Patent Application
20140299761
The present invention relates to a mass or mass to charge ratio selective ion trap. The preferred embodiment relates to ion guiding and trapping systems and methodology for use in mass spectrometry systems.
It is well known that the time averaged force on a charged particle or ion due to an AC inhomogeneous electric field is such as to accelerate the charged particle or ion to a region where the electric field is weaker. A minimum in the electric field is commonly referred to as a pseudo-potential well or valley. Correspondingly, a maximum is commonly referred to as a pseudo-potential hill or barrier.
Paul traps, also known as 3D ion traps, are designed to exploit this phenomenon by causing a pseudo-potential well to be formed in the centre of the ion trap. The pseudo-potential well is then used to confine a population of ions. Due to its symmetric nature the 3D ion trap acts to confine ions to a single point in space as shown in
There is a finite space charge capacity for any ion confining device beyond which its performance begins to degrade and where ultimately the device cannot hold any further charges. For example, overfilling an ion trap leads to a loss of mass resolution and of mass accuracy, a result of the electric field becoming distorted by the presence of the large number of charges being focussed into close proximity. It is generally the case that the space charge limit for storage of ions is significantly greater than the spectral or analytical space charge limit which is the maximum number of ions which can be confined whilst retaining a given mass resolution and mass accuracy.
For mass spectrometry applications it is necessary to detect the mass to charge ratio (m/z) of the confined ions. For example, ions may be ejected in a mass selective manner towards an ion detector (although many other detection methods exist). There are several known methods of ejecting ions either resonantly or non-resonantly to achieve this goal.
It is often necessary to introduce gas into ion trapping devices. The gas may be used for cooling purposes or ion fragmentation via Collision Induced Decomposition (“CID”). Ion Mobility Separation (“IMS”) has also been performed either with a static volume of gas or with a flow of gas. The use of pulsed gas valves to introduce gas into ion traps is also known.
Recently, there has been increased interest in 2D or Linear Ion Traps (“LIT”) because of the increased volume which the confined ions are able to occupy. Linear ion traps allow a greater number of ions, or more correctly a greater number of charges, to be confined and then detected. Such ion traps are generally based on multipolar RF ion guides such as quadrupoles, hexapoles or octopoles. A pseudo-potential well is formed within the rod set ion trap around the central axis of the ion guide so that ions are confined radially within the ion trap. The ions are normally confined axially using DC fields although methods of using RF fields to axially confine ions are also known.
The radial pseudo potential of a 2D ion trap acts to focus the confined ions to a line through the central axis of the ion trap as shown in
Ion ejection has been demonstrated both radially and axially using 2D ion traps by resonantly exciting the ions within the confining radial pseudo potential. Radial ejection has been achieved by allowing the ions to resonate until their radial excursions reach the quadrupole electrodes at which point they pass through narrow slots in the electrodes. Axial ejection has been achieved by resonantly exciting the ions into the naturally occurring fringing fields which exist at the exit of a quadrupole at which point it is possible for the ions to gain sufficient axial kinetic energy to overcome the confining DC barrier. Both of these methods are inherently non-adiabatic in nature and lead to large ejection energies and large energy spreads which makes them generally unsuitable for coupling with other devices such as other mass analysers.
Another form of axial ejection from a 2D ion trap is known and comprises superimposing an axial harmonic DC potential upon a radial confining RF of an ion guide. Such approaches are schematically represented in
With respect to the 2D ion traps shown in
U
z(t)=(a+b·cos(Ωt))z2 (1)
where b is the electric field constant of the axial quadratic potential, a is the amplitude and Ω is the frequency of the modulation of the axial potential.
According to an aspect of the present invention there is provided a mass or mass to charge ratio selective ion trap comprising:
a first device arranged and adapted to generate a radially asymmetric pseudo-potential barrier or well which acts to confine ions in a first (y) and a second (x) direction within the ion trap;
a second device arranged and adapted to generate a substantially DC quadratic potential well which acts to confine ions in a third (z) direction within the ion trap; and
a third device arranged and adapted to excite ions in the third (z) direction so as to mass or mass to charge ratio selectively eject ions in the third (z) direction.
