Ion mobility spectrometry detection method

Abstract

A method of operating an ion mobility spectrometer to detect the presence of a target in a solid or liquid sample, the target type existing as a first class of substances and a second class of substan

Description

Field of Invention The present invention relates to methods and apparatus, and more particularly to methods and apparatus for detecting a target agent by ion mobility spectrometry, still more particularly these methods and apparatus may be for detecting a target agent having a variety of forms provided by different classes of substances, such as both organic and inorganic substances. Background 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 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 graphically or numerically as a plasmagram. As noted above, 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. Often there is a need to detect a presence of an agent which may present a particular type of threat or risk associated with agents of that type, such an agent which is a target of the detection method may be referred to as a "target agent". An example of a type of agent which may be a target for detection is explosives. A number of different explosive agents exist. Different explosives may have different characteristics and so methods used to detect one explosive may not work for other explosives. For example, both organic substances and inorganic substances may be explosive and detection methods designed for one class of substances (e.g. organic substances) may be insensitive to another class of substances (e.g. inorganic substances). Thus, detection methods may fail to detect some explosives because the detection method may be sensitive only to one class of explosives but may fail to detect another class of explosives unless specifically adapted for that purpose. For example, methods for detecting inorganic explosives may not detect organic explosives. Some attempts to mitigate these problems may give rise to high false alarm rate. The same problem may occur for other types of agents, where different classes of those agents exist. Examples of types of agent where this may be an issue include chemical warfare agents, toxic industrial chemicals, and narcotics. Prior approaches to this question have required personnel to use different detection methods to detect different classes of the same type of agent. This can be effective, but it places an additional burden on personnel and requires specialist training. Smiths Detection's IONSCAN 600 responds quickly to trace levels of many organic explosive materials, such as RDX and TNT, and indeed to select inorganic explosives, such as Ammonium nitrate and Urea nitrate. Summary In an aspect there is provided a method of operating an ion mobility spectrometer to detect presence of a target agent of a particular type, such as an explosive, in a solid or liquid sample. The target agent type may exist in forms provided by a first class of substances and forms provided by a second class of substances. The first class of substances may be organic substances. The second class of substances may be inorganic substances. The first class of substances may have a melting point lower than a melting point of the second class of substances. An aspect of the disclosure provides a method comprising: operating a desorber at a first desorber temperature to desorb a first gaseous fluid sample from a sample; operating the ion mobility spectrometer to obtain first ion mobility spectrometry data by ion mobility spectrometry of the first gaseous fluid sample; operating the desorber at a second desorber temperature to desorb a second gaseous fluid sample from the sample; operating the ion mobility spectrometer to obtain second ion mobility spectrometry data by ion mobility spectrometry of the second gaseous fluid sample; and providing an indication of the presence of target agent in the event that either: (a) the first ion mobility spectrometry data indicates the presence of target agent of the first class; or (b) the second ion mobility spectrometry data indicates the presence of target agent of the second class, wherein presence of target agent of the first class is determined by applying a first type of detection processing to the first ion mobility spectrometry data and presence of target agent of the second class is determined by applying a second type of detection processing to the second ion mobility spectrometry data, wherein (i) the second type of detection processing is different from the first type of detection processing and / or (ii) the first ion mobility spectrometry data relates to a different range of reduced mobility, Ko, values from the second ion mobility spectrometry data. Another aspect provides a method comprising: operating a desorber at a first desorber temperature to desorb a first gaseous fluid sample from the a sample; operating the ion mobility spectrometer to obtain first ion mobility spectrometry data by ion mobility spectrometry of the first gaseous fluid sample using a first gate delay; operating the desorber at a second desorber temperature to desorb a second gaseous fluid sample from the sample; operating the ion mobility spectrometer to obtain second ion mobility spectrometry data by ion mobility spectrometry of the second gaseous fluid sample using a second gate delay, different from the first gate delay; and determining presence of a target agent in the solid or liquid sample based on the first ion mobility spectrometry