ANALYSIS OF VOLATILE LIQUIDS OR SOLUTIONS

Abstract: There is disclosed a method for introducing the volatile components of a liquid or a solution to at least one gas sensor, comprising the steps of: nebulising the liquid or solution; entraining the nebulised liquid or solution in a gas stream; and flowing the gas stream over the at least one gas sensor.

Description: ANALYSTS OF VOLATILE LIQUIDS OR SOLUTIONS This invention relates to sample introduction techniques and apparatuses for conveying the volatile components of a liquid or a solution to a gas sensor or sensors. The field of gas sensing is one of ever growing importance and sophistication. One approach is to employ an array of gas sensors which display broad and overlapping sensitivities towards a range of molecules. In this way, identification of a gas is achieved by recognising the characteristic pattern of responses across the array; such patterns can be regarded as species specific, molecular "fingerprints". Sensors based on semiconducting organic polymers, such as derivatised polypyrroles, are in many ways ideally suited for such purposes since they display a number of advantageous properties, such as broad sensitivities and rapid and reversible adsorption/desorption kinetics. "Gas" sensors can, of course, be used to detect vapours emanating from volatile liquids or volatile components of solutions. The performance of such semiconducting organic polymer gas sensors - indeed, of any gas sensor - is highly dependent upon the sample introduction technique employed to pass gas over the sensor or sensors. Judicious selection of the sample introduction technique can result in improvements in reproductibility, sensitivity and sampling speeds. The present invention concerns a sample introduction technique which can provide dramatic increases in gas sensing sensitivity. According to a first aspect of the invention there is provided a method for introducing the volatile components of a liquid or a solution to at least one gas sensor, comprising the steps of: nebulising the liquid or solution; entraining the nebulised liquid or solution in a gas stream; and flowing the gas stream over the at least one gas sensor. The at least one gas sensor may comprise an array of semiconducting organic polymer gas sensors. Other types of gas sensors are also within the scope of the invention; however, for the purposes of the present invention, and for reasons which are discussed fully below, mass spectrometers are not considered to be "gas sensors." After entraining the nebulised liquid or solution in the gas stream, said gas stream may be flowed through water removing means, which may be condensing means. The humidity of the gas stream may be controlled. The nebulised liquid or solution may be entrained in a gas stream of nitrogen or an inert gas. Geosmin may be introduced to the gas sensor or sensors. According to the second aspect of the invention there is provided an apparatus for introducing the volatile components of a liquid or a solution to at least one gas sensor comprising : a vessel containing the liquid or solution and having nebulising means; gas supply means connected to the vessel and thence connected to a chamber containing the at least one gas sensor; and pumping means for pumping gas from the gas supply means through the vessel and over the at least one gas sensor. The at least one gas sensor may comprise an array of semiconducting organic polymer gas sensors. The apparatus may further comprise water removing means positioned downstream of the nebulising means. The water removing means may comprise condenser means. The apparatus may further comprise humidity controlling means. A method and apparatus for introducing volatile components of a liquid or a solution to at least one gas sensor will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram of a first embodiment of an apparatus of the present invention; Figure 2 is a three dimensional plot of data obtained from a 1000 ppt Geosmin solution and a blank solution; and Figure 3 is a schematic diagram of a second embodiment of the present invention. Figure 1 is a schematic diagram of a first embodiment of an apparatus for introducing the volatile components of a liquid or a solution to at least one gas sensor comprising: a vessel 10 containing the liquid or solution 12 and having nebulising means 14a, 14b; gas supply means 16 connected to the vessel 10 and thence connected to a chamber containing the at least one gas sensor; and pumping means 18 for pumping gas from the gas supply means through the vessel 10 and over the at least one gas sensor. The apparatus further comprises humidity controlling means 20, 22. The at least one gas sensor is, in fact, an array of thirty two semiconducting organic polymer gas sensors, which is housed within a commercially available analysis unit 24 (A32S, manufactured by the assignees, AromaScan pic, UK). The pumping means 18 is a KNF Neuberger pump which pumps nitrogen from a nitrogen reservoir - the gas supply means 18 - through the vessel 10 and the humidity controlling means 20 to the array of gas sensors. The nebulising means is home built, and comprises a piezo ceramic transducer 14a and a driver board 14b. Clearly, other forms of nebulising means are within the scope of the invention. As described more fully below, enormous enhancements in sensitivity (3 orders of magnitude, or greater) are observed when nebulisation of a liquid or a solution is performed as a precursor to analysis of the volatile components thereof by an array of gas sensors. It is believed that the reason for this enhancement is essentially due to kinetics. By using a nebuliser, an aerosol or aerosol-like dispersion of liquid droplets is produced. Due to the accompanying increase in liquid surface area/volume ratio, gas phase equilibration proceeds at a greatly enhanced rate. As a consequence, on the time scale of a gas sensing experiment (a number of minutes) the gas phase concentrations of any volatile components present in the liquid or solution are dramatically increased. It is also possible that the nebulisation process assists in breaking up molecular clusters present in the solution phase. Both vapour and liquid droplets are entrained in the gas stream and carried to the gas sensor array, via a water trap 26 and the humidity controlling means 20, 22. The latter comprises a 100 ml Duran bottle 20 having Nafian R tubing coiled inside. The gas stream flows through the Nafion R tubing. A commercially available "sample station" 22 (A8S, manufactured by the assignees, AromaScan pic, UK) pumps an air stream of controlled relative humidity into the Duran bottle 20, and the ensuing equilibration processes result in the gas stream acquiring a relative humidity equal to that of the controlled air stream emanating from the sample station 22. Nebulisation techniques are well known in mass spectrometric applications; however the purpose of such procedures, as well as the physical mechanisms involved, are very different to the present invention. In mass spectrometric applications, nebulisation is performed to produce ionised droplets : the ions are extracted for mass spectrometric analysis. Such procedures do not make use of, or even teach the possible use of, any enhancement in gas £has£ concentration occurring as a result of the more rapid equilibration between gas and liquid phases, this rapid equilibration being induced by the formation of droplets upon nebulisation. For the avoidance of doubt, we emphasise that mass spectrometers are not considered to be gas sensors for the purposes of the present invention. In any event, of course, mass spectrometry involves the extraction and focusing of ions with electric and magnetic fields, whereas the present invention involves, simply, gas flow over the sensor or sensors. Figure 3 depicts a second embodiment of the invention. Numerous items are common to the embodiments of Figure 1 and Figure 3 : these are denoted by shared reference numerals. A gas stream 30 of clean, dry air entrains nebulised liquid or solution therein. The nebulising means 14 is an ultrasonic nebuliser. The apparatus further comprises water removing means 32 positioned downstream of the nebulising means 14. The gas stream flows through the water removing means 32 and thence to an array of semiconducting organic polymer gas sensors 34 via a valve 36 and a gas manifold 38. Other forms of gas sensing arrays may be employed. The gas stream 30 is monitored by a station 40 containing temperature, humidity and flow sensors before exhausted from the system by pumping means 18. The gas manifold 38 permits purge and reference cycles to be performed via valves 42, 44. In the present embodiment the water removing means 32 is a Peltier condenser. It will be apparent to those skilled in the art that other forms of condenser may also be used. Furthermore, other forms of water removal means may be advantageously employed. One such example comprises a Nafion ® based drying system similar to the controlled humidity system described with reference to Figure 1. Nafion ® is a commercially available material, primarily noted for its exceptional highly selective water absorbing abilities. In this example, gas stream 30 is flowed through a section of Nafion ® tubing, and the outside of this tubing is exposed to a second gas stream of substantially dry air or nitrogen rather than a gas stream of controlled humidity. In this instance, water from the 'wet' gas stream 30 is continually removed due to the water concentration gradient which exists across the Nafion ® tubing. Other materials which exhibit similar water absorbing properties to Nafion ® are also within the scope of