GB2147140A - Mass spectrometers - Google Patents

Abstract

A time-of-flight mass spectrometer, having means 16 for accelerating and directing ions from a sample 10 to form a beam with a predicted trajectory along a time-of-flight tube 14 to a detector 32, includes wire guide means 38, 40 extending along the trajectory and maintained at a potential of typically about +/- 15 volts, of the opposite polarity from that of the sample ions, for guiding the ions to the detector to improve the intensity and resolution of individual mass peaks detected thereby. The tube shown is of the reflectron type, with an ion reflector 30 which, in reflecting the ion beam back to the detector, compensates for any variations in energy of ions of the same mass. <IMAGE>

Description

SPECIFICATION Mass spectrometers The present invention relates to mass spectrometers.

In one known form of mass spectrometer, known as a time of flight mass spectrometer, a pulse of electromagnetic radiation, specifically but not exclusively laser light, or a beam of ions generated by a primary ion beam, is focussed on a sample to be analysed causing evaporation and ionisation of a small part of the sample. The ions so produced are accelerated and deflected by suitable electrostatic devices into a time of flight drift tube, in which they become separated according to their mass, and they arrive at an ion detector, where they are sequentially detected by an electron multiplier and recorded with a transient recorder.

A problem encountered with such time of flight detection systems is that instead of all of the ions leaving the sample having the same energy imparted to them by the pulse of laser light and the accelerator, they have a spread of energy which causes a spread in times at which ions of the same mass arrive at the detector, which is in addition to the separation between ions of different masses caused due to the time of flight between the sample and the detector. This limits the mass resolution of the system.

A recent innovation, described by Mamyrin and Shmikk in the publication Sovient Physics - JETP (English Translation) 1979,49,762, has been a device now known as a mass reflectron, which compensates for the difference in time of flight of ions of different energies, and thus improves the mass resolution of the system. The mass reflectron uses a system of electrostatic fields and an ion reflector which focuses the ions in space and time at a detector. Simply stated, the ion reflector allows ions with greater velocities to penetrate a greater distance into the reflector region than ions with slower velocities, effectively delaying the faster ions and compensating for the initial spread of ion velocities.

A problem with this type of design is that the path of the ions is now V-shaped as they pass down the tube from sample to the reflector and back along the tube to the detector.

This presents difficulties in focussing of the ion beam from the sample to the detector. In addition, the angular or spatial distribution (and hence the trajectory along the time of flight tube) of ions produced from a given laser pulse, varies from pulse to pulse, dependent on such factors as the sample composition, topography and reflectivity, and variation in laser power and pulse shape.

These problems manifest themseleves in a typical mass analyser, spectrum as variations in the intensity, resolution and position of individual mass peaks. In addition, the average intensity and the resolution is generally decreased.

It is an object of the present invention to provide a mass spectrometer in which these problems are alleviated.

According to the present invention there is provided a mass spectrometer in which electromagnetic radiation, or an ion beam, is focussed onto a sample to ionise a portion of the sample, and which includes means for accelerating and directing a beam of ions from the sample into a time flight tube and towards a detector, characterised by a wire guide which is maintained at an electrical potential which is of opposite polarity to the charge on the ions, said wire extending along the path of the predicted trajectory of the ions between the accelerating and directing means and the detector.

The potential on the wire guide is dependent upon the mean energy of the ions entering the time of flight tube, and for the typical acclerator voltages used, is likely to lie in the range 10-30 volts. A typical wire potential for a mean ion energy of + 2500 volts is ç 1 5 volts. It is a simple matter of experiment however, to determine the optimum value of the wire potential for maximum intensity and resolution of the system.

The wire guide may comprise more than one wire. In a simple time of flight mass spectrometer, i.e. without a reflectron to compensate for energy variations, a single wire extending from one end of the time of flight tube to the other, may suffice to improve the resolution of the system by reducing the spatial divergence of the ions as they pass down the tube. A plurality of wires may, however, be used in array, the exact shape of which, and the number of wires contained therein, being dependent upon the ion trajectories in the time of flight tube.

Possible arrays, for example, would include wires spaced around the surface of notional cylinder, or cone, with the wires extending parallel to the axis of the cylinder or cone, which in turn is colinear with the theoretical desired trajectory of the ions between the accelerator and the detector.

Where the time of flight mass spectrometer includes a reflectron deivce, two wires, forming a vee-shaped trajectory between the accelerator and the detector via the ion reflector would be needed.

The wires forming the guide are preferably made to have the smallest practical diameter to avoid obscuring the number of ions reaching the detector, and to ensure that relatively few ions are collected on it. A typical diameter would be 50 microns.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows in diagrammatic form the basic components of a time of flight mass spectrometer with which the present invention is concerned: Figure 2 shows in more detail the wire guide system which forms the basis of the present invention.

