Gas sensing apparatus and method

Abstract

Disclosed is a gas sensing apparatus which includes a gas sensor, a modulator, and a processor. A gas sensitive property affects an output signal of the gas sensor and is sensitive to a gas of interes

Description

[0001] Gas sensing apparatus and method

[0002] The invention relates to a gas sensing apparatus and related methods, including a method of detecting a gas, for example a method for fast gas detection.

[0003] Aspects of the present invention seek to provide an improved gas sensing apparatus and related methods.

[0004] According to an aspect of the invention, there is provided a gas sensing apparatus as in claim 1.

[0005] Optional features are set out in the dependent claims.

[0006] According to an aspect of the invention, there is provided a method as in claim 15 or 17.

[0007] Optional features of the gas sensing apparatus apply also to the aspects of the invention relating to methods.

[0008] In some embodiments of determining or calculating the modulation frequency, optimizing the modulation frequency includes: sweeping, optionally using a signal generator with reference frequency scanning, modulation and / or reference frequencies greater than or approximately equal to 1 / T, wherein the processor produces a processed signal identifying or selectively emphasising main and / or higher harmonics of the modulation frequency in the output signal of the gas sensor; measuring a frequency response characteristic, the frequency response characteristic being a difference between the processed signal amplitude dependence on the modulation frequency for the gas of interest and the processed signal amplitude dependence on the modulation frequency for a reference gas (for example clean air); and achieving a minimal response time of sensing the gas of interest by choosing the modulation frequency at the highest frequency where the amplitude of the frequency response characteristic exceeds three times the measurement noise; or achieving the highest sensitivity of sensing the gas of interest by choosing the modulation frequency at the maximum amplitude of the frequency response characteristic.

[0009] In some embodiments of the apparatus, the modulation frequency is calculable and / or determined by the method of any of claims 17 to 20 optionally including any further optional features of the method.

[0010] In some embodiments of the apparatus, it is configured to perform the method of any of claims 15 to According to an aspect of the invention, there is provided computer program code configured to implement any of the methods when executed, for example on an apparatus as recited, and / or a computer program product incorporating the said code.

[0011] The method of embodiments disclosed hereby relates to an improvement of the sensing rate of a gas sensor such as the resistive sensor.

[0012] In one example, the method for fast gas detection comprises measuring a sensor property, such as its resistance or capacitance, that is sensitive to the gas or its concentration being measured, and simultaneously cyclically changing the temperature of the sensor. According to this example, the gas-sensitive property of the sensor is continuously measured with a lock-in amplifier to extract the signal component associated with the frequency of the heating cycle. The extracted signal component is used as the actual output signal of the sensor. The cyclical temperature modulation frequency is selected near the maximum of the sensor's frequency response range. I n this way, the lock-in amplifier filters out from the whole heterogeneous ensemble of gas molecules adsorbing to the sensor's sensitive layer, only the species with the shortest adsorption / desorption time constant. The response rate of the gas detector is thereby increased and the baseline drift is diminished.

[0013] Embodiments of the present invention relate to a new method of increasing the gas-sensing rate of the resistive gas sensor by using cyclic thermal modulation. The resistive gas sensors operate on the principle that gas adsorption affects the resistance and / or capacitance of the gas sensitive resistive layer. The method of embodiments of the invention relies on the heterogeneity of gas adsorption sites of the resistive sensing layer. The heterogeneity is a common characteristic that can arise from inhomogeneous surface morphology, structural features, irregularities of chemical composition, and the presence of various defects, impurities, cavities, pores, different functional groups or adsorbed molecules on the surface [1,2], The adsorption sites of different types are characterised by different adsorption and desorption time constants. The sites of high adsorption rates are characterised by high affinity in respect of the adsorbing gas molecules. On the other hand, the high desorption rate is a feature of weakly bound molecules with the shortest retention time. The ratio of the fast and slow adsorbing processes' time constants can be as large as three orders of magnitude [3], spanning from 10 s to 104s. The adsorption and desorption processes depend on the temperature of the sensing surface. In embodiments, periodically heating the sensing surface at the frequency corresponding to the time constants of the fastest adsorbing / desorbing gas molecules, the temperature modulation induces the AC signal of the fastest molecules. Registering this AC signal with a lock-in amplifier at this modulation frequency, only the fastest-reacting gas molecules contribute to the response time of the lock-in amplifier output signals - amplitude and phase. Based on the readings of amplitude and / or phase and using the stored calibration data, the concentration of sensed gas can be calculated.

