WO1998036272A1

WO1998036272A1 - Gas sampling method and apparatus

Info

Publication number: WO1998036272A1
Application number: PCT/GB1998/000404
Authority: WO
Inventors: Paul James Travers; Martin Henery; David Grindrod
Original assignee: Aromascan Plc
Priority date: 1997-02-14
Filing date: 1998-02-09
Publication date: 1998-08-20
Also published as: GB9703061D0

Abstract

There is disclosed a method for sampling a headspace comprising the steps of continuously flowing a gas stream taken from said headspace over at least one gas sensor; and continuously returning said gas stream to said headspace.

Description

GAS SAMPLTNG METHOD AND APPARATUS

This invention relates to the field of gas sensing, in particular to gas sampling techniques for introducing gaseous samples to a gas sensor or sensors. The gas sensor(s) preferably, but by no means exclusively, comprise semiconducting organic polymers.

The field of gas sensing is one of ever increasing importance, with a myriad of possible applications. In recent years, broad band gas sensing devices, colloquially termed "electronic noses", have become commercially available. Electronic noses purposely function in a mariner bearing similarities to the human nose: namely, an array comprising a plurality of sensors is provided, and the presence of a gas is recognised by detecting the pattern of sensor response across the array. Although the use of highly selective - possibly gas specific - sensors is possible, the technique works to best advantage when the sensors each exhibit broad and overlapping sensitivities towards a range of gases. In this way, a single device may be produced which is capable of detecting a large number of gases.

A family of gas sensors based upon the use of semiconducting organic polymers has proved particularly useful in this regard. Examples of suitable polymers include polypyrrole, polyindole, polyaniline and derivatives thereof. Polymers of this type exhibit numerous advantageous features such as broad band selectivities, high sensitivities and rapid and reversible adsorption/desorption kinetics. The presence of a gas is usually monitored by detecting accompanying variations in the dc resistance of the polymer (see, for example, Persaud K C, Bartlett J G and Pelosi P, in 'Robots and Biological Systems: Towards a new bionics?', Eds. Dario P, Sandini G and Aebisher P, NATO ASI Series F: Computer and Systems Sciences 102 (1993) 579 and references therein), although alternative interrogation techniques are possible (see, for example, UK Patent GB 2 203 553). An important component of gas sensing is the gas sampling technique utilised. The selection and subsequent optimisation of a gas sampling technique is important with regard to repeatability, reproducibility and sensitivity. Furthermore, the sampling technique employed should be compatible with i) the time scale within which it is desired that measurements are made, and ii) with the working conditions required by the sensors. An example of the latter consideration is the sampling of vapour from a volatile liquid. In this instance, the technique of sparging, wherein a carrier gas is bubbled through the liquid and vapour is thus entrained in the carrier gas flow, is well known. However, a problem encountered with sparging is that the carrier gas flow can produce liquid films or bubbles which are blown into the region housing the gas sensors. The performance of semiconducting organic polymer gas sensors - in common with many other types of gas sensor - is badly affected by contact with such films.

The present invention addresses the above mentioned problems and considerations.

For the avoidance of doubt, the term "gas" is understood to comprise all chemical species present in the gas phase, including vapours emanating from volatile liquids or from solids subject to sublimnation. The term "headspace" is understood to represent a reservoir of gas, at least a portion of which is sampled, for the present purposes, by a gas sensing device.

According to a first aspect of the invention there is provided a method for sampling a headspace comprising the steps of:

continuously flowing a gas stream taken from said headspace over at least one gas sensor; and

continuously returning said gas stream to said headspace. A plurality of semiconducting organic polymer gas sensors may be used.

The headspace may comprise the gas to be detected diluted in a carrier gas. The gas to be detected may emanate from a solid or a liquid sample disposed within a container, the container having an atmosphere of the carrier gas.

Alternatively, the gas to be detected may emanate from a liquid sample and the gas stream may be produced by sparging said sample with the carrier gas. A sparging apparatus according to the second aspect of the invention may be used.

According to a second aspect of the invention there is provided an apparatus for sparging a liquid sample comprising:

container means in which the liquid sample is retained;

gas inlet means through which a carrier gas is flowed, the inlet means being disposed in the liquid sample so that carrier gas emerges from the inlet means directly into the liquid sample; and

gas outlet means disposed above the liquid sample;

in which the inlet and/or the outlet means are provided with shaped portions adapted to burst any films or bubbles produced by the sparging.

