Method and apparatus for mass spectrometry

Abstract

A method for analyzing ions according to their mass-to-charge ratio and mass spectrometer for performing the method, comprising directing a collimated ion beam along an ion path from an ion source to an ion detector, causing a portion of the ion beam to contact one or more surfaces prior to reaching the ion detector, wherein the method comprises providing a coating on and/or heating the one or more surfaces to reduce variation in their surface patch potentials. The method is applicable to multi-reflection time-of-flight (MR TOF) mass spectrometry.

Claims

1. A method of analyzing ions according to their mass-to-charge ratio comprising directing a collimated ion beam along an ion path from an ion source to an ion detector, causing a portion of the ion beam to pass one or more electrically conductive surfaces prior to reaching the ion detector, wherein the one or more surfaces form collimating apertures to maintain the collimated ion beam, wherein the method further comprises reducing variation in surface patch potentials of the one or more surfaces by performing at least one of: (i) providing a coating on the one or more surfaces, wherein the coatings have a lower variation in surface patch potentials than a surface material on which it is coated, and (ii) heating the one or more surfaces.

2. A method as claimed in claim 1, wherein the ion beam is generated as a pulsed ion beam from a pulsed ion source.

3. A method as claimed in claim 1, wherein the method further comprises separating the ions according to their time of flight along the ion path.

4. A method as claimed in claim 1, wherein the ion beam undergoes multiple changes of direction between the ion source and the detector.

5. A method as claimed in claim 4, wherein the ion beam undergoes multiple reflections in ion mirrors.

6. A method as claimed in claim 5, further comprising providing two opposing elongated planar ion mirrors, wherein the collimated ion beam is repeatedly reflected between the mirrors whilst undergoing displacement in the direction of mirror elongation, the shift direction Z.

7. A method as claimed in claim 6, wherein the ion beam is collimated in the Z direction.

8. A method as claimed in claim 1, further comprising collimating the ion beam downstream of the ion source.

9. A method as claimed in claim 1, wherein the collimating apertures are periodically spaced apart.

10. A method as claimed in claim 1, wherein the divergence of the collimated beam is 1 mrad or less.

11. A method as claimed in claim 1, wherein the coating comprises a coating of an amorphous or polycrystalline material.

12. A method as claimed in claim 1, wherein the coating comprises a coating of graphite, gold, or molybdenum.

13. A method as claimed in claim 1, wherein the heating of the one or more surfaces comprises heating the one or more surfaces at a temperature in the range of 100 to 300? C.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) In order to more fully understand the invention, various embodiments will now be described in more detail by way of examples with reference to the accompanying Figures in which:

(2) FIG. 1 shows schematically an embodiment of the present invention; and

(3) FIG. 2 shows schematically a specific example of the structure and voltages of an ion mirror for use in the embodiment of the present invention shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(4) One preferred embodiment of the present invention is presented in FIG. 1. It is a multiple reflection time-of-flight mass spectrometer comprising two parallel planar mirrors 50 opposing each other as known in prior art. Improvements provided in accordance with the present invention are now described.

(5) Ions generated (from a device not shown but which could be any conventional device such as an electrospray ionisation) enter a linear RF-only storage trap or multipole 10 of a type described in described in WO 2008/081334 perpendicularly to the plane of the drawing and are initially stored within it. Whilst stored in the multipole, the ions lose energy in collisions with a bath gas therein (preferably nitrogen). After the ions are thermalized in this way, the RF is switched off from the multipole and the ions are radially extracted from it as a pulsed beam as described in WO 2008/081334. In the case of implementation in a TOF spectrometer, it will be appreciated that the ion source will be a pulsed ion source, i.e. to produce a pulsed beam of ions comprising short ion packets. A preferred pulsed ion source comprises an ion storage device, such as an ion storage trap, providing pulsed extraction of an ion beam therefrom, an example being the multipole arrangement 10 and more specifically such as the device of WO 2008/081334. The pulsed extraction may be radial or axial pulsed extraction from the storage device, preferably radial as described, for example, in WO 2008/081334.

(6) The pulsed beam from the storage trap 10 is extracted into a lens system 20. This lens system could include a deflector, or alternatively be tilted together with multipole 10, to define the initial angle of ion trajectory as it enters the first of the mirrors and thus its rate of drift in the shift direction Z. After that, the ion beam enters field-free region 30 and is allowed to diverge until it enters focusing lens 40 (indicated schematically by the double headed arrow). This lens 40 transforms the original beam extracted from the multipole into a parallel one with low divergence of preferably <1 mrad with corresponding increase of its width (i.e. its dimension in the direction perpendicular to Z).

(7) Thus, a low divergence along Z direction is achieved by transforming the initially thermalized ion beam from a small-diameter thread having a thermal spread of radial velocities into a wide ribbon with an ultra-low spread of transverse velocities (i.e. in the shift direction Z). For example, the transverse velocity v.sub.t could be presented as orthogonal energy: E.sub.t=mv.sub.t.sup.2/2. Then, if ions stored in the linear RF-only trap are radially extracted after removal of RF their initial E.sub.t can be limited, for example, by 25 to 50 meV and their initial radius by 0.1 to 0.2 mm. After acceleration by 10 kV voltage (presumed aberration-free), this corresponds to phase volume of 0.2 to 0.4 ?*mm*mrad. Using a lens with a focal length of F=200 mm located at the point corresponding to effective length F from the beam starting point, such a beam could be transformed into a beam of less than 10 mm full width and angular divergence of less than 0.2 mrad in the shift direction.

