ion trap 20 has been used. This enables a large population of ions to be injected into the IMS device 24 without degrading the resolution of the ion mobility spectrometer. The separated ions may be injected into the IMS device 24 together in the same pulse into the IMS device 24. The injection may be performed in a manner that maintains the separation of the ions, at least to some degree, during the injection.

(76) As described above, ions may be spatially separated in the ion trap 24 using a combination a pseudo-potential electric field and a DC electric field. As also described above, with optimisation of the DC potential parameters, d.sub.1 and d.sub.2, and the RF potential parameters, p.sub.1 and p.sub.2, it is possible to obtain reasonable voltage levels and good spatial separation of the ions within the ion trap. In the embodiments below, optimisation of the parameters assumes an ion trap length of 0.1 m and a distribution of ions in the trap for ions having a range of mass to charge ratios of 100 to 2000 Da.

(77) FIG. 8 shows the DC voltage potential profile V.sub.dc(z) along the ion trap 20 achieved using values of d.sub.1=1254, d.sub.2=2280, a maximum V.sub.dc=150 V, and =100 kHz.

(78) FIG. 9 shows the RF voltage potential profile V.sub.x(z) along the ion trap 20 achieved using parameters p.sub.1=560.96 and p.sub.2=43519.8 and a maximum V.sub.x=400 V.

(79) FIG. 10 shows a graph of the position z at which the total potential V.sub.tot is minimum as a function of ion mass to charge ratio, for ions of mass to charge m between 100 and 2000 Da, and for the combination voltage profiles shown in FIGS. 8 and 9.

(80) The ions are cooled by the buffer or background gas in the ion trap 20 and they reach thermal energy. A worst case assumption would be that the residual axial energy V of the ions was ten times this value, say approximately V=0.25 eV. This assumption then allows the spatial width z(m) along the z-direction of the ion trap that any given ion will reside within to be determined from equation 9 above, i.e. +/0.25 V from the central potential position.

(81) FIG. 11 shows this spatial width z(m) as a function of the mass to charge ratio of the ions.

(82) Once the ions have been separated in the ion trap 20, the separated ions are injected into the IMS device 24. This may be performed by travelling a voltage along the ion trap 20. For example, a controller and associated circuitry may control a voltage supply so as to apply one or more voltage (e.g., a DC voltage) to the ion trap 20 that travels along the ion trap 20 so as to pulse ions from the ion trap 20 into the IMS device 24. The velocity v of the ions exiting the ion trap 20 may be controlled, e.g., by setting or controlling the velocity of the travelling voltage. For example, the velocity v is typically set to the order of 300 m/s. Therefore, the spatial spread z of ions having any given mass to charge ratio may be mapped into a temporal spread t of these ions leaving the ion trap 20, wherein t=z/v.

(83) FIG. 12 shows a plot of the temporal spread of the ions t as a function of mass to charge ratios of the ions.

(84) The drift time tin equation 11 above is dependent upon the mass to charge ratio of the ions. The drift time t(m) may be calculated using the following equation:

(85) 0$\begin{array}{cc}t\left(m\right)=\frac{L_d}{E_d\left[\frac{\sqrt{18}}{16}\frac{q}{\sqrt{\mathrm{kT}}}\sqrt{\frac{1}{\mathrm{mM}}+\frac{1}{m_{b}M}}\frac{\mathrm{kT}}{p}\frac{1}{\left(m\right)}\right]}& \left(\mathrm{equation}12\right)\end{array}$
where L.sub.d is the length of the ion mobility drift tube, E.sub.d is the electric field along the drift tube, p is the pressure, m.sub.b is the molecular mass of the buffer gas, and (m) is the collision cross-sectional area. For simplicity, only ions of single charge have been considered here.

(86) The collision cross-sectional area (m) may be estimated by the following equation, which is based upon the molecular radii of the ion and buffer gas molecules:

(87) $\begin{array}{cc}\left(m\right)=\left[\frac{1.436\left(\sqrt[3]{m}+\sqrt[3]{m_b}\right)}{2}\right]& \left(\mathrm{equation}13\right]\end{array}$

(88) FIG. 13 shows a plot of drift time Dt as a function of mass to charge ratio. This plot was obtained using the equation for t(m) above and substituting reasonable values for the operational parameters, which were an electric field along the drift tube E.sub.d of 3 kV/m, a length of the ion mobility drift tube L.sub.d of 1 m, a temperature T of 293 K, and a pressure p of 3 mbar nitrogen.

