COMBINATION OF PHOTOACOUSTIC DETECTOR WITH GAS DIFFUSION PROBES FOR THE MEASUREMENT OF METHANE CONCENTRATION GRADIENTS IN SUBMERGED PADDY SOIL

Dissolved methane was monitored by means of a diffusion probe in combination with a photoacoustic (PA) detector cell placed in the cavity of a liquid nitrogen-cooled CO laser. The detection limit of the photoacoustic detector was 1 ppbv methane (= 2 uM in aqueous solution), the time response was 60 s, the spatial resolution was 1.36 mm. These limits were determined by the acoustic noise and the configuration of the diffusion probe. The combination of PA detector with gas diffusion probes was found to be useful for monitoring gaseous compounds. However, the membrane material of the diffusion probe was critical. Silicone as membrane material was useful only for measurement of CH4. Goretex as membrane material was applicable to measurement of dimethylsulfide (DMS), but did not give a stable signal for trimethylamine (TMA). Vertical concentration profiles of CH4 in anoxic paddy soil agreed well with earlier results obtained with a gas chromatograph as detector. Methane was produced in anoxic soil layers below 8-l 0 mm depth and diffused upwards to the surface through a layer of CH4-consuming bacteria situated at about 2 mm depth. In the oxic upper 2 mm soil layer the concentration of CH4 decreased below the detection limit of our system. Methane-containing gas bubbles that were embedded in the soil were detected by a steep increase of the CH4 signal. The combination of PA detector and gas diffusion probe was found to be a useful tool to measure CH4 gradients in submerged soil or sediment with high temporal and spatial resolution, thus allowing the localization and quantification of CH4 production and CH4 oxidation rates within the soil profile. Copyright 0 1996 Elsevier Science Ltd