Photoacoustic trace gas sensing : application to fruit and insects

Contains fulltext : 19010_phottrgas.pdf (publisher's version ) (Open Access)A novel photoacoustic spectrometer has been applied to study trace gas emissions by fruit and insects. The spectrometer is based on a newly designed CO laser that can operate on 400 laser lines between 5.1-8.0 and 2.8-4.1 micrometer (delta v=1 and 2 mode, respectively). The spectrometer is equipped with three intracavity photoacoustic cells which introduces additional losses in the laser cavity, but still a proper operation in the delta v=2 mode with its low gain is possible. Fast switching between the two operation modes is achieved using a newly designed grating holder. A number of applications show the versatility of this spectrometer: The trace gase emissions by avocado fruit were studied under anaerobic and post-anaerobic conditions. Release of ethanol and acetaldehyde under anaerobic conditions followed a pattern that is similar to that of many other fruits. Returning the fruit to aerobic conditions yielded an unprecedentedly high upsurge in acetaldehyde. A similar response in acetaldehyde is observed when initial conditions are hypoxic instead of anaerobic. Another application deals with water loss recordings of tiny insects called thrips (50 microgram), the smallest of all flying insects. We were able to record the water loss of a single individual. Interestingly, they could stand arid conditions for very prolonged periods. As a potential insecticide, we tested the effect of elevated CO2 levels. Only very high CO2 levels (34 %) showed a clear effect on the mortality of the insects. Finally, with a different photoacoustic spectrometer based on a CO2 laser we determined the tracheal volume of insects using a novel SF6 wash-out technique. In addition, the respiration pattern of the insects was resolved in detail from both CO2 and SF6 release. The high sensitivity of the spectrometer for SF6 detector (5 ppt) opens new ways to study respiration in even the smallest insects104 p