(89) The parameter t is equivalent to the initial ion pulse width W.sub.o in equation 10 above. The diffusion broadened peak width W.sub.d may be calculated using the estimated drift times t(m) from equation 13 above. Accordingly, the resolution R may be determined from equation 10 above.

(90) FIG. 14 shows two plots of resolution R against mass to charge ratio of the ions. The lower plot corresponds to that of a prior art technique that does not spatially separate the ions in the ion trap 20 (i.e. using the instrument shown in FIG. 7A) and which gates ions into the IMS device 24 using a gate time of 250 s. In other words, the initial ion pulse width W.sub.o is 250 s. The upper plot corresponds to an embodiment of the present invention that spatially separates the ions in the ion trap 20. FIG. 14 indicates that the embodiments of the present invention provide a significant increase in the resolution of the ion mobility spectrometer over the prior art spectrometer. The enhancement in resolution increases with decreasing ion mobility drift time. For mass to charge ratios up to 1000 Da, there is a greater than two fold enhancement in resolution. The embodiments allow ion mobility measurements to be made with high resolution, even though a large ion trap 20 is utilised to inject a large ion population into the IMS device 24 at any one time.

(91) The embodiments introduce a mass to charge ratio dependent shift in the measured drift time, since ions of different mass to charge ratios are separated and stored in the ion trap 20 at different distances from the entrance to the IMS device 24. However, this may be easily taken care of by calibration. In addition, the pre-separation may be ion mobility dependent and may result in an increase in temporal separation.

(92) The IMS device 24 may comprise a drift length having a static DC field arranged across it for forcing ions to separate within the IMS device 24 according to ion mobility. Alternatively, an electric potential barrier may be travelled along the drift length of the IMS device 24 in order to force the ions to separate according to ion mobility in the IMS device 24.

(93) Various embodiments of said third set of embodiments will now be described, in which the ion trap is used to separate ions and then eject separated groups of ions into a discontinuous ion analyser at different times.

(94) FIG. 15 shows a schematic of part of a conventional Time of Flight (ToF) mass spectrometer. The apparatus comprises an ion trap 32 of length Z, an ion transfer region 34 of length L, and a pusher region 36 of an orthogonal acceleration time of flight (oa-ToF) mass analyser having a length L.

(95) In use, ions are trapped in the ion trap 32 and are pulsed into the ion transfer region 34. Ion optics in the transfer region 34 guide the ions to the pusher region 36. The pusher region 36 pulses an orthogonal acceleration electric field such that the ions are accelerated orthogonally from their flight path and into the time of flight region of the ToF mass analyser. In order to achieve the optimum duty cycle of the oa-ToF spectrometer, all of the ions released from the trap 32 must be spatially located within the pusher region 36 when the orthogonal acceleration field is applied. The following calculations can be made to determine the mass to charge ratios of the ions that would fulfil this condition.

(96) The time of flight T.sub.1 for an ion of mass to charge ratio m.sub.1 to travel from the exit of the ion trap 32 (at z=0) to the end of the pusher region 36 is as follows:

(97) $\begin{array}{cc}{T}_1=\sqrt{\frac{M{m}_1}{2\mathrm{qVz}}}\left(L+L\right)& \mathrm{equation}\left(14\right)\end{array}$
where M is the atomic mass unit, q is the electronic charge constant, V.sub.z is the potential that the ion experiences on its journey through the ion transfer optics in the transfer region 34, and (L+L) is the distance from the exit of the ion trapping region 32 to the end of the pusher region 36.