the invention. The nebulisation step has the side effect of introducing large amounts of water into the gas stream. By employing such water removing means, sensitivity is improved, since water generally gives rise to interfering and fluctuating responses from the gas sensor or sensors. Example Experiments were performed on aqueous solutions of Geosmin. The apparatus of Figure 1 was used, and the relative humidity was set at 20%. Each sample was analysed for four minutes, and a wash method was run between each sample analysis. The results were compared to similar experiments with a blank solution of 0.1% methanol in water. For ease of comparison, the outputs of the array - which comprise the pattern of responses across the thirty two sensors - are reduced into 2 dimensional space by the well known Sammon non-linear mapping technique (see, for example, Hatfield J V, Neaves P, Hicks P J, Persaud K and Travers P, Sensors and Actuators B, 18-19 (1994) 221). Figure 2 shows the results of a series of experiments on a 1000 ppt (part per trillion, or 10 12 ) Geosmin solution in comparison to patterns obtained using the blank solution. Each square represents a pattern obtained with the Geosmin solution, whilst each circle represents a pattern obtained with the blank solution. Clearly, it is possible to discriminate between vapour associated with the two solutions. To place this result in context, it has been found that "conventional" measurements of the headspace above Geosmin solutions, without nebulisation. cannot discriminate between Geosmin and blank solutions below Geosmin concentration of 100- 1000 ppb. Therefore, the present invention provides, at a conservative estimate, a three order of magnitude increase in sensitivity. The "best" sensitivity obtained using conventional headspace measurements was 100 ppb Geosmin; however this required £ 18 hours equilibration before measurement, a restriction which is clearly undesirable if rapid results are required. Reduction of the equilibration time resulted, unsurprisingly, in inferior sensitivities toward Geosmin. Therefore, in addition to the enhanced sensitivity, another advantage of the present invention is the short timescale of measurements : no prior equilibration is necessary. The scatter in the data of Figure 2 is likely to be due, at least in part, to a number of uncontrolled variables in the experiments. These variables were : a small amount of leakage of the sample gas stream from an as yet unidentified portion of the apparatus; some variation in pumping volume during each measurement; a small leak in the nebuliser which resulted in an inconsistent headspace; and variations in the flow rate of the pump. It is likely that control over at least some of these variations would lead to a further reduction in the detection limit for Geosmin.

Claims: C AIMS 1. A method for introducing the volatile components of a liquid or a solution to at least one gas sensor, comprising the steps of : nebulising the liquid or solution; entraining the nebulised liquid or solution in a gas stream; and flowing the gas stream over the at least one gas sensor. 2. A method according to claim 2 in which the at least one gas sensor comprises an array of semiconducting organic polymer gas sensors. 3. A method according to claim 1 or claim 2 in which, after entraining the nebulised liquid or solution in the gas stream, said gas stream is flowed through water removing means. 4. A method according to claim 3 in which the water removing means comprises condenser means. 5. A method according to any of the previous claims in which the humidity of the gas stream is controlled. 6. A method according to any of the previous claims in which the nebulised liquid or solution is entrained in a gas stream of nitrogen or an inert gas. 7. Apparatus for introducing the volatile components of a liquid or a solution to at least one gas sensor comprising : a vessel containing the liquid or solution and having nebulising means; gas supply means connected to the vessel and thence connected to a chamber containing the at least one gas sensor; and pumping means for pumping gas from the gas supply means through the vessel and over the at least one gas sensor. 8. An apparatus according to claim 7 in which the at least one gas sensor comprise an array of semiconducting organic polymer gas sensors. 9. An apparatus according to claim 7 or claim 8 further comprising water removing means positioned downstream of the nebulising means. 10. An apparatus according to claim 9 in which the water removing means comprises condenser means. 11. An apparatus according to any of claims 6 to 8 further comprising humidity controlling means.

Inventors: GRINDROD DAVID, GRINDROD, DAVID
Applicants: AROMASCAN PLC, GRINDROD DAVID, GRINDROD, DAVID
Application year: 1997
Main classification: G01N1
IPC codes: G01N1, G01N27