GB2315864A

GB2315864A - Detecting volatile organic compounds

Info

Publication number: GB2315864A
Application number: GB9616089A
Authority: GB
Inventors: Pankaj Madganlal Vadgama; Ian Mcintyre Christie
Original assignee: Victoria University of Manchester; University of Manchester
Current assignee: Victoria University of Manchester; University of Manchester
Priority date: 1996-07-31
Filing date: 1996-07-31
Publication date: 1998-02-11
Also published as: GB9616089D0

Abstract

Compounds, particularly volatile organics (VOCS), are detected using a sensor where a PVC membrane barrier containing no plasticiser is interposed between the sample and detector. The sensor typically comprises an electrode system with the unplasticised PVC membrane forming an interface between the sample and the active electrode of the sensor. Utilty is in measurement of the ethanol content of alcoholic liquors or beverages during their manufacture. Other VOCS which are detectable are alkanols, aldehydes, ketones or benzenes.

Description

ANALYTICAL METHOD AND APPARATUS.

This invention relates to an analytical method and apparatus useful for the determination of volatile organic compounds, and especially ethanol, in a fluid sample.

Many forms of analytical methods and apparatus have been proposed for the detection and measurement of various components in fluid samples, and commonly these rely on some form of membrane to control the extent to which the components present in a sample under examination can gain access to a detector (e.g. an electrode) at which they can then be detected and determined. Especially in electrolytic methods it is well known to make sensors using membranes to separate the media being analysed from the active electrode itself.

The main function of the membrane is to separate, as far as possible, those components which are desirable - i.e. can take part in the reactions at an electrode on which the desired determination depends - from interferents - i.e.

compounds which may be present but are undesirable because they either interfere with the progress of the desired determination reactions or take part in reactions of their own which compete with those of the component sought and distort or overwhelm the signals which are to be measured.

The forms of construction have much in common with each other, and mainly differ in the nature of the membrane or media within it or combined with it in some way.

Some forms of sensor devices rely on the components used to make the membrane, while others rely on the mode of fabrication of the membrane, selecting its physical properties (for example its porosity) or treatments given to it, as these factors can control its effectiveness and selectivity in use.

Other forms of sensors incorporate an enzyme, which converts one substrate compound or analyte into another which may then be more easily measured. Especially, it is known to use oxidase enzymes, which generate hydrogen peroxide -- a substance which can be measured very conveniently and accurately by electrolytic methods, especially amperometric methods, and thereby provide a measure of the compound giving rise to the hydrogen peroxide. An example is European Patent No. 216577 (of ICI PLC) which specifies an enzyme electrode sensor with membrane barrier of low ( > 5%) porosity.

Polyvinyl chloride (PVC) has been proposed as a material for the fabrication of membranes for sensors. It has been proposed for use in plasticised form, as described in European Patent Application No. 575412, where the plasticiser performs the necessary part of the function of the membrane and the PVC serves to "carry" the plasticiser.

PVC in un-plasticised form has also been proposed for use as a membrane in European Patent Application No. 652942, where a PVC membrane is shown to possess a selective permeability towards hydrogen peroxide and oxalate. The reason for this selective permeability of un-plasticised PVC itself is not understood, as it is not logically related to molecular size and/or charge. There has been no indication therein of any other species to which the PVC might be permeable, or why it should be permeable at all.

Such known sensor devices, utilising various membrane materials, have been aimed principally at the analysis of fluids for the presence of sugars, especially glucose, and other substances which may be oxidisable by enzyme action to form hydrogen peroxide. Examples include biological media for example blood and fruit.

We have now found that membrane made of un-plasticised PVC has also a very surprising permeability towards volatile organic compounds and especially towards ethanol. This discovery is unexpected, as the permeability of unplasticised PVC is so unpredictable, and very valuable as it can provide the basis for analytical methods and apparatus for detection and determination of volatile organic compounds of low volatility by selective diffusion of this compound from a fluid sample. This is of value in the examination and monitoring of a wide range of products and processes in which volatile organic compounds are present. Especially it is of value in the examination and monitoring of products and process in which ethanol is produced or present - e.g.

fermentation processes such as brewing and wine making, and in the analysis of beer, wines, and other alcoholic liquids, drinks and the like, and also in process liquids, effluents and other media in which ethanol may be desirable, undesirable, or requiring monitoring and control, e.g. for legal purposes.

Thus according to our invention we provide an improved method for detecting and/or determining a component in fluid samples which comprises interposing a membrane barrier composed of polyvinyl chloride (PVC) in un-plasticised form between the the sample to be analysed and a detecting means providing an output representative of the content of said component and allowing the said component to diffuse through the said barrier membrane and then measuring its presence at the detecting means characterised in that the said component detected or determined is an organic compound which is uncharged, soluble in the un-plasticised PVC membrane material and mobile within the PVC sufficiently to exert a vapour pressure on the opposite side of the membrane from that at which it entered the membrane and thereby pass though the membrane material and make itself accessible to a detector device adapted to detect it and produce an output signal representative of the said component.

Alternatively stated, our invention comprises a method for detecting and/or determining a volatile organic compound in fluid samples by allowing the said volatile organic compound to diffuse from a sample under examination through a barrier to a detector means responsive to said volatile organic compound, characterised in that the barrier membrane is composed of polyvinyl chloride (PVC) in un-plasticised form and the volatile organic compound is uncharged, soluble in the un-plasticised PVC membrane material and mobile within the PVC sufficiently to exert a vapour pressure on the opposite side of the membrane from that at which it entered the membrane and thereby pass though the membrane material and make itself accessible to a detector device.

According to our invention we also provide an improved sensor device for detecting a volatile organic compound present in fluid samples and providing an output representative of the content of said volatile organic compound in said sample, comprising a detector means responsive to the volatile organic compound and a membrane barrier between the said detector means and the sample to be analysed, characterised in that the membrane barrier is composed of polyvinyl chloride (PVC) itself, in unplasticised form and the volatile organic compound is uncharged, soluble in the un-plasticised PVC membrane material and mobile within the PVC sufficiently to exert a vapour pressure on the opposite side of the membrane from that at which it entered the membrane and thereby pass though the membrane material and make itself accessible to a detector device.

For these purposes, the un-plasticised PVC membrane allows the passage of the volatile organic compound through the PVC itself and not through any additive (e.g. a plasticiser) incorporated in it. Likewise, it does not operate by reliance on porosity (pores or open channels) through a PVC matrix, and therefore we describe the PVC as substantially non-porous.

The volatile organic compound to be detected and determined according to the present invention may be polar or non-polar, and its molecule should be uncharged. By the term "polar" we mean a compound whose molecule has a finite dipole moment. Preferably it is a non-polar compound. Its degree of volatility is important, and we find that compounds of low volatility do not permeate readily through the PVC membrane.

In most respects, the volatility which is appropriate for the operation of our invention is found in organic compounds which have vapour pressures which give the compound a boiling point at normal atmospheric pressure (760 mm. of mercury) of not greater than 150 degrees C., and preferably not greater than 80 degrees C. The compound may be solid, liquid or gaseous, but provided it has have the required volatility, in terms described above, the invention is applicable to it. As a guide, the compounds molecular weight is relevant, and this should be less than 400, and preferably substantially less - especially less than 100.

The invention is especially applicable to the detection and determination of ethanol, partly because the ability of this compound to pass through the PVC membrane is so surprising and partly because a facility to analyse reliably for this compound is of such high commercial and industrial importance and exceptionally wide applicability.

The PVC (polyvinyl chloride) may be any polymer of vinyl chloride, as for example those made and available commercially, but should be free from any added plasticiser (an ingredient which is often present in some commercial products intended for uses such as moulding). Such "unplasticised" PVC polymers are readily obtainable in commerce, however, and it is necessary only for the quality and purity of any polymer to be checked, whether by its specification or labelling. The molecular weight of the PVC is relatively non-critical, and most commercial grades will be satisfactory in use. A typical molecular weight is in the range 10,000 to 200,000, but others may be used if desired.

