WO1997001753A1

WO1997001753A1 - Gas sensor arrangement

Info

Publication number: WO1997001753A1
Application number: PCT/GB1996/001554
Authority: WO
Inventors: Peter Alfred Payne; Krishna Chandra Persaud; Richard Mark Dowdeswell; Mohammed El Hassan Amrani
Original assignee: Aromascan Plc
Priority date: 1995-06-28
Filing date: 1996-06-28
Publication date: 1997-01-16
Also published as: JPH11509621A; AU6236896A; EP0835441A1; GB9513217D0

Abstract

There is disclosed a gas detection apparatus comprising two sets of gas sensors which produce electrical output therefrom for detection purposes incorporated into a differential measurement arrangement.

Description

GAS SENSOR ARRANGEMENT

This invention relates to differential measurement arrangements, such as Wheatstone bridge type arrangements, for gas sensors.

The detection of gases and odours is a rapidly expanding and important field. One particularly useful form of gas sensor involves the use of a pair of electrodes between which is deposited a semiconducting organic polymer (SOP) such as ' polypyrrole (see, for example, K C Persaud, Trends in Anal. Chem.Ji (1992) 61, and references therein). SOP sensors behave as chemoresistors which may be used to detect volatile chemicals that are reversibly adsorbed on the surface ofthe sensing materials. Commonly the gas sensor is connected to a dc electrical supply, and detection ofthe gas or odour is accomplished by detecting the change in the dc resistance of the sensor on exposure of the sensor to the gas or odour. The change in resistance is caused by changes in charge distribution within the polymer induced by adsoφtion of the gas onto the surface of the polymer.

A problem with SOP based gas sensors is the lack of selectivity: a sensor will typically be sensitive toward a range of gases or odours, although the sensor characteristics can be tailored to enhance the response to particular chemical families. The nature of the chemical interaction may determine the polarity of the observed response and the size, shape and charge characteristics ofthe adsorbed molecule together with the binding site on the SOP may determine the affinity of the binding and the intensity ofthe resulting signal. One solution to the lack of selectivity is to produce a gas sensing device which incoφorates a plurality of gas sensors wherein the individual gas sensors display broadly overlapping responses to a range of gases. The pattern of responses from such a plurality of sensors may be regarded as a characteristic 'fingeφrint' of the gas in question. Realisation of such a device generally requires complex electronics and pattern recognition circuitry. Another solution, disclosed in British Patent No. GB 2 203 553B, is to apply a high frequency ac signal across a sensor and to detect changes in various impedance characteristics, such as conductance, as a function of the ac frequency, this response function again providing a gas specific 'fmgeφrint'. Although this is potentially an extremely powerful means of extracting information from a single sensor, the method requires very careful sensor design and substantial investment in electronic components such as an impedance analyser.

However, there are many examples of measurement situations in which the measurement problem is clearly defined, it being desired, for instance, to distinguish a change in the concentration of a volatile chemical or a mixture thereof in the presence of a background that may be complex but that is relatively constant. For reasons of cost, it would be desirable in such scenarios to employ as simple a method of detection as possible. The present invention provides a solution to this problem.

Additionally, and notwithstanding the fact that SOP based gas sensors display sensitivities which are considered high within the field, it is clearly always desirable to devise means by which sensitivity may be increased. Since SOP based gas sensors rely upon the measurement of a change in an electrical property on exposure of the sensor to the gas, and since this change is often small, one factor which hampers sensitivity is the problem of detecting a small difference in a relatively large background signal. The present invention can provide an increase in sensitivity over prior art methods of interrogating gas sensors by enhancing the measured change in the electrical property detected.

It is noted that whilst the foregoing discussion refers only to SOP based gas sensors, the present invention may be applied to other types of gas sensor which provide an electrical output for detection puφoses. Furthermore, it is understood that gas detection in the present context encompasses the detection of volatile species. According to the present invention there is provided a gas detection apparatus comprising two sets of gas sensors which produce electrical output therefrom for detection puφoses incoφorated into a differential measurement arrangement.

Said arrangement may monitor changes in the ratio of an electrical property ofthe two sets of sensors. The two sets of gas sensors may be incoφorated into separate arms of a Wheatstone bridge type arrangement. Two gas sensors only may be employed, which may each comprise at least one semiconducting organic polymer. A dc electrical supply may be applied across the gas sensors and gas detection may be accomplished by monitoring the change in the ratio of resistances ofthe two gas sensors. The gas sensors may display changes in resistance on exposure to a gas or a mixture of gases which differ in sign.

The gas detection apparatus may comprise rectification means for rejecting changes in differential measurements ofa defined polarity. The rectification means may comprise a diode.

The variation in response of the two gas sensors as a function of temperature may be substantially similar.

An ac electrical supply may be applied across the gas sensors.

