WO1998033203A1

WO1998033203A1 - Gate for eliminating charged particles in time of flight spectrometers

Info

Publication number: WO1998033203A1
Application number: PCT/AU1998/000041
Authority: WO
Inventors: Stephen Davis; Dmytro Bandura
Original assignee: Gbc Scientific Equipment Pty. Ltd.
Priority date: 1997-01-28
Filing date: 1998-01-28
Publication date: 1998-07-30
Also published as: AUPO481097A0

Abstract

An analysis device such as a time of flight mass spectrometer is disclosed which includes an ICP torch (6) for producing a plasma of bath gas and sample material which is to be analysed. Beam forming optics (13) are provided in order to create a beam of ions of bath gas and ions for delivery to an orthogonal accelerator (17) which pushes out ion packets from the beam. An ion mirror (21) flex the beam towards a detector (24). Arranged between the orthogonal accelerator (17) and the ion mirror (21) is a gate (19) which is formed from a plurality of elements A, B, C, D which are arranged parallel to the direction of beam travel from the orthogonal accelerator to the ion mirror (21). The power supply (28) supplies voltages to the gate (19) so that the elements A, B, C, D create deflection fields for deflecting charged particles sideways so they are not received via the detector (24). The power supply (28) momentarily changes the state of the elements A, B, C, D from time to time so as to selectively remove various charged particles from the beam which will be detected by the detector (24).

Description

GATE FOR ELIMINATING CHARGED PARTICLES IN TIME OF FLIGHT SPECTROMETERS

This invention relates to a gate for eliminating charged particles in time of flight analysis devices and to a time of flight analysis device including the gate. The analysis device is typically a time of flight mass spectrometer but the invention may be embodied in other types of analysis devices in which it is necessary to eliminate some charged particles from other charged particles in order to analyse a sample.

Selective removal of ion packets of predetermined characteristics from an ion beam may be used in time of flight mass spectrometry (TOF MS) for detector dynamic range enhancement and protection. To date, TOF MS have employed either micro channel plate or discrete dynode electron multipliers for primary ion detection. These instruments are not able to analyse samples in which the species of interest at low concentrations are accompanied by matrix ions at concentrations many orders of magnitude higher. Typical example of such a situation is matrix assisted LASER desorption (MALDI) TOF MS, where light atomic weight ions are present in higher abundance than heavy molecular species of interest. Another typical example is a TOF MS with inductively or capacitively coupled plasma ion source or glow discharge ion source, where analyte ions are usually accompanied by much more abundant ions of a bath gas and products of solvent ionisation. The same situation appears for any TOF instrument if one wants to measure the concentration of ultratrace impurities in any highly ionisable matrix. In all of those cases the detector, if not protected, is swamped by the intense pulse of matrix ions and effectively blinded to the far less abundant analyte species following it. As distinct from other types of mass analysers, conventional TOF mass analysers, being truly simultaneous analysers, have no means of filtering a particular mass or mass range of ions from the input beam to prevent their striking the detector. At best such a situation leads to the detector being temporarily saturated by the high intensity pulse and unable to detect the species of interest. At worst, amplification of the pulse leads to excessive currents being drawn through the device and results in physical damage. The recovery times of the detectors used are such that it is not practicable to switch the detector off for the duration of the carrier ion pulse and then return it to a stable operating condition in time to record the remainder of the mass spectrum. A better approach has been established by several groups and employs devices positioned within the mass analyser which act as a selective ion gate.

Elimination of low molecular weight components from MALDI TOF spectra (C D Hanson, C L Just An Chem 66(1994), 3676) has been done by pulsed electrostatic particle guide (EPG) in order to prevent an ion detector from saturation by ions undesirable for detection. In the devices like pulsed EPG, ions of interest are "guided" to the detector by electrostatic field of an isolated wire electrode situated along the central axis of the flight tube, and undesirable ions are deflected from the beam by supplying a voltage pulse of suitable duration and magnitude at a predetermined time after acquisition starts. This helps to improve dynamic range of microchannel plate ion detection. The limitation of this method is poor ability to eliminate ions of single mass without effecting ions of adjacent mass.

