Quantum cascade laser based spectrometer for in situ stable carbