Detection of CO2 in N2 and H2O using photoacoustics

Abstract

Photoacoustic spectroscopy is a technique where absorbed modulated light is released as heat, causing thermal expansion which can be detected using an acoustic transducer. It can be used to determine the absorption spectra or the concentration of a material. In this project, photoacoustic spectroscopy is performed on CO2 in N2 gas and on CO2 dissolved in water in the 2003 - 2006 nm range. Studies on CO2 concentrations can be used in environmental research and fish industries, to mention some applications. Linearity in the signal is demonstrated for concentrations over several orders of magnitude for the gas mixture. The effect of water vapour in the sample is investigated, and the optimal modulation frequency and other relevant factors are determined. For the water sample, different measurement cell configurations are investigated before achieving a photoacoustic signal. With the final configuration used, high concentrations of CO2 in water can be detected indirectly through a small layer of inert gas above the water. Smaller concentrations can not be detected due to a high noise level. Together with instruments for generating and modulating laser light and for detecting the acoustic signal, a gas sample measurement cell and a water sample measurement cell form the experimental setup. LabVIEW from National Instruments is used to develop a software for instrument control, data acquisition and data analysis.