The demonstrated selectivity of the method was 15 amu at m/z 300.

Another known way of selective ions removal from the beam in TOF MS is deflecting them from an original path in a TOF drift tube by a pulsed electric field formed by a pair of deflection plates (see for example Myers D P, Li G, Yang P, Hieftje G M JASMS, 1994, 5, 1008-1016) . In conventional straight tube TOF, where iso-mass ion packets are sufficiently resolved from each other only at the detector, the selectivity of such a method is quite poor.

A more selective method of removal of ions of predetermined mass-to-charge ratio is deflecting ions at the first spatial focus (See W C Wiley, I H McLaren, Rev Sci Instrum 1955, 26,1150-1157 for definition of spatial focus) in so called reflectron TOF spectrometers (See for example B A Mamyrin et al, Sov Phys -JETP, 1973,37,45-48) by use of either deflection plates or an interleaved comb ion deflection gate (See P R Vlasak et al, Rev Sci Instrum 67(1996), 68). The interleaved comb device has been demonstrated to be more effective in achieving high selectivity in ions removal, mainly due to the much shorter length of deflection field needed to achieve the same deflection effect. As a result, very good mass selectivity is achievable. With ions at 167 amu being completely eliminated from the spectra, the intensity of an adjacent peak at 168 amu is diminished only to 88% of the original value.

Both deflection plates and interleaved comb devices employ short duration (order of 100 ns) high voltage (order of 200 V) pulses supplied to alternative electrodes of the gate. This puts a limitation on the method, mainly in repetition frequency at which the gate can be switched on and off. The limitation becomes particularly important if one wants to use TOF MS for elemental analysis at a high duty cycle in order to get sensitivities competitive with other types of mass spectrometers. In that case typical repetition frequencies of 10-50 kHz need to be used, with ions accelerated to 3-4 keV of energy and full single mass spectrum collected in 20-100 uS. In this case the typical iso mass packet temporal width at the first spatial focus is of the order of <10 ns and time intervals between adjacent masses are of the order of 100 ns. In order to be able to filter out one mass per each single scan, one would need a gate producing 10-50 ns pulses at 50 kHz. When integrating multiple mass spectra is used, time jitter of gate pulses should be of the order of 1 ns or less.

Up to date, the techniques of generating very short duration square high voltage pulses have been based on the use of bipolar transistors in avalanche mode (see for example R J Baker, Rev Sci Instrum 62(1991), 1031), high voltage relays (see for example A Schuette, H Rodrigo, Rev Sci Instrum 67(1966), 3759), or power MOSFETs (see for example R J Baker, M D Pocha, Rev Sci Instrum 61(1990), 2211) as high voltage switching devices, and coaxial charge lines for a pulse shape forming. Another way recently developed is totem-pole power MOSFET configurations (see for example M T Bernius, A Shut ian, Rev Sci Instrum 60(1989), 779), with improvements made for better timing adjustments (M T Bernius, A Chutjian, Rev Sci Instrum, 61(1990), 925) and stacking MOSFETs for higher voltage switching (R J Baker, B P Johnson, Rev Sci Instruments, 63(1992), 5799). None of these approaches is able to produce a pulse generator with parameters needed. The main reasons are: for avalanche transistors - repetition frequency limitations due to power dissipation factors; for relays - high jitter and low repetition frequency; for single MOSFET as switching device - high switch off impedance; for totem-pole MOSFET configuration - sensitivity of peak shape to changes in duty cycle due to temperature effects on unevenly loaded transistors.

The interleaved comb ion deflection gate made out of wires has one more disadvantage, namely sensitivity of gate resolution on geometrical alignment of individual wires, which is physically not easy to achieve.

The object of the present invention is to provide a gate for charged particles which would be able to selectively remove charged particles from the beam at elevated frequencies typical for TOF MS . Although being aimed at usage in TOF MS, the invention may be used in any other fields where beams of charged particles need temporal encoding or modulation, for example for selective filtering of the beams of elementary particles in nuclear physics, and for selective removal of hypervelocity macroparticles from the beam in experimental simulation of hypervelocity projectile impact etc.

