GB2371143A - Reflectron comprising plurality of electrodes each with a curved surface - Google Patents

Abstract

A reflectron for use with a mass spectrometer, the reflectron including a plurality of electrodes 11 each having a hyperbolic surface. The electrodes may comprise series of nested hyperboloid plates disposed along an axis of the reflectron about which they are centered. Some or all of the electrodes may include an aperture 12 to allow for the passage of ions therethrough. Where there are n<SB>o</SB> electrodes the shape of the surface of the n<SP>th</SP> electrode is given by <F>z<SP>2</SP>-r<SP>2</SP>/2=(n/n<SB>o</SB>)a<SP>2</SP></F> where z is the distance along the axis of the reflectron from its entry plane where z =0, r is the radial distance from the axis, a is a constant denoting the position along the axis at which the final electrode n<SB>o</SB> crosses the axis and o < n < n<SB>o</SB>.

Description

REFLECTRON

The present invention relates to a reflectron for use with a mass spectrometer.

It is known to use reflectrons, also referred to as ion mirrors, with a time of flight mass spectrometers in order to improve their resolution.

Examples are disclosed in US 5464985, GB 2153139A and GB 2303962A.

A reflectron typically has an axis of cylindrical symmetry and is arranged to retard and reflect ions by electrostatic forces travelling along the axis such that their trajectories are reversed. In practice ions are generally introduced at a slight angle to the axis so that they are reflected back along a path which is slightly displaced from the original path, towards a detector.

It is preferable that, as near as possible, an ion entering a reflectron executes one half cycle of simple harmonic motion before reaching the detector. As such the period of oscillation of the ion is substantially independent of injection velocity enabling the reflectron to accept a wide range of kinetic energies without loss of temporal focus.

To obtain the desired harmonic reflection it is necessary for the reflectron to have a potential distribution which is proportional to the square of the distance from the injection point along its axis, and rotationally symmetrical about its axis.

GB 2303962A discloses that a suitable potential distribution may be

generated by an electrode structure comprising an electrode with a conical surface facing an electrode with a hyperbolic or spherical surface, with the hyperbolic or spherical electrode maintained at a retarding potential relative to the conical electrode. There is, however, a major disadvantage with this approach for a practical apparatus. The two electrodes converge slowly as their radius increases. In order to prevent fringe field effects from distorting the potential distribution near the axis of the reflectron the electrodes must extend radially until they almost converge. This results in a device with an inconveniently large radial dimension, much larger than the typical distance of ion paths from its axis.

An alternative approach is disclosed in GB 2153139A, using a large number of ring-like electrodes as equipotential surfaces. This gives a close approximation to the desired potential, at least where the distance along the axis of the device is large, but because the ring-like electrodes are not the correct shape they perturb the potential distribution where the distance along the axis is small.

It is an object of the present invention to provide a reflectron which addresses the problems associated with the known arrangements discussed above.

According to the present invention there is provided a reflectron for use with a mass spectrometer comprising a plurality of electrodes each having a hyperbolic surface.

The electrodes provide equipotential surfaces of the correct shape to provide the desired potential distribution.

The electrodes preferably comprise hyperboloid plates. The electrodes are preferably nested. The electrodes are preferably disposed along an axis of the reflectron, about which they are each centered. Some or all of the electrodes preferably include an aperture, to allow for the passage of ions therethrough; the axis preferably extends through the apertures. The apertures preferably extend over less than 2% of the surface area of the electrodes. Preferably there are at least ten electrodes having a hyperbolic surface.

The reflectron preferably has an entry plane.

The desired potential distribution is given by: V (r, z) =Vo (z-r/2)/a for z in the range O < z < a, where z is the distance along the axis of the device from its entry plane (at z=0), r is the radial distance from the axis and a is a constant and denotes the position along the z axis at which the final electrode crosses the z-axis. This electrode corresponds to n = no.

For no electrodes the shape of the surface of the nth electrode is preferably given by: z2-r2/2 = (n/no) a2 where 0 < n < no It is preferable that the electrode located closest to the entry plane of the device crosses the z-axis at z=O, it will be appreciated that this

electrode will be conical in shape. This electrode corresponds to n=0.

In use the potential on the nah electrode is preferably given by : V (n) = (n/no) Vo where n = 0 to no The electrodes are preferably distributed along the axis according to the formula: z (n) =a (n/n The distance over which electrodes are distributed along the axis preferably exceeds the radius of the largest electrode.

