Analytical Apparatus

Abstract

An IMS or other analytical instrument has a corona discharge needle (20) to ionize sample gases or vapours. A gate (3) is opened or closed to admit or prevent entry of the ions produced by the corona discharge to a drift chamber (4). The operation of the corona discharge needle (20) and the gate (3) are controlled such that the gate is open during at least two discharges, to admit faster ions produced by the most recent discharge together with slower ions produced by an earlier discharge.

Description

This invention relates to analytical apparatus of the kind including an ionization source in an ionization region, a chamber, a gate at one end of the chamber adjacent the ionization region arranged to control passage of ions into the chamber, the ionization source including a corona discharge source and a control for controlling operation of the corona discharge source and the gate.

The invention is more particularly, but not exclusively concerned with ion mobility spectrometers (IMSs).

Ion mobility spectrometers (IMSs) are used to detect the presence of small quantities of airborne chemicals in vapours or gases at atmospheric pressure. An IMS has some means to ionize the sample chemicals, such as a corona discharge or a radioactive source. Where a corona discharge is used this may operate continuously or it may be pulsed. A gate admits the ions into one end of a drift chamber across which a voltage is applied to cause the ions to drift to the opposite end where they are collected on a collector plate. Where the corona discharge is pulsed, the gate is operated in synchronisation once every corona discharge event or group of discharge events. The time taken for an ion to pass from the gate to the collector plate is dependent on the mass, size, shape and charge on the ion. By measuring this time an indication can be provided of the nature of the chemical. The problems in handling radioactive sources has resulted in moves away from these to corona discharge ionisation sources, which may be either continuous or pulsed. A problem with pulsed ionisation sources arises because the period for which the gate is open only allows certain mobilities of ions to pass into the drift region. If the gate is held open for longer periods to admit a wider range of ion mobilities from the discharge, the resolution of the apparatus is substantially reduced.

It is an object of the present invention to provide alternative analytical apparatus.

According to one aspect of the present invention there is provided analytical apparatus of the above-specified kind, characterised in that the corona source is discharged at least twice for every time that the gate is opened and that the gate is arranged to pass faster ions produced by one discharge together with slower ions produced by a preceding discharge.

The apparatus is preferably arranged such that three corona discharges are produced during each period that the gate is open. The apparatus may be an ion mobility spectrometer and the chamber may be a drift chamber. The corona discharge source may include a corona needle extending within a metal tube. The gate preferably includes a first grid to which a pulsed voltage is applied. The apparatus may include a second grid adjacent the first grid to which a constant voltage is applied such as to draw ions through the gate. The switching period of the gate is preferably approximately 200 μs.

According to another aspect of the present invention there is provided a method of analyzing a sample gas or vapour comprising the steps of producing at least two corona discharges to ionize substances in the gas or vapour, alternately enabling and preventing admission of some of the ions produced by both discharges to a chamber, applying a field to the chamber to cause the admitted ions to move to a detector and providing an output from the detector.

An IMS spectrometer according to the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows the spectrometer schematically; and

FIG. 2 illustrates ion production and gate switching.

The spectrometer has an inlet 1 by which airborne chemicals and vapours enter the instrument and pass to an ionization chamber 2, where they are ionized. A gate 3 admits the ions to the left-hand end of a drift chamber 4 where they are caused to flow to the right-hand end by a voltage field applied to electrodes 5. Ions are collected on a collector plate 6 where they are detected to provide an output to a processing unit 7, which in turn provides an output representative of the nature of the chemical to a display 8 or other utilisation means.

The ionization chamber 2 has a metal corona needle 20 supported by an electrically-insulative ferrule 21 at one end to project co-axially along an electrically-insulative housing 22 and at right-angles to the inlet 1. The sharp point 23 of the needle 20 locates within a brass or nickel tube 24. A corona drive circuit 25 is connected between the needle 20 and the tube 24 to apply a high voltage of around 5 kV between them sufficient to produce a corona discharge effective to ionize chemicals and vapours admitted through the inlet 1. The right-hand end of the ionization chamber 2 opens into the left-hand end of the drift chamber 4 via the gate 3.

The gate 3 consists of two grid arrays 30 and 31 of fine electrical wires connected to a gating grid circuit 32. The gating grid circuit 32 applies a pulsed voltage to the first grid 30 and a constant voltage to the second grid 31. The second grid 31 helps define the field that draws the ions through the open gate. Operation of the gating grid circuit 32 is controlled by signals from the processing unit 7. The gate circuit 32 is able to float up to a few hundred volts with respect to the ionization source tube 24 in order to produce an accelerating potential that pulls ions from the corona point 23.

The drift chamber 4 takes the form of an electrically-insulative cylindrical tube or cell 40 having eight ring electrodes 5 extending coaxially around the outside of the tube and equally spaced from one another along the length of the tube. The electrodes 5 are connected to an electrode supply unit 42 that provides a field of about 0.24 kV/cm along the tube 40.

