WO2004102178A1

WO2004102178A1 - Spectrometer systems

Info

Publication number: WO2004102178A1
Application number: PCT/GB2004/002038
Authority: WO
Inventors: James Andrew Breach; John Patrick Fitzgerald; Bruce Alec Colin Grant
Original assignee: Smiths Group Plc
Priority date: 2003-05-17
Filing date: 2004-05-12
Publication date: 2004-11-25
Also published as: GB0311419D0

Abstract

An ion mobility spectrometer has a detection cell 1 with a gate (8) at one end formed by spaced grids (81) and (82) across which a driver circuit (83) applies a potential difference. The potential difference is varied between different values such that the gate (8) admits either substantially all the ions to the detection cell (1), or substantially no ions, or only some ions. The potential difference may be varied such as to produce a sinusoidal variation in the ion current, in which case a Fourier transform technique can be used to analyse the output spectra.

Description

SPECTROMETER SYSTEMS

This invention relates to spectrometer systems of the kind including a detection cell including control means operable to control passage of charged particles along the cell.

Spectrometer systems, such as ion mobility spectrometers (IMS), are often used to detect substances such as explosives, drugs, blister and nerve agents or the like. An IMS system typically includes a detector cell to which a sample of air containing a suspected substance is supplied as a gas or vapour. The cell operates at or near atmospheric pressure and contains electrodes that are energized to produce a voltage gradient across the cell. Molecules in the sample of air are ionized, such as by means of a radioactive source or by corona discharge, and are admitted into the drift region of the cell by an electrostatic gate at one end. The ionized molecules drift to the opposite end of the cell at a speed dependent on the size of the ion. By measuring the time of flight across the cell it is possible to identify the ion. The electrostatic gate used to control entry of ions into the drift region is usually a Bradbury Nielson gate. This consists of two parallel or near parallel grids, each grid being made up of a series of conductive wires spaced from one another. A drive circuit connected to the two grids applies a potential difference between them. The drive circuit switches the potential difference between two fixed voltages so that ions are either enabled to pass through the grids into the drift region or are prevented from passing.

It is an object of the present invention to provide an alternative spectrometer system.

According to one aspect of the present invention there is provided a spectrometer system of the above-specified kind, characterised in that the control means is selectively operable to enable passage of substantially all particles, to prevent passage of substantially all particles, and to enable passage of only some of the particles.

The control means preferably includes a gate having two spaced grids, the system being arranged to control passage of the particles by varying a potential difference applied across the grids. The control means may be switchable between different discrete states or it may be arranged to provide a sinusoidal variation in the flow of particles in which case the system may employ a Fourier transform technique to analyse the output of the cell. The system may be an ion mobility spectrometer including an ionization source, the charged particles being ions.

According to another aspect of the present invention there is provided a spectrometer system including a detection cell having a gate towards one end to control passage of charged particles along the cell, characterised in that the gate is operable between at least three different states: fully open, fully closed and partially open.

According to a third aspect of the present invention there is provided a spectrometer system including a detection cell having a gate towards one end to control passage of charged particles along the cell, characterised in that the gate has an admittance that is variable between at least three different levels.

According to a fourth aspect of the present invention there is provided a spectrometer system including a detection cell having a gate towards one end to control passage of charged particles along the cell, characterised in that the system includes a control for controlling a voltage applied to the gate such that the applied voltage can be selected either to enable passage of substantially all charged particles, or to prevent passage of substantially all charged particles, or to enable passage of only some of the charged particles.

An IMS system according to the present invention, will now be described, by way of example, with reference to the accompanying drawing, which shows the system schematically.

The system includes a detection cell in the form of an LMS drift cell 1 having an inlet manifold 2 with an inlet port 3 and an exhaust port 4. Sample air to be analysed is drawn into the inlet port 3 by means of a pump 5 connected to the exhaust port 4. The interior of the manifold 2 opens into the left-hand end of the interior of the cell 1 via a selective barrier 6. The barrier 6 may be a pinhole, as described in WO93/01485, or a semi-permeable membrane, or of any other form that allows passage of the molecules of interest whilst excluding the majority of other molecules. Instead of a barrier, the sample to be analysed may be supplied to the cell 1 by some other interface, such as of the kind described in EP 596978.

