Browsing by Author "Dakin, John P."
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Item Open Access Comparison of self-referencing techniques for photothermal detection of trace compounds in water(Elsevier Science, 2000-09) Hodgkinson, Jane; Johnson, Mark; Dakin, John P.Self-referencing techniques are compared for a closed-cell photothermal detector that uses a water meniscus as a pressure sensor. Deflection of the meniscus was measured using an optical fibre Fabry–Perot interferometer. For long measurement integration times, interference fringe drift was a serious limitation on the detection repeatability for non self-referenced measurements. Two self-referencing techniques were compared for measurements of optical absorption. The first technique used a simultaneous reference absorption signal at a second wavelength, and the second used a simultaneous volumetric modulation within the cell. Both methods have been evaluated with photothermal excitation by a 658-nm LED, a 478-nm LED and a UV discharge lamp. For the detection of absorption in aqueous solutions, the two methods had similar performance. However, the volumetric method could be used for detection of any absorbing compound, regardless of its absorption spectrum and was more convenient to use.Item Open Access Performance of a photothermal detector with turbid liquids(Optical Society of America, 2005) Hodgkinson, Jane; Johnson, Mark; Dakin, John P.A closed-cell photothermal detector for aqueous analytes has been evaluated at 254nm and 678nm. The detector used a water meniscus as a pressure sensor, whose periodic deflection was measured using a low finesse optical fibre Fabry-Perot interferometer. Performance was compared with a commercial diode-array spectrometer and found to be similar for absorption measurements in non-turbid samples, but the results were affected up to 60 times less by scattered light. Finally the photothermal cell was converted into an integrating cavity using ceramic inserts, showing freedom from scattering related errors at 678nm but a degradation in performance at 254nm.Item Open Access Photothermal detection of trace compounds in water, using the deflection of a water meniscus(Iop Publishing Ltd, 1998-01-01T00:00:00Z) Hodgkinson, Jane; Johnson, Mark; Dakin, John P.Anovelphotothermaldetectorisdescribed,basedonaclosedcellsuitableforusewithlow-frequencymodulatedcontinuous-wavelightsources.Photothermalexpansioninaqueoussamplescausedthedeflectionofawatermeniscusheldacrossa200m-radiuspinhole.Displacementofthewatermeniscuswasmeasuredusingfibreopticinterferometry.Amercurydischargelamp(254nm)andalaserdiode(678nm)wereusedtodetecttoabsorptionby2ppbanthraceneand0.5ppmpotassiumpermanganateinaqueoussolution,respectively.Thetechniquewasusedtodetectdifferencesbetweenabsorptioncoefficientsinaqueoussolutionsdowntoapproximately20%ofthebackgroundabsorptionofthewateritself.Item Open Access Quantitative analysis of a closed photoacoustic cell that uses a high compliance piezoelectric transducer(American Institute of Physics, 2005-01-01T00:00:00Z) Hodgkinson, Jane; Johnson, Mark; Dakin, John P.A treatment of the operation of low-frequency closed photoacoustic cells is presented, which considers the compliance of various cell elements. The optimum detector, according to our predictions, has a sensor whose compliance is large compared to that of the fluid enclosed by the cell. A simple photoacoustic cell has been built, incorporating a thin-walled piezoelectric cylinder. It was used to test the theory, and demonstrate photoacoustic detection of an absorber in aqueous solution, excited by a low power 678nm laser diode modulated at 110Hz. We have detected absorbances as low as approximately 3ï ´10 -3 cm -1 (SNR=1), corresponding to the absorption of light with a modulated power of 50ï ÂWItem Open Access Very efficient sampling technique for fiber-remote optical emission spectroscopy of aqueous solutions(1997-01-01T00:00:00Z) MacKenzie, Steven J.; Hodgkinson, Jane; Johnson, Mark; Dakin, John P.; Cecchi, G.; Lamp, T.; Reuter, R.; Weber, K.We have measured fluorescence from a waveguide formed by a falling cylindrical stream of liquid. The configuration is suited to taking measurements from liquids with any refractive index, such as aqueous solutions. The parameters which determine the stream stability have been investigated, and the optical collection efficiency has been mathematically modelled. We have produced streams up to 350 mm long with a 2.5 mm diameter, and measured a fluorescence collection enhancement factor of 9 from a 100 mm long, 1 mm diameter stream.