data and / or the second ion mobility spectrometry data. Methods of the present disclosure may be used in an ion mobility spectrometer, such as a time-of-flight ion mobility spectrometer (TOF-IMS) and a differential ion mobility spectrometer, including field asymmetric ion mobility spectrometers (FAIMS) and other types of detectors. Other examples of the present disclosure may be used in mass spectrometry systems and in so-called "hyphenated" IMS-MS systems where ion mobility spectrometry methods and mass spectrometry methods are used together to perform trace detection. For example, in an aspect there is provided a method of operating a detector to detect presence of a target agent type in a solid or liquid sample, the target agent type existing in forms provided by a first class of substances and forms provided by a second class of substances, the method comprising: operating a desorber at a first desorber temperature to desorb a first gaseous fluid sample from the sample; operating the detector to obtain first spectrometry data by spectrometry of the first gaseous fluid sample; operating the desorber at a second desorber temperature to desorb a second gaseous fluid sample from the solid or liquid sample; operating the detector to obtain spectrometry data by spectrometry of the second gaseous fluid sample; and providing an indication of the presence of target agent in the event that either: (a) the first spectrometry data indicates the presence of target agent of the first class; or (b) the second spectrometry data indicates the presence of target agent of the second class, wherein presence of target agent of the first class is determined by applying a first type of detection processing to the first spectrometry data and presence of target agent of the second class is determined by applying a second type of detection processing to the second spectrometry data, wherein the second type of detection processing is different from the first type of detection processing. The detector may comprise at least one of an ion mobility spectrometer and a mass spectrometer. For example the spectrometry data may comprise and / or be based on mass-to-charge data obtained by mass spectrometry of the respective gaseous fluid samples. Examples of mass spectrometers include time-of-flight mass spectrometers (TOF-MS), ion-trap mass spectrometers, quadra-pole mass spectrometers and other spectrometers capable of measuring mass-to-charge. In some embodiments the desorber may be operated at the second desorber temperature before it is operated at the first desorber temperature. In some embodiments the desorber may be operated at the first desorber temperature before it is operated at the second desorber temperature. Typically, where the sample is a solid or liquid sample carried by a swab, the desorber is operated at the first desorber temperature before it is operated at the second desorber temperature. These and other methods may comprise determining presence of a target agent of the first class in the sample based on the first ion mobility spectrometry data and / or determining presence of a target agent of the second class in the based on the second ion mobility spectrometry data. These and other methods may comprise determining presence of target agent of the first class in the sample by applying a first type of detection processing to the first ion mobility spectrometry data; and determining presence of target agent of the second class in the sample by applying a second type of detection processing to the second ion mobility spectrometry data, wherein (a) the second type of detection processing is different from the first type of detection processing and / or (b) the first ion mobility spectrometry data relates to a different range of reduced mobility, Ko, values from the second ion mobility spectrometry data. The first class of substances may comprise organic substances and the second class of substances may comprise inorganic substances. For example, the second class of substances may predominantly comprise inorganic substances or consist essentially of such substances. By contrast the first class may predominantly comprise organic substances or both organic and inorganic substances of lower melting point than the second class. Where the target agent is explosives, the first class of target agents may include low melting point inorganic substances such as ammonia nitrate and urea nitrate. The inventors in the present case have found that, where the first desorber temperature is 200°c sodium chlorate (an inorganic explosive) is not detectable in the first ion mobility spectrometry data but is detectable in the second ion mobility spectrometry data. Irrespective of the organic or inorganic make-up of the two classes, the first class of substances may have melting points which are lower than melting points of the second class of substances. For example, a first range of melting points may be associated with the first class of substances and a second and different range may be associated with the second class. In some embodiments, the two ranges of melting points do not overlap. The first desorber temperature and / or the dwell time for which the desorber is held at the first desorber temperature may be selected to desorb the majority of, for example substantially all, target agents of the first class. The first desorber temperature may al; so be selected so that target agents of the second class are not desorbed in the dwell time for which the desorber is