[0014] In addition to diminishing of the response time of a gas sensor, the technique of sensor parameter modulation and synchronous detection with lock-in amplifier provides a benefit of remarkably diminishing of the baseline drift and increasing the operation stability. According to an aspect of the invention, there is provided a method for fast gas detection comprising the steps of passing a gas containing the molecules of interest over a gas sensor electrically connected to a modulator; said modulator electrically connected to a signal generator; said signal generator producing continuously a cyclical electrical signal waveform at a modulation frequency close to the high end of the frequency response band of the combination of said gas sensor and detectable gas molecules of interest; said modulator continuously modulating said gas sensor parameter(s) affecting said gas sensor sensitivity in respect of sensing said gas molecules of interest; applying the modulated electrical signal of said gas sensor to the lock-in amplifier; applying said signal generator electrical signal waveform to said lock-in amplifier; said lock-in amplifier continuously synchronously filtering out the frequency component(s) at said modulation frequency and / or at its higher harmonics of the modulated output signal of said gas sensor; the lock-in amplifier continuously producing its measurement data - an amplitude and a phase shift, wherein the improvement comprises generating the said measurement data at shortest response time possible for said gas molecules of interest and said gas sensor combination; continuously calculating by a processor the concentration of the gas molecules of interest from the said measurement data of said lock-in amplifier - having previously performed calibration of concentration of the gas molecules of interest.

[0015] In some embodiments, said gas sensor is a chemiresistive gas sensor.

[0016] In some embodiments, a property of said gas sensor sensitive togas concentration is a resistance and / or a capacitance.

[0017] In some embodiments, a modulated parameter of said gas sensor is temperature.

[0018] In some embodiments, said modulator is a heater.

[0019] In some embodiments, said heater is electrically powered (micro)heater in contact with said sensor.

[0020] In some embodiments, said heater is a non-contact electromagnetic radiation emitter (LED, laser).

[0021] In some embodiments, the modulated parameter of said gas sensor is gas pressure.

[0022] In some embodiments, said modulator is a miniature electronic gas pressure modulator.

[0023] In some embodiments, the modulated parameter of said gas sensor is a concentration of a gas of interest.

[0024] In some embodiments, the modulated parameter of said gas sensor is a gas flow rate. In some embodiments, said modulator is a gas flow controller.

[0025] In some embodiments, the modulated parameter of said gas sensor is light.

[0026] In some embodiments, said modulator is light emitting diode or laser.

[0027] In some embodiments, a gas sensor is a Field Effect Transistor (FET) gas sensor.

[0028] In some embodiments, the modulated parameter of said FET gas sensor is electric field.

[0029] In some embodiments, said modulator is said signal generator providing voltage waveform applied to a gate of said FET.

[0030] Some embodiments of the aspect of claim 17 include optimizing the modulation frequency by measuring the frequency response characteristic of the gas sensor at frequencies determined in claim 19; using a signal generator with frequency scanning (sweeping) capability measuring the lock-in signal amplitude dependence on the modulation frequency (frequency response characteristic); measuring the frequency response characteristic for both the gas of interest and the reference gas (clean air), their difference providing the frequency response characteristic for the gas of interest; achieving a minimal response time of sensing the gas of interest by choosing the modulation frequency at the highest frequency where amplitude of the frequency response characteristic exceeds three times the measurement noise; achieving the highest sensitivity of sensing the gas of interest by choosing the modulation frequency at the maximum amplitude of the frequency response characteristic.