The invention may be said to reside in a gate for eliminating charged particles in a time of flight analysis device, said gate including: a first deflection means for deflecting the charged particles; a second deflection means for deflecting the charged particles; and field control means coupled to the first and second deflection means for causing the first and second deflection means to create deflection fields and for momentarily changing the state of the deflection field of one or the other of the first and second deflection means .

The invention may also be said to reside in a time of flight analysis device, including: means for producing a beam of charged particles; a detector for detecting the charged particles and for enabling a parameter of the charged particles to be determined; and a gate for eliminating charged particles from the beam so that selected ones of the charged particles arrive at the detector for detection, said gate having; i) a first deflection means for deflecting the charged particles; ii) a second deflection means for deflecting the charged particles; and iii) field control means coupled to the first and second detection means for causing the first and second detection means to create deflection fields and for momentarily changing the state of the deflection field of one or the other of the first and second deflection means.

According to the present invention, the provision of the first and second deflection means enables two sets of stepped voltages to be supplied to the first and second deflection means to create the deflection fields. In order to allow all charged particles to pass through the gate, the deflection fields are merely arranged in opposite directions with respect to one another so that a deflection caused by the first field is compensated for by the deflection caused by the second field so all the particles pass through the gate. In order to selectively remove some of the particles from the beam, one of the deflection fields is changed to effectively provide a closed gate to eliminate some of the charged particles. In order to provide the deflection fields, stepped voltages can be applied to the first and second deflection means with the duration of closure of the gate being defined by the delay between the two sets of stepped voltages. This time delay can be much easier controlled and established at much smaller values than the width of short duration pulses.

Thus, the present invention does not rely on short duration pulses in order to produce deflection fields which eliminate some of the charged particulars, but enables stepped voltages to be applied with the delay time between two stepped voltages being used to create a deflection field to eliminate some of the charged particles which are not required for detection. Thus, high repetition frequency can be obtained with the present invention without the inherent difficulties of the prior art techniques which rely upon short duration pulses.

Thus, the elimination of particles is achieved by the momentary switching of deflection fields of different deflection means. Because the switching between the states is done by a change of field direction of physically different deflection means and stepped voltages can be used instead of conventional very short duration high pulses, much higher switching frequencies are achieved.

Preferably the field control means causes the first and second deflection means to create deflection fields and for momentarily changing the state of the deflection field of one or the other of the first and second deflection means in such a manner that a duration of the gate being in an opened or closed state for the charged particles is defined by a time delay between changes applied to the states of the deflection fields of the first and second deflection means .

Preferably the first and second deflection means are spaced apart in the direction of travel of particles passing through the gate. However, in the other embodiments, the first and second deflection means may be at the same position with the second deflection means arranged with the first deflection means.

In the above embodiments, the first and second deflection means are parallel to one another. However, in other embodiments the ions may be deflected by a magnetic field so the ions travel in a curved path and the first and second deflection means may be provided in a non-parallel arrangement along the curved path.

In the preferred embodiment of the invention, two deflection means are provided. However, in other embodiments, more than two deflection means could be utilised.

Preferably the deflection means are arranged such that at least some of the particles pass through at least one of the deflection means whilst others of the particles pass through another of the deflection means.

Preferably the deflection means are positioned parallel to each other.

Preferably the main component of the deflection fields produced by the deflection means are orthogonal to the direction of travel of the particles.

Preferably in order to maintain particle energy distortion at a minimum, the main component of the confined deflection field of the deflection means is parallel to the direction in which the velocity spread of the particles is minimal.

Preferably the deflection means comprise pairs of parallel plates.

In other embodiments, the deflection means may be formed by parallel wires. The parallel wires may be interleaved with one another.

In other embodiments of the invention, the first and second deflection means comprise three or more deflection elements defining a plurality of deflection gaps with at least one of the elements forming the plurality of gaps being common for adjacent gaps.

Preferably the control means coupled to the deflection means comprises means for applying stepped voltages to the deflection means .

Preferably the magnitude of the stepped voltages is the same.