held at the first desorber temperature. The second desorber temperature is generally higher than the first desorber temperature, for example wherein the first temperature is between 150°C and 250°C, for example wherein the second temperature is at least 500°C, for example at least 590°C, for example 600°C. The target agent may be selected from explosives, narcotics, toxic industrial chemicals, chemical warfare agents, and other types of agent. The first type of detection processing may comprise a first set of peak detection criteria, and the second type of detection processing may comprise a second set of peak detection criteria, different from the first set. The peak detection criteria may comprise a peak detection window and a threshold level, corresponding to (to be applied in) the peak detection window. Each peak detection window may correspond to a time of flight along the drift chamber. To provide such windows in an "instrument independent" fashion they may be provided as a reduced mobility Ko range . Some indicative figures for parameters which may be used in embodiments of the present disclosure are set out in the table below. It can be seen that the Ko windows used for the first type of detection processing may comprise Ko a range of values of at least 0.4 cnfV^s^1. for example at least 0.6 cnfV^s^1, preferably at least 0.8 cm2V"1s"1. These Ko windows may comprise a range of Ko values less than 3 cm2V 1s 1 for example less than 2.5 cm2V 2s 1, preferably less than 2.1 cm2V-1s’1. It can be seen that the Ko windows used for the second type of detection processing may comprise Ko a range of values of at least 1.5 cm2V-1s-1. for example at least 1.7 cm2V-1s-1, preferably at least 1.9 cm2V“1s“1. These Ko windows may comprise a range of Ko values less than 3 cm2V-1s_1 for example less than 2.5 cm2V-1s“l, preferably less than 2.1 cm2V-1s_1. The system may be configured to administer dopant into the reaction region with the first gaseous fluid sample but not with the second gaseous fluid sample. The dopant may comprise a chloride based dopant. The gate delay used for the IMS of the first gaseous fluid sample may be at least 1300 microseconds, for example at least 1400 microseconds, preferably about 1500 microseconds, for example less than 1700 microseconds, for example less than 1600 microseconds. The gate delay used for the IMS of the first gaseous fluid sample may be at least 600 microseconds, for example at least 700 microseconds, preferably between800 and 1200 microseconds, for example less than 1400 microseconds, for example less than 1300 microseconds. It will be appreciated in the context of the present disclosure that the absolute value of the gate delay used in any particular circumstances may depend on the size of the reaction region and the electric potential profile applied in the reaction region. Generally, the gate delays used for the second class of target agent may be less than those used for the first class of target agent. First gaseous fluid sample Second gaseous fluid sample Ko range 0.8 - 2.1 cm2V-1s-1 1.9 - 2.1 cm2V-1s-1 Chemical dopants Generally chloride based Maybe none Gate delay Around 1500 microseconds 800 - 1200 microseconds The first type of detection processing may comprise detecting a pattern of peaks corresponding to a signature of a known target agent of the first class. For example, such detection may be based on a Boolean combination of detecting suprathreshold IMS signal in a selected plurality of detection windows. For example the presence of suprathreshold signal in one or more first windows and the absence of peaks in one or more second windows. The second type of detection processing may also comprise detecting a pattern of peaks corresponding to a signature of a known target agent of the second class. These too may be combined using Boolean combinations wherein peaks in certain windows and not in other windows may be used to detect presence of a particular target agent. In an embodiment the two analyses are combined so that, presence of a particular type of agent is indicated only in the event that a peak is detected in a predetermined detection window in the second IMS data and not in a window in the first IMS data. The fact that an ion was not present in gaseous fluid desorbed at a lower temperature but was present in the gaseous fluid desorbed at the higher temperature may be used to infer information about the substance in question (e.g. that it is very involatile) and so to eliminate certain types of false alarm. The second type of detection processing may comprise a pattern of peaks corresponding to a signature of a known target agent of the second class. Operating the ion mobility spectrometer to obtain second ion mobility spectrometry data may comprise using a shorter gate delay than for obtaining the first ion mobility spectrometry data. Embodiments of the disclosure provide computer readable storage media which may carry program instructions configured to program a controller of an ion mobility spectrometer to cause the ion mobility spectrometer to perform any one or more of the methods described herein. These computer readable storage media may be tangible and non-transitory. Embodiments provide computer program products which may be provided as network messages, electronic files and downloads, including libraries and firmware, and computer readable signals. Embodiments of the disclosure provide a controller for a detector such as an ion mobility spectrometer or a mass spectrometer or a