Methane monitoring in faecal headspace gas using mid-IR laser spectroscopy with a hollow waveguide gas cell

The feasibility of using laser spectroscopy for analysis of the headspace gas emanating from biological samples is demonstrated by monitoring the concentration of methane produced by bovine faeces. The spectrometer consists of a 7.8 µm quantum cascade laser operating in the intra-pulse mode and a 4.79 m pathlength hollow silica waveguide gas cell. Concentration measurements using known concentrations of methane demonstrate a limit of detection of 116 ppb, and Allan-Werle deviation analysis indicates a minimum detectable absorbance of 9.5 × 10^-5 at an integration time of 45 s. A custom-designed PTFE enclosure with an embedded heating element was designed to contain the faecal samples. Filtered ambient air pumped through the instrument at a controlled rate of 5 ml/min was used to transfer the headspace gas from the sample container to the gas cell. Methane production was found to vary depending on the sample, with one sample reaching an approximate equilibrium of 13.9 ppm after 40 minutes and another, measured for longer, reaching an equilibrium of 11.4 ppm after 2 hours. These findings demonstrate the potential of laser spectroscopy for volatile biomarker analysis, with significant promise for non-invasive point-of-care diagnostics.