Preferably the time at which switching of the gate from an open state allowing particles to pass through the gate to a closed state eliminating at least some of the particles takes place is related to the velocity or mass to charge ratio of particles to be deflected or allowed to pass through the gate.

Preferably a particle collector is provided for collecting particles deflected by the gate.

Preferably the collector comprises at least two plates positioned behind the gate.

Preferably, in order to shield particles leaving the gate, from gate potentials, said collector is kept at the potential of the drift space surrounding said gate thereby acting as a shield.

Preferably, electronic means is connected to the collector for charge measurements in order to measure charge of particles deflected by the gate or passing through the gate.

Preferably an electrical shield is positioned before the gate.

Preferably the shield is kept at a potential of the drift space surrounding the gate.

Preferably electronic means for charge measurement is connected to the shield for measuring the charge of particles passing through the shield towards the gate.

In the preferred embodiment of the invention, the deflection means are formed by electrodes via the metallised surfaces of printed circuit boards.

The invention may also be said to reside in a method of deflecting particles from a beam of charged particles, including the steps of: providing a first deflection means; providing a second deflection means; energising the first and second deflection means to create deflection fields for deflecting the charged particles and momentarily changing the direction of the deflection field of at least one of the deflection means to deflect some of the particles out of the beam.

Preferably the change of direction of the deflection field is caused by applying stepped voltages to the deflection means and momentarily switching the stepped voltage applied to one of the deflection means.

Preferably the charge of particles or groups of particles is monitored prior to the deflection means with the momentary change of direction of at least one of the deflection fields occurring when the measured charge exceeds a preset value.

Preferably the time at which said deflection means is switched to a closed state is defined by the time particles of predetermined characteristic travel within certain predetermined drift space prior to entering said gaps.

Preferably the potentials supplied to the deflection means are chosen according to the degree of attenuation desirable.

The invention may also be said to reside in a gate for deflecting charged particles, including: a plurality of spaced apart printed circuit boards, each circuit board having; a) a first region defining a shielding electrode; b) a second region, electrically separated from - li ¬

the first region, defining an element for creating a first deflection field; c) a third region, electrically separated from the first and second regions, defining an element for creating a second deflection field; and d) a fourth region electrically separated from the first to third regions, defining a collector.

Preferably the regions are electrically separated by opening through the printed circuit board.

Preferably the board has first and second sides and the first to fourth regions are provided on both sides of the board.

Preferably the regions are formed by metallic layers or coatings on the board.

Preferably the openings have metallic coatings on sides thereof to electrically connect the first regions on both sides of the board, the second regions on both sides of the board, the third regions on both sides of the board, and the fourth regions on both sides of the board.

A preferred embodiment of the invention will be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a time of flight mass spectrometer with which the present invention is used;

Figure 2a is a diagram showing deflection fields of the gate embodying the invention in an open state;

Figure 2b shows deflection fields of the gate in the closed state; Figure 2c shows deflection fields in an open state;

Figure 2d shows deflection fields in a closed state ;

Figure 2e shows the deflection fields in an open state.

Figure 3 shows a timing diagram of potential supply to the gate of the preferred embodiment;

Figure 4 shows the structure of the gate according to one embodiment of the invention;

Figure 5 shows the structure of the gate according to a further embodiment; Figure 6 shows the structure of the gate in yet a further embodiment;

Figure 7 shows the structure of the gate according to yet another embodiment;

Figure 8 is a plan view of a gate according to a still further embodiment of the invention; and

Figure 9 is a cross-sectional view along the line IX-IX of Figure 8.

The preferred embodiment of the invention will be described with reference to a time of flight mass spectrometer.

However, it should be understood that the invention could be utilised in other environments.