According to an aspect of the present invention there is provided a mass or mass to charge ratio selective ion trap comprising:
a first device arranged and adapted to generate a pseudo-potential barrier or well which acts to confine ions in a first (y) direction and a DC potential barrier or well which acts to confine ions in a second (x) direction within the ion trap;
a second device arranged and adapted to generate a substantially DC quadratic potential well which acts to confine ions in a third (z) direction within the ion trap; and
a third device arranged and adapted to excite ions in the third (z) direction so as to mass or mass to charge ratio selectively eject ions in the third (z) direction.
The first (y) direction and/or the second (x) direction and/or the third (z) direction are preferably substantially orthogonal.
The ion trap preferably comprises a plurality of electrodes.
The plurality of electrodes preferably comprise:
(i) a muitipole rod set or a segmented multipole rod set comprising a plurality of or at least 4, 5, 6, 7, 8, 9, 10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100 or >100 rod sets or segmented rod sets; and/or
(ii) an ion tunnel or ion funnel comprising a plurality of or at least 4, 5, 6, 7, 8, 9, 10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100 or >100 annular, ring or oval electrodes having one or more apertures through which ions are transmitted in use; and/or
(iii) a plurality of or at least 4, 5, 6, 7, 8, 9, 10, 10-20, 20-30, 30-40, 40-50, 50-60, 60-70, 70-80, 80-90, 90-100 or >100 half annular, half ring, half oval or C-shaped electrodes; and/or
(iv) a stack or array of planar, plate or mesh electrodes arranged generally in the plane in which ions travel in use.
The first device is preferably arranged and adapted to apply an RF voltage to at least some of the electrodes.
The ion trap is preferably arranged and adapted so that there is a full and/or direct line of sight through the ion trap in the third (z) direction.
The ion trap is preferably arranged and adapted so that there is a full and/or direct line of sight through the ion trap in the second (x) direction.
The second device is preferably arranged and adapted to form the substantially quadratic DC potential well so that either: (i) a minimum of the substantially quadratic DC potential well is along a central axis of the ion trap; or (ii) a minimum of the substantially quadratic DC potential well is offset from a central axis of the ion trap.
According to an aspect of the present invention there is provided a mass or mass to charge ratio selective ion trap having a substantially toroidal ion trapping region, the ion trap comprising:
a first device arranged and adapted to generate a pseudo-potential barrier or well which acts to confine ions in a first (y) direction within the ion trap;
a second device arranged and adapted to generate a substantially DC quadratic well which acts to confine ions radially within the ion trap; and
a third device arranged and adapted to excite ions in a radial (r) direction so as to mass or mass to charge ratio selectively eject ions in the radial (r) direction.
The first (y) direction is preferably substantially orthogonal to the radial (r) direction.
The ion trap preferably comprises a plurality of electrodes.
The plurality of electrodes preferably comprise:
(i) a first group of electrodes and a second group of electrodes, wherein the first group of electrodes comprises a first plurality of concentric closed loop, circular or oval electrodes arranged at different radial displacements and wherein the second group of electrodes comprises a second plurality of concentric closed loop, circular or oval electrodes arranged at different radial displacements, wherein the first and second groups of electrodes are arranged at different displacements in the first (y) direction; or
(ii) a first group of electrodes and a second group of electrodes, wherein the first group of electrodes comprises a first plurality of annular groups of electrodes wherein each of the first annular groups of electrodes is arranged at different radial displacements and wherein the second group of electrodes comprises a second plurality of annular groups of electrodes wherein each of the second annular groups of electrodes is arranged at different radial displacements, wherein the first and second groups of electrodes are arranged at different displacements in the first (y) direction.