The un-plasticised polyvinyl chloride may be made into membranes by any conventional method. Most conveniently, this can be done by solution casting techniques, using solvents to dissolve the polymer and then forming the PVC film from the solution of PVC. This can be done very conveniently by spreading the solution on a plate or flat surface and allowing the solvent to evaporate. This spreading and evaporation can be adjusted appropriately, by simple trial, to allow the PVC to form a film of the desired thickness, degree of regularity or uniformity, and lack of porosity. Other known film-forming techniques may be used if desired, for example casting, spin-coating, screen printing, or any combination of such techniques. A convenient solvent for the purpose is tetrahydrofuran (THF), but other solvents or mixtures of solvents which are known to be able to dissolve PVC be used if desired.

The thickness of the membranes can be of the order already used conventionally in the art, but may be varied as found most appropriate having regard for the particular mixed polymer composition being used and the conditions under which it is to be used. Thus, convenient thicknesses are those in the range 10 to 40fur, though larger or smaller thicknesses can be used if desired.

The detector means may be any known in the art, and may be based on any property of the volatile organic compound which is detectable and measurable. For example, it may be based on an electrochemical, optical (e.g. spectrophotometric) or chemical detecting system, or any other known system - alone or in combination. Likewise, the detecting means may be such as to detect the volatile organic compound directly (i.e. as the compound itself) or indirectly, by detection and measurement of another product derived from it, for example by the action of an appropriate condition and/or reagent. Such indirect means may include an enzyme-based reaction system (for example using an oxidase or dehydrogenase enzyme), electrochemical systems (usually electrochemical oxidation), chemical mediation using reagents which can interact with the volatile organic compound to form another product which may be determined more readily or conveniently, optical techniques (e.g. those based on the optical absorption characteristics of the compound itself or some product derived from it, or combinations of one or more of these techniques.

Examples of chemical mediators include oxidising agents, for example dichromate, permanganate and like reagents, or mixtures thereof. Optical methods may include those in which the absorption characteristics measured may be in the visible part of the spectrum (e.g. as a colour visible to the eye) or in the ultraviolet or infrared parts of the spectrum. Such measurements may be made in known and conventional manner using conventional apparatus. Appropriate systems are known in the art, for example those used in devices for the detection of ethanol in a person's breath (the so-called "breathalyser"), in which ethanol reacts with a chemical reagent to produce a change of colour, the depth of which can be quantified.

Most conveniently, the detecting means is one of an electrochemical nature. This may be based on potentiometric or amperometric measurement (or even a combination of these) of which amperometric measurement is usually found to be preferable.

The detecting means will usually comprise an electrode system and a liquid or gel phase electrolyte-containing medium. In most applications the electrolyte will be aqueous (i.e. aqueous or aqueous-based) but the use of non-aqueous electrolyte media (for example organic-based media) is not excluded.

Especially, in the devices and method of our invention, the sensor device comprises a detecting means in contact with an electrolyte medium and both associated with a membrane of polyvinyl chloride (PVC) itself, in un-plasticised form, which provides an interface for contact with a sample to be analysed and interposed between the active electrode (anode) of the cell used as detector. Alternative forms of construction may have (a) both the electrodes of the detecting means (cell) enclosed within the PVC membrane (so that both are separated from the sample) or (b) only the anode of the detecting means (cell) enclosed within the PVC membrane (so that the PVC membrane is between the electrodes and the cathode is not separated from the sample). Of these, the former is very convenient and compact, but the latter is more simple and is made practicable by the fact that PVC in un-plasticised form is less insulating in its properties, and H+ ions can pass through it.

The governing factor is the fact that un-plasticised PVC, in substantially non-porous form, has an excellent permeability to volatile organic compounds but is relatively impermeable to most (or almost all) of the other compounds commonly associated with them in the fluids and other media in which it is to be determined.