[0031] Literature

[0032] 1. Rudzinski W., Steele W.A. and Zgrablich G. (1997) Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces, Elsevier, Amsterdam.

[0033] 2. Rudzinski W. and Everett D.H. (1992) Adsorption of Gases on Heterogeneous Surfaces, Academic Press, London.

[0034] 3. X. Xia, J. Strunk, W. Busser et al. Probing the Surface Heterogeneity of Polycrystalline Zinc Oxide by Static Adsorption Microcalorimetry. 1. The Influence of the Thermal Pretreatment on the Adsorption of Carbon Dioxide, The Journal of Physical Chemistry C 2008 112 (29), 10938- 10942. DOI: 10.1021 / jp8007464

[0035] US Pat. No. 6564633 discloses a measurement method and system for humidity or gas concentration sensors based on measuring a parameter of the sensor (capacitance or resistance) and simultaneously cyclically modulating the temperature of the sensor. The measured parameter is filtered by an electronic filter or by Fourier analysis in order to extract the signal component, which frequency is associated with the frequency of thermal cycling. The thermal modulation frequency is kept so low that the sensor parameter, which is measured, can stabilise during gas concentration changes.

[0036] According to the logic of the patent, in the case of heterogeneity of the adsorption sites of gas molecules and heterogeneity of response times, the temperature modulation frequency should be as low as the response frequency of the slowest adsorption sites. This determines the slow sensing rate of the gas sensor, unlike embodiments described herein, which enable maximising the sensing rate. The sensing rate of embodiments described herein is determined by the fastest- absorbing / -desorbing sites of the sensing layer.

[0037] US Pat. No. 4627269 discloses the method and apparatus for detecting reducing gases in a gas mixture. The temperature of the gas sensor is periodically cycled up and down according to a certain pattern. The periodically varying gas sensing property (for example, electric conductivity) is detected by an electronic circuit and evaluated for determining the gas concentration and / or the nature of the reducing gases. Simultaneously therewith, the gas mixture to be investigated, which is present in the measuring chamber, is periodically exchanged against a pure reference gas which is contained in a reference chamber.

[0038] The described requirement to periodically replace the content of the measuring chamber complicates the construction, causes a slow response of the sensor, and is incompatible with the energy efficiency and miniaturisation of the sensor.

[0039] US Pat. No. 11275046B2 discloses a humidity meter, which includes a humidity sensor, a heating device for the heating of the humidity sensor, a signal generator for controlling the modulation of the heating device, and a lock-in amplifier for demodulating the alternating response signal induced by modulated heating. The heating device is a radiative device, e.g., a light emitting diode (LED), a laser, an incandescent light bulb, or another electromagnetic (EM) radiation-emitting device.

[0040] The description of the humidity meter provides no information on how to select the heating modulation frequency that is beneficial for achieving a fast response time of the humidity meter. The patent does not address the issue of increasing the response rate of the gas sensor.

[0041] US Pat. No. 5047352 discloses a method and instrument for identifying an unknown component of a gas sample which uses temperature modulation to produce the modulated reactive response in the unknown component of the sample and uses an electrochemical cell to produce a modulated sensor signal, which is converted to concentration independent value. Comparing this value with the analogous values of known gases allows identifying the unknown component.

[0042] Although the method of temperature modulation is used for analytical purposes, the patent does not contain any information concerning the possibility of enhancing the gas detection rate.

[0043] Embodiments of the invention are described below, by way of example only, with reference to the accompanying figures, in which:

[0044] Fig. 1 is a block diagram of an apparatus for application of a fast gas detection method according to an embodiment of the invention; Fig. 2 is a schematic diagram of an apparatus for measuring gas sensor response according to an embodiment of the invention;

[0045] Fig. 3 shows a graphene based chemiresistor type gas sensor functionalised with vanadium pentoxide V2O5. Chemiresistor response and recovery kinetics for 50 ppm of NH3 gas, measured with apparatus of Fig. 2;

[0046] Fig. 4 shows transient responses of a graphene-based sensor to 10 ppm of NH3.

