Photochemical method for removing methane interference for improved gas analysis

Abstract

Abstract. The development of laser spectroscopy has made it possible to measure minute changes in the concentrations of trace gases and their isotopic analogs. These single or even multiply substituted species occur at ratios from percent to below parts per million and contain important information concerning trace gas sources and transformations. Due to their low abundance, minimizing spectral interference from other gases in a mixture is essential. Options including traps and membranes are available to remove many specific impurities. Methods for removing CH4, however, are extremely limited as methane has low reactivity and adsorbs poorly to most materials. Here we demonstrate a novel method for CH4 removal via chlorine-initiated oxidation. Our motivation in developing the technique was to overcome methane interference in measurements of N2O isotopic analogs when using a cavity ring-down spectrometer. We describe the design and validation of a proof-of-concept device and a kinetic model to predict the dependence of the methane removal efficiency on the methane concentration [CH4], chlorine photolysis rate JCl2, chlorine concentration [Cl2] and residence time tR. The model was validated by comparison to experimental data and then used to predict the possible formation of troublesome side products and by-products including CCl4 and HCl. The removal of methane could be maintained with a peak removal efficiency >98 % for ambient levels of methane at a flow rate of 7.5 mL min−1 with [Cl2] at 50 ppm. These tests show that our method is a viable option for continuous methane scrubbing. Additional measures may be needed to avoid complications due to the introduction of Cl2 and formation of HCl. Note that the method will also oxidize most other common volatile organic compounds. The system was tested in combination with a cavity ring-down methane spectrometer, and the developed method was shown to be successful at removing methane interference.