hyphenated IMS-MS system. The controller may be configured to control the detector to perform any one or more of the methods described herein. The controller may comprise an i / o interface for controlling the detector and a processor having processing logic configured to control detector via the interface to perform any one or more of the methods described herein. For example, the controller of an ion mobility spectrometer may comprise a processor having processing logic for executing one or more such instructions and an i / o interface operable to control a desorber, an ion shutter, and an ionisation source of the spectrometer. The interface may be further operable to obtain detection signals from a detector of the IMS. An aspect of the disclosure provides an ion mobility spectrometer configured to perform any one of the methods described herein. An aspect of the disclosure provides a method comprising operating a desorber at a first desorber temperature to desorb a first gaseous fluid sample from a solid or liquid sample; operating the ion mobility spectrometer to obtain first ion mobility spectrometry data by ion mobility spectrometry of the first gaseous fluid sample; operating the desorber at a second desorber temperature to desorb a second gaseous fluid sample from the solid or liquid sample; operating the ion mobility spectrometer to obtain second ion mobility spectrometry data by ion mobility spectrometry of the second gaseous fluid sample; determining presence of target agent of the first class in the solid or liquid sample by applying a first type of detection processing to the first ion mobility spectrometry data; and determining presence of target agent of the second class in the solid or liquid sample by applying a second type of detection processing to the second ion mobility spectrometry data, wherein (a) the second type of detection processing is different from the first type of detection processing and / or (b) the first ion mobility spectrometry data relates to a different range of reduced mobility, Ko, values from the second ion mobility spectrometry data. Embodiments of the disclosure provide trace detection of many inorganic explosive materials, such as chlorates and perchlorates. Embodiments of the disclosure provide trace detection of higher melting point inorganic explosive materials, such as potassium chlorate and potassium perchlorate. Embodiments may enable detection of these high melting point inorganic explosives, optionally without compromising the current detection capability. This may extend the coverage of current Explosive Trace Detectors (ETDs) and their technology. Embodiments may permit the use of current ETDs to detect new classes of explosive without changing the concept of operation (so called "CONOPS"). The target agent type may be explosives. The target agent may exist in forms provided by a first class of substances, such as organic substances. Organic explosives may include most military and commercial-grade organic compounds such as RDX, PETN and HMX. The target agent may exist in forms provided by a second class of substances, such as inorganic substances. Inorganic explosives may include potassium nitrate (KNO3), potassium chlorate (KC1O3), sodium perchlorate (NaC104) and potassium perchlorate (KC1O4). Such a class of substances may have higher melting points than the first class of substances. Some organic explosives have melting points between ~80°C and ~300°C, whereas inorganic explosives typically transition to the gas phase between ~300°C and ~700°C. 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 flow chart indicating a method of the present disclosure and also includes a desorber temperature diagram indicating desorber temperatures which may be used in some embodiments; and, Figure 2 illustrates an IMS device which may be configured to implement methods of the present disclosure such as those described with reference to Figure 1. In the drawings like reference numerals are used to indicate like elements . Specific Description Figure 1 shows, in Fig. 1A, a plot of temperature against time for a desorber of an ion mobility spectrometer and in Fig. IB, a flow chart illustrating a method of operating an ion mobility spectrometer. In summary, the method illustrated in Figure 1 allows a single sample to be introduced to an ion mobility spectrometer and the presence of a target agent in that sample to be detected whether it relates to a first class of the target agent or a second class of the target agent. The target agent may be an explosive. The first class of target agent may have melting points below a selected temperature and the second class may have melting points above the selected temperature. Typically, the melting point ranges of the two classes of target agent do not overlap. Generally, a sample is provided to the instrument and heat is applied to raise temperature of a sample to a first temperature. This may generate vapour which is ionised and analysed by the ion mobility spectrometer to generate first IMS data. Heat is also applied to raise temperature of the sample to a second temperature. This may generate vapour which is ionised and analysed by the ion mobility spectrometer to generate second IMS data. The second temperature is higher than the first temperature. Usually, the first temperature is applied before the second temperature. The sample may be a solid or liquid sample carried by a carrier such as a swab. The sample may also be in the form of an aerosol which may be