The reflectron may form part of a mass spectrometer apparatus also comprising a means for generating ions from a sample, means for accelerating those ions and a field free drift space. The apparatus preferably also includes means for delaying the acceleration of ions after their generation, such that ions with the same mass to charge ratio are brought substantially to a time focus plane despite having differing initial velocities. This time focus plane preferably coincides with the entry plane of the reflectron. The apparatus preferably also includes an ion selector for selecting ions according to their arrival time at the end of the field free drift space.

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings of which: Figure 1 is a schematic drawing of a tandem time of flight mass

spectrometer incorporating a reflectron ; and Figure 2 is a schematic drawing of the electrodes of the reflectron of Figure 1 with z and r axes drawn on.

Referring to Figure 1 the illustrated apparatus comprises a first, time of flight, mass spectrometer A and a second, harmonic ion mirror reflectron, mass spectrometer B. A comprises an ion source 1, means for accelerating ions 2, a field free drift space 3, a collision cell 4 and an ion selector 5, all disposed in a vacuum chamber.

The means for accelerating ions 2 comprises three electrodes. The first electrode comprises a sample holder 6. The second electrode 7 is displaced from the first and includes an aperture through which ions may pass. The third electrode 8 is displaced from the second electrode and also includes an aperture through which ions may pass.

The ion source 1 further comprises means 9 for directing laser light onto the sample holder 6, to enable ionisation of a sample by matrix assisted laser desorption (MALDI).

In use, the field between the first 6 and second 7 electrodes is initially kept at zero. Laser light is then directed onto a sample in the sample holder 6. This releases matrix molecules and analyte molecules into the vacuum chamber as a rapidly expanding vapour cloud. Molecules within the expanding cloud typically take a range of velocities from 200 to 2,000 meters per second. During formation of the cloud a small fraction of the

molecules are ionised ; this includes the matrix molecules as well as the analyte molecules. lonisation of analyte molecules continues during the expansion of the vapour cloud by ion-molecule reactions at the expense of the matrix ions.

A short time afterwards an electric field is suddenly applied between the first 6 and second 7 electrodes to accelerate the ions in the cloud through the aperture in the second electrode 7 and a constant accelerating field is maintained between the second 7 and third 8 electrodes to accelerate the ions through the aperture in the third electrode 8 and into the field free drift space 3.

The delay between the formation of a vapour cloud by laser light and application of a field serves to increase the mass resolving power of the spectrometer, through so-called"time-lag"focussing. The principle of this method is simple: the ions of the vapour cloud are allowed to fly apart at first for a brief time in a field free region. The faster ions thereby separate further from the sample support electrode 6 than the slower ions, and from the velocity distribution of the ions, a location distribution results. Then the accelerating field is applied. As the field is applied, the faster ions, having drifted further from the sample electrode 6, find themselves at a lower potential than the slower ions. As the faster ions leave the ion source region they have a lower ultimate velocity than the slower ions. At some point in the field free drift space the ions which were slow but are now fast

will overtake the ions which were originally fast. This plane is a plane of first order time focus for ions of a given charge to mass ratio. The position of the time focus plane can be adjusted by selection of the magnitude of the accelerating field and the delay time before it is applied. With the present apparatus the time focus plane is arranged to coincide with the entrance plane of B.

After acceleration, ions travel down the field free drift space 3 to the collision cell 4, in which ion molecules may be caused to dissociate through collision with a gas atom. In one arrangement the source of collision gas atoms comprises a pulsed gas valve arranged to release gas atoms into the collision cell 4 such that the local pressure is highest when the ion of interest is drifting through the collision cell 4.

Alternatively, dissociation may occur via unimolecular dissociation.

Unimolecular dissociation describes the process of molecular dissociation in the absence of inter-molecular collisions. This process is more likely to occur when a neutral molecule becomes ionised as the molecule then has to accommodate a perturbed electronic configuration and the process of reorganisation can result in the molecule becoming metastable. Dissociation can then occur within a few microseconds. If dissociation occurs in the field free drift region of a time of flight mass spectrometer the dissociation products, fragment ions and neutral sub-molecules, will continue to drift with the same velocity. If the kinetic energy of a precursor molecular ion

of mass Mo, is given as Eo, then the kinetic energy Em of the fragment of mass m, is given by : Em = (m/MO) Eo Molecular ions such as peptides containing a series of amino acid residues which undergo dissociation can lead to considerable array of dissociation products with masses ranging down to a few percent of the parent ion mass. The result is a collection of dissociation products, all drifting with the same velocity as the precursor ion, but with a very large range of kinetic energies. Efficient detection of the full range of dissociation products demands that B is capable of accepting this great range of kinetic energies without loss of temporal or spatial focus.