(8) After that, the collimated ion beam repeatedly reflects in ion mirrors 50 which comprise a plurality of electrode sections 52, 54, 56 and 58 to which suitable voltages are applied. It will be appreciated that four electrode sections are shown in Figure for simplicity but a greater or lesser number of electrode sections could be used as described further with reference to FIG. 2 below. As the ion beam repeatedly reflects between ion mirrors 50 it passes through the diaphragms 60 which define apertures 65 therein, i.e. collimating apertures. The diaphragms 60 are made of conductive material, typically a metal such as stainless steel, nickel coated aluminium or Invar. As the collimated ion beam continues to expand due to higher-order aberrations, its wings are increasingly clipped by diaphragms 60 and this is where surface patch potentials could be formed and could vary, thereby perturbing the beam. In accordance with the present invention, the surfaces of the diaphragms 60 forming the collimating apertures 65 are coated with material having low patch potential such as graphite or polycrystalline gold. Alternatively to such coating, or in addition, the diaphragms 60 may be heated, e.g. at a surface temperature from 100 to 300? C. to reduce the variation in surface path potentials. Thus, the collimated ion beam remains highly collimated and only its outer wings or edges become clipped.

(9) As the beam reaches the end of mirrors 50 at the maximum extent of travel in the shift direction Z, it may be detected by a detector. Alternatively, as shown in FIG. 1, the beam is sent on a return path by a deflector 70 thereby doubling the ion flight path length and increasing the resolution of the mass spectrometric separation. The deflector 70 could also be made as a multi-deflector using double-sided printed-circuit boards (PCBs), so that chromatic aberrations are reduced. Some of the ions may also be clipped by metal surfaces of the deflector 70, which if necessary could also be coated and/or heated as described above to reduce surface patch potentials of the deflector.

(10) After returning back on the return path along the trajectory shown by the dashed lines, the ions continue to get clipped by diaphragms 60 until they reach ion detector 80 and are detected. The detector may be any conventional type of ion detector, for example such as an electron multiplier or MCP.

(11) In FIG. 2 there is shown a specific example of the design and voltages for the ion mirrors in the embodiment of the present invention shown in FIG. 1. One of the mirrors is shown in side cross-sectional view in the X-Y plane, i.e. orthogonal to the shift direction Z in FIG. 1. The entrance to the ion mirror is located at the right hand side of the drawing and comprises an aperture 105 of reduced diameter compared to the internal diameter of the ion mirror. The rear of the mirror, which the ions do not quite reach as they penetrate into the mirror, is located at the left hand side of the drawing and shown by the vertical line 110. The central axis (i.e. Z axis) located mid-way between the two ion mirrors 50 in FIG. 1 is shown by the vertical line 115 at the right hand side of the mirror. The dimensions shown on the mirror in the drawing are given in millimeters. Whereas the ion mirrors 50 in FIG. 1 are shown comprising four electrode sections 52, 54, 56 and 58 for simplicity, the mirror in FIG. 2 comprises more than four sections. Each section comprises one or more conductive rods shown in cross-section by the circles. The rods are preferably made from a metal such as stainless steel, Invar, molybdenum, or nickel-coated aluminium. As an alternative to rods, plates or printed circuit boards could be used to form electrodes. The rod diameter in the example is 5 mm and the rod spacing (i.e. spacing between adjacent rods) is 8 mm. The mirror comprises a first electrode section closest the mirror entrance of 4 rods wherein the rods carry a voltage of 0 V in use. The next electrode section after that consists of 6 rods and carries a voltage U1x. The next electrode section after that consists of 8 rods and carries a voltage U2x. The next electrode section after that consists of 2 rods and carries a voltage 0 V. The next electrode section after that consists of 4 rods and carries a voltage U3x, followed by another section that consists of 6 rods and carries a voltage U4x and finally a last electrode section that consists of 6 rods and carries a voltage U5x. Examples of the voltages are shown (in volts) in the Table in FIG. 2 for ions initially accelerated by 2 kV. It will be appreciated that the voltages may be applied by a suitable power supply (not shown).

(12) Herein, the term mass-to-charge ratio (m/z) also includes parameters which can be converted into m/z, for example time-of-flight.

(13) Herein, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as a or an means one or more.

(14) Herein, the words comprise, including, having and contain and variations of the words, for example comprising and comprises etc., mean including but not limited to, and are not intended to (and do not) exclude other components.

(15) It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(16) The use of any and all examples, or exemplary language (for instance, such as, for example and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(17) Any steps described herein may be performed in any order or simultaneously unless stated or the context requires otherwise.

(18) All of the features disclosed herein may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non-essential combinations may be used separately (not in combination).