Quantitative Mid-Infrared Cavity Ringdown Detection of Methyl Iodide for Monitoring Applications

Methyl iodide is a toxic halocarbon with diverse industrial and agricultural applications, and it is an important ocean-derived trace gas that contributes to the iodine burden of the atmosphere. Quantitative analysis of CH₃I is mostly based on gas chromatography coupled with mass spectrometry or electron capture detection (GC-MS/ECD) as of yet, which often limits the ability to conduct in situ high-frequency monitoring studies. This work presents an alternative detection scheme based on mid-infrared continuous wave cavity ringdown spectroscopy (mid-IR cw-CRDS). CH₃I was detected at the ^RR₂(15) rovibrational absorption transition at ṽ = 3090.4289 cm^-1; part of the corresponding v₄ vibration band has been measured with Doppler-limited resolution for the first time. A line strength of S(T = 295 K) = (545 ± 20) cm/mol, corresponding to a line center absorption cross-section σ_c(p = 0 bar) = (1.60 ± 0.06) × 10⁵ cm²/mol, and pressure-broadening coefficients γ_p(Ar) = (0.094 ± 0.002) cm^-1/bar and γ_p(N₂) = (0.112 ± 0.003) cm^-1/bar have been determined. The performance of the detection system has been demonstrated with a tank-purging experiment and has been directly compared with a conventional GC-MS/ECD detection system. Quantitative detection with high reproducibility and continuous sampling is possible with a current noise-equivalent limit of detection of 15 ppb at 20 mbar absorption-cell pressure and 70 s averaging time. This limit of detection is suitable for practical applications in the ppm mixing ratio level range such as workplace monitoring, leak detection, and process studies. Natural environmental abundances are much lower, therefore possibilities for future improvement of the detection limit are discussed.