GB2638111A - Method and apparatus - Google Patents

Abstract

A method of reducing electrical measurement noise in a detection apparatus (e.g., an ion mobility spectrometer or a mass spectrometer) configured to detect the arrival of ions at a collector electrode 118. The detection apparatus comprises an air mover 122 (e.g., a fan, pump or blower) to provide a flow of air in the apparatus. The method comprises: operating the air mover at an operating level to provide the flow of air; reducing operation of the air mover for a quiet period; and operating the detection apparatus to allow ions to travel to be detected by the collector electrode during the quiet period. The collector electrode may be subject to variations in capacitance due to mechanical vibration. The air mover may be a source of mechanical vibration. Mechanical vibration of the collector electrode may be reduced during the quiet period. A detection apparatus is also claimed.

Description

Method and Apparatus Field of Invention

The present invention relates to methods and apparatus, and in particular methods and apparatus for detecting ions, which may be used in systems for detecting substances of interest and/or analysing substances. Still more particularly the present disclosure relates to methods and apparatus for reducing measurement noise in the detection of ions at a collector electrode.

Background

A variety of systems, such as physical measurement apparatus and trace detection apparatus, spectrometers and other types of physical instruments rely on the detection of electrical charge to sense the arrival of ions at an electrode of a detector. Such an electrode may be referred to as a collector electrode.

Examples of such systems include ion mobility spectrometers, and mass spectrometers. One example of a type of mass spectrometer is a so-called "time of flight" mass spectrometer, TOF-MS. A variety of such systems exist.

Ion mobility spectrometers (IMS) can identify material from a sample of interest by ionising the material (e.g., molecules, atoms, and so forth) and measuring the time it takes the resulting ions to travel a known distance through a counterflow of drift gas under a known electric field. Typically this time is measured from the time an ion gate (which may also be referred to as an ion shutter) is opened, to the time that ion arrives at a detector such as a Faraday cup.

Each ion's time of flight is associated with the ion's mobility. An ion's mobility relates to its mass and geometry. Therefore, by measuring the time of flight of an ion it is possible to infer its identity. These times of flight may be displayed numerically and/or graphically as a spectrum.

Some IMS cells include detectors which collect ions to measure their time of flight so they can be identified, this may be done in the presence of a drift gas so that mobility effects can separate the ions. Ion mobility spectrometers typically comprise at least some electromechanical actuators and other types of actuators, including motors. These may be used for things like operating fans and blowers and other types of air movers. These can be used to provide the flow of drift gas through the drift chamber and to operate pressure pulsers, which are used to take samples using a pinhole or other types of sampling inlet. A variety of types of IMS devices exist -including some types of differential mobility spectrometers (DMS) and field asymmetric IMS devices FAIMS, in which the mechanism of resolving mobility characteristics is different. Many of these also have mechanical components for reasons similar to those outlined above.

Time-of-flight mass spectrometers (TOF-MS) may use travel time through a chamber under electric field in the absence of drift gas to measure mass to charge ratio. Typically, mass spectrometers comprise at least some electromechanical actuators and other types of actuators, including motors for air moving functions such as the operation of vacuum pumps, providing sample flows, and other applications.

Summary

Aspects and examples of the present invention are set out in the 30 claims and aim to address the problem of measurement noise due to mechanical vibration in detection devices, such as ion mobility spectrometers and mass spectrometers.

In an aspect there is provided a method of reducing electrical measurement noise in a detection apparatus configured to detect the arrival of ions at a collector electrode, wherein the detection apparatus comprises an air mover to provide a flow of air in the apparatus, the method comprising: operating the air mover, at an operating level, to provide the flow of air; reducing operation of the air mover for a quiet period; and operating the detection apparatus to allow ions to travel to be detected by a collector electrode during the quiet period.

The collector electrode may have a capacitance which is affected by mechanical vibration caused by the air mover. For example, one or more capacitive elements (e.g., conductive and/or charge carrying elements) may be assembled with the collector electrode. Mechanical vibration of this assembly may cause corresponding variations in a capacitance of the collector electrode. The flow of air may be associated with operation of the detection apparatus, for example it may perform a function upon which the detection method used by the apparatus depends. As an example it may provide a flow of air through a pneumatic system of the apparatus such as may be used in providing a flow of drift gas in an IMS system. As another example it may operate a pump to provide a reduced pressure region, such as a vacuum chamber of a TOF-IMS system.

