Development of an Ultra-Low Concentration Vapour Detection System Implemented in Microfluidics
Davies, M. J.
This thesis discusses the preliminary development of a microfluidic system for low concentration vapour analysis incorporating a novel analyte preconcentration method to extend vapour detection limits. The topicality of this subject is evidenced by the urgent requirement to detect vapours released by explosives or their manufacturing byproducts, allied to recent reports of gas phase detection of pathogen-related chemical markers. Commercially available, non-microfluidic, sensitive, delayed response, broadly specific, gasphase analysis methods have been developed recently. However microfluidic analysis offers the prospect of both the improved specificity of liquid phase analytical methods and increased sensitivity with fast response times. The necessary conditions to achieve a viable microfluidic vapour analysis system are discussed from collection, sampling, assay and measurement perspectives. Efficient, rapid, vapour collection into a liquid phase is predicated by large surface area to volume ratio phase-interfaces, as occur within microfluidic devices. Accordingly, research has focussed on stable, segmented gas and liquid microflows. The literature has concentrated on fixed structures and precise flow rate control to produce such segmented flow. In contrast, we have investigated pressure driven flow and small active valves in combination with precision patterned passive valves to provide deterministic control over flow and thus define gas and liquid segment sizes. This has allowed introduction of larger, precise gas volumes and hence gas/liquid ratios while still maintaining more stable flow patterns than those previously reported in the literature. Ethanol was employed as a completely soluble, volatile, ‘model’ analyte to assess collection efficiency. Research into detection focussed on a number of optical methods utilising either ‘wet’ or enzymatic chemistries. The Phase-to-Phase Extraction via a Chemical Reaction to give Lower Limits of Chemical Detection hypothesis (for the purpose of brevity this is shortened to ‘Chemical Amplification’ within this dissertation) was proposed. Thorough testing of the hypothesis using an enzyme catalysed reaction scheme has demonstrated its validity, and potential value if applied to ‘real world’ systems, particularly those for detecting low solubility analytes such as the explosive 2,4,6-trinitrotoluene (TNT) or its byproduct 2,4- dinitrotoluene (2,4-DNT).