[0047] An embodiment of the invention relates to a method of fast gas detection and associated gas sensing apparatus, which will be described referring to the block diagram depicted in Fig. 1. The gas sensing apparatus, also referred to as a fast gas detector, comprises two blocks: the sensor block and the electronics block. The sensor block contains gas sensor 1 and modulator 2. Gas sensor 1 is the gas-sensitive element, which has at least one measurable gas sensitive property, sensitive to the adsorption of the ambient gas molecules of interest, with a sensitivity dependent on a gas sensor parameter, the gas sensor parameter in this embodiment being operating temperature. In this embodiment, the gas sensitive property is an electrical property of the gas sensor, specifically resistance and / or capacitance, and affects an electrical output signal of the gas sensor 1, the electrical output signal of the gas sensor in this example also being an output signal of the sensor block. The sensitivity of gas sensor parameter, for example the resistance, in respect of gas concentration changes, depends on operating temperature.

[0048] The gas sensing properties of the gas sensor 1 are cyclically modified by modulator 2 modulating the gas sensor parameter, which is the operating temperature of the gas sensor in this embodiment, resulting in the modulated output signal of the sensor block. The preferred embodiment of modulator 2 is a heater connected to and configured to heat gas sensor 1, which modulates its temperature and stimulates the adsorption / desorption of gas molecules to / from the sensor's active layer and results in the modulation of the gas sensor 1 resistance and / or capacitance. The heater can be an electrical resistor powered by the signal generator 3. According to another embodiment, contactless mode heating can be performed by infrared or visible light emitted by a LED or a laser.

[0049] According to some other embodiments, the gas sensor parameter can be gas pressure, a gas flow rate, or a concentration of a gas of interest. For example, modulator 2 can be a gas flow or pressure modulator, for example an electronic gas pressure modulator. In some embodiments, the gas sensor and modulator can be a Field Effect Transistor (FET) and the gas sensor parameter can be the variable electric field induced by FET sensor gate voltage; the FET as a modulator can be configured by the signal generator being configured to provide a voltage waveform to a gate of the FET.

[0050] The signal generator 3 supplies modulator 2 with a reference signal in the form of voltage waveform at angular frequency co causing the gas sensor parameter to be modulated at the frequency co and / or at its higher harmonics 2 co, 3co .... The waveform can be sinusoidal, rectangular, sawtooth, trapezoidal or any other cyclically variable voltage. The gas sensitive property of the gas sensor 1 (resistance and / or capacity in this embodiment) are / is sensitive in respect of modulated temperature-stimulated adsorption / desorption of gas molecules, and therefore the cyclic heating of sensor 1 induces its output electrical signal that is modulated at a modulation frequency; specifically it is modulated at the frequency of cyclic heating at frequency co and / or at its higher harmonics 2 co, 3co etc. A processor includes a lock-in amplifier 4. The modulated output signal of the gas sensor 1 is received and amplified by the lock-in amplifier 4, which selectively amplifies (and thereby emphasises) and demodulates the main harmonic co and / or higher harmonics of its input signal. Lock-in amplifier 4 is also supplied with the reference signal of frequency co from signal generator 3. The concentration of the gas of interest can be unambiguously derived from the output data (amplitude and phase shift) of the lock-in amplifier 4, having previously calibrated the apparatus.

[0051] The gas sensor parameter modulation has a period less than an average gas adsorption time constant of the gas sensor. Preferably, the modulation frequency corresponds to a shortest adsorption time constant of the gas sensor, in particular corresponding to the gas adsorption sites with the shortest adsorption time constant in the entire ensemble.

[0052] An apparatus used for the assessment of rapid gas detection method is shown in Figure 2. The apparatus of Figure 2 corresponds to the apparatus of Figure 1, with additional details as identified below.