The start of the quiet period may precede the operating of the detection apparatus by a wait period. The wait period may be selected to allow variations in a capacitance of the collector due to vibration of the collector by the air mover to be reduced below a threshold level.

A pressure pulser may be used to move a sample of gaseous fluid through a sampling inlet, such as a pinhole, into the detector. The amplitude of operation of the pressure pulser may be reduced during the quiet period as compared to its operation when the air mover is operating at its usual operating level.

After the end of the quiet period the air mover may be increased to return to the operating level.

The method may comprise performing a series of cycles of operation of the detection apparatus during the quiet period. For example, a sample of gaseous fluid may be obtained via a sampling inlet (e.g. by operation of a pressure pulser) prior to the series of cycles. An ioniser may be operated to generate ions from the sample at the start of each cycle. The method may comprise maintaining the air mover in reduced operation throughout the series of cycles and operating the air mover at the operating level before any subsequent sample is obtained through the sampling inlet.

The method may comprise performing only a single cycle of operating the detection apparatus during the quiet period and then operating the air mover at the operating level before a subsequent cycle of operation of the detection apparatus.

This may comprise obtaining a sample of gaseous fluid through the sampling inlet prior to a series of cycles of operating the ion mobility spectrometer, and providing a series of quiet periods corresponding to the series of cycles of operation of the ion mobility spectrometer. The air mover may be returned to operating at the operating level between each cycle of operation. The air mover at the operating level before any subsequent sample is obtained through the sampling inlet.

In an aspect there is provided a controller of a detection apparatus, the controller comprising: a control interface for controlling an air mover of the detection apparatus and for operating the detection apparatus to allow ions to travel to be detected by a collector electrode. The controller may be configured to perform any one or more of the methods described herein.

An aspect of the disclosure comprises configuring such a controller of a detection apparatus to perform any one or more of the methods described herein. Embodiments of the disclosure provide a computer program product configured to program a programmable processor, such as a controller of detection apparatus to perform any of the methods described or claimed herein. Such computer programs may be encoded in tangible non-transitory computer readable storage media.

An aspect of the disclosure provides a detection apparatus comprising such a controller.

The disclosure also provides a detection apparatus comprising: a collector electrode assembly for detecting ions, wherein the collector electrode assembly is subject to variations in 25 capacitance due to mechanical vibration; a chamber along which ions travel to reach the collector electrode for detection; a controller configured to operate the detection apparatus to provide ions to be detected by the collector apparatus during a quiet period, wherein mechanical vibration of the collector electrode is reduced during the quiet period.

The controller may be configured to inhibit operation of at least one vibration source of the detection apparatus to provide said reduced mechanical vibration. The vibration source comprises an air mover, such as a fan, pump, or blower.

The air mover may operate to provide air flow into and/or out of the chamber.

The detection apparatus may comprise an ion mobility spectrometer (IMS) cell, the flow of air is provided along a drift chamber of the IMS cell, and the ions travel in the drift chamber to be detected by the collector electrode.

The detection apparatus may comprise a mass spectrometer such as a time of flight mass spectrometer (TOF-MS) comprising a reduced pressure chamber, the air mover operates to reduce air pressure in the chamber, and the ions travel in the reduced pressure chamber to the collector electrode.

The controller may be configured to reduce operation of the air mover to provide the quiet period. For example, the controller may be configured so that the start of the quiet period precedes the operating of the detection apparatus to provide the ions by a wait period. The controller may be configured so that the wait period is long enough to allow variations in a capacitance of the collector due to vibration of the collector by the air mover to be reduced below a threshold level.

The controller may be configured to increase operation of the air 30 mover after the quiet period. For example, it may return operation of the air mover to the operating level at which it is usually operated to provide function of the detection apparatus.