The first device is preferably arranged and adapted to apply an RF voltage to at least some of the electrodes.
The ion trap is preferably arranged and adapted so that there is a full and/or direct line of sight through the ion trap in the radial (r) direction.
The third device is preferably arranged and adapted to excite ions in a radial (r) direction so as to mass or mass to charge ratio selectively eject ions towards the centre of the ion trap.
The pseudo-potential barrier or well preferably comprises a non-quadrupolar pseudo-potential barrier or well.
The second device is preferably arranged and adapted to maintain the substantially DC quadratic potential well across some but not all electrodes arranged in the third (z) or radial (r) direction.
The second device is preferably arranged and adapted to maintain a substantially DC quadratic potential well across x % of the width of the ion trap in the third (z) or radial (r) direction, wherein x is selected from the group consisting of: (i)<10; (ii) 10-20; (iii) 20-30; (iv) 30-40; (v) 40-50; (vi) 50-60; (vii) 60-70: (viii) 70-80; (ix) 80-90; (x) 90-95; and (xi) 95-99.
The second device is preferably arranged and adapted to maintain a DC potential profile in the third (z) or radial (r) direction across the ion trap wherein the DC potential profile comprises a first region and one or more second regions, wherein the DC potential profile in the first region is substantially quadratic and wherein the DC potential profile in the one or more second regions is substantially linear, constant or non-quadratic.
The second device is preferably arranged and adapted to maintain a DC potential profile in the third (z) or radial (r) direction which is asymmetric preferably about a central axis of the ion trap, wherein the central axis is preferably in the second (x) direction.
The second device is preferably arranged and adapted to maintain a DC potential profile in the third (z) or radial (r) direction which results in ions being ejected from the substantially DC quadratic well in one direction only.
The third device is preferably arranged and adapted so that ions are mass or mass selectively ejected from the ion trap either: (i) in a first direction only, or (ii) both in a first direction and a second direction, wherein the second direction is different to or opposed to the first direction.
The third device is preferably arranged and adapted to excite ions resonantly in the third (z) or radial (r) direction.
The third device is preferably arranged and adapted to apply a supplemental AC voltage or potential to at least some of the electrodes having a frequency σ which is equal to ω, wherein ω is the fundamental or resonance frequency of ions which are desired to be ejected from the ion trap.
The third device is preferably arranged and adapted to excite ions parametrically in the third (z) or radial (r) direction.
The third device is preferably arranged and adapted to apply a supplemental AC voltage or potential to at least some of the electrodes having a frequency σ equal to 2ω, 0.667ω, 0.5ω, 0.4ω, 0.33ω, 0.286ω, 0.25ω or <0.25ω, wherein ω is the fundamental or resonance frequency of ions which are desired to be ejected from the ion trap.
The third device is preferably arranged and adapted to scan, vary, alter, increase, progressively increase, decrease or progressively decrease the frequency σ of the supplemental AC voltage or potential.
The third device is preferably arranged and adapted: (i) in a mode of operation to eject ions from the ion trap in order of their mass to charge ratio; and/or (II) in a mode of operation to eject ions from the ion trap in reverse order of their mass to charge ratio.
The third device is preferably arranged and adapted to cause ions to be ejected from the ion trap in a substantially adiabatic manner.
The third device is preferably arranged and adapted to cause ions to be ejected from the ion trap with an ion energy selected from the group consisting of: (i)<0.5 eV; (ii) 0.5-1.0 eV; (iii) 1.0-1.5 eV; (iv) 1.5-2.0 eV; (v) 2.0-2.5 eV; (vi) 2.5-3.0 eV; (vii) 3.0-3.5 eV; (viii) 3.5-4.0 eV: (ix) 4.0 eV-4.5 eV; (x) 4.5-5.0 eV; and (xi)>5.0 eV.
