Influence of ethylene spectral lines on methane concentration measurements with a diode laser methane sensor in the 1.65 μm region

Abstract

Spectra measurements around 1.65 μm have revealed that spectral lines of ethylene are present in this range along with methane spectral lines. Furthermore, the HITRAN database has no parameters of these ethylene lines. In particular, it was found that several ethylene absorption lines are located near the methane feature often used for quantitative spectroscopy (R3 triplet of the 2ν₃ band). Ethylene can be present together with methane in the samples under study, perturbing the recorded spectrum of CH₄. This perturbation by an interfering species can produce systematic errors in the derived CH₄ concentration.

We have found in this work by numerical modelling that when measuring the methane concentration by a diode laser methane sensor operating in the 6046–6048 cm^-1 range, the systematic error on the retrieved CH₄ concentration decreases, as a rule, when decreasing the width of the fitted spectral window. At equal concentrations of ethylene and methane in the sample under study, the error on the concentration of CH₄ determined by the correlation technique can reach 15%. When using the technique based on the 2nd or 3rd derivative of the spectrum, interfering or impurity lines induce a systematic error on the derived CH₄ concentration, which is less than 2%.