Rapid and Sensitive Quantification of Isotopic Mixtures Using a Rapidly-Swept External Cavity Quantum Cascade Laser

A rapidly-swept external-cavity quantum cascade laser with an open-path Herriott cell is used to quantify gas-phase chemical mixtures of D2O and HDO at a rate of 40 Hz (25-ms measurement time). The chemical mixtures were generated by evaporating D2O liquid near the open-path Herriott cell, allowing the H/D exchange reaction with ambient H2O to produce HDO. Fluctuations in the ratio of D20 and HDO on timescales of <1 s due to the combined effects of plume transport and the H/D exchange chemical reaction are observed. Noise -equivalent concentrations (16) (NEC) of 147.0 ppbv and 151.6 ppbv in a 25 -ms measurement time are determined for D20 and HDO, respectively, with a 127-m optical path. These NECs are improved to 23.0 and 24.0 ppbv with a 1-s averaging time for D2O and HDO, respectively. NECs <200 ppbv are also estimated for N20, 1,1,1,2 tetrafluoroethane (F134A), CH4, acetone and SO2 for a 25 -ms measurement time. The isotopic precision for measurement of the [D20]/ [HDO] concentration ratio of 33 % and 5& is calculated for the current experimental conditions for measurement times of 25 ms and 1 s, respectively.