As the un-plasticised PVC is known to be permeable to some compounds, for example oxalate, and the mechanisms affecting its permeability are not understood, some other components present in a sample under examination may be able to pass through the PVC - even in only a small amount. If this is found to occur they may not interfere with the detection of the organic compound sought (e.g. ethanol) but, if they do have a tendency to interfere, this can be remedied by appropriate selection or adjustment of the detection system and detecting means. Alternatively, correction for possible interferents can be achieved by appropriate processing of the data from the detecting means. If such adjustment is not convenient, then the possible interference may be remedied by addition of a reagent to the sample or to the medium on the detector side of the PVC membrane to react with the potential interferents (e.g. chemically and/or enzymatically) to reduce or eliminate its effect. Other means known in the art for distinguishing different analytes (e.g. ethanol from other compounds) may be used if desired; for example, these can include one or more additional membranes which can impart an appropriate extra degree of selectivity of access of species to the detector means and/or voltage modulation when electrochemical detector means are used.

The temperature of use of the sensor is most conveniently around ambient temperatures, e.g. in the range 5 to 40 degrees C.

The invention can also be applied to the detection and determination of compounds which cannot permeate through the PVC membrane. This can be done when the analyte sought can be made to generate the organic compound sought (e.g.

ethanol) in the sample by any appropriate agent or conditions. Thus, for example, an analyte which can generate ethanol by enzymic action can be determined indirectly by way of the detection and determination of the ethanol derived from it. For this, the appropriate reagent to bring about the generation of the ethanol may be added to the sample under examination and then, when reagent has acted, the resulting ethanol can pass through the PVC membrane and be detected as such and the presence of the original analyte can be deduced from this.

The sensor device of our invention can have a single membrane or, if desired, multiple layers of membrane material. When multiple layers of membrane are used, these may be the same or different. The preference for the position to be occupied by the un-plasticised PVC membrane differs to some degree according to the particular use to which the sensor is being applied and what substrate compound is to be detected by the sensor (e.g. whether the volatile organic compound, e.g. ethanol, is to be determined directly or indirectly.

When multiple membrane layers are used, any membrane layer or layers other than any comprised of un-plasticised PVC may be made of any of the wide variety of materials known in the art. Examples of these include dialysis membranes, and in general are preferably non-diffusion limiting membranes, at least to the extent that they do not limit diffusion and passage of desired species towards the detecting means. When un-plasticised PVC is used as in inner membrane (i.e. a layer which is not the outermost), then one or more outer layers may be used which are of material which protects the sensor assembly in a mechanical manner (e.g.

from mechanical damage) or in a chemical or any other manner considered appropriate for the use to which it is to be applied. Thus for example, there may be used an outer layer comprising a polycarbonate (especially in a porous form).

The active electrode may be any of those known in the art, for example a metal electrode, but especially a platinum anode. This is most conveniently made in combination with a silver/silver chloride counter electrode, as for example in the so-called Clark electrode, which comprises a platinum electrode surrounded, wholly or partially, by a silver/silver chloride ring.

The principal forms of construction of sensors for use in putting the present invention into practice are those well known in the art, with the difference being the membrane used and the compound sought for determination. For these, the PVC membrane is assembled in conjunction with the detecting means so that the un-plasticised PVC membrane is interposed between the sample and the detecting means. The components (apart from the un-plasticised PVC membrane) are mainly the conventional ones, and the many variants known in the art may be used.

One practical and convenient form of construction is that in which the un-plasticised PVC membrane is put directly on to the detector means when this is an electrode, and relying on the external wetting of this combination with the sample to produce the electrolyte contact with the electrode as required to make the electrochemical operation of the detector means function.

An un-plasticised PVC membrane possesses the unexpected advantage over many other membrane materials in that (unlike the plasticised form) it is permeable to the volatile organic compound (e.g. ethanol) without having to rely on any pores in the membrane material to allow this passage to take place.

Consequently, it can be used for the determination of the chosen volatile organic compound (e.g. ethanol) in a wide variety of media. For example, it can be used to determine the amount of ethanol in alcoholic liquids containing highmolecular weight compounds, sugars, and many other components which would otherwise be expected to interfere with ethanol determination. This selectivity in favour of a volatile organic compound is a valuable property which is not easily found and it very useful in clinical and related analytical, diagnostic and monitoring work.