A gas inlet 50 opens into the right-hand end of the cell 40 by which a drift gas is admitted to the cell to flow from right to left against the direction of ion flow. The inlet 50 connects with piping 51 extending to the output of a pump 52 via two molecular sieves 53 and 54 and a valve 55. The inlet of the pump 52 connects via a third molecular sieve 56 to an exhaust outlet 57 at the left-hand end of the cell 40. The pneumatic circuit further includes a branch pipe 58 extending between the inlet 1 and the outlet of the pump 52 via a fourth molecular sieve 59 and a valve 60. The branch pipe 58 further includes an opening 61 between the sieve 59 and the inlet 1 by which external air for sampling is admitted to the spectrometer. Additional air can be admitted to the pneumatic system via a fifth molecular sieve 62 and a breather restriction 63. It can be seen that air flows along the cell 40 from the inlet 50 at its right-hand end to the exhaust outlet 57 at its left-hand end and that this exhausted air is filtered by the sieves 56, 54 and 53 before being re-circulated back to the inlet 50. The external air to be sampled is supplied to the spectrometer with a stream of filtered carrier air. The sieve 59 may include a chemical dopant selected to enhance identification of ion species of interest, as described in WO00/79261. The interior of the cell 40 is at substantially atmospheric pressure.

The collector plate 6 at the right-hand end of the cell 40 is connected with current amplifier 70, which in turn provides an output to the processing unit 7. The processing unit 7 produces a spectrum of the ions arriving at the collector plate 6 in the usual way. The spectrum need not take the form of a graphical representation but could be processed to provide a display of the name of the identified compound or an alarm when a specified compound was identified.

The pump 52 is energized to draw airborne molecules in through the opening 61 where they flow with carrier gas, which may include dopant, to the inlet 1 of the ionization chamber 2. The molecules are ionized by the discharge at the corona point 23 and, when the gate 3 is opened by the gating circuit 32, the ions pass into the left-hand end of the drift chamber 4. The field established in the cell 40 by the voltage on the electrodes 5 causes the ions to drift to the right along the cell against the flow of gas along the cell. The speed at which the ions move along the cell is determined by the size, shape and charge on the ions. When the ions reach the far end of the cell 40 they fall on the collector plate 6, thereby producing a small electrical charge on the plate, which is amplified by the amplifier 70 and supplied to the processing unit 7. The processing circuit 7 measures the time of flight of the ions, that is the time between opening the gate 3 and reception of a charge on the collector plate 6, and produces spectra against time with peaks representing different ions.

The processing unit 7 also controls operation of the corona drive circuit 25. In particular, the corona discharge pulses produced are linked to the opening and closing of the gate 3. As can be seen in FIG. 2, at any one time the ionization region between the corona discharge 23 and the gate 3 will contain a mixture of slow and fast moving ions from different discharges. In FIG. 2, ions from three different discharges are illustrated, labelled “1”, “2” and “3” respectively. The switching on and off of the corona discharge is sufficiently rapid that the slow-moving ions of one discharge will be close to or be overlapped by the fast-moving ions of the next, subsequent discharge. The gate 3 is opened for repeated periods of time T. The start of the open gate period is synchronised with the corona discharge such that it occurs when the slower ions of the first discharge, the fast and medium ions of the second discharge and the fast ions of the third discharge approach the gate 3. By appropriate switching of the corona the open-gate period can be relatively short, typically around 200 μs, to produce an acceptable resolution in the spectra, whilst still ensuring that the complete range of fast, medium and slow moving ions are admitted. It will be appreciated that the apparatus could be arranged such that ions from only two successive discharges were admitted during each gate-open period. Alternatively, the apparatus could be arranged so that ions from four or more discharges were admitted.

Claims

1. Analytical apparatus including an ionization source (20, 23, 25) in an ionization region (2), a chamber (4), a gate (3) at one end of the chamber (4) adjacent the ionization region (2) arranged to control passage of ions into the chamber, the ionization source including a corona discharge source (20, 23) and a control (7, 25) for controlling operation of the corona discharge source and the gate, characterised in that the corona source (20, 23) is discharged at least twice for every time that the gate (3) is opened and that the gate (3) is arranged to pass faster ions produced by one discharge together with slower ions produced by a preceding discharge.

2. Apparatus according to claim 1, characterised in that the apparatus is arranged such that three corona discharges are produced during each period that the gate (3) is open.

3. Apparatus according to claim 1, characterised in that the apparatus is an ion mobility spectrometer and that the chamber is a drift chamber (4).

4. Apparatus according to claim 1, characterised in that the corona discharge source includes a corona needle (23) extending within a metal tube (24).

5. Apparatus according to claim 1, characterised in that the gate (3) includes a first grid (30) to which a pulsed voltage is applied.

6. Apparatus according to claim 5, characterised in that the apparatus includes a second grid (31) adjacent the first grid (30) to which a constant voltage is applied such as to draw ions through the gate (3).

7. Apparatus according to claim 1, characterised in that the switching period of the gate (3) is approximately 200 μs.

8. A method of analyzing a sample gas or vapour comprising the steps of producing at least two corona discharges to ionize substances in the gas or vapour, alternately enabling and preventing admission of some of the ions produced by both discharges to a chamber (4), applying a field to the chamber (4) to cause the admitted ions to move to a detector (6) and providing an output from the detector (6).