This thesis presents an. investigation of the feasibility of exploiting the pulsed indirect photoacoustic spectroscopy (pIPAS) with respect to the remote detection and identification of hazardous liquid chemical species at ranges up to 10m. 2 / Modelling of the pulsed photoacoustic generation mechanism relevant to the case of its generation by pulsed laser irradiation of liquid samples at locations remote from the laser source and acoustic detection equipment was conducted to provide insight into the efficiency of the photoacoustic mechanism and the principle factors which determine the photoacoustic signal magnitude. The peak-to-peak pressure of the photoacoustic signal has been shown to be l.inearly dependent on the absorption coefficient of the sample and the laser pulse energy, and to vary inversely with the distance from the sampie to the detector. The duration of the photoacoustic signal is also dependent on a temporal characteristic which is defmed in terms of a combination of the laser pulse ,.-/ length and microphone impulse response. To demonstrate the feasibility of this method as a remote technique implemented outside the confmes of a photoacoustic cell, it has been necessary to produce a significant enhancement in the photoacoustic signal magnitude over that demonstrated in preliminary 'short range' laboratory experiments. Modifications to the experimental system design were observed to produce an increase in the photoacoustic signal by a factor of -10 and the detector sensitivity by a factor of -30, which together were sufficient to allow the laser-generated acoustic signal to be measurable over ranges of several metres. Using a pulsed C02 laser, tunable from 9.2 - 1O.9J.lm, the photoacoustic spectrum of simulant hazardous liquid chemicals has been measured at ranges up to 8m in the laboratory. Excellent agreement was obtained between the photoacoustic spectrum obtained using the PIPAS technique and the spectrum measured using a conventional transmission spectrometer. Using these system enhancements, positive identification of simulant liquids and unambiguous discrimination between the spectrum of typical background materials and simulant hazardous liquids has been demonstrated repeatedly and reliably.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:486871 |
| Date | January 2007 |
| Creators | Perrett, Brian James |
| Publisher | Heriot-Watt University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10399/2089 |
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