[0053] The apparatus (Fig. 2) for measuring the response time of a gas sensor in DC and AC modes performs as follows. Gas sensor 14 with gas-sensitive resistance Rgasis located in gas chamber 13. The gas flow controller 16 (which can be a gas mixer) allows rapid switching between the gas of interest 17 (such as one or more of O2, N2, CO2, CO, H2, NH3, H2S, NO, NO2, O3 or volatile organic compounds from one or more gas supplies; in one embodiment gas supplies for the gas of interest are O2 N2 CO2 NH3) and the reference gas and controls the concentration of the gas of interest in the gas chamber.

[0054] In DC mode, heater 15, connected to the gas sensor, creates a constant temperature of the sensor. The signal of the gas sensor 14 resistor Rgasis measured by Keithley 2450 Sourcemeter 18, its reading reflecting changes in the concentration of the gas of interest in the gas chamber 13. The output signal reflects all the different adsorption sites present on the sensitive surface characterised by different adsorption / desorption time constants. The measurement results are stored in a data logger (not shown) connected to the Keithley 2450 Sourcemeter 18 for subsequent analysis.

[0055] The modulation frequency is determined as follows. The gas of interest is excluded from the gas chamber to avoid exposure of the gas sensor to the gas of interest in a first time period and a baseline level of the sensor's electric output signal is measured. The time dependence of the sensor's electric output signal, preferably at an average working temperature, is then measured while turning on the flow of the gas of interest into the gas chamber for a preferably short moment only, in other words while the gas sensor is exposed to the gas of interest in a second time period and then while exposure of the gas sensor to the gas of interest is avoided in a third time period (when temperature modulation is used the temperature changes (periodically) between a minimal and maximal value. The average working temperature is between; it is the average during the modulation period). The signal jumps abruptly to a new value due to some amount of gas adsorption and then starts to recover to the previous baseline level because of gas desorption. The fastest recovery time T is determined by drawing the tangent line to the initial part of the recovery curve in the third time period. Time constant T equals the time interval between the first moment of avoiding gas exposure in the third time period and the moment when the tangent line crosses the signal baseline. The modulation frequency to be chosen approximately equal or larger than 1 / T.

[0056] The modulation frequency can be further optimised by measuring the frequency response characteristic of the gas sensor at frequencies around 1 / T as follows: As a signal generator 3 (Figure 1), we use a sweep generator capable of scanning (sweeping) the frequency of its electrical output signal. This variable frequency signal is used by the modulator 2 to modulate gas sensor 1 temperature, resulting in the modulation frequency being swept at frequencies greater than or approximately equal to 1 / T. Synchronously with the heating frequency, gas sensor 1 signal is measured with the lock-in amplifier 4. This measurement results in a gas sensor frequency response characteristic (lock-in signal amplitude dependence on modulation frequency). The frequency response characteristic is measured for both the gas of interest and the reference gas (clean air) and the difference gives the gas sensor frequency response characteristic for the gas of interest. The optimal frequency is determined at the point where the amplitude of the characteristic has a maximum value (for the highest sensitivity) or as the highest frequency at which the amplitude of the characteristic still exceeds three times the measurement noise (for the highest rate).

[0057] Referring to Figure 2, in the AC mode, the gas sensor heater is fed from signal generator in the form of HP3245A waveform generator 12 with sinusoidal bipolar voltage (reference signal) at frequency f: U(t)-Uo sin(2nft). As during one period of sinusoidal supply voltage, the heater warms up and cools down twice, the sensor's temperature is modulated (oscillates) at the modulation frequency of 2f. The oscillating temperature induces the adsorption / desorption of gas molecules on the surface of the gas sensor, causing the gas sensor resistance Rgasto oscillate at 2f. The AC voltage signal (output signal of the gas sensor) at frequency 2f is received and measured synchronously by the lock-in amplifier SR850 11, which provides amplitude and phase 9 as output data. The lock-in amplifier 11 filters out from the plurality of the adsorption sites preferably those capable of temperature-induced adsorption / desorption at the high frequency of temperature modulation by selectively amplifying and thereby emphasising main and / or higher harmonics of the modulation frequency in the output signal of the gas sensor, hence the sites with the fastest response among the whole ensemble of adsorbing / desorbing gas molecules.

