Monitoring trace gases in the biological sciences and petrochemistry by photoacoustic and Raman spectroscopy

Photoacoustic spectroscopy (PAS) and Cavity-Enhanced Raman Spectroscopy (CERS) can detect headspace gases above a microbiological culture in a closed system. PAS and CERS have a detection limit of few ppm per volume. These techniques were used to investigate the aerobic respiration of Escherichia coli (E. coli). Both techniques are able to monitor O2 and CO2 and its isotopomers with excellent sensitivity and time resolution to characterise bacterial growth and metabolism. CO2 can be detected using CERS and a differential Helmholtz Resonator (DHR) because it has Raman and IR active vibrations. However, homonuclear diatomic molecules, such as O2, have only symmetric stretching vibrations that are Raman active but not IR active. In PAS, O2 can be detected by exciting a formally forbidden electronic absorption band in the red, the b 1Σg+ (ν = 0) ← X 3Σg- (ν = 0) band (the "A band") near 760 nm. Identification of different growth phases and changes in the aerobic metabolic activity of E. coli was possible by taking simultaneous measurements of O2 consumption and CO2 production using CERS and DHR in PAS, including optical density (OD) measurements. We demonstrate how 13C isotopic labelling of sugars combined with spectroscopic detection allows the study of bacterial mixed sugar metabolism, to establish whether sugars are sequentially or simultaneously metabolised. For E. coli, we have characterised the shift from glucose to lactose metabolism without a classic diauxic lag phase. DHR and CERS are shown to be cost-effective and highly selective analytical tools in the biosciences and in biotechnology, complementing and superseding existing, conventional techniques. They also provide new capabilities for mechanistic investigations in biochemistry and show a great deal of promise for use in stable isotope bioassays. Finally, PAS in a differential Helmholtz resonator has been employed with near-IR detection of CO2 and H2S in natural gas, in static and flow cell measurements. The set-up has also been used for simultaneous in situ monitoring of O2, CO2 and H2S in the cysteine metabolism of microbes (E.coli), and for the analysis of CO2 and H2S impurities in natural gas.