A TC/WMS-TDLAS mid-infrared detection method for ultra-low concentration carbon isotope methane

Abstract

There is a great demand for online high-precision monitoring of carbon isotope methane in the fields of atmospheric monitoring, geological exploration, oil and natural gas extraction, etc. However, the current detection method has some problems, such as the difficulty to ensure a wide concentration detection range while detecting an ultra-low concentration of carbon isotope methane with high accuracy, and a long detection period. To address these issues, in this paper, we propose TC/WMS-TDLAS, temperature-controlled/wavelength-modulated spectroscopy of tunable diode laser absorption spectrometry for the detection of ultra-low concentration carbon isotope methane. Also, we analyze the effect of temperature variation on the absorption signal of the infrared spectrum of carbon isotope methane and further investigate the interaction effect of the two isotope detection when the concentrations of ¹²CH₄ and ¹³CH₄ differed greatly. In addition, we develop a calibration model for detection when the difference between ¹²CH₄ and ¹³CH₄ concentrations is large. The experimental results show that TC/WMS-TDLAS can achieve an efficient analysis of ¹²CH₄ and ¹³CH₄ concentrations when the gas volume is less than 200 mL with a detection period less than 2 s. The detection error for ultra-low concentration carbon isotope methane is less than 1%, and the lower limit of detection for the concentration of ¹²CH₄ and ¹³CH₄ is 0.5 × 10⁻⁶ (0.5 ppmv) and 0.1 × 10⁻⁶ (0.1 ppmv), respectively. It also demonstrates that TC/WMS-TDLAS has significantly improved the detection performance of TDLAS for ultra-low concentration carbon isotope methane and it is promising for future application.