Quantitative isotopic measurements of gas-phase alcohol mixtures using a broadly tunable swept external cavity quantum cascade laser

A swept-ECQCL is used for broadband IR spectroscopy of isotopic mixtures of CH₃OH, CH₃OD, CH₃CH₂OH, and CH₃CH₂OD in a static gas cell over a wavelength range of 9.5 to 10.4 μm. A weighted least squares fitting approach with quantitative library spectra illustrates that significant spectral congestion does not negatively impact the ability for in situ quantification of large isotopic species in a mixture. The noise equivalent concentrations for CH₃OH, CH₃OD, CH₃CH₂OH, and CH₃CH₂OD are 19 ppbv m, 28 ppbv m, 450 ppbv m, and 350 ppbv m respectively for a 50 seconds integration time. Based on the observed NECs, isotopic precisions of 0.07‰ and 0.79‰ for a 50 s integration time are calculated for measurements of the [MeOD]/[MeOH] and [EtOD]/[EtOH] isotope ratios, respectively, for the species concentrations in the gas cell.