Thus, our invention can be used for the analysis of a wide variety of fluid samples. In the case of methanol, examples of these include alcoholic beverages (for example beer, wines and other fermentation products) - in their final form or at intermediate stages in their manufacture. It is especially useful for the determination of the ethanol content of stocks of alcoholic liquids which are in storage or in bulk, and for which it may be difficult to take samples, as all our invention requires is to bring a small specimen of the liquid to be tested into contact with the sensor device as specified herein. The sensor can be very small and portable, and it can be used with the minimum of disturbance to the material being tested, for example stocks of alcoholic liquids being stored in casks or barrels in cellars. Obtaining a reliable measurement of ethanol content easily and accurately is of great importance for purposes of assessing and monitoring it for legal reasons, especially for determining the tax or duty payable as required by law.

An especial advantage of this invention is that the sensor device and method can be used very conveniently and effectively for the determination of ethanol over a very wide range of concentrations -- from small fractions of a percent to 100% ethanol - without having to alter that part of the sensor device which contacts the specimen or sample under examination, but only having to adjust the processing of the data from that contact. The different concentrations of ethanol do not affect the performance of the PVC membrane, whereas many of the prior art membranes can be severely affected by concentration of analyte and are usable only for restricted ranges of analyte - which limits their usefulness in practice and in commercial conditions. Thus, it can be used for examination of weakly alcoholic media (e.g. contents of brewing vessels and low-alcohol beverages) and strongly alcoholic liquids (e.g. distilled spirits and high-alcohol beverages), and also the waste or unusable liquid portions of such liquids so that appropriate repayments of taxes can be obtained.

Applicability to other volatile organic compounds can be similarly valuable and widely applicable. Examples of such compounds include benzene and alkylated derivatives of benzene, and various other alkanols (e.g. methanol, propanol) and low-boiling point aldehydes and ketones (e.g. acetone, acetaldehyde, formaldehyde) and the like. If more than one of these is present in the sample under examination, then it is quite practicable to distinguish them from each other by an appropriate modification of the detector and/or the detection procedure, as discussed above.

It may appear, at first sight, that the permeability of the membrane to more than one compound can be a source of difficulty, but this is not so. The advantage remains that the membrane is selective enough, even in the presence of a complex mixture of compounds, to let only very few compounds pass through. This is very valuable in dealing with a sample specimen which contains many other compounds which could interfere with the determination, because the un-plasticised PVC membrane can enable many of the potentially conflicting compounds to be separated from each other and, by holding back a large proportion of the undesirable components from the detector, any task of separating or distinguishing between those which do pass through the membrane can be made very much more simple and practical. This can be of value, for example, in dealing with a complex mixture containing many high-molecular weight components - for example in monitoring of process liquids and complex products, fluid wastes (for example process and plant effluents, emissions and the like) where the volatile components may be the ones of greatest concern but often the most difficult ones to monitor satisfactorily by the previously known methods and apparatus.

Claims

CLAIMS:

1. A method for detecting and/or determining a component in fluid samples which comprises interposing a membrane barrier composed of polyvinyl chloride (PVC) in un plasticised form between the the sample to be analysed and a detecting means providing an output representative of the content of said component and allowing the said component to diffuse through the said barrier membrane and then measuring its presence at the detecting means characterised in that the said component detected or determined is an organic compound which is uncharged, soluble in the un plasticised PVC membrane material and mobile within the PVC sufficiently to exert a vapour pressure on the opposite side of the membrane from that at which it entered the membrane and thereby pass though the membrane material and make itself accessible to a detector device adapted to detect it and produce an output signal representative of the said component.

2. A method for detecting and/or determining a volatile organic compound in fluid samples by allowing the said volatile organic compound to diffuse from a sample under examination through a barrier to a detector means responsive to said volatile organic compound, characterised in that the barrier membrane is composed of polyvinyl chloride (PVC) in un-plasticised form and the volatile organic compound is uncharged, soluble in the un plasticised PVC membrane material and mobile within the PVC sufficiently to exert a vapour pressure on the opposite side of the membrane from that at which it entered the membrane and thereby pass though the membrane material

3. A method as claimed in Claim 1 or Claim 2 wherein the PVC is substantially non-porous.

4. A method as claimed in any of Claims 1 to 3 wherein the molecular weight of the PVC is in the range 10,000 to 200,000.