collected on a pre-concentrator. Where a pre concentrator is used the sample may be provided to the preconcentrator by the accumulation of material over a period of time, for example by the accumulation of aerosol. The desorber may be a heater for heating a swab, or may be a heater arranged for heating the pre-concentrator. In more detail to detect presence of a target agent type in a sample, the sample is provided to a heater of an ion mobility spectrometer, such as a desorber. The desorber may be arranged to heat a swab carrying a solid or liquid sample or may be provided by the heating of a pre-concentrator arranged to accumulate solid or liquid aerosol. As illustrated in Fig 1A, the desorber is then operated 2 at a first desorber temperature to desorb a first gaseous fluid sample from the sample. The first desorber temperature may be selected to desorb the first class of target agent and this temperature may be selected so that the first class of target agent is desorbed without desorbtion of the second class of target agent. Then to perform 4 IMS on the first gaseous fluid sample, that sample of gaseous fluid is ionised, and gated into a drift chamber of an ion mobility spectrometer. Times of flights of the ions along the drift chamber are detected to obtain first ion mobility spectrometry data of the first gaseous fluid sample. The so called "detection windows" applied (see step 10) to this first ion mobility spectrometry data may be chosen according to the expected drift times of the first class of target agent. The desorber is then operated 6 at a second desorber temperature, higher than the first desorber temperature, to desorb a second gaseous fluid sample from the sample. At step 8, the second gaseous fluid sample is ionised, and gated into the drift chamber of the ion mobility spectrometer. Times of flights of the ions along the drift chamber are detected to obtain second ion mobility spectrometry data of the second gaseous fluid sample . The so called "detection windows" applied to this second ion mobility spectrometry data (see to the expected drift times of and generally comprise different step 14) may be chosen according the second class of target agent detection windows from those which are used for the first ion mobility spectrometry data. Either or both of these analyses are then used (determination steps 12, 16) to provide an indication 18 of the presence of target agent. The indication may be provided in the form of an alert to an operator of the device and / or in the form of an electronic signal. The indication may be provided in the event that either: (a) the first ion mobility spectrometry data indicates 12 the presence of target agent of the first class; or (b) the second ion mobility spectrometry data indicates 16 the presence of target agent of the second class. Presence of target agent of the first class is determined by applying a first type of detection processing to the first ion mobility spectrometry data. As noted above, this may comprise determining whether the IMS signal exceeds a threshold in at least one detection window, corresponding to a range of reduced mobility, Ko, values of a species of ion associated with a target agent of the first class. The first type of detection processing may comprise determining whether the IMS signal exceeds threshold in a plurality of such windows, and a corresponding one of a plurality of thresholds may be applied in each of the detection windows. In the event that IMS signal in such a detection window exceeds the threshold for that window, the presence of target agent of the first class may be indicated. For example, the IMS system may provide a signal, such as an audible or visible alert, to an operator of the device. It may also send a signal, such as a wireless signal and / or a network message indicating that target agent has been detected. The indication may also indicate the class of the target agent in question and / or a likely identity of the agent. Presence of target agent of the second class is determined in a corresponding way but by applying a second type of detection processing to the second ion mobility spectrometry data. As noted above, this may comprise detecting peaks in a detection window (or windows) selected for target agent of the second class. Accordingly, the second type of detection processing may comprise determining whether the IMS signal exceeds threshold in at least one detection window, which may be different from a detection window used in the first type of detection processing. In the event that IMS signal in a detection window of the second type of detection processing exceeds the threshold for that window, the presence of target agent of the second class may be indicated. For example, the IMS system may provide a signal, such as an audible or visible alert, to an operator of the device. It may also send a signal, such as a wireless signal and / or a network message indicating that target agent has been detected. The indication may also indicate the class of the target agent in question and / or a likely identity of the agent. In addition, or as an alternative windows, different gate delays may spectrometry of the first gaseous to using different detection be used in the ion mobility fluid sample and the second gaseous fluid sample respectively. For example, the gate delays used for the first gaseous fluid sample may be selected to favour the transmission of ions associated first class of target agent into the drift chamber. The gate delays