The ion trap is preferably arranged and adapted to contain N ion charges within the ion trap, wherein N is selected from the group consisting of: (i) <5×104; (ii) 5×104-1×105; (iii) 1×105-2×105; (iv) 2×105-3×105; (v) 3×105-4×105; (vi) 4×105-5×105; (vii) 5×105-6×105; (viii) 6×105-7×105; (ix) 7×106-8×105; (x) 8×105-9×105: (xi) 9×105-1×106; and (xii)>1×106.
In a mode of operation at least a region or substantially the whole of the ion trap is preferably arranged and adapted to be operated:
(i) as an ion guide; and/or
(ii) as a collision or fragmentation cell; and/or
(iii) as a reaction cell; and/or
(ii) as a mass filter; and/or
(iii) as a time of flight separator; and/or
(iv) as an ion mobility separator; and/or
(v) as a differential ion mobility separator.
In a mode of operation the ion trap is preferably arranged and adapted to be maintained at a pressure selected from the group consisting of (i)<1.0×10−7 mbar; (ii) 1.0×10−7-1.0×10−6 mbar; (iii) 1.0×10−6-1.0×10−5 mbar; (iv) 1.0×10−5-1.0×10−4 mbar; (v) 1.0×10−4-1.0×10−3 mbar; (vi) 0.001-0.01 mbar; (vii) 0.01-0.1 mbar; (viii) 0.1-1 mbar, (ix) 1-10 mbar; (x) 10-100 mbar; and (xi) 100-1000 mbar.
According to an aspect of the present invention there is provided a mass spectrometer comprising a mass or mass to charge ratio selective ion trap as described above.
According to an aspect of the present invention there is provided a method of mass or mass to charge ratio selective ejection of ions from an ion trap comprising:
generating a radially asymmetric pseudo-potential barrier or well which acts to confine ions in a first (y) and a second (x) direction within the ion trap;
generating a substantially DC quadratic potential well which acts to confine ions in a third (z) direction within the ion trap; and
exciting ions in the third (z) direction so as to mass or mass to charge ratio selectively eject ions in the third (z) direction.
According to an aspect of the present invention there is provided a method of mass or mass to charge ratio selective ejection of ions from an ion trap comprising:
generating a pseudo-potential barrier or well which acts to confine ions in a first (y) direction and a DC potential barrier or well which acts to confine ions in a second (x) direction within the ion trap;
generating a substantially DC quadratic potential well which acts to confine ions in a third (z) direction within the ion trap; and
exciting ions in the third (z) direction so as to mass or mass to charge ratio selectively eject ions from the ion trap in the third (z) direction.
According to an aspect of the present invention there is provided a method of mass or mass to charge ratio selective ejection of ions from an ion trap having a substantially toroidal ion trapping region, comprising:
generating a pseudo-potential barrier or well which acts to confine ions in a first (y) direction within the ion trap;
generating a substantially DC quadratic well which acts to confine ions radially (r) within the ion trap; and
exciting ions in a radial (r) direction so that ions are mass or mass to charge ratio selectively ejected in the radial (r) direction.
According to an aspect of the present invention there is provided a method of mass spectrometry comprising a method as described above.
According to an aspect of the present invention there is provided an ion trap with a trapping volume which is spatially extended in two spatial dimensions from which ions may be ejected in a substantially mass to charge ratio dependent manner.
Although 2D ion traps have a larger ion capacity than 3D ion traps, the need for ion traps with yet further increased ion capacity continues to grow as instruments become every increasingly more sensitive and ion sources become brighter.
The preferred embodiment of the present invention relates to an ion trap or ion transmission device with an enlarged trapping or transmitting volume wherein the ion trap comprises a 1D ion trap which is arranged and adapted to confine and eject ions and which has a greater ion charge capacity than conventional 3D and 2D ion traps.