After passing through the collision cell 4 ions pass through the ion selector 5. This comprises timed ion gate selector and may take the form of a pair of electrostatic deflection plates to which positive and negative voltages may be applied. When an ion of interest approaches the ion gate these voltages are suddenly and simultaneously brought to zero. As the ion passes through the gate region the voltages are restored to their initial values. The gate may be made inactive by holding the voltages on the plates to zero.

The ion gate selector is used to deflect ions, so that they do not enter B, until a time from the issue of an ion pulse equal to the expected drift time of ions of interest has elapsed. The potential on the selector plates is then

reduced to zero to allow the ions to pass into B.

B comprises a plurality of nested hyperboloid plate electrodes 11, shown in Figure 2, each having an aperture 12 through their centre. The electrodes are disposed along and centered about an axis, the z-axis, of the instrument. The final electrode crosses the z-axis at z=a. A conical electrode crosses the z axis at z=0.

Where there are no electrodes the shape of the surface of the nth electrode is given by: z2-r2/2 = (n/no) a2 where 0 < n < no.

Where z is the distance along the axis of B from its entry plane and r is the radial distance from the axis. a is the position at which the electrode for which n = n0 crosses the z-axis.

In use, the potential of each electrode is given by: V (n) = (n/no) Vo where n==0 to no.

The electrodes are distributed along the axis according to the formula: z (n) =a (n/no) The electrodes provide equipotential surfaces and generate a potential about the axis of B which corresponds to: V (r, z) = Vo (z/2)/a2 for z in the range O < z < a, where a is a constant. This potential causes ions entering B to perform one half cycle of harmonic motion and be reflected towards detector 10.

The potential distribution produced by the electrodes of the reflectron is closer to the ideal potential distribution for such an instrument than those of prior art devices. This leads to improved resolution of the instrument.

The shaded region 12 of Figure 2 shows the ion trajectory range. 13 denotes the entrance aperture into the electrode system.

The above embodiment is described by way of example only, many variations are possible without departing from the invention.

Claims (18)

1. A reflectron for use with a mass spectrometer comprising a plurality of electrodes each having a hyperbolic surface.

2. A reflectron as claimed in claim 1 comprising at least ten electrodes having a hyperbolic surface.

3. A reflectron as claimed in either claim 1 or 2, wherein the electrodes comprise hyperboloid plates.

4. A reflectron as claimed in any preceding claim, wherein the electrodes are nested.

5. A reflectron as claimed in any preceding claim, wherein the electrodes are disposed along an axis of the reflectron, about which they are each centered.

6. A reflectron as claimed in claim 5, wherein some or all of the electrodes include an aperture, to allow for the passage of ions therethrough and the axis of the reflectron extends through the aperture (s).

7. A reflectron as claimed in claim 6, wherein the aperture (s) preferably extend (s) over less than 2% of the surface area of the electrode (s).

8. A reflectron as claimed in any of claims 5 to 7 having no electrodes wherein the shape of the surface of the nth electrode is given by Z2~ r2/2 = (n/no) a2 where z is the distance along the axis of the reflectron from its entry plane where z=O, r is the radial distance from the axis,

a is a constant denoting the position along the axis at which the final electrode no crosses the axis and 0 < n < no.

9. A reflectron as claimed in claim 8, wherein the electrode located closest to the entry plane crosses the axis at z = O.

10. A reflectron as claimed in either claim 7 or 8, wherein in use the potential on the nah electrode is given by: V (n) = (n/no) Vo where n = 0 to no.

11. A reflectron as claimed in either claim 9 or 10, wherein the electrodes are distributed along the axis according to the formula : z (n) =a (n/no) .

12. A reflectron as claimed in any of claims 8 to 11, wherein the distance over which electrodes are distributed along the axis exceeds the radius of the largest electrode.

13. Mass spectrometer apparatus comprising a reflectron as claimed in any preceding claim.

14. Mass spectrometer apparatus as claimed in claim 13 and further comprising a means for generating ions from a sample, means for accelerating those ions, a field free drift space and means for delaying the acceleration of ions after their generation such that ions with the same mass to charge ratio are brought substantially to a time focus plane despite having differing initial velocities.

15. Mass spectrometer apparatus as claimed in claim 14, wherein this time focus plane coincides with the entry plane of the reflectron.

16. Mass spectrometer apparatus as claimed in either claim 14 or 15 and further comprising an ion selector for selecting ions according to their arrival time at the end of the field free drift space.

17. A reflectron substantially as herein described with reference to the accompanying drawings.

18. A tandem time of flight mass spectrometer substantially as herein described with reference to the accompanying drawings.