(98) The time of flight T.sub.2 of a second ion of higher mass to charge ratio m.sub.2 to travel the distance L from the exit of the ion trapping region 32 (at z=0) to the entrance of the pusher region 36 is as follows:

(99) $\begin{array}{cc}{T}_2=\sqrt{\frac{M{m}_2}{2\mathrm{qVz}}}\left(L\right)& \mathrm{equation}\left(15\right)\end{array}$

(100) By setting time of flight T.sub.1 to be equal to the time of flight T.sub.2 one can determine the mass to charge ratio m.sub.2 of the ions that have reached the entrance to the pusher region 36 at the same time that ions of mass to charge ratio m.sub.1 have reached the exit of the pusher region 36. This results in the following equation:

(101) $\begin{array}{cc}{m}_2={\left(1+\frac{L}{L}\right)}^2{m}_1& \mathrm{equation}\left(16\right)\end{array}$

(102) In order to increase the duty cycle of the ToF mass analyser it is required that the pusher is presented with a restricted range of mass to charge ratios, otherwise all of the ions will not be located within the pusher region 36 at the time that the orthogonal acceleration extraction pulse is applied. The required range of mass to charge ratios m.sub.i, m.sub.i-1 as a function of the sequential push number i, is as follows:

(103) $\begin{array}{cc}{m}_i={\left(1+\frac{L}{L}\right)}^2{m}_{i-1}={\left(1+\frac{L}{L}\right)}^{2\left(i-1\right)}{m}_1& \mathrm{equation}\left(17\right)\end{array}$

(104) FIG. 16 shows the values of m.sub.i as a function of the pulse number i for an instrument having a typical ion transfer region 34 length L and a typical pusher region 36 length L. In this example the ion transfer region length L is 0.13 m, the pusher region length L is 0.033 m, and the range of mass to charge ratios pulsed in the first pulse is m.sub.1=100 Da.

(105) The third set of embodiments of the present invention provide an improvement over such conventional discontinuous ion analysers by spatially separating the ions according to a physicochemical property (e.g., mass to charge ratio) in an ion trap upstream of the discontinuous ion analyser.

(106) As described above, according to the embodiments of the present invention, ions may be spatially separated in the ion trap using a combination a pseudo-potential electric field and a DC electric field so as to provide the relationship in equation 7 above, from which can be determined the position z at which the pseudo-potential minimum for any given ion is located, and hence the position at which that ion will remain trapped.

(107) As the ions are separated along the ion trap according to mass to charge ratio, ions may be swept out of the ion trap from different positions within the ion trap in order to eject different ranges of mass to charge ratio into the downstream ion analyser. In order to do this, a controller and associated circuitry may control a voltage supply so as to apply a DC voltage to the ion trap that travels along the ion trap so as to eject the ions into the downstream ion analyser.

(108) Substituting m.sub.i from equation 17 above into equation 7, gives the distance along the ion trap that must be swept out by the travelling voltage in the i.sub.th pulse. This distance z.sub.i is given by the following equation:

(109) $\begin{array}{cc}{z}_i=-\frac{1}{2}\frac{d_1}{d_2}\left[\frac{4\left(\right)\frac{p_1{p}_2}{d_1}+{\left(1+\frac{L}{L}\right)}^{2\left(i-1\right)}{m}_1}{4\left(\right)\frac{p_2^2}{d_2}+{\left(1+\frac{L}{L}\right)}^{2\left(i-1\right)}{m}_1}\right]& \left(\mathrm{equation}18\right)\end{array}$

(110) As described above, with optimisation of the DC potential parameters, d.sub.1 and d.sub.2, and the RF potential parameters, p.sub.1 and p.sub.2, it is possible to obtain reasonable voltage levels and good spatial separation of the ions within the ion trap.

(111) FIG. 17 shows the position z.sub.i along the ion trap from which the travelling voltage sweeps ions towards the exit of the ion trap (wherein the exit is at z=0) as a function of push number i, for pushes i between i=1 and i=7. In this model, the length of the ion trapping device is 0.2 m and =100 kHz. Optimised values for the DC voltage gave d.sub.1=2425 and d.sub.2=2280, with a maximum of 300 V and the RF voltage p.sub.1=270 and p.sub.2=43272 and shows a maximum of approximately 400 V.

(112) As can be seen from FIG. 17, the distance that the travelling voltage must travel along the ion trap increases in subsequent push numbers. This enables ions from different depths within the ion trap to be received in the pusher region 36 of the ToF mass analyser when different orthogonal acceleration pulses are applied. As ions having different ranges of mass to charge ratio are trapped at different depths in the ion trap (i.e. different values of z), ions having different ranges of mass to charge ratio are analysed in the different orthogonal acceleration pulses.