used for the second gaseous fluid sample may be selected to favour the transmission of ions associated second class of target agent into the drift chamber. The gate delays used for the second sample (which may be a associated with target agent with higher melting point) may be shorter than those used for the first sample. Figure 2 illustrates an example of an ion mobility spectrometer which may be configured to perform a method such as that described above with reference to Figure 1. The ion mobility spectrometer comprises a desorber 200, onto which a swab 202 carrying a liquid or solid sample 204 may be provided. The desorber 200 comprises a heater for heating the swab 202 to desorb the solid or liquid sample 204 from the swab 202 to generate a gaseous fluid, such as a vapour or a gas. The desorber may comprise a resistive heater. Other heating methods could be used e.g. IR lamps, lasers, induction heating and so forth. The apparatus may be used in a variety of circumstances and for detecting target agents of a variety of types. A conduit provides an inlet 206 from the desorber 204 to a reaction region 102 of the ion mobility spectrometer. An air mover 208, such as a fan or blower is configured in the conduit for generating a flow of air along the inlet from the desorber into the reaction region. This flow of air can carry a sample of gaseous fluid into the reaction region. A supply of cleaned dried air may be provided at the desorber 200 for this purpose. At the reaction region 102 an ionisation source 104, such as a corona point or dielectric barrier discharge (DBD) source is provided for ionising the sample of gaseous fluid in the reaction region 102. The ion mobility spectrometer also comprises an ion shutter 105, a detector 118, and a controller 120. 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 105 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. In the example illustrated in Figure the drift region 103 lies between the reaction region 102 and the detector 118. Any appropriate arrangement for detecting the arrival of ions may serve as the detector 118. Examples of detectors include a collector electrode, for example a Faraday cup for detecting the arrival of ions . A voltage profile may be provided in the drift region 103 using a series of drift electrodes 103a, 103b spaced apart along the drift region. 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 detector 118. The electric field may be uniform and / or known along the drift region 103 and / or the reaction region 102. A guard grid 123 may be interposed between the drift region and the detector 118 so as to shield the detector from approaching ions, to prevent "image effect" from approaching ions detracting from timing accuracy. The controller 120 comprises a programmable processor, an input / output interface such as an ADC / DAC (not shown in the drawings) which is able to control the provision of appropriate electrical control signals and / or power supply to the desorber, the ionisation source, and the ion shutter. It is also configured to receive detection signals from the detector 118 indicating the arrival of ions at the detector. It is therefore operable to control the temperature of the desorber, and the operation of the shutter, and to determine the time of arrival of ions at the detector following their being gated into the drift region by operation of the shutter. The IMS cell 100 illustrated in Figure 1 may also comprise a drift gas inlet 122 into the drift region near the detector, and a drift gas outlet 125 near the shutter so that a flow of (cleaned, dried) drift gas can be provided against the direction of travel of the ions towards the detector. In operation, a sample is provided to the desorber. The controller then controls the IMS system to perform the method described above with reference to Figure 1. It will be appreciated in the context of the present disclosure that the term "gate delay" is understood in the art to mean the interval between the operation of the ion source to generate ions from a neutral sample of gaseous fluid and the subsequent opening of the gate. It will be appreciated from the explanation above that embodiments of the disclosure the use of thermal desorption at two of more temperature dwell points in conjunction with temperature-controlled (or time-controlled) detection algorithms during those dwell points may provide a detection capability for a wider range of explosive materials. In particular, this approach could be used to add the class of high melting point inorganic explosives to the detection library of an ETD instrument. At each desorber temperature level, a corresponding set of detection algorithms specific to that dwell level are operated to detect compounds expected to evaporate from the swab at that dwell level. In this way, false alarm rate can be controlled. In particular, the risk of cross-talk between IMS signatures of organic and inorganic explosives will be minimised, thus including the case of hang-up of vapours from the organic compounds. With this scheme, a wider range of explosive materials could be detected, crucially from a single swab. Use of a single swab avoids retraining costs for users that have current ETD instruments. 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 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, FPGA, 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 employed without departing from the scope of the invention, which is defined in the accompanying claims .

