Miniaturization and integration of photoacoustic detection

Abstract

Photoacoustic spectroscopy (PAS) is an absorption spectroscopy technique that is currently used for low-level gas detection, biological studies, and catalyst characterization. In PAS, the incident radiation is modulated at an acoustic frequency. In the presence of an absorbing species, the periodic radiation is translated into periodic heat generation, which launches an acoustic signal. PAS is a promising technique for chemical analysis in mesoscale analysis systems because the detection limit scales favorably with miniaturization. This work focuses on the miniaturization of gas-phase photoacoustic detection of propane in a nitrogen or carbon dioxide ambient. This work could be expanded to other chemical species or to spectroscopy with a change of light source. The detection system was modeled with a transmission line analogy, which was verified with a set of experiments. The model includes the effects of acoustic leaks and absorption saturation. These two phenomena degrade the performance of the PA detector and must be controlled to realize the scaling advantages of PA systems. The miniature brass, cells used to verify the model used hearing aid microphones and optical excitation from a mechanically-chopped, 3.39-[mu]m He-Ne laser, transmitted into the cells with an optical fiber. I was able to detect 10 ppm of propane in nitrogen with these cells (a signal level of - 1 Pa/W). This thesis describes the development of two more miniaturized PA systems-- one formed by microfabrication and another in which PA detection is integrated with another miniaturized system. The microfabricated PA detector used the He-Ne laser and a less-sensitive optical microphone. I was able to detect 5% propane in nitrogen with this detector, which had a signal level of about 100 Pa/W. In the integrated system, a PA cell was machined into the mounting chuck of a microfabricated chemical reactor. The detector used a hearing aid microphone and a 3.4 [mu]m infra;red diode that was modulated at the first acoustic resonance of the cell. The integrated system was not as well characterized as the others, but demonstrates the potential for portability and integration of PA detection systems. These systems have shown that PA detection may be inappropriate for systems with large variations in gas concentration because of absorption saturation and changing gas acoustic properties. However, it is a promising technique for the analysis of dilute mixtures in miniature chemical systems and may also be useful in miniaturized multi-phase systems.