The detection apparatus may comprise one or more capacitive elements, for example conductive or charge carrying elements. Such elements may be capacitively coupled to the collector electrode. For example these elements may be assembled around the 5 collector electrode sufficiently closely as to make an appreciable contribution to the capacitance of the collector electrode. One example of such an element is a screen electrode arranged to shield the collector electrode from electric field from the ions and the variation In capacitance is associated with 10 vibrational changes in a spacing between the screen electrode and the collector electrode.

The quiet period described herein may be provided by reducing operation of the air mover, this may be done by reducing a power 15 or speed of operation of the air mover. This may be done by switching the air mover off or otherwise ceasing operation.

Brief Description of Drawings

Embodiments of the disclosure will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows a cut away view of an ion mobility spectrometer; Figure 2 shows a flow chart of a method of reducing measurement noise in a detection apparatus such as that ilhusLraLed in Figure 1 or Figure 5; Figure 3 shows a further flow chart of a method of reducing measurement noise in a detection apparatus such as that illustrated in Figure 1 or Figure 5; Figure 4 shows yet a further flow chart of a method of reducing measurement noise in a detection apparatus such as that illustrated in Figure 1 or Figure 5; Figure 5 shows a very schematic view of a different detection apparatus.

In the drawings like reference numerals are used to indicate like elements.

Specific Description

The embodiments described below present different types of detection apparatus in which the methods of the present disclosure can be implemented. These methods offer particular advantages in detection devices which include ion mobility spectrometry capability, where a collector electrode maybe used to detect the arrival of ions and mechanical vibration of the collector electrode may create a particular problem of vibration induced capacitance fluctuation. This may arise particularly in systems where the assembly around the collector electrode comprises conductive elements. Embodiments also offer significant advantages in mass spectrometry devices, where comparable structures may be present and may be subject to comparable mechanical vibration.

Accordingly, embodiments of the disclosure provide a quiet period, during which mechanical vibration of the collector electrode assembly is reduced and operate the detection apparatus to allow ions to travel to be detected by the collector electrode during the quiet period. A variety of implementations are contemplated, as will become apparent from the disclosure which follows.

The ion mobility spectrometer of Figure 1 comprises a reaction 30 region 102, an ionisation source 104 for ionising gaseous fluid in the reaction region 102, an ion shutter 105, a collector electrode 118 such as a Faraday cup, and a controller 120. The ion mobility spectrometer also comprises a drift region 103, drift electrodes 103a, 103b, a screen grid 117, an air mover 122, a drift gas inlet 119, and a drift gas outlet 121.

The spectrometer comprises a housing, such as a tube 101. The reaction region 102 is at one end inside this housing 101, and separated from the collector electrode 118 by a drift region 103. The reaction region 102 is separated from the drift region 103 by the ion shutter 105. The housinc 101 comprises an inlet 108 for enabling a sample of gaseous fluid (such as vapour, and/or gas, and/or an aerosol) to be introduced into the reaction region 102.

The ionisation source 104 is disposed in the housing for generating ions to ionise the sample of gaseous fluid in the reaction region 102. In the example illustrated in Figure 1, the ionisation source 104 comprises a corona point. Embodiments of the disclosure may use a dual point corona ionisation source.

A voltage profile may be provided in the drift region 103 using a series of drift electrodes 103a, 103b spaced apart along the 20 drift region 103. Although not illustrated in Figure 1, a repeller plate or other electrode may be arranged for extending this voltage profile into the reaction region 102. Between the reaction region 102 and the detector 118 the profile voltage varies spatially (e.g. as a function of displacement along the cell in the drift direction) to provide an electric field that moves ions along the cell 100 towards the collector 118. The electric field may be uniform and/or known along the drift region 103 and/or the reaction region 102.

The screen grid 117 is disposed between the drift region and the collector electrode and typically closely adjacent to the collector electrode. It is connected to the controller 120. The controller is configured to hold the voltage of the screen grid 117 at a potential selected to shield the collector electrode 118 -10 -from image charge effects associated with ions approaching the collector electrode from the drift region 103. To help prevent peak broadening the screen grid 117 is generally positioned closely adjacent to the collector electrode 118, and typically it is positioned parallel to the collector electrode. The screen grid may extend across a portion of the centre of the collector electrode on the axis of the drift chamber. For example, the screen grid may cover substantially all of the collector electrode to shield it from image charge effects from approaching ions.