In the same way that a 3D ion trap fundamentally confines ions to a point and a 2D ion trap fundamentally confines ions to a line, the 1D ion trap according to the preferred embodiment fundamentally confines ions to a plane as shown in
A preferred embodiment of the invention comprises an array of electrodes defining an extended volume to which various combinations of RF, AC and DC voltages are applied. The device may act as either a transmission device or as an ion trap which may be used to hold, accumulate, store, process, isolate, fragment, detect and eject ions. In operation some or all of the ions are distributed within the extended trapping structure and may be moved in a mass to charge ratio dependent manner towards a specific region of the device from which the ions may be subsequently ejected. Ion ejection may be effected by exciting the ions within a substantially DC quadratic potential leading to low energy ion ejection with a consequent low energy spread to the ejected ions.
The ion trap may be operated as a mass analyser or may be used in conjunction with mass analysers or other devices within a mass spectrometer.
According to an embodiment the mass spectrometer may further comprise:
(a) an ion source 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 ion source; (xviii) a Thermospray ion source; (xix) an Atmospheric Sampling Glow Discharge Ionisation (“ASGDI”) ion source; and (xx) a Glow Discharge (“GD”) ion source; and/or
(b) one or more continuous or pulsed ion sources; and/or
(c) one or more ion guides; and/or
(d) one or more ion mobility separation devices and/or one or more Field Asymmetric Ion Mobility Spectrometer devices; and/or
(e) one or more ion traps or one or more ion trapping regions; and/or
(f) one or more collision, fragmentation or reaction cells selected from the group consisting of: (i) a Collisional Induced Dissociation (“CID”) fragmentation device; (ii) a Surface Induced Dissociation (“SID”) fragmentation device; (iii) an Electron Transfer Dissociation (“ETD”) fragmentation device; (iv) an Electron Capture Dissociation (“ECD”) fragmentation device; (v) an Electron Collision or Impact Dissociation fragmentation device; (vi) a Photo Induced Dissociation (“PID”) fragmentation device; (vii) a Laser Induced Dissociation fragmentation device; (viii) an infrared radiation induced dissociation device; (ix) an ultraviolet radiation induced dissociation device; (x) a nozzle-skimmer interface fragmentation device; (xi) an in-source fragmentation device; (xii) an in-source Collision Induced Dissociation fragmentation device; (xiii) a thermal or temperature source fragmentation device; (xiv) an electric field induced fragmentation device: (xv) a magnetic field induced fragmentation device; (xvi) an enzyme digestion or enzyme degradation fragmentation device; (xvii) an ion-ion reaction fragmentation device; (xviii) an on-molecule reaction fragmentation device; (xix) an ion-atom reaction fragmentation device; (xx) an ion-metastable ion reaction fragmentation device; (xxi) an ion-metastable molecule reaction fragmentation device; (xxii) an ion-metastable atom reaction fragmentation device; (xxiii) an ion-ion reaction device for reacting ions to form adduct or product ions; (xxiv) an ion-molecule reaction device for reacting ions to form adduct or product ions; (xxv) an ion-atom reaction device for reacting ions to form adduct or product ions; (xxvi) an on-metastable ion reaction device for reacting ions to form adduct or product ions; (xxvii) an ion-metastable molecule reaction device for reacting ions to form adduct or product ions; (xxviii) an ion-metastable atom reaction device for reacting ions to form adduct or product ions; and (xxix) an Electron Ionisation Dissociation (“EID”) fragmentation device; and/or
(g) a mass analyser 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; and (xiv) a linear acceleration Time of Flight mass analyser; and/or
(h) one or more energy analysers or electrostatic energy analysers; and/or
(i) one or more ion detectors; and/or
(j) one or more mass filters selected from the group consisting of: (i) a quadrupole mass filter, (ii) a 2D or linear quadrupole ion trap; (iii) a Paul or 3D quadrupole ion trap; (iv) a Penning ion trap; (v) an ion trap; (vi) a magnetic sector mass filter; (vii) a Time of Flight mass filter, and (viii) a Wein filter and/or
(k) a device or ion gate for pulsing ions; and/or
(l) a device for converting a substantially continuous ion beam into a pulsed ion beam.