The two sets of gas sensors may sample different atmospheres.

A gas detection apparatus in accordance with the invention will now be discussed with reference to the accompanying drawings, in which:

Figure 1 shows a circuit diagram of a gas detection apparatus; Figure 2 shows the response of a number of sensors to methanol vapour;

Figure 3 shows the response of a number of sensors to acetic acid vapour;

Figure 4 shows the response of two sensors to water and ethyl acetate vapour;

and Figure 5 shows a differential measurement arrangement.

The present invention comprises two sets of gas sensors which produce electrical output therefrom for detection puφoses incoφorated into a differential measurement arrangement. The differential measurement arrangement may monitor changes in the ratio of an electrical property of the two sets of sensors : it is this ratio which is obtained from a bridge type measuring arrangement such as a Wheatstone bridge.

Example 1

Figure 1 shows a gas detection apparatus of the present invention comprising two gas sensors 10, 12 which produce electrical output therefrom for detection puφoses incoφorated into separate arms of a Wheatstone bridge arrangement 14. In a preferred embodiment, both gas sensors 10, 12 comprise at least one SOP. Examples of SOPs employed in gas sensors include various substituted polypyrroles.

In keeping with a traditional Wheatstone bridge arrangement a dc electrical supply (in this instance 2.5V) is provided by electrical supply means 16, and this potential is applied across the sensors 10,12. A fixed value resistor 18 and a variable resistor 20 comprise the two remaining arms of the Wheatstone Bridge arrangement. Detection circuitry 22 measures the potential difference E1-E2 where El and E2 are the potentials at bridge mid-points 24 and 26. The resistance of the variable resistor 20 is varied until the potential difference E1-E2 is zero, whereupon the balancing condition of equation 1 is achieved: where S, and S₂ are the resistances of the gas sensors 10, 12 respectively, R, is the

resistance of fixed value resistor 18 and R_v is the resistance of variable resistor 20. Equation 1 may, of course, be rearranged to produce:

As mentioned above, exposure ofthe gas sensors to certain gases alters the resistances of the gas sensors. In the present invention the presence of such gases is detected by detecting the change in the value of R_v required to maintain the potential difference El - E2 at zero and thereby satisfy equation 2. From the point of view of sensitivity, the optimal combination of sensors is, of course, when the changes in resistance of the individual sensors on exposure to a gas are of opposite sign, i.e. the resistance of one sensor increases whilst the resistance of the other decreases. In this instance, the modulus of the percentage change in R_v after exposure to the gas will be greater than either ofthe moduli of the percentage changes in sensor resistances S,, S_:.

The value of resistance R, may be selected so as to provide vaues of R which are more conveniently measured than certain sensor resistances. Example 2

An important aspect of the present invention is that it can provide selectivity toward gases or classes of gases. Figure 2 shows the concentration response curves of four SOP based sensors to methanol vapour, the response being measured in conventional manner by determining the variation in the dc resistance of the sensor occurring on exposure of the sensor to the methanol vapour. Figure 3 shows the concentration response curves ofthe same sensors to acetic acid vapour. In Figure 2 all ofthe sensors display an increase in dc resistance on exposure to methanol, whereas in Figure 3 all ofthe sensors experience a fall in resistance on exposure to acetic acid, this difference in response being due to the different types of charge interactions occurring on adsoφtion. Sensor 4 shows the lowest sensitivity toward methanol and acetic acid, whilst sensor 2 shows the highest sensitivity ofthe four sensors towards methanol but the second poorest sensitivity towards acetic acid. [The sensitivity is given by the ratio of the modulus of the fractional resistance change to the vapour concentration.] By selecting sensors for differential measurement that have different characteristics in terms of selectivity to chemical groups, the cross sensitivity of both sensors to a number of chemicals may be substantially eliminated. For example, if the sensors 2 and 4 are utilised in the configuration of Figure 1 (ie they comprise sensors 10, 12) and the device is exposed to methanol vapour, the ratioed signal ofthe two sensors would minimise the effect of background gases to which both sensors 2 and 4 respond. Any registered change in the ratio of sensor signals would be mainly due to methanol. and thus the selectivity ofthe device thereto is substantially enhanced. If the same device is exposed to acetic acid vapour, the magnitude of response of sensor 2 would again exceed that of sensor 4, but the polarity of the response is now reversed (ie. there is a resistance decrease rather than an increase). Therefore changes in the ratioed signal due to acetic acid vapour would have an opposing polarity to those registered with methanol vapour. A diode 28 may be employed as a simple means of rectification ofthe ratioed signal so that only ratios ofa certain polarity are subsequently amplified. Therefore the device may be tailored to reject acetic acid and accept methanol or vice versa. Another embodiment, for rejection of water sensitivity, utilises hydrophilic and hydrophobic sensors as the sensors 10, 12 in the arrangement of Figure 1.