As a result the screen grid is one of the largest contributions to the capacitance of the collector electrode. However, there may be other contributions to this capacitance such as those associated with any other conductive and/or charge carrying elements in the assembly around the collector electrode but which are not at the same DC potential. This assembly may be referred to herein as the collector electrode assembly. The capacitance of the collector electrode assembly may arise from the screen grid and/or such other other conductive and/or charge carrying elements as are assembled sufficiently closely to the collector electrode assembly that the contribute appreciably to the apparent capacitance of the collector electrode.

The controller 120 is also connected to control the air mover 122. The air mover 122 typically comprises a fan, pump, or blower or other appropriate air movement means. It is connected by an air circulation system, such as appropriate conduits, to the drift gas inlet. Typically, the air mover is mounted in or to the detector such that mechanical vibration produced by operation of the air mover ca be communicated to the collector assembly through the detection apparatus itself.

The controller 120 operates the air mover 122 to provide a flow of drift gas into the drift region via the drift gas inlet. The drift gas then flows down the drift region 103 away from the collector 118 towards the reaction region 102. It exits the drift region via the drift gas outlet, from where it may be recirculated to the air mover. The drift gas may also be passed through cleaning and/or drying stages such as molecular sieves and so forth.

The operation of the air mover 122 may be one source of mechanical vibrations, which may be transmitted through the body of the ion mobility spectrometer to the collector electrode 118 and/or the surrounding assembly comprising conductive or charge carrying elements. These vibrations may cause relative displacement of such elements and the collector electrode from one another. As a result, the capacitance of the collector electrode may exhibit corresponding variations because of the dependence of capacitance on spacing.

The ion shutter 105 comprises two electrodes 106, 107, which are coupled to the controller 120 to enable a barrier voltage to be provided between the two electrodes 106, 107. When the shutter is "closed" this barrier voltage acts to prevent ions from travelling from the reaction region into a drift region of the IMS, and an open state in which ions can travel into the drift region towards the detector. The ion shutter 105 may comprise a Tyndall-Powell, Bradbury-Nielsen shutter, or other type of shutter. The shutter electrodes 106, 107 may each comprise elongate conductors, and the elongate conductors of the first shutter electrode 106 may be aligned in the drift direction with the elongate conductors of the second shutter electrode 107. The elongate conductors of each shutter electrode 106, 107 may be arranged as a grid, such as a mesh, for example a triangular, rectangular, hexagonal, or other regular or irregular mesh. The -12 -shutter electrodes 106, 107 need not be separated in the drift direction. For example they may be coplanar, in which case the elongate conductors may be interdigitated, for example they may be interleaved or interwoven.

The controller 120 may comprise a control interface for controlling the air mover and other parts of the 1MS cell. Examples of control interfaces may include serial or parallel interfaces and synchronous and asynchronous interfaces. By these or other means, the controller 120 is connected ro the ion shutter 105 and to the collector electrode for sensing arrival of ions at the collector electrode. The controller 120 may also be connected for operating the ionisation source 104.

The operation of this apparatus will now be described with reference to Figure 2. A sample of gaseous fluid (such as vapour) is provided into the reaction region. This may be done by operation of a pressure pulser or other means or taking a sample. the controller 120 provides a pulse or series of pulses of the ionisation source 104 to generate ions. These ions are mixed with the sample to generate sample ions. During this process, the controller 120 operates 200 the air mover, at an operating level, to provide the flow of air along the drift chamber 103. This flow of air may act to keep the drift chamber 'clean' for example to inhibit the flow or diffusion of neutral sample into the drift chamber from the reaction region.

The controller 120 then provides 202 a quiet period, in which mechanical vibration of the collector electrode (and/or any surrounding components which contribute to its apparent capacitance) is reduced. This may be done by reducing operation of the air mover and/or other sources of mechanical vibration. One way to do this is to reduce the speed or power of operation of the air mover. This may be achieved by simply switching it -13 -off, but mere reduction in speed/power of operation may be enough to reduce vibration, so the air mover need not always be completely switched off. This reduced operation of the air mover provides a quiet period.

