Analysis of isotopic CO(2) mixtures by laser photoacoustic spectroscopy

Acoustic resonance laser calorimetry for measurements of low optical absorption

A novel method of acoustic resonance laser calorimetry (ARLC) for the determination of low optical absorption coefficients of crystals and glasses is introduced. It is based on measurements of the kinetics of the equivalent temperature of the sample irradiated by the pulse laser radiation. The equivalent temperature of the sample is directly obtained by measuring the frequency changes of its temperature-calibrated acoustic resonances excited by the laser radiation with a corresponding pulse repetition rate. In contrast to the conventional resonant photoacoustic spectroscopy, where the resonance is used for the enhancement of the registered signal, in ARLC, the laser-excited acoustic resonance of the sample acts as a high-sensitivity temperature probe for measurements of its heating kinetics. The ARLC approach was verified by measuring the optical absorption coefficient of the potassium dihydrogen phosphate crystal (α=4.2⋅10^-2cm^-1) at 1064 nm wavelength.

Ammonia monitoring at trace level using photoacoustic spectroscopy in industrial and environmental applications

An ammonia traces analyser based on photoacoustic spectroscopy is described. The system uses a CO(2) laser and a properly designed resonant photoacoustic cell to achieve ammonia detection at sub-parts-per-billion (ppb) level. The instrument features unattended automatic on-line monitoring of ammonia with a detection limit of 0.1 ppb. Interferences from atmospheric CO(2) and H(2)O are efficiently suppressed by a careful selection of the laser wavelength and a compensation of the water vapour signal made with a high-precision hygrometer. The cell design enables continuous measurement at high flow rates (up to 5 l/min), which guarantees a fast response time of the system for the monitoring of ammonia, a sticky polar molecule that adheres to most surfaces. Various examples of applications of the instrument in the semiconductor industry and for atmospheric pollution monitoring are presented. They demonstrate the excellent performances of the system and its suitability for these applications.

Application of pulsed photoacoustics in water at high pressure

The application of pulsed photoacoustics to the study of liquids at pressures of up to 350 bars is discussed. The design and development of an in-line sensor for the subsea monitoring of crude oil concentrations in water is reported. Crude oil detection sensitivities at parts per million concentrations were achieved with prototype instrumentation. A comparison of experimental results and a theoretical prediction of the pressure dependence of the pulsed photoacoustic response from water is outlined. The results demonstrate that existing models that describe pulsed photoacoustic generation in liquids are applicable to high-pressure conditions.