5. A method as claimed in any of Claims 1 to 4 wherein the un-plasticised polyvinyl chloride may be made into membranes by solution casting techniques, using solvents to dissolve the polymer and then forming the PVC film from the solution of PVC.

6. A method as claimed in any of Claims 1 to 5 wherein the thickness of the membrane is in the range 10 to 40vim,

7. A method as claimed in any of Claims 1 to 6 wherein the detecting means is one of an electrochemical nature, e.g.

potentiometric or amperometric, preferably amperometric.

8. A method as claimed in any of Claims 1 to 7 wherein the detecting comprises an electrode system and a liquid or gel phase electrolyte-containing medium, preferably aqueous or aqueous-based.

9. A method as claimed in any of Claims 1 to 8 wherein the temperature of use of the sensor is around ambient temperatures, e.g. in the range 5 to 40 degrees C.

10. A method as claimed in any of Claims 1 to 9 wherein the un-plasticised PVC membrane is used in conjunction with one or more other membranes, e.g. a dialysis membrane.

11. A method as claimed in any of Claims 1 to 10 wherein the volatile organic compound is a non-polar compound.

12. A method as claimed in any of Claims 1 to 11 wherein the volatile organic compound has a vapour pressure which gives it a boiling point at normal atmospheric pressure (760 mm.

of mercury) of not greater than 150 degrees C., and preferably not greater than 80 degrees C.

13. A method as claimed in any of Claims 1 to 12 wherein the volatile organic compound has a molecular weight less than 400, and especially less than 100.

14. A method as claimed in any of Claims 1 to 13 wherein the volatile organic compound is ethanol.

15. A method for detecting or determining a volatile organic compound, especially ethanol, substantially as described.

16. A sensor device for detecting a volatile organic compound present in fluid samples, using a method as claimed in any of Claims 1 to 15.

17. A sensor device for detecting a volatile organic compound present in fluid samples and providing an output representative of the content of said volatile organic compound in said sample, comprising a detector means responsive to the volatile organic compound and a membrane barrier between the said detector means and the sample to be analysed, characterised in that the membrane barrier is composed of polyvinyl chloride (PVC) itself, in un plasticised form and the volatile organic compound is uncharged, soluble in the un-plasticised PVC membrane material and mobile within the PVC sufficiently to exert a vapour pressure on the opposite side of the membrane from that at which it entered the membrane and thereby pass though the membrane material and make itself accessible to a detector device.

18. A sensor device as claimed in Claim 16 or Claim 17 which comprises comprises a detecting means in contact with an electrolyte medium and both associated with a membrane of polyvinyl chloride (PVC) itself, in un-plasticised form, which provides an interface for contact with a sample to be analysed and interposed between the active electrode (anode) of the cell used as detector.

19. A sensor device as claimed in Claim 18 wherein the active electrode is a metal electrode, especially a platinum anode, conveniently made in combination with a silver/silver chloride counter electrode, as for example in the so-called Clark electrode, which comprises a platinum electrode surrounded, wholly or partially, by a silver/silver chloride ring.

20. A sensor device as claimed in any of Claims 16 to 19 in which the un-plasticised PVC membrane is put directly on to the detector means when this is an electrode, and relying on the external wetting of this combination with the sample to produce the electrolyte contact with the electrode as required to make the electrochemical operation of the detector means function.

21. A sensor device for detecting and/or determining a volatile organic compound, especially ethanol, in fluid samples substantially as described.

22. Use of a method or sensor as claimed in any Claims 1 to 21 for the analysis of products, process liquors, effluents and the like, in their final form or at intermediate stages in their production or storage, to determine their content of a selected volatile organic compound.

23. Use of a method or sensor as claimed in any Claims 1 to 21 for the analysis of alcoholic liquors or beverages (for example beer, wines and other fermentation products), in their final form or at intermediate stages in their manufacture or storage, to determine their ethanol content.