Figure 1 shows general layout of a time of flight mass spectrometer using inductively coupled plasma as an ion source. The mass spectrometer of Figure 1 includes a radio frequency generator 1 for supplying radio frequency power via a matching network 4 to an inductance coil 8, and a gas control unit 2 for supplying bath gas such as argon or helium to an ICP torch 6 and a sample introduction system

3. The sample introduction system 3 supplies gas flow with particles of a sample dispersed in it for injection into plasma. The ICP torch 6 and the inductance coil 8, located within a torch compartment 7, produce plasma consisting of species of bath gas and sample material which is to be analysed which plasma is schematically shown by reference numeral 9. Plasma is sampled through orifice of a sampler cone 10a into an evacuated by a rotary pump 11 interface chamber 10 and then further into an intermediate vacuum chamber 12 through an orifice in a skimmer cone 10b. The intermediate chamber 12 is pumped by a turbo-molecular pump 14. A beam forming optics 13 is provided within a side vacuum chamber 15 behind third orifice 15a along the path of the sampled plasma jet for creating a beam of ions of bath gas and ions of species representing the sample. Reference number 5 represents a beam forming optics power supply, and reference number 16 represents said ion beam which is to be analysed by means of a time of flight analyser located further downstream within a main vacuum chamber 29. A turbo-molecular pump 30 provided for pumping the main chamber down to acceptable pressure for analysis pressure.

An orthogonal accelerator 17 is provided for pushing out a multi-mass ion packet from the beam 16. A push out pulse supply 18 is coupled to the accelerator 17 for providing repetitive push-out voltage at typical frequency of 40 kHz.

The dropped out ions or ion packets which are moved out of the beam 16 travel then within field free space of liner 20 towards ion gate 19, being partially separated in time into iso-mass ion packets shown schematically by reference number 31. The ion gate 19 and ion gate power supply 28 provided for powering the ion gate 19 will be described in more detail hereinafter. The potential of liner 20 is supplied to it by a liner power supply 23. An ion mirror 21 provided at the end of the liner 20, and an ion mirror power supply 22 is provided. The ions passing through the gate 19 travel down the field free space of the liner 20 and are turned about by the ion mirror 21 to travel back towards a detector 24. The detector 24 detects the ions and is coupled to an amplifier 25 which in turn is coupled to a detection system 26 which in turn can be coupled to a computer 27 for analysing and displaying results. The sample ions which are to be analysed by the mass analyser are entrained in an excess of bath gas ions usually prevailing in the plasma. If the sample ons have a mass-to-charge ratio which is close to that of bath gas ions, it is likely the sample ions will not be detected if the bath gas ions also arrive at detector 24. Also, if the sample to be analysed consists of the elements of adjacent atomic weights, one of which represents a matrix of the sample and much more concentrated than another, it is likely the ions of less abundant element will not be separated from the tails of adjacent peak of more abundant element. That is, the ions of less abundant species in the beam will simply be seen as forming part of the more abundant matrix peak and not be resolved from it. Moreover, an excessive ion current striking the detector will gradually reduce its gain, influencing detector response for further measurements and shortening its life time. In order to overcome these problems, unwanted ions are removed from the beam received by the detector 24 by means of the gate 19. If the gate 19 is able to remove all of the unwanted ions, but leave the ions of interest to pass through the gate, then the ions of interest will reach the detector and therefore the ions of interest, even if extremely low in concentration, will be identified.

Figures 2a to 2e show the gate 19 according to a preferred embodiment of the present invention in schematic form. The gate 19 comprises a first pair of elements A and B for creating a deflection field and a second pair of elements C and D which are spaced from the first pair of elements A and B in the direction of travel of the ions for creating a deflection field. If the elements A and B and C and D are creating deflection fields which have field vectors, as shown by the arrows in Figure 2a, so that the two fields are directed opposite to each other, ions deflected from the beam by the first field (that is, by the elements A and B) will be returned back by the deflection field created by the elements C and D. Thus, all of the ions will effectively pass through the gate 19 and the gate may be said to be in an open state.

Referring now to Figure 3, the potentials applied to the elements A to D are shown in Figure 3 and the potentials applied to create the opposite fields as shown in Figure 2a are the potentials shown at time t₀ in Figure 3.

If at time ti (see Figure 3) the electrical potentials of elements C and D are changed as shown in Figure 3 so that the deflection field of the elements C and D is directed in the opposite direction as shown in Figure 2b, most of the particles are deflected from the beam by the combined action of the two deflection fields produced by the elements A and B and elements C and D and the gate may be said to be in a closed state so effectively none of the ions would be received by the detector 24.