The barrier 6 communicates with an ionization region 7 provided by an ionization source such as a radiation source or a corona discharge. To the right of the ionization region 7 a gate 8 controls passage of ionized molecules into a drift region 9 formed by a series of drift electrodes 10. A collector plate 11 at the right-hand end of the cell 1 collects ions passed through the drift region 9. The cell 1 may include an additional drift electrode in the form of a screen grid 12 located just in front of the collector plate 11. The collector plate 11 provides an output to a processor 20, which, in turn, provides an output to a display 21 or other utilisation means indicative of the nature of the sample.

At its right-hand end, the cell 1 has an inlet 30, by which air containing a dopant or reagent is supplied to the interior of the cell where it travels from right to left and flows out to atmosphere via an exhaust outlet 31 close to the gate 8 in the ionization region 7.

The gate 8 consists of two closely spaced, parallel or near parallel grids 81 and 82, each made up of a number of parallel, spaced, conductive wires. The two grids 81 and 82 are connected to a gate driver circuit or control 83, which applies a potential difference across the two grids. The potential difference applied by the driver circuit 83 can be varied continuously between a first value at which substantially all ions are prevented from passing to the drift region 9 and a second value at which substantially all ions are enabled to pass through the gate 8. Alternatively, the driver circuit 83 can be switched between the first value, the second value and a third value between the first and second values at which some, but not all, ions pass to the drift region 9. The driver circuit 83 could be switchable between more than one value between the "open" and "closed" values.

The driver circuit 83 is connected with the processor 20 so that the processor can correlate the input from the collector plate 11 with the gate potential.

The system of the present invention can generate IMS spectra using coded modulation techniques having a higher order than binary. Three-valued or ternary sequences have better autocorrelation properties than previous binary sequences, where the gate is switchable between only the fully open and fully closed states. This can be used to improve the reliability of detection.

Alternatively, the gate voltage can be controlled to give a sinusoidal ion current modulation and Fourier Transform techniques can be used to analyse the spectra. This has an advantage over previous binary gates because binary gates give rise to square wave modulation of the ion current, which results in additional and unwanted harmonics in the analysis.

The invention is not confined to use with LMS spectrometers but could be used in other spectrometers having gated admission of any form of charged particles (including gaseous and liquid particles), such as time-of-flight mass spectrometers, gas chromatography (GC), high pressure liquid chromatography (HPLC) or capillary electrophoresis (CE) spectrometers.

Claims

1. A spectrometer system including a detection cell (1) including control means (8, 83) operable to control passage of charged particles along the cell, characterised in that the control means (8, 83) is selectively operable to enable passage of substantially all particles, to prevent passage of substantially all particles, and to enable passage of only some of the particles.

2. A spectrometer system according to Claim 1, characterised in that the control means includes a gate (8) having two spaced grids (81 and 82) and that the system is arranged to control passage of the particles by varying a potential difference applied across the grids.

3. A spectrometer system according to Claim 1 or 2, characterised in that the control means (8, 83) is switchable between different discrete states.

4. A spectrometer system according to Claim 1 or 2, characterised in that the control means (8, 83) is arranged to provide a sinusoidal variation in the flow of particles.

5. A spectrometer system according to Claim 4, characterised in that system employs a Fourier transform technique to analyse the output of the cell (1).

6. A spectrometer system according to any one of the preceding claims, characterised in that the system is an ion mobility spectrometer including an ionization source (7), and that the charged particles are ions.

7. A spectrometer system including a detection cell (1) having a gate (8) towards one end to control passage of charged particles along the cell, characterised in that the gate (8) is operable between at least three different states: fully open, fully closed and partially open.

8. A spectrometer system including a detection cell (1) having a gate (8) towards one end to control passage of charged particles along the cell, characterised in that the gate (8) has an admittance that is variable between at least three different levels.

9. A spectrometer system including a detection cell (1) having a gate (8) towards one end to control passage of charged particles along the cell, characterised in that the system includes a control (83) for controlling a voltage applied to the gate (8) such that the applied voltage can be selected either to enable passage of substantially all charged particles, or to prevent passage of substantially all charged particles, or to enable passage of only some of the charged particles.