The gas inlet means may comprise a tubular member and the shaped portion may be a ferrule attached thereto and positioned above the surface of the liquid sample.

The gas outlet means may comprise a tubular member having an entrance sectioned to produce a sharply tipped end. Preferably the entrance is sectioned at an angle less than 55° with respect to the longitudinal axis of the tubular member. Most preferably, the angle is also greater than 35° with respect to this axis, i.e. the angle lies in the range 35-55°.

The gas flow from the apparatus may be flowed across at least one gas sensor. The gas flow may be flowed across a plurality of semiconducting organic polymer gas sensors.

Methods and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of gas recycling apparatus;

Figure 2 shows sensor response to a sample of mature cheese; and

Figure 3 shows a sparging apparatus.

The invention provides a method for sampling a headspace comprising the steps of:

continuously returning said gas stream to said headspace.

Figure 1 depicts schematically an apparatus suitable for performing the method. The apparatus comprises a container 10, containing the headspace, a gas analyser 12, containing a plurality of gas sensors and a pump (not shown), and a three way valve 14. These components are connected by suitable conduits 16, such as PTFE tubing.

The analyser 12 is a commercially available device (A32S, manufactured by AromaScan pic, UK) which contains an array of thirty two different semiconducting polymer gas sensors. A commercially available "sample station" 18 (A8S, manufactured by AromaScan pic, UK) supplies a reference gas flow of air-nitrogen at a defined humidity to the analyser 12 in order to provide a well defined reference environment.

When the reference gas is being used, the pump inside the analyser 12 pumps a flow of reference gas over the sensor array. The reference flow exits the analyser 12 at an exhaust 12a, and the three way valve 14 is set at position A so as to vent this reference flow.

When the headspace contained within container 10 is sampled, the three way valve 14 is moved to position B so as to recycle the exhaust gas streams back through the container 10. This procedure is in contrast to conventional sampling techniques - either continuous or stop-flow - in which the gas stream is vented once it has passed over the gas sensor or sensors. Recycling the gas stream provides a number of advantages:

1. A relatively high flow rate can be used (ca. 250mlmin ^"1) thereby minimising the loss of signal caused by adsorption of gas onto walls.

2. A saturated headspace is eventually established in the closed recycling loop. With a conventional stripping technique - wherein a gas stream is passed across the surface of a sample - or sparging technique the gas stream is continuously vented, and this equilibrium is never attained. In addition to improved signal levels, improved discrimination between samples may be obtained.

3. The gas sample is conserved, rather than being continually depleted through venting.

The gas to be detected might emanate from a solid sample or a liquid sample disposed within the container 10 having an atmosphere of the carrier gas. However, it is understood that the term "headspace" is to be construed broadly to include any reservoir of gas that might be recycled in a closed loop system. For example, a sparging apparatus might be incorporated for use with a liquid sample.

The sampling method discussed above in relation to the apparatus of Figure 1 has been successfully used to measure geosmin taints in water. It has also been possible to achieve discrimination between samples of mild and mature cheddar cheese- such discrimination was not possible using a conventional stripping technique. The cheeses have relatively high humidity and were run at maximum reference humidity (50% at 30°C) available from the sample station 18. Both cheeses still display a residual water signal which is responsible for an initial, rapid increase in sensor response. Thereafter, there is a fanning out of the sensor traces as the semiconducting organic polymers of the analyser 12 respond to the other volatile species in the headspace. A much larger response is observed for the mature cheddar - where a greater concentration of odour molecules would be expected. The response with the mature cheddar is shown in Figure 2. The fanning out of the sensor response is much smaller with the mild cheddar, and thus the pattern of response is quite similar to that of pure water. It is anticipated that the recirculation method described above can be advantageously employed with a wide range of gas sensors and gas sensing arrays.

Figure 3 shows apparatus for sparging a liquid sample 30 comprising: container means 32 in which the liquid sample 30 is retained;

gas inlet means 34 through which a carrier gas is flowed, the inlet means 34 being disposed in the liquid sample 30 so that carrier gas emerges from the inlet means 34 directly into the liquid sample 30; and

gas outlet means 36 disposed above the liquid sample 30;

in which the inlet 34 and/or the outlet 36 means are provided with shaped portions 34a, 36a adapted to burst any films or bubbles produced by the sparging.