At time t₂ the potentials on the electrodes A and B are momentarily switched as shown in Figure 3 so that the deflection field created by the elements A and B is reversed from that shown in Figures 2a and 2b to that shown in Figure 2c. In this configuration, most of the ions deflected by the elements A and B are returned by the elements C and D and the gate may again be said to be in an open state.

At time t₃ the potentials applied to the elements C and D are switched as is shown in Figure 3 so that the field created by the elements C and D is reverse to that shown in Figure 2c as is shown by the arrows in Figure 2d. Once again, the gate 19 may be said to be in the closed state because all of the ions are deflected out by the combined effect of the two fields created by the elements A and B, and C and D. At time _u the field created by the elements A and B is reversed as is shown in Figure 2e and once again the gate may be said to be in an open state where the ions can pass to the detector 24.

As can be seen from Figure 3, the duration of the gate closed states are defined by the time delay between the two sets of voltages which are applied to the elements A and B, and C and D. The switching of the voltages applied to the elements A and B, and C and D is performed by applying stepped voltages to the elements and to open and close the gate, the time delay between switching of the two sets of elements is utilised. Thus, rather than applying short duration pulses which are much harder to generate and control in order to open and close the gate, the gate is opened and closed by applying two sets of stepped voltages to two different sets of elements and by overlapping the stepped voltages, as shown in Figure 3, to create either oppositely directed deflection fields or deflection fields in the same direction, ions can be selectively deflected or allowed to pass through the gate 19. The time delay between the two sets of stepped voltages can be much easier controlled and established to much smaller values than the width of short duration pulses and stepping high voltages at high repetition frequency (in the order of 1 MHz) technologically is much easier than generating short duration (such as 10 ns) voltage pulses.

Thus, the preferred embodiment of the invention provides an extremely high degree of control of opening and closing the gate and high repetition frequency so that unwanted ions which are of very similar atomic weight to those which are desired to be detected, can be eliminated from the electron beam 16 so that desired ions only will arrive at the detector 24 for detection.

In the embodiment of Figure 2, the elements A, B and C, D define a single deflection gap and all of the beam passes through the elements and therefore the deflection gap when the gate 19 is in the open state.

In the embodiment of Figure 4, a plurality of elements A and B create a plurality of gaps for the first deflection field and a plurality of elements C and D create a plurality of gaps for the second deflection field. Thus, some of the ions will pass through a particular gap between one of the elements A and one of the elements B and others will pass through a different gap between another of the elements A and B. Similarly, ions will pass through one or other of the gaps created by the elements C and D. Thus, Figure 4 shows an arrangement where parallel arrangements of elements A and B, and C and D produce sets of complementary deflection gaps rather than merely a single deflection gap between a pair of elements as in the embodiment of Figure 2.

Figure 5 is an arrangement similar to Figure 4, except that two parallel sets of elements A and B, and C and D are provided. The arrangement of Figure 4 effectively creates more than one gate in series and by doing so creates more particle deflection effect per unit of stepped voltage.

The elements A, B, C and D may be in the form of plates, wire electrodes or the like. The' elements A and B, and C and D may be of identical shape and stepped voltages applied to the elements A, B, C and D may have the same absolute value or magnitude. Due to "check order" of identical potentials applied to adjacent elements of identical shape, the combined deflection field of the gate becomes sharply defined in space with opposite potentials cancelling each other in the vicinity of the gate and not influencing the trajectories of particles far from the gate. Figure 6 shows an embodiment of the invention in which the elements A, B, C and D are formed from a plurality of parallel wires which are coupled together.

Figure 7 shows an arrangement in which individual wires form the elements A, B, C and D.

Figures 8 and 9 show a further embodiment of the invention in which the elements A, B, C and D are formed from metal coating layers on printed circuit boards. Figure 8 shows a plan view of the gate 19 according to this embodiment of the invention and Figure 9 shows a view along the line IX- IX of Figure 8. Particles travel in the direction of the arrows shown, in both Figures 8 and 9.