The mass spectrometer may further comprise either:
(i) a C-trap and an Orbitrap® mass analyser comprising an outer barrel-like electrode and a coaxial inner spindle-like electrode, wherein in a first mode of operation ions are transmitted to the C-trap and are then injected into the Orbitrap® mass analyser and wherein in a second mode of operation ions are transmitted to the C-trap and then to a collision cell or Electron Transfer Dissociation device wherein at least some ions are fragmented into fragment ions, and wherein the fragment ions are then transmitted to the C-trap before being injected into the Orbitrap® mass analyser; and/or
(ii) a stacked ring ion guide comprising a plurality of electrodes each having an aperture through which ions are transmitted in use and wherein the spacing of the electrodes increases along the length of the ion path, and wherein the apertures in the electrodes in an upstream section of the ion guide have a first diameter and wherein the apertures in the electrodes in a downstream section of the ion guide have a second diameter which is smaller than the first diameter, and wherein opposite phases of an AC or RF voltage are applied, in use, to successive electrodes.
An RF voltage is preferably applied to the electrodes of the preferred ion trap and preferably has 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; (xi) 500-550 V peak to peak; (xxii) 550-600 V peak to peak; (xxiii) 600-650 V peak to peak; (xxiv) 650-700 V peak to peak; (xxv) 700-750 V peak to peak; (xxvi) 750-800 V peak to peak; (xxvii) 800-850 V peak to peak; (xxviii) 850-900 V peak to peak; (xxix) 900-950 V peak to peak; (xxx) 950-1000 V peak to peak; and (xxxi)>1000 V peak to peak.
The RF voltage preferably has 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 ion trap is preferably maintained at a pressure selected from the group comprising: (i)>0.001 mbar; (ii)>0.01 mbar; (iii)>0.1 mbar; (iv)>1 mbar; (v)>10 mbar; (vi)>100 mbar, (vii) 0.001-0.01 mbar; (viii) 0.01-0.1 mbar; (ix) 0.1-1 mbar; (x) 1-10 mbar and (xi) 10-100 mbar.
Various embodiments of the present invention together with other arrangements given for illustrative purposes only will now be described, by way of example only, and with reference to the accompanying drawings in which:
An ion trap according to a preferred embodiment of the present invention will now be described with reference to
The preferred ion trap can be considered as comprising two horizontal layers of electrodes. Ions are confined in the vertical (y) direction (i.e. between the two horizontal layers of electrodes) by applying an RF voltage to the electrodes. Ions are confined in the vertical (y) direction by a non-quadrupolar pseudo-potential barrier or well.
With reference to
A quadratic DC potential is preferably maintained in the z-direction by applying a quadratic DC potential to the electrodes in the z-direction. As a result, ions are preferably confined in an ion volume 302 which is shown in
Ions may initially enter the ion trap in the z-direction and then the quadratic DC potential may be applied to the electrodes in the z-direction. Alternatively, the quadratic DC potential may be applied to the electrodes in the z-direction and ions may enter the ion trap in the x-direction.
With reference to
According to a preferred embodiment a DC quadratic potential is preferably superimposed on the RF voltages applied to the electrodes in the z-direction such that a DC potential well is formed in the z-direction as shown in
According to an embodiment a distributed cloud of ions may enter the volume of the ion trap through either open end (x-y plane) of the ion trap in the z-direction. The ions preferably move towards the DC potential minimum under the influence of the DC field and are confined in an ion confining volume which preferably comprises a rectangular prism as shown in
According to an embodiment a background gas may be provided in the ion trap volume in order to collisionally cool the ion cloud such that the ions are confined at the DC potential minimum in the z-direction and by the confining RF voltage in the y-direction. Ions are confined in the x-direction by applying confining potentials to the end electrodes in a manner as described above with reference to
The DC quadratic potential which is applied to the electrodes in the z-direction may be maintained on the end electrodes through matching segmentation in the z-direction with the main array of electrodes and applying the appropriate DC voltages.
