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 σc(p = 0 bar) = (1.60 ± 0.06) × 105 cm2/mol, and pressure-broadening coefficients γp(Ar) = (0.094 ± 0.002) cm-1/bar and γp(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.