During this quiet period, the controller 120 can operate 204 the detection apparatus to allow ions to travel to be detected by the collector electrode 118.

In a typical cycle of operation, after a gate delay (following operation of the ioniser) the controller 120 opens the ion shutter for a gate width, which allows a cloud of ions to enter the drift region 103 where they travel, against the flow of drift gas to the collector electrode 118. As the ions travel along the drift chamber they become separated due to their differing mobilities so that they arrive at the collector electrode 118 at different times. The collector electrode 118 is connected to the controller so that the controller can detect 206 the charge associated with the arrival of ions at the collector electrode 118. This signal is used to provide an ion spectrum which characterises that cycle of operation of the IMS device. The controller 120 may operate the IMS device to provide a series of such cycles, for example a number of cycles of operation, for each sample. The resultant spectra may be combined, for example by averaging.

The start of the quiet period may precede the operating of the detection apparatus by a wait period. Typically, the wait period is selected to be long enough for vibration of the collector by the vibration sources, such as the air mover, to be reduced below a threshold level. The relevant threshold may be selected based on the specific details of the device and/or the desired accuracy of measurement. After the cycle (or series of cycles) of operation have been completed, the controller 120 may return 208 -14 -the operation of the air mover 122 to the operating level. This may assist in preventing diffusion or flow of neutral sample vapour into the drift region.

Figure 3 illustrates a scheme of operation for a device such as that illustrated in Figure 1. In this method, the pressure pulser is operated 300 to obtain a sample, and to ionise the sample. Operation of any vibration source (such as the air mover) is then reduced 302 to provide a quiet period. The spectrometer is then controlled to perform a series of cycles 305 of operation, before the vibration source(s) are returned 308 to their normal operating level. As they are all taken from a single sample, the spectra obtained from this series of cycles of operation may be combined by summing them, for example to provide an average.

Figure 4 illustrates a further method of operation. In this method, the pressure pulser is operated 400 to obtain a sample, and then the operation of any vibration source (such as the air mover) is reduced 402. The spectrometer is then controlled to perform 405 one cycle of operation before the vibration source (such as the air mover) is returned 408 to its operating level. The controller may then reduce operation 402 of the vibration source again, perform another cycle of operation 405, and then return 408 the vibration source to its operating level before each subsequent cycle of operation. In the intervals between cycles, further neutral sample may be taken into the reaction region to be ionised. When the pressure pulser is used between successive quiet periods, the power applied to the pressure pulser may be reduced as compared to its use when the air mover is working at its operating level. Embodiments of the disclosure may therefore relate to systems in which operation of the air mover which provides flow of drift gas is reduced for certain periods, and during these quiet periods, the power level of each -15 -operation of the pressure pulser is also reduced as compared to the level used when the air mover is operating normally.

Figure 5 illustrates a further detection apparatus. The detection apparatus 500 illustrated in Figure 5 is a time of flight mass spectrometer (TOF-MS). The apparatus comprises a reduced pressure chamber 503, an air mover 522, a collector electrode 518, a controller 520, an ion source 506, and an inlet 508. The apparatus 500 also comprises an assembly of conductive and/or charge carrying elements 517 adjacent to the collector electrode.

The ion source 506 comprises a source of ions to be analysed and is coupled to the reduced pressure chamber 503 by the inlet 508, which may comprise a capillary inlet or ion funnel or other appropriate means of conveying ions to the reduced pressure chamber 503.

The air mover 522 is connected to the controller 520 and is arranged in fluid communication with the reduced pressure chamber 503. The air mover 522 is operable to reduce the air pressure in the reduced pressure chamber 503, for example in the manner of a vacuum pump.

The collector electrode 518 is arranged at the opposite end of the chamber 503 from the inlet 508 for detecting ions. The collector electrode and assembly 518, 517 is subject to mechanical vibration due to operation of the air mover 522. This may cause variations in capacitance.

The chamber 503 may comprise a vacuum chamber and one or more electrodes may be provided along the chamber to cause ions to travel from the inlet to reach the collector electrode for detection. These and other components, such as a screen grid, may provide the assembly 517.