According to the preferred embodiment the DC quadratic potential is preferably modulated in the z-direction in such a manner as to cause mass to charge ratio selective excitation and ejection of ions through the open ends of the ion trap in the x-y plane. Ions are therefore preferably ejected from the ion trap in the z-direction.
Ions ejected from the ion trap may be subjected to further analytical steps or the ions may pass to a detection system.
Embodiments of the present invention are contemplated wherein ions are mass or mass to charge ratio selectively ejected from the preferred ion trap in the z-direction in one direction only. According to an embodiment the quadratic potential which is maintained in the z-direction may be asymmetric in the sense that a quadratic potential may be maintained across a majority of the electrodes but some of the electrodes on one side of the ion trap may be maintained at a constant potential. As a result, a quadratic potential may be maintained which is effectively truncated on one side of the potential well in the z-direction. It will be apparent, therefore, that the maximum potential on one side of the potential well may be greater than the maximum potential on the other side of the potential well.
A DC quadratic potential well was modelled as being maintained in the z-direction along the length of the ion trap with 10 V at peak and with a half length of 9.5 mm. The ion trap was modelled as being 30 mm long in the x-direction. An RF excitation voltage of 8.5V (0-peak) was modelled as being applied to the electrodes. The excitation frequency was scanned downwards to eject ions in increasing mass order. A frequency ramp was calculated to give a linear 5000 Da/s mass scan. A singly charged ion having a mass of 500 was simulated. The buffer gas was modelled as being helium gas at a pressure of 4×10−3 mbar and a hard sphere collision model was used. Space charge was modelled using a super-ion approximation wherein each ion in the system represents a cloud of ions of a given total charge. The total charge in the system is thus the product of the number of ions flying simultaneously and the charge on each super-Ion.
Calculations were performed for: (i) no space charge; (ii) 30 ions with 10,000 charges each (i.e. 300,000 charges in total); (iii) 50 ions with 20,000 charges each (i.e. 1 million charges in total); and (iv) 60 ions with 50,000 charges each (i.e. 3 million charges in total).
It is apparent from
When the total number of charges was modelled as being 3×106 then the resolution dropped to about 14 i.e. about half of the resolution observed in the absence of space charge. The peak also shifts to higher mass.
A person skilled in the art will appreciate that the ion capacity of the ion trap according to the preferred embodiment is significantly larger than conventional 2D and 3D ion traps. For example, for comparison purposes, conventional 2D ion traps see a degradation in performance (i.e. a reduction in resolution and a shift in apparent mass position) when there are only of the order of 50,000 ions present in the ion trap.
It is apparent, therefore, that the on trap according to the preferred embodiment represents a significant improvement in the art compared to conventional 2D and 3D ion traps in terms of increased ion storage capacity.
An ion trap according to an alternative less preferred embodiment is contemplated and will be described in more detail with reference to
With the ion trap according to the alternative embodiment as shown in
According to an embodiment, which is not shown in
If the top and bottom rods are continuous or interconnected at both ends then the top and bottom rods form an oval shape or elongated ring arrangement. If the top and bottom rods are continuous or interconnected at just one end then the top and bottom rods form a C-shape, or half oval shape or half ring arrangement. According to these embodiments ions are preferably confined in the z-direction by a pseudo-potential well.
The ions are observed as being confined in a relatively large ion confinement volume which is elongated in two spatial directions.
An ion trap according to the preferred embodiment may be used in several different modes of operation.
In a mode of operation, the ion trap may be used as an ion transmission device and/or as a collision cell. This may be achieved by applying appropriate DC potentials to the electrodes so that one or more transmission channels exist through which ions my pass.