Example 3

In this example the utility of a Wheatstone bridge arrangement in minimising the effects of background water signal in the measurement of ethyl acetate vapour is demonstrated.

A sensor array comprising thirty two SOP sensors was separately exposed to water vapour and ethyl acetate vapour at "relative humidities" of 30% and 50%. The sensor responses (defined as the percentage change in resistance measured upon exposure of a sensor to a vapour) were measured. The absolute differences in each sensor response between water vapour and ethyl acetate at each of the two humidities were calculated. For each relative humidity measurement set, the absolute differences were compiled in the order of increasing magnitude and each sensor (corresponding to a different SOP) ranked accordingly. The rankings for the two relative humidities were combined to produce an overall ranking, and the SOPs 40,42 having the highest and lowest rankings selected. Figure 4 shows the responses of the selected SOPs 40.42 to water vapour and ethyl acetate vapour at relative humidities of 30% and 50%. SOP 40 shows very little difference in response to water and ethyl acetate at either relative humidity, whilst SOP 42 exhibits a relatively large change.

This combination of SOPs 40,42 can be used to extract an ethyl acetate response even in the presence of large and fluctuating background water humidity levels. Table 1 illustrates that the ratio of the responses of SOPs 40,42 is a quite distinct characteristic ofthe vapour. Clearly a bridge arrangement balanced for a response ratio of 0.9 could be used; in the presence of ethyl acetate - or possibly another non polar species - this ratio would fall to a value of CJL. 0.6.

Table 1 Ratios of SOP Responses

Vapour Ratio of Response (SOP 40:SOP42) 30% RH 50% RH

Water 9.54/10.50 = 0.91 11.55/12.89 = 0.90

Ethyl Acetate 9.67/15.00 = 0.60 1 1.15/17.23 = 0.65

Example 4

The use of differential measurement arrangements does not necessarily involve a Wheatstone bridge type arrangement. In this example, a thirty two sensor array of the type described in the previous example was used to analyse saturated vapour samples of water (100% relative humidity), ethyl acetate, methanol, ethanol, butanol, propanol and toluene. Responses were recorded with respect to the resistance in dry air as ΔR/R where ΔR is the change in measured resistance between analyte and dry air and R is the basal resistance in dry air. The concentration of volatile in mole I^'1 was calculated from the vapour pressure and the ideal gas equation in order to calculate sensor sensitivities (as previously defined). Table 2 shows the results for two sensors which employ different SOPs 4a and 14a. Table 2. Responses of two SOP sensors to a variety of vapours

Vapour

Humidity Methanol Ethanol Propanol Butanol Toluene Ethyl Acetate

Response 10.68 25.08 15.87 8.21 5.5 10.51 9.15

^ΔR % (4a) R

Sensitivity 38454 4794 6506 4531 23976 2603 7659

-^ % mol-'l R

(4a)

Response 9.4 22.43 13.78 6.86 4.59 9.22 7.99

^ΔR % (14a) R

Sensitivity 33831 4288 5649 3787 20021 2284 6691

^ΔR % mol 'l R

(14a)

After examination of these data, SOPs 22a and 23 a were selected as having greater responses to the range of non-polar volatiles, whilst SOPs 4a and 14a were selected as exhibiting the smallest responses to non-polar volatiles. The following description leads to a form of differential measurement which enhances selectivity by reducing cross-sensitivities towards the range of volatiles detected.

From Table 2 it is possible to construct a response function for SOP 4a in terms ofthe concentrations (XJ ofthe volatile sampled in a single measurement. SOP4aresponse=38454 X,+4794 X₂+6506 X₃+4531 X₄ +23976X₅+2603 X,+7659 X₇

(1)

SOP 14aresponse=33831 X,+4288 X₂+5649X₃+ 3787 X₄ +20021 X₅+2284X^6691 X₇

(2)

The response of SOP 4a is scaled by a factor of 0.86 so as to produce a substantially identical humidity response to that of SOP 14a :

Scaled SOP 4a response = 33070 X, + 4123 X₂ + 5595 X₃ + 3897 X₄ + 20619 X₅ + 2239 X^ 6587 X₇

(3)

A differential response is then calculated : (2)-(3) = (4)

Differential response (14a-4a) = 761 X, + 165 X₂ + 54 X₃ - 1 10 X₄ - 598 X₅ + 45 X₆+ 104 X₇

(4)

In similar fashion a differential response for SOPs 23a and 22a can be calculated, involving the application ofa 0.93 scaling factor to the response of SOP 23a.