-16 -The controller 520 is connected to operate the ion source 506 band/or the inlet 508 for controlling the inlet. The controller is thus operable to allow or prevent ions to enter the reduced pressure chamber to provide ions to be detected by the collector electrode. The controller is also connected to the air mover and is operable to control operation of the air mover -e.g., to reduce or increase the power/speed of operation of the air mover and/or to switch it on and off.

In operation, the air mover 522 is operated at an operating level to provide a reduced air pressure in the chamber. The controller then reduces the operation of the air mover as compared to the operating level, for example by switching it off to provide a quiet period. During the quiet period, while mechanical vibration of the collector electrode is reduced, the controller operates the inlet to allow ions to enter the reduced pressure chamber so that they can travel along the reduced pressure chamber to the collector electrode. As the chamber is evacuated, or at least at reduced pressure, the time of flight of ions to the collector provides an indication of their mass to charge ratio. A series of such cycles of operation may be performed before the controller returns the air mover to its operating level.

It will be appreciated in the context of the foregoing disclosure that a variety of embodiments are contemplated. The principles of the disclosure have been explained with reference to ion mobility spectrometers and time of flight mass spectrometers, but they may find application in other types of detection apparatus, such as differential mobility spectrometers, field asymmetric IMS (FAIMS) devices and any other device in which a collector electrode is used to detect the arrival of ions. It is particularly advantageous where time of arrival of ions needs to be measured -17 -sensitively. The use of pressure pulsers and pinhole inlets has been described, but other kinds of inlet may also be used such as membrane inlets. The quiet period has been described as being achieved through reducing operation of a vibration source but in some embodiments, vibration of the collector electrode may be reduced by applying a counteracting vibration to the collector electrode assembly. For example, the controller may be connected to a vibration provider and configured to detect vibration of the collector assembly and further configured to control the vibration provider to apply a compensating vibration, such as a vibration in antiphase with the detected vibration. Such embodiments may employ the principles of active noise cancellation and other active methods to reduce vibration of the collector electrode and/or any surrounding conductive or charge carrying elements with which it is assembled.

A variety of other embodiments will be apparent to the skilled addressee in the context of the present disclosure. Any feature of any one of the examples disclosed herein may be combined with any selected features of any of the other examples described herein. For example, features of methods may be implemented in suitably configured hardware, and the configuration of the specific hardware described herein may be employed in methods implemented using other hardware.

It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of -18 -hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and/or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit.

In some examples the functionality of the controller may be provided by a general purpose processor, which may be configured to perform a method according to any one of those described herein. In some examples the controller may comprise digital logic, such as field programmable gate arrays, FPCA, application specific integrated circuits, ASIC, a digital signal processor, DSP, or by any other appropriate hardware. In some examples, one or more memory elements can store data and/or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and/or claimed herein and/or to provide data processing apparatus as described and/or claimed herein. The controller may comprise an analogue control circuit which provides at least a part of this control functionality. An embodiment provides an analogue control circuit configured to perform any one or more of the methods described herein.

The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be -19 -employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims (25)