Claims

1. A method of operating an ion mobility spectrometer to detect presence of a target agent type in a solid or liquid sample, the target agent type existing in forms provided by a first class of substances and forms provided by a second class of substances, the method comprising:operating a desorber at a first desorber temperature to desorb a first gaseous fluid sample from the sample;operating the ion mobility spectrometer to obtain first ion mobility spectrometry data by ion mobility spectrometry of the first gaseous fluid sample;operating the desorber at a second desorber temperature to desorb a second gaseous fluid sample from the solid or liquid sample;operating the ion mobility spectrometer to obtain second ion mobility spectrometry data by ion mobility spectrometry of the second gaseous fluid sample; andproviding an indication of the presence of target agent in the event that either:(a) the first ion mobility spectrometry data indicates the presence of target agent of the first class; or(b) the second ion mobility spectrometry data indicates the presence of target agent of the second class,wherein presence of target agent of the first class is determined by applying a first type of detection processing to the first ion mobility spectrometry data and presence of target agent of the second class is determined by applying a second type of detection processing to the second ion mobility spectrometry data, wherein(i) the second type of detection processing is different from the first type of detection processing and / or(ii) the first ion mobility spectrometry data relates to a different range of reduced mobility, Ko, values from the second ion mobility spectrometry data.

2. A method of operating an ion mobility spectrometer to detect presence of a target agent type in a sample, the target agent type existing in forms provided by a first class of substances and forms provided by a second class of substances, the method comprising:operating a desorber at a first desorber temperature to desorba first gaseous fluid sample from the sample;operating the ion mobility spectrometer to obtain first ion mobility spectrometry data by ion mobility spectrometry of the first gaseous fluid sample using a first gate delay;operating the desorber at asecond desorber temperature todesorb a second gaseous fluid sample from the sample;operating the ion mobility spectrometer to obtain second ion mobility spectrometry data by ion mobility spectrometry of the second gaseous fluid sample using a second gate delay, different from the first gate delay; anddetermining presence of a target agent in the sample based on the first ion mobility spectrometry data and / or the second ion mobility spectrometry data.

3. The method of claim 2 comprising determining presence of a target agent of the first class in the sample based on the first ion mobility spectrometry data.

4. The method of claim 2 or 3 comprising determining presence of a target agent of the second class in the sample based on the second ion mobility spectrometry data.

5. The method of claim 2, 3 or 4 comprising determining presenceof target agent of the first class in the sample by applying a first type of detection processing to the first ion mobility spectrometry data; anddetermining presence of target agent of the second class in the sample by applying a second type of detection processing to the second ion mobility spectrometry data, wherein(c) the second type of detection processing is different from the first type of detection processing and / or(d) the first ion mobility spectrometry data relates to a different range of reduced mobility, Ko, values from the second ion mobility spectrometry data.

6. The method of any preceding claim wherein the first class of substances comprises organic substances and the second class of substances comprises inorganic substances.

7. The method of any preceding claim wherein the first class of substances have melting points which are lower than melting points of the second class of substances.

8. The method of any preceding claim wherein the target agenttype is explosives.

9. The method of any preceding claim wherein the second temperature is higher than the first temperature, for example wherein the first temperature is between 150°C and 250°C, for example wherein the second temperature is at least 500 °C, for example at least 590°C, for example 600°C.

10. The method of any preceding claim wherein the first type of detection processing comprises a first set of peak detection criteria, and the second type of detection processing comprises a second set of peak detection criteria.

11. The method of claim 10 wherein the peak detection criteria comprise a peak window and a threshold level, corresponding to the peak window.

12. The method of claim 11 wherein a peak window of first set of peak detection criteria corresponds to a reduced mobility Ko range of between 0.8 cm2v-is-i to 2.1 cnryV1.

13. The method of claim 11 or 12 wherein a peak window of secondset of peak detection criteria corresponds to a reduced mobilityKo range of between 1.9 cm2V xs 1 to 2.1 cm2V 2s 114. The method of any preceding claim wherein the sample comprises a solid or liquid sample carried by a swab and the desorber comprises a heater configured for heating the swab.

15. The method of any preceding claim wherein the sample comprises an aerosol accumulated on a pre-concentrator and the desorber comprises a heater configured to heat the pre-concentrator.

16. The method of any preceding claim wherein the first type of detection processing comprises a pattern of peaks corresponding to a signature of a known target agent of the first class.

17. The method of any preceding claim wherein the second type of detection processing comprises a pattern of peaks corresponding to a signature of a known target agent of the second class.

18. The method of claim 1, or any of claims 6 to 17 as dependent upon claim 1 wherein operating the ion mobility spectrometer to obtain second ion mobility spectrometry data comprises using a shorter gate delay than for obtaining the first ion mobility spectrometry data.

19. The method of any of claims 2 to 5, or any of claims 6 to 17 as dependent upon any of claims 2 to 5, wherein the second gate delay is shorter than the first gate delay.

20. A computer program product configured to program a controller of an ion mobility spectrometer to cause the ion mobility spectrometer to perform the method of any preceding claim.

21. An ion mobility spectrometer configured to perform the method of any of claims 1 to 19.

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