Laser Absorption Spectroscopy of Gas in Scattering Media

University dissertation from Lund University

Abstract: Absorption spectroscopy constitutes a chemical analysis tool which can be applied to various samples and application fields. This thesis focuses on gas absorption spectroscopy with the means of diode lasers - tunable diode laser absorption spectroscopy, TDLAS. In particular, the absorption of gases inside porous scattering solids and liquids, referred to as GASMAS - gas in scattering media absorption spectroscopy, has been studied. The spectrally sharp wavelength from a diode laser is scanned over the absorption fingerprint of the studied gas, and the transmitted intensity is detected. Although the light is heavily absorbed and scattered by the bulk material, the gas absorption imprint can still be distinguished. The free gas, in contrast to the bulk material which is made out of perturbed molecules, exhibits narrow absorption lines and its absorption can thus be isolated although the light has been heavily absorbed by the solids and liquids. The diffuse light propagation demands alternative solutions to extract gas concentration, which otherwise is obtained through the Beer-Lambert law using the interaction distance. Water vapor sensing in samples with liquid water present has shown to be a feasible way to achieve information about the effective distance through gas inside the sample even though the light is heavily scattered. Applications studied in this thesis work include gas sensing within the human body for medical diagnostics, gas monitoring inside food packages for quality assurance, and fundamental studied of gas in nanoporous ceramics. Investigations of GASMAS as a diagnostic tool for the paranasal sinuses, subject to the common rhinosinusitis have been carried out. Correlation between obstruction and ventilation of the sinuses diagnosed by computer tomography and GASMAS data has been demonstrated. Diagnostic useful data from the air cell system in the mastoid bone, located behind the ears, have also been obtained. Furthermore, possibilities of gas sensing in the lungs of premature born babies have been demonstrated in a feasibility study on a realistic model made out of animal lung tissue and gelatin based phantoms. Sensing of the gas non-intrusively by GASMAS has been demonstrated on packages of minced meat, bread as well as on the headspace of translucent containers with milk or orange juice. Food packages are to an increasing extent filled with a modified atmosphere where the O2 concentration is suppressed. Traditional gas detectors for food packages are intrusive or demand direct optical access. GASMAS constitutes an alternative interesting approach with high potential. Gas detection in nano-porous samples allows fundamental studies of the gas molecules. Broadening of absorption lines of O2 and H2O due to tight confinement in nano pores in ceramics has been studied, as well as diffusion of the gases. In addition to fundamental physical interest the study of broadening and diffusion allows for assessment of material parameters.

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