Referring now to Fig. 1 the basic elements of a known time of flight mass spectrometer are illustrated. A sample 10 of material being analysed is supported in a vacuum chamber 1 2 which is evacuated by a pump (not shown).

Attached to the vacuum chamber 1 2 and evacuated therwith is a time of flight drift tube 14, into which ions from the sample are directed by an electrostatic accelerator 16.

Mounted on top of the tube 14 is a laser device 1 8 which is capable of directing a high-power pulse of light onto the sample via an optical focussing system which includes mirrors 20, 22 and 24. The particular system of mirrors shown in this specification is described in more detail in our co-pending patent application No. 8309456 and is therefore only briefly described with reference to Fig. 2.

It is to be understood that the type of optical focussing system used is not a feature of this particular invention and alternative systems may be used.

To assist in setting up the device an optical viewing system 26 may be used in conjunction with a low-power pilot laser (not shown).

These and other features of the device which form no part of the invention herein described, such as the sample transport and mounting system, are not described in detail.

Referring now to Fig. 2, laser light which is reflected into the chamber from the laser 1 8 is directed by an inclined annular plane mirror 20 onto an annular convex mirror 22 from which it is reflected onto an annular concave mirror 24 which focussed the light past the mirrors 20 and 22 onto the sample 12. The central apertures of all of the mirrors are aligned to provide a passage through which the beam of ions 13 produced at the sample can be accelerated and directed to the time of flight drift tube 14.

At the opposite end 28 of the drift tube 14 is a reflectron device 30, described in the above-referenced paper to Mamynin and Shmikk, which reflects the ions back towards a detector 32 while at the same time compensating for the variations in energy of the ions.

The detector signal is amplified by an amplifier 34 and then passed to a recorder 36 for storage or display.

In order to accurately direct the ion beam from the accelerator 1 6 to the reflectron 30 and from the reflectron 30 to the detector 32 a pair of guide wires 38 and 40 are respectively provided in the drift tube 14 and extend along the predicted trajectory of the beam.

The wires are connected internally and maintained at a positive or negative potential depending whether negative or positive ions respectively are released from the sample.

Each wire is of the smallest practical diameter and the potential on the wires is kept low to avoid capturing any significant number of ions on the wire surface. The wires are typically of a diameter of 50 microns and their potential is typically 1 5 volts. The potential of the wires is maintained by connecting them to any convenient D.C. supply indicated at 42.

The electronic field created by the wires exerts a force on the ions which acts radially inwards towards the wire while having a negligible influence on the component of their velocity along the wire.

Ions which emerge from the ion accelerator 1 6 with velocities which would otherwise cause them to diverge from the desired path towards the reflectron 30 or the detector, are caused to follow the course of the wires in a spiral path.

It may be beneficial, to optimise the system, to provide a plurality of wires which may, for example, take the form of a circular array which lies on the surface of a notional cylinder, or cone, which surrounds the desired trajectory of the ions. The wires would in such an embodiment lie parallel to the axis of the notional cylinder or cone which in turn would be arranged to lie along the desired ion trajectory.

However, it is to be understood that the number of wires in the array, and the shape of the array, will depend upon the ion trajectories in the time of flight tube, which in turn will depend upon the radiation source used to generate the ions and the input to the accelerating and focussing system.

The preferred embodiment described above utilises laser light to energise the sample, but it is to be understood that other forms of electromagnetic energy may be used, or indeed a beam of ions generated by such energy, for example, as is known in secondary ion mass spectrometry.

Claims (8)

1. A mass spectrometer in which electromagnetic radiation or an ion beam is focussed onto a sample to ionise a portion of the sample, and which includes means for accelerating and directing a beam of ions from the sample into a time of flight tube and towards a detector, characterised by a wire guide which is maintained at an electrical potential which is of opposite polarity to the charge on the ions, said wire extending along the path of the predicted trajectory of the ions between the accelerating and directing means and the detector.

2. A mass spectrometer as claimed in Claim 1 and in which said wire guide extends along substantially the whole of the path of the ions.

3. A mass spectrometer as claimed in Claim 1 and in which the electrical potential at which the wire guide is maintained lies in the range 10-30 volts.

4. A mass spectrometer as claimed in Claim 3 and in which the wire guide is maintained at an electrical potential of 1 5 volts.

5. A mass spectrometer as claimed in any preceding claim and in which the diameter of the wire guide is about 50 microns.

6. A mass spectrometer according to any preceding claim and in which the ions enter the time of flight tube in a first direction and are reflected back in the opposite direction by means of a reflecting device towards the detector, and the wire guide comprises two wires extending between the accelerator and the reflector, and between the reflector and the detector respectively.

7. A mass spectrometer according to any preceding claim and in which a plurality of wire guides are used and which are formed into an array around the expected trajectory of the ions in the time of flight tube.

8. A mass spectrometer substantially as herein described with reference to and as shown in the accompanying drawings.