This invention relates to apparatus of the kind for providing an indication of the concentration of an analyte in a gas or vapour sample including means for supplying the sample containing the analyte to detector means, the detector means being arranged to provide an output indication when the amount of analyte in the sample exceeds a threshold.

It is well known that certain diseases and medical conditions can be detected by analysing the breath exhaled by a person or animal. This breath analysis can be carried out in various ways such as using gas chromatography, ion mobility spectrometry or by means of an array of vapour-sensitive resistive elements in a device known as an ENOSE® (ENOSE is a Registered Trade Mark of Smiths Detection, Inc). These detectors employ an array of two or more chemiresistor elements made up of a nonconductive material such as a polymer incorporating a conductive material such as fine particles or nanoparticles of carbon. When the elements are exposed to a vapour, the vapour permeates the polymer causing it to expand and thereby increasing the resistance of the element. By measuring the change in resistance of elements made of different materials it is possible to identify specific substances. Detectors with greater numbers of elements enable a greater number of analytes to be detected reliably. The Cyranose 320 detector sold by Smiths Detection, Inc has an array of thirty two elements and can be trained to recognise many different substances. Details of this form of detector are given, for example, in US6746960, US6537498, US5571401, US5698089, US5788833, US591 1872, US6093308, US6331244, US6010616, US6017440, US5959191, US5891398, US6013229, US6752964, US6244096, US6467333, US6319724, US6841391, US6571603, US71 13069, US7471185, US6290911, US6085576, US6234006, US6418783, US6883364, US6170318, US6387329, US6759010, US7144553, US6610367, US7527821, US7255677, US7819803 and US7129095. Where a detector is required only to identify one specific analyte it may be sufficient to employ a fewer number of chemiresistor elements selected to have a relatively high response to the particular analyte. These detectors can be used in various applications other than medical applications. These chemiresistor analyte detectors can be very effective at identifying specific analytes. They are, however, less effective at providing an accurate quantitative indication of the concentration or amount of an analyte in a specific sample.

It is an object of the present invention to provide an alternative apparatus and method for use in detecting gas or vapour analytes.

According to one aspect of the present invention there is provided apparatus of the above-specified kind, characterised in that the apparatus includes means for mixing the analyte gas or vapour sample with a reference gas or vapour free from the analyte, and means for varying the concentration of analyte gas or vapour in the mixture and identifying when a threshold indication is produced thereby to provide an indication of the concentration of the analyte in the sample.

The detector means preferably includes a plurality of chemiresistor elements. The means for mixing may include a mixer with a first inlet connected to receive the gas or vapour sample, a second inlet connected to receive the reference gas or vapour, an outlet connected to an inlet of the detector means and a connection between the detector means and the mixer by which the concentration of the mixture of the gas or vapour supplied to the first and second inlets is varied for supply to the inlet of the detector means. The reference gas or vapour may be provided by a source being one of the following: a container of compressed gas or vapour, a hospital oxygen or air supply, and filtered ambient air. The apparatus may be arranged such that the mixing means varies the concentration of analyte gas or vapour until the concentration passes a threshold level at which the analyte can be detected and that the concentration of analyte in the sample is derived from knowledge of the concentration of the mixture equivalent to the threshold level. The mixing means may vary the concentration of the analyte gas or vapour in discrete steps or continuously and gradually.

According to another aspect of the present invention there is provided a method of providing a quantitative indication of the relative amount of an analyte in a sample, including the step of providing a sample of a gas or vapour containing the analyte to detection means operable to detect the presence of the analyte above a threshold concentration, characterised in that, in response to detection of the analyte by the detection means, the analyte sample is mixed with a reference gas or vapour free from the analyte in varying concentrations so as to determine the concentrations above and below the threshold, and that an indication of the relative amount of analyte in the sample is thereby provided.

Detection apparatus and its method of operation, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawing ^' , in which:

Figure 1 shows the apparatus schematically; and

Figure 2 is a chart illustrating operation of the apparatus.