The printed circuit boards 30 have a first portion 32 which forms a shielding electrode. The portion 32 is bounded by an opening 34. The element A is arranged adjacent the opening 34 and is bounded on its opposite side by an opening 36.

Arranged adjacent the opening 36 is the element C of the second deflection means and the element C is bordered by a space 38. The right hand side 40 of the circuit board 30 forms a collector.

Metal coatings are provided to the' portions 32, A, C and 40 and also longer sides of the openings 34, 36 and 38 are also metallised to protect insulating surfaces from charge accumulation and to make electrical connection between the two sides of the printed circuit board 30. Thus, each circuit board carries one of the elements A (or B) for creating the first deflection field and the element C (or D) of the second deflection field. The circuit board also carries the portions 32 and 40 which form shielding electrodes and collector respectively. As is clearly shown in Figure 9 by arranging a number of the circuit boards 30 in vertical array, groups of elements A and B for forming first deflection fields and groups of elements C and D for forming second deflection fields are provided. The shielding electrodes SH formed by the portions 32 provide shielding situated prior to the deflection gaps between the elements A and B, and C and D for shielding the primary beam of charged particles from distortion by the gate fields. Trapping electrodes TR provided by the portions 40 are provided down stream of the elements C and D for both shielding the beam travelling behind the gate from the gate potentials and also for collecting particles deflected by the gate when in the closed state. The shielding electrodes SH and trapping electrodes TR are kept at the potential of the field-free space where the beam of particles drift before and after the gate 19.

In the preferred embodiment of Figures 8 and 9, the printed circuit boards 30 may be of 0.8 mm thickness and stacked parallel with a 2 mm pitch to form the gate 19 which would have a transparency in the gate open state of about 70%.

In other embodiments, the shielding electrodes SH and trapping electrodes TR may be left floating with the signal induced on them measured by electronic means for further accounting for the intensity of the stream of charged particles passing through or hitting the shielding electrodes or trapping electrodes. In some other embodiments of the invention, the signal induced on the SH electrodes may be used as a measure of whether the gate should be switched to closed state in order to automatically deflect the beam if the intensity detected by the SH electrodes exceeds a certain preset value.

In most embodiments of the invention, attenuation α = ID ÷ IO if the gate is desirable to be close to 1 where ID is the amount of particles deflected from initial direction and IO is the amount of particles which were meant to be deflected. However, in certain other embodiments of the inventions when particles to be deflected have characteristics spread within certain ranges (for example, ion velocity in iso-mass ion packets in time of flight mass spectrometers usually have finite spread) the potentials applied to the gate may be varied in order to achieve variable degrees of attenuation.

In other embodiments of the invention, the gate 19 instead of being formed from printed circuit boards as described with reference to Figures 8 and 9, could be formed from etched plates in which material is etched from the plates, the plates are then stacked on top of one another and spaced apart by spacers (not shown) . Edges of the plates can be cut to provide electric isolation between various parts of the plate in order to provide the various deflection regions.

Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiments described by way of example hereinabove.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A gate for eliminating charged particles in a time of flight analysis device, said gate including: a first deflection means for deflecting the charged particles; a second deflection means for deflecting the charged particles; and field control means coupled to the first and second deflection means for causing the first and second deflection means to create deflection fields and for momentarily changing the state of the deflection field of one or the other of the first and second deflection means.

2. A time of flight analysis device, including: means for producing a beam of charged particles; a detector for detecting the charged particles and for enabling a parameter of the charged particles to be determined; and a gate for eliminating charged particles from the beam so that selected ones of the charged particles arrive at the detector for detection, said gate having; i) a first deflection means for deflecting the charged particles; ii) a second deflection means for deflecting the charged particles; and iii) field control means coupled to the first and second detection means for causing the first and second detection means to create deflection fields and for momentarily changing the state of the deflection field of one or the other of the first and second deflection means.