Differential response (23a-22a) = -240 X, - 80 X₂ - 248 X₃ - 238 X₄

(5) Differential responses (4) and (5) are summed to produce a combined response (6) :

Combined response = 521 X, + 85 X_: - 194 X₃ - 348 X₄ - 3281 X₅ + 18 Xg - 257 X₇

(6)

The above described differential measuring process results in a response which can be positive or negative - in contrast to the raw data, which is always positive. This increases selectivity: for example, if a positive combined response is observed, then the dominant contribution is likely to be water vapour. Only methanol and/or toluene, present in significantly higher concentrations than any water vapour present, could produce a positive response. Similarly, a large negative response would almost certainly indicate the presence of butanol. Therefore, selectivity towards water vapour and butanol is enhanced. There is an approximately equal probability ofthe combined response with respect to a given vapour being positive or negative in sign. As a consequence, in the event that a gas sample comprises a range of background contaminants, the responses from these contaminants will likely substantially cancel themselves out.

Figure 5 shows a differential measuring arrangement capable of performing operations ofthe type described above. The outputs from a pair of gas sensors 50,52 and 54,56 fed into differential amplifiers 58,60. The outputs therefrom are inputted into a summing amplifier 62 to produce the combined response. It should be noted that this arrangement measures changes in resistance ΔR, rather than fractional resistance changes ΔR/R. The approach will still succeed if the base resistances of the gas sensors 50.52,54.56 are substantially identical. Alternatively, the approach embodied in equations ( 1) to (6) could be modified to employ absolute resistance changes ΔR, rather than fractional resistance changes. Pairs of sensors in a differential arrangement may advantageously comprise SOPs having substantially similar variations in response as a function of temperature. In this manner thermal drift due to the temperature dependence of individual sensors may be substantially reduced.

Signals from pairs of sensors in a differential arrangement may be processed by hardware thresholding to indicate when a signal is above a certain level or by software embedded in a microcontroller circuit to trigger alarms if the signal is above a certain threshold.

Arrangements ofthe present invention may be employed in applications where it is necessary to distinguish the appearance of, or a change in the concentration of, a volatile chemical or a mixture thereof in the presence ofa background that may be complex in composition but relatively invariant over a period of time. It is not necessary that the two sensors are positioned so as to sample identical atmospheres : indeed, it may be desirable to sample different atmospheres. For instance, the two sensors may be situated upstream and downstream from an air filter. Blockage of the air filter due to, for instance, aggregation of dust thereon would be detectable through the increase in upstream concentration of various volatile components. In this case the two sensors used would normally be ofthe same sensor type.

The arrangement ofthe Wheatstone Bridge and the associated detection circuitry may be as displayed in Figure 1. However, many other arrangements will be apparent to one skilled in the art which also fall within the ambit of the invention. For example, operational amplifier feedback control may be used to reduce output non¬ linearity by adding op-amp feedback control to produce a constant-current bias: feedback modulation of the bridge bias voltage may be employed to cancel bridge non-linearity; and current feedback may be used to provide non-linearity correction. Other types of bridge arrangement, including arrangements permitting ac interrogation of the sensors. are also within the scope ofthe invention. It is also possible to combine a plurality of differential arrangements ofthe present invention to produce a composite device. In a conventional SOP array the outputs of individual sensors are used for pattern recognition puφoses; with a composite device comprising differential arrangements it is the output of pairs of sensors which are used for pattern recognition. Since in this instance each output is more selective toward particular groups of chemicals, the result is a device more tightly tuned towards any particular application.

Claims

1. A gas detection apparatus comprising two sets of gas sensors which produce electrical output therefrom for detection puφoses incoφorated into a differential measurement arrangement.

2. A gas detection apparatus according to claim 1 in which the differential measurement arrangement monitors changes in the ratio of an electrical property ofthe two sets of sensors.

3. A gas detection apparatus according to claim 2 in which the two sets of gas sensors are incoφorated into separate arms of a Wheatstone bridge type arrangement.

4. A gas detection apparatus according to claim 3 in which two gas sensors are employed.

5. A gas detection apparatus according to claim 4 in which the gas sensors comprise at least one semiconducting organic polymer.

6. A gas detection apparatus according to claim 5 in which a dc electrical supply is applied across the gas sensors and the change in the ratio of resistances of the two gas sensors is monitored.

7. A gas detection apparatus according to claim 6 in which the gas sensors display changes resistance on exposure to a gas or a mixture of gases which differ in sign.

8. A gas detection apparatus according to any of the previous claims comprising rectification means for rejecting changes in differential measurements ofa defined polarity.

9. A gas detection apparatus according to claim 8 in which the rectification means comprises a diode.

10. A gas detection apparatus according to any one of claims 3-9 in which the variation in response ofthe two gas sensors as a function of temperature is substantially similar.

1 1. A gas detection apparatus according to any of claims 1 -5 in which an ac electrical supply is applied across the gas sensors.

12. A gas detection apparatus according to any ofthe previous claims in which the two sets of gas sensors sample different atmospheres.