In another mode of operation DC potentials may be applied as discussed above with reference to the preferred embodiment. In an embodiment as shown in
According to an embodiment a device may be situated downstream of the preferred ion trap and may be used to collect and/or capture and/or focus the spatially extended beam of ions which is preferably ejected from the ion trap.
Alternatively, the collision cell and the preferred ion trap may be used separately i.e. the collision cell may be used to fragment and/or accumulate ions and the preferred ion trap may be used to hold and eject ions accumulated in the stacked ring ion guide.
As shown in
A substantially DC quadratic potential is preferably maintained in a radial direction across the electrodes in the first or upper layer of electrodes and/or across the electrodes in the second or lower layer of electrodes as shown in
According to the embodiment shown and described above with reference to
According to this embodiment the ion trap comprises a first group of electrodes and a second group of electrodes. The first group of electrodes comprises a first plurality of concentric closed loop, circular or oval electrodes arranged at different radial displacements. The second group of electrodes comprises a second plurality of concentric closed loop, circular or oval electrodes arranged at different radial displacements. The first and second groups of electrodes are arranged at different displacements in the first (y) direction.
According to this embodiment the ion trap comprises a first group of electrodes and a second group of electrodes. The first group of electrodes comprises a first plurality of annular groups of electrodes wherein each of the first annular groups of electrodes is arranged at different radial displacements. The second group of electrodes comprises a second plurality of annular groups of electrodes wherein each of the second annular groups of electrodes is arranged at different radial displacements. The first and second groups of electrodes are arranged at different displacements in the first (y) direction.
According to the embodiments shown and described above with reference to
A particularly advantageous feature of both embodiments is that the quadratic potential maintained in the radial (r) direction may be asymmetric such that when ions are excited and ejected from the ion trap in the radial (r) direction the ions are ejected towards the centre of the ion trap. As a result, ions may be ejected from a localised region of the ion trap and the geometry of the ion trap preferably enables ions to be either detected or onwardly transmitted using non-complex ion optics.
A further particular advantage of the ion trap as shown and described with reference to
Ions may be resonantly or parametrically ejected from the ion trap by applying a supplemental AC voltage to the first or upper layer of electrodes and/or to the second or lower layer of electrodes in order to cause ions to be ejected radially from the ion trap.
It will be apparent that various modifications may be made to the particular embodiments discussed above without departing from the scope of the invention.
For example, embodiments are contemplated wherein the electrodes comprising the ion trap may comprise electrodes which are not rod shaped. For example, the electrodes may comprise a plurality of stacked plate electrodes, a plurality of stacked ring or oval electrodes, a plurality of half ring or half oval electrodes or a plurality of C-shaped electrodes. Embodiments comprising ring or oval electrodes, half ring or half oval electrodes or C-shaped electrodes have been described above with reference to
According to a less preferred embodiment the applied DC potential may be non-quadratic.
According to an embodiment, the DC potential well may be deeper on one side of the ion trap than on the other side of the ion trap. As a result, ions are preferably ejected in one direction rather than being ejected in two directions.
According to an embodiment, the direction of exit of ions from the ion trap may be changed by changing the depth of the DC well appropriately such that all or a selection of ions preferably exit one way or all or a selection of ions preferably exit the other way.
According to an embodiment, the ion trap may be operated in a linked scanning mode of operation with the mass to charge ratio ejection of ions from the DC well linked with the mass to charge ratio scan of an adjacent mass analyser.
According to an embodiment, there may be more than one ejection region.
According to an embodiment, ions may be injected in one place and either ejected from the same location or from another spatially distinct region.
Although the present invention has been described with reference to preferred 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1114735.2 | Aug 2011 | GB | national |
This application claims priority from and the benefit of U.S. Provisional Patent Application Ser. No. 61/528,891 filed on 30 Aug. 2011 and United Kingdom Patent Application No. 1114735.2 filed on 25 Aug. 2011. The entire contents of these applications are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2012/052053 | 8/22/2012 | WO | 00 | 4/29/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61528891 | Aug 2011 | US |