1. -20 -Claims: 1. A method of reducing electrical measurement noise in a detection apparatus configured to detect the arrival of ions at a collector electrode, wherein the detection apparatus comprises an air mover to provide a flow of air in the apparatus, the method comprising: operating the air mover, at an operating level, to provide the flow of air; reducing operation of the air mover for a quiet period; and operating the detection apparatus to allow ions to travel to be detected by a collector electrode during the quiet period.
2. 2. The method of claim 1 wherein the start of the quiet period precedes the operating of the detection apparatus by a wait period.
3. 3. The method of claim 2 wherein the wait period is selected to allow variations in a capacitance of the collector due to vibration of the collector by the air mover to be reduced below a threshold level.
4. 4. The method of claim 2 or 3 wherein during the quiet period a pressure pulses, used to move sample into the detector, is operated at reduced amplitude.
5. 5. The method of any preceding claim comprising increasing operation of the air mover to the operating level after the quiet 30 period.
6. 6. The method of any of claims 1 to 5 wherein the detection apparatus comprises an ion mobility spectrometer (IMS) cell, the flow of air is provided along a drift chamber of the ion mobility -21 -spectrometer, and the ions travel in the drift chamber to be detected by the collector electrode.
7. 7. The method of any of claims 1 to 5 wherein the detection apparatus comprises a mass spectrometer, such as a time of flight mass spectrometer (TOF-MS) comprising a reduced pressure chamber, the air mover operates to reduce air pressure in the chamber, and the ions travel in the reduced pressure chamber to the collector electrode.
8. 8. The method of any of claims 1 to 7 comprising performing a series of cycles of operating the detection apparatus during the quiet period.
9. 9. The method of claim 8 comprising operating a sampling inlet to obtain a sample of gaseous fluid to be ionised to provide the ions prior to the series of cycles.
10. 10. The method of claim 9 comprising maintaining the air mover 20 in reduced operation throughout the series of cycles and operating the air mover at the operating level before any subsequent operation of the sampling inlet.
11. 11. The method of any of claims 1 to 7 comprising performing only a single cycle of operating the detection apparatus during the quiet period and operating the air mover at the operating level before a subsequent cycle of operation of the detection apparatus.
12. 12. The method of claim 11 comprising operating a sampling inlet to obtain a sample of gaseous fluid to be ionised to provide the ions prior to a series of cycles of operating the ion mobility spectrometer, and -22 -providing a series of quiet periods corresponding to the series of cycles of operation of the ion mobility spectrometer, and optionally operating the air mover at the operating level between each cycle of operation.
13. 13. The method of claim 9, 10, 11 or 12 comprising operating the air mover at the operating level before any subsequent operation of the sampling inlet.
14. 14. A controller of a detection apparatus, the controller comprising: a control interface for controlling an air mover of the detection apparatus and an 'MS cell of the spectrometer and for operating the detection apparatus to allow ions to travel to be 15 detected by a collector electrode; and control logic configured to perform the method of any preceding claim.
15. 15. A detection apparatus comprising the controller of claim 14. 20
16. 16. A detection apparatus comprising: a collector electrode assembly for detecting ions, wherein the collector electrode assembly is subject to variations in capacitance due to mechanical vibration; a chamber along which ions travel to reach the collector electrode for detection; a controller configured to operate the detection apparatus to provide ions to be detected by the collector apparatus during a quiet period, wherein mechanical vibration of the collector electrode is reduced during the quiet period.
17. 17. The detection apparatus of claim 16 wherein the controller is configured to inhibit operation of at least one vibration -23 -source of the detection apparatus to provide said reduced mechanical vibration for example wherein the vibration source comprises an air mover, such as a fan.
18. 18. The detection apparatus of claim 17 wherein the air mover operates to provide air flow into and/or out of the chamber.
19. 19. The detection apparatus of claim 18 wherein the detection apparatus comprises an ion mobility spectrometer (IMS) cell, the flow of air is provided along a drift chamber of the EMS cell, and the ions travel in the drift chamber to be detected by the collector electrode.
20. 20. The detection apparatus of claim 19 wherein the detection apparatus comprises a mass spectrometer such as a time of flight mass spectrometer (TOF-MS) comprising a reduced pressure chamber, the air mover operates to reduce air pressure in the chamber, and the ions travel in the reduced pressure chamber to the collector electrode.
21. 21. The detection apparatus of claim 19 or 20 wherein the controller is configured to reduce operation of the air mover to provide the quiet period.
22. 22. The detection apparatus of claim 21 wherein the start of the quiet period precedes the operating of the detection apparatus to provide the ions by a wait period, for example wherein the wait period is selected to allow variations in a capacitance of the collector due to vibration of the collector by the air mover to be reduced below a threshold level for example wherein the controller is configured to increase operation of the air mover to an operating level after the quiet period.-24 -
23. 23. The detection apparatus of any of claims 16 to 22 comprising a screen electrode arranged to shield the collector electrode from electric field from the ions and the variation in capacitance is associated with vibrational changes in a spacing between the screen electrode and the collector electrode.
24. 24. The method of any of claims 1 to 13, or the detection apparatus of any of claims 14 to 23 wherein reducing operation of the air mover comprises ceasing operation.
25. 25. A computer program product, configured to program a programmable controller of detection apparatus to perform the method of any of claims 1 to 13 or 23.