The apparatus includes a conventional chemiresistor detector unit 1, such as of the kind sold by Smiths Detection under the name Cyranose 320, including an array of thirty two . detection elements 10 based on mixtures of polymers and conductive material. The detector unit 1 is programmed to detect the presence of one or more specific analytes from the pattern of outputs produced from the element array 10. In particular, the detector 1 produces an output indication to utilisation means, such as a display 2, to indicate the presence of the analyte when the concentration of the analyte exceeds a threshold at which the recognition processing output exceeds a predetermined confidence level.

The detector unit 1 is modified to produce a control output on line 3 when the analyte is detected. This control output is used, effectively, slightly to dilute the analyte gas sample supplied to the detector. This is achieved by connecting a variable gas mixer 4 to the inlet of the detector unit 1. The mixer 4 has two inlets, one of which 5 connects with the analyte sampling inlet 6, which is connected with, or is held up to, the source of the suspected analyte. The outlet 9 of the mixer 4 connects with the inlet 1 1 of the detector unit 1. Gas is drawn into the detector unit 1 by means of a pump in the detector unit. The other inlet 7 extends to a source 8 of gas or vapour that is known to be free from the analyte. This source 8 could be provided by a container of a compressed gas or vapour or, in a hospital environment, it could be connected to the hospital oxygen or air supply. Alternatively, the source 8 could be provided by ambient air away from the suspected analyte source and could be filtered such as by means of a molecular sieve. The mixer 4 is connected to line 3 and controlled in a variable manner by signals supplied from the detector unit 1.

In normal operation, when no analyte is detected, the mixer 4 is switched to close or isolate the reference inlet 7 so that the detector unit 1 is supplied exclusively with the nominal analyte gas sample from inlet 6, that is, at its maximum possible concentration. When the level of analyte in the gas sampled at the inlet 6 exceeds a threshold level, the detector unit I provides an output indication to the display 2 to indicate the presence of the analyte. The detector unit 1 also supplies an output signal on line 3 to the mixer 4 to open the reference inlet 7 and admit an amount of reference gas to dilute the analyte gas sample from inlet 6. Figure 2 shows one way in which this dilution could be arranged, that is, by progressively opening the reference inlet 7, gradually to admit more of the reference gas or vapour so that the mixture gradually becomes less concentrated. Column A indicates the initial state where the detector unit 1 is supplied with 100% gas from the analyte inlet 6, which has an analyte concentration above the threshold indicated by the broken horizontal line. Columns B to F indicate subsequent dilutions of the analyte gas flow with the reference gas in discrete 10% steps. For example, column B shows where the gas supplied to the detector inlet 1 1 is made up of 90% analyte-containing gas from the inlet 6 plus 10% of analyte-free gas from the source 8. It can be seen that the dilutions E and F at 60% and 50% respectively are both below the threshold level so the gas mixture at these concentrations cannot be detected by the detector unit 1. The dilution D, however, at 70% is just above the threshold level so can be detected. This indicates that the concentration of the analyte in the sample gas is such that it passes the threshold level when between a dilution of 60% and 70%. It will be appreciated that the mixer 4 need not be controlled to reduce the concentration progressively in the manner described above but could instead, for example, change concentration between levels on either side of the threshold such as: 100%, 50%, 90%, 60%, 80%, 70%, 65%. Instead of stepwise changes in concentration, the change could be gradual and continuous.

The detector unit 1 converts the mixed gas concentration estimate to a figure for the concentration of analyte in the sample gas stream. This may be done by use of a look-up table or algorithm using information about the concentration of analyte equivalent to the threshold level obtained during set up of the detector unit I when the level of analyte is gradually increased until the threshold is triggered.

Alternative ways of varying the concentration of the analyte sample gas with an ambient or reference gas include drawing the gas into the detector in a pulsed fashion where it mixes with ambient air within the detector. By altering the frequency or length of the pulses, the amount of analyte sample gas mixed with a reference or ambient gas can be varied in a controlled manner.

The present invention enables a more accurate quantitative measurement of analyte concentration. This can be particularly useful in medical applications where the detector is employed to detect particular analytes in a patient's breath indicative of the presence of a disease. The quantitative measurement possible could enable a more accurate assessment of the severity or progress of a disease and could enable changes in the state of the disease over time to be monitored more reliably.

Although the invention is particularly useful for chemiresistor array detectors it could also be used in other detectors.