Quantum cascade laser absorption spectrometer with a low temperature multipass cell for precision clumped CO 2 measurement

Abstract

We present a quantum cascade laser-based absorption spectrometer deploying a compact (145 mL volume) segmented circular multipass cell (SC-MPC) with 6 m optical path length. This SC-MPC is embedded into an effective cooling system to facilitate operation at cryogenic temperatures. For CO 2 , the sample is cooled to 153 K, i.e. close to the sublimation point at 10 mbar. This enables efficient suppression of interfering hot-band transitions of the more abundant isotopic species and thereby enhances analytical precision. As a demonstration, the amount fractions of all three CO 2 isotopologues involved in the kinetic isotope exchange reaction of 12 C 16 O 2 + 12 C 18 O 2 ⇌ 2· 12 C 16 O 18 O are measured. The precision in the ratios [ 12 C 18 O 2 ]/[ 12 C 16 O 2 ] and [ 12 C 16 O 18 O]/[ 12 C 16 O 2 ] is 0.05 ‰ with 25 s integration time. In addition, we determine the variation of the equilibrium constant, K, of the above exchange reaction for carbon-dioxide samples equilibrated at 300 K and 1273 K, respectively.