[0058] The sensing element of the gas sensor 14 consists of a microelectrode substrate and a gassensitive CVD graphene layer on top of it. Single-layer graphene was functionalised using the pulsed laser deposition method with a 0.6 nm V2O5 layer. The sensor contains a resistance microheater under the sensing layer. Results of experiments

[0059] The results of an experimental assessment of the gas rapid detection method are depicted in Fig.3.

[0060] Measured in DC mode (no temperature modulation), the gas sensor resistance response to the NH3 concentration step increase is depicted in Panel 1. Fitting with single exponential y=yo+A*exp(-(x-xO) / r) (red lines)gave r - 14.6 s and r - 45.5 s as response and recovery kinetics time constants, respectively.

[0061] Measured in AC mode (temperature modulation at a frequency of 46 Hz) lock-in amplifier output amplitude (Panel 2) and phase shift (Panel 3) kinetics. The exponential fit time constants for amplitude are 6.4 s and 27.2 s for response and recovery kinetics, whereas for phase shift the respective time constants are 3.2 s and 105.1 s.

[0062] The main result of the experimental assessment of the gas rapid detection method proposed in this disclosure is: the ammonia detection response rate (inverse response time) increased 4.6 times compared to the standard (DC) measurement method.

[0063] Another example of experimental assessment is given in Figure 4, which shows transient responses of a graphene-based sensor to 10 ppm of NH3. The microheater's power was modulated at 10 Hz, and two different responses were measured in parallel: averaged DC conductance (upper panel); signal detected at the modulation frequency (lower panel). Response curves (1) and (2) were recorded at precisely similar conditions. AC response is not only much faster but is much more stable and the baseline has a much lower drift. Between the responses (1) and (2) the sensor was exposed to 10 ppm of NH3 in different temperature regimes.

[0064] In some embodiments, a portable sensor or gas sensing apparatus is provided as described above in which the components are miniaturized devices. For example, the lock-in amplifier is available as chip as well as the signal source can be in the form of chip.

[0065] All optional and preferred features and modifications of the described embodiments and dependent claims are usable in all aspects of the invention taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

[0066] The features of UK patent application number GB 2308069.0, from which this application claims priority, and in the abstract accompanying this application, are incorporated herein by reference in their entirety.

[0067] The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under Graphene Flagship's FPA.

Claims

CLAIMS1. A gas sensing apparatus, including: a gas sensor, wherein a gas sensitive property affects an output signal of the gas sensor and is sensitive to a gas of interest with a sensitivity dependent on a gas sensor parameter; a modulator configured in operation to modulate the gas sensor parameter at a modulation frequency; a processor configured in operation to receive the output signal of the gas sensor and to identify or selectively emphasise main and / or higher harmonics of the modulation frequency in the output signal of the gas sensor for sensing of the gas of interest; wherein the modulation has a period less than an average gas adsorption time constant of the gas sensor.

2. The apparatus of any preceding claim, including a signal generator configured in operation to provide a reference signal to the modulator and to the processor, wherein the modulation frequency corresponds to a frequency of the reference signal or a harmonic thereof.

3. The apparatus of any preceding claim, wherein the gas sensitive property is an electrical property of the gas sensor, for example resistance and / or capacitance.

4. The apparatus of any preceding claim, wherein the output signal is an electrical output signal.

5. The apparatus of any preceding claim, wherein the processor includes a lock-in amplifier configured in operation to selectively amplify the main and / or higher harmonics of the modulation frequency in the output signal of the gas sensor, the lock-in amplifier optionally producing a lock-in signal.

6. The apparatus of any preceding claim, wherein the modulation frequency corresponds to a shortest adsorption time constant of the gas sensor.