(113) FIG. 18 shows the length z of the ion trap that both contains trapped ions and from which ion are extracted, as a function of the ToF push number i. It can be seen that the ions trapped over a small length (e.g. 6 mm) of the ion trap are swept into the pusher region for the first push at i=1. The next sweep extracts ions trapped over a larger length of the ion trap, such that these ions are swept into the pusher region for the second push at i=2. It will be seen that subsequent sweeps extract ions that were trapped over progressively increasing lengths of the ions trap. FIG. 18 shows data that would cover a mass range between 1 and approximately 2000 Da, requiring a minimum sweep of approximately 6 mm of ion guide and thus is entirely possible with current technologies.

(114) The third set of embodiments of the invention enables ions having a relatively large range of mass to charge ratios to be trapped in the ion trap 32, without the ions being ejected from the ion trap 32 and into the pusher region 36 in a manner that overfills the pusher region 36 at the time the orthogonal acceleration extraction pulse is applied. By spatially separating the ions within the ion trap, ions of any given range of mass to charge ratios become confined within a sub-region (which may be a relatively small region) of the ion trap. Different ranges of mass to charge ratios may then be swept out of the ion trap and into the mass analyser at different times by ejecting ions from different regions of the ion trap at different times.

(115) FIG. 19 shows an embodiment of the ion trap 40 that may be used in the various embodiments described herein. The ion trap 40 may be a linear ion trap and comprises a plurality of apertured electrodes 42. An AC or RF voltage supply 44 may apply AC or RF voltages to the electrodes 42 so as to radially confine ions within the ion trap 40. Opposite phases of an AC or RF voltage may be applied to axially adjacent electrodes. Different AC or RF voltages (e.g., different magnitudes) may be applied to different electrodes 42 along the ion trap 40 so as to generate a first force on the ions in a first direction along the axial length of the ion trap 40. A DC voltage supply 46 may apply DC voltages to the electrodes 42. Different DC voltages (e.g., different magnitudes) may be applied to different electrodes 42 along the ion trap 40 so as to generate a second force on the ions in a second direction along the axial length of the ion trap 40, opposite to the first direction. Additionally, or alternatively, a pump 48 may be provided to generate a gas flow through the ion trap 40 that generates a force on the ions in the second direction.

(116) A controller 50 is provided that comprises an ion separator. The controller and ion separator contain a processor and electronic circuitry that are configured to control the voltage supplies 44,46 (and/or pump 48) so as to apply the voltages to the electrodes 42 (and/or pump the gas through the ion trap 42) that cause the first and second forces to be generated on the ions. This causes the ions to separate along the axial length of the ion trap 40 according to mass to charge ratio, as described above.

(117) The controller 50 also comprises an ion driving or pulsing circuit that contain a processor and electronic circuitry configured to control the voltage supplies 44,46 (and/or pump 48) so as to apply voltages to the electrodes 42 (and/or control the gas supply) to cause ions to be driven or pulsed out of the ion trap, after they have been separated.

(118) Although the present invention has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.

(119) For example, although a stacked ring ion guide has been described as being used in the ion trap, other geometries of electrodes may be used.

(120) Although a travelling wave has been described as the means by which ions are extracted from the trap, alternative methods of releasing the ions from the ion trap in a controlled manner may be used. For example, the axial DC potential or pseudo-potential gradient may be ramped so as to force ions out of the ion trap, or may be altered along different lengths of the ion trap at different times so as to eject ions.

(121) Although the spatial separation has been described as being achieved by using opposing forces on the ions generated by pseudo-potential and an opposing DC potential, the spatial separation may be achieved by other methods. For example, one of the opposing forces may be applied from a gas flow instead of the DC potential or pseudo-potential gradient.

(122) Although various values of the RF and DC voltages have been described herein, these parameters and other operational parameters of the ion trap may be varied according to the desired mode of operation and/or the ions trapped therein.

(123) The ions may be caused to separate in the ion trap by ion mobility instead of mass to charge ratio, or by a combination of mass to charge ratio and ion mobility.

(124) The ion trap of the various embodiments may be a discrete device or may be an ion trapping region, e.g., within a larger device.