3. The device of claim 2, wherein the field control means causes the first and second deflection means to create deflection fields and for momentarily changing the state of the deflection field of one or the other of the first and second deflection means in such a manner that a duration of the gate being in an opened or closed state for the charged particles is defined by a time delay between changes applied to the states of the deflection fields of the first and second deflection means.

4. The device of claim 2 or 3, wherein the first and second deflection means are spaced apart in the direction of travel of particles passing through the gate.

5. The device of claim 2, 3 or 4, wherein two deflection means are provided.

6. The device of claim 5, wherein the deflection means are arranged such that at least some of the particles pass through at least one of the deflection means whilst others of the particles pass through another of the deflection means.

7. The device of claim 5, wherein the deflection means are positioned parallel to each other.

8. The device of any one of claims 2 to 7, wherein the main component of the deflection fields produced by the deflection means are orthogonal to the direction of travel of the particles.

9. The device of any one of claims 2 to 8, wherein in order to maintain particle energy distortion at a minimum, the main component of the confined deflection field of the deflection means is parallel to the direction in which the velocity spread of the particles is minimal.

10. The device of any one of claims 2 to 9, wherein the deflection means comprise pairs of parallel plates.

11. The device of any of claims 2 to 10, wherein the control means coupled to the deflection means comprises means for applying stepped voltages to the deflection means .

12. The device of claim 11, wherein the magnitude of the stepped voltages is the same.

13. The device of any one of claims 2 to 12, wherein the time at which switching of the gate from an open state allowing particles to pass through the gate to a closed state eliminating at least some of the particles takes place is related to the velocity or mass to charge ratio of particles to be deflected or allowed to pass through the gate.

14. The device of any one of claims 2 to 13, wherein at least one particle collector is provided for collecting particles deflected by the gate.

15. The device of claim 14, wherein the collector comprises at least two plates positioned behind the gate.

16. The device of claim 14, wherein in order to shield particles leaving the gate, from gate potentials, said collector is kept at the potential of the drift space surrounding said gate thereby acting as a shield.

17. The device of claim 14, wherein electronic means is connected to the collector for charge measurements in order to measure charge of particles deflected by the gate or passing through the gate.

18. The device of any one of claims 2 to 17, wherein at least one electrical shield is positioned before the gate.

19. The device of claim 18, wherein the shield is kept at a potential of the drift space surrounding the gate.

20. The device of claim 18, wherein electronic means for charge measurement is connected to the shield for measuring the charge of particles passing through the shield towards the gate.

21. A method of deflecting particles from a beam of charged particles, including the steps of: providing a first deflection means; providing a second deflection means; energising the first and second deflection means to create deflection fields for deflecting the charged particles and momentarily changing the direction of the deflection field of at least one of the deflection means to deflect some of the particles out of the beam.

22. The method of claim 21, wherein the change of direction of the deflection field is caused by applying stepped voltages to the deflection means and momentarily switching the stepped voltage applied to one of the deflection means.

23. The method of claim 21 or 22, wherein the charge of particles or groups of particles is monitored prior to the deflection means with the momentary change of direction of at least one of the deflection fields occurring when the measured charge exceeds a preset value.

24. The method of claim 21 or 22, wherein the time at which said deflection means is switched to a closed state is defined by the time particles of predetermined characteristic travel within certain predetermined drift space prior to entering said gaps.

25. A gate for deflecting charged particles, including: a plurality of spaced apart printed circuit boards, each circuit board having; a) a first region defining a shielding electrode; b) a second region, electrically separated from the first region, defining an element for creating a first deflection field; c) a third region, electrically separated from the first and second regions, defining an element for creating a second deflection field; and d) a fourth region electrically separated from the first to third regions, defining a collector.

26. The gate of claim 25, wherein the regions are electrically separated by opening through the printed circuit board.

27. The gate of claim 25, wherein the board has first and second sides and the first to fourth regions are provided on both sides of the board.

28. The gate of claim 25, wherein the regions are formed by metallic layers or coatings on the board.

29. The gate of claim 25, wherein the openings have metallic coatings on sides thereof to electrically connect the first regions on both sides of the board, the second regions on both sides of the board, the third regions on both sides of the board, and the fourth regions on both sides of the board.