7. The apparatus of any preceding claim, wherein the gas sensor is a chemiresistive gas sensor.

8. The apparatus of any preceding claim, wherein the gas sensor parameter is temperature and the modulator includes a heater configured in operation to heat the gas sensor.

9. The apparatus of any preceding claim, wherein the modulator includes an electrically powered microheater in contact with the gas sensor and / or a non-contact electromagnetic radiation emitter such as a LED or laser.

10. The apparatus of any of claims 1 to 7, wherein the gas sensor parameter is gas pressure and the modulator includes a gas pressure modulator, optionally an electronic gas pressure modulator.

11. The apparatus of any of claims 1 to 7, wherein the gas sensor parameter is a concentration of the gas of interest or a gas flow rate.

12. The apparatus of claim 11, wherein the modulator includes a gas flow controller.

13. The apparatus of any preceding claim, wherein the gas sensor includes a Field Effect Transistor (FET) gas sensor; optionally wherein the gas sensor parameter is electric field; optionally wherein the modulator is the FET, a signal generator optionally being configured to provide a voltage waveform to a gate of the FET.

14. The apparatus of any preceding claim, wherein the modulation frequency is greater than or approximately equal to 1 / T, wherein T is calculable by the following method: avoiding exposure of the gas sensor to the gas of interest in a first time period; exposing the gas sensor to the gas of interest in a second time period; avoiding exposure of the gas sensor to the gas of interest in a third time period; measuring a time dependence of the output signal of the gas sensor in the third time period; measuring a baseline level of the output signal in the first time period; determining a tangent line to an initial part of a recovery curve of the output signal in the third time period; wherein T is a time interval from a first moment of avoiding gas exposure in the third time period to a time represented by when the tangent line crosses the baseline level.

15. A method of gas detection with a gas sensor, wherein a gas sensitive property of the gas sensor affects the output signal of the gas sensor and is sensitive to a gas of interest with a sensitivity dependent on a gas sensor parameter; the method including: modulating the gas sensor parameter at a modulation frequency; identifying or selectively emphasising main and / or higher harmonics of the modulation frequency in the output signal of the gas sensor for detection of the gas of interest; wherein the modulation frequency has a period less than an average gas adsorption time constant of the gas sensor.

16. The method of claim 15, including determining a concentration of gas molecules of interest from the identified or emphasised main and / or higher harmonics.

17. A method of determining the modulation frequency for the apparatus of any of claims 1 to 14, including: avoiding exposure of the gas sensor to the gas of interest in a first time period; exposing the gas sensor to the gas of interest in a second time period; avoiding exposure of the gas sensor to the gas of interest in a third time period; measuring a time dependence of the output signal of the gas sensor; determining the modulation frequency from the time dependence.

18. The method of claim 17, wherein measuring the time dependence of the output signal of the gas sensor includes measuring the timedependence of the output signal of the gas sensor in the third time period.

19. The method of claim 18, including measuring a baseline level of the output signal in the first time period; determining a tangent line to an initial part of a recovery curve of the output signal in the third time period; determining a time interval T from a first moment of avoiding gas exposure in the third time period to a time when the tangent line crosses the baseline level; determining the modulation frequency greater than or approximately equal to 1 / T.

20. The method of claim 19, including optimizing the modulation frequency including: sweeping, optionally using a signal generator with reference frequency scanning, modulation and / or reference frequencies greater than or approximately equal to 1 / T; measuring a frequency response characteristic, the frequency response characteristic being a difference between the lock-in signal amplitude dependence on the modulation frequency for the gas of interest and the lock-in signal amplitude dependence on the modulation frequency for a reference gas (for example clean air); and achieving a minimal response time of sensing the gas of interest by choosing the modulation frequency at the highest frequency where the amplitude of the frequency response characteristic exceeds three times the measurement noise; orachieving the highest sensitivity of sensing the gas of interest by choosing the modulation frequency at the maximum amplitude of the frequency response characteristic.

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