The gas inlet means 34 comprises a tubular member and the shaped portion 34a is conveniently a ferrule attached thereto, and positioned about 10mm above the surface of the liquid sample 30.

The gas outlet means 36 comprises a tubular member having an entrance sectioned to produce a sharply tipped end 36a. Preferably the entrance is sectioned to produce a sharply tipped end 36a. Preferably the entrance is section at an angle of less than 55° with respect to the longitudinal axis of the tubular member 36, most preferably this angle is also greater than 35°, i.e. the angle lies in the range 35 to 55° with respect to the longitudinal axis of the tubular member 36. In the apparatus of Figure 3 the angle is 45°.

The sectional arrangement described above has the advantage of being extremely easy to produce. However, other configurations for the outlet means might be envisaged, such as a bevelled tubular member having a circular knife-edge like entrance. Similarly, many shaped portions might perform the role of the ferrule on the inlet means 32. However the ferrule is convenient and cheap. The inlet and outlet means 34, 36 comprise PTFE tubing. Luer fittings 38, 40 permit connection of the sparging apparatus to the rest of the gas sampling system. The container means 32 comprises a vessel 32a and screw on cap 32b (both supplied by Omnifit, UK). Other suitable embodiments would suggest themselves to one skilled in the art.

The sparging apparatus is primarily directed towards providing a gas flow which is flowed across a gas sensor or sensors. In the present example the gas flow is introduced to an array of semiconducting organic polymer sensors. It has been observed that conventional sparging techniques can result in the formation of bubbles or films, which can travel through the outlet means and onto the gas sensor or sensors. Such an occurrence is, at best, irksome, since misleading data can result. Furthermore, it is possible that contact with the liquid will have a deleterious effect on sensor lifetime and reproducibility. It should be noted that the prevention of bubbles and films entering a gas flow may have wider application.

Using a conventional sparging arrangement, the formation of film is a problem when ethylacetate is used as the liquid sample. However, the ferrule 32a significantly reduced film formation at a gas flow rate of 200mlmin^"1 Use of the sharply tipped outlet tube is recommended, to burst any film that was not eliminated by the ferrule. It is possible, although less preferable, to use the sharply tipped outlet tube as the only means of bursting bubbles and films.

Claims

1. A method for sampling a headspace comprising the steps of:

continuously returning said gas stream to said headspace.

2. A method according to claim 1 in which a plurality of semiconducting organic gas sensors is used.

3. A method according to claim 1 or claim 2 in which the headspace comprises the gas to be detected in a carrier gas.

4. A method according to claim 3 in which the gas to be detected emanates from a solid sample disposed within a container, the container having an atmosphere of the carrier gas.

5. A method according to claim 3 in which the gas to be detected emanates from a liquid sample disposed within a container, the container having an atmosphere of the carrier gas.

6. A method according to claim 3 in which the gas to be detected emanates from a liquid sample and the gas stream is produced by sparging said sample with the carrier gas.

7. Apparatus for sparging a liquid sample comprising: container means in which the liquid sample is retained;

gas outlet means disposed above the liquid sample;

8. Apparatus according to claim 7 in which the gas inlet means comprises a tubular member and the shaped portion is a ferrule attached thereto and positioned above the surface of the liquid of the liquid sample.

9. Apparatus according to claim 7 or claim 8 in which the gas outlet means comprises a tubular member having an entrance sectioned to produce a sha╧åly tipped end.

10. Apparatus according to claim 9 in which the entrance is sectioned at an angle of less than 55┬░ with respect to the longitudinal axis of the tubular member.

11. Apparatus according to claim 10 in which the entrance is sectioned at an angle greater than 35 ┬░ with respect to the longitudinal axis of the tubular member.

12. Apparatus according to any of claims 7 to 11 in which the gas flow from the apparatus is flowed across at least one gas sensor.

13. Apparatus according to claim 12 in which the gas flow is flowed across a plurality of semiconducting organic polymer gas sensors.

14. A method according to claim 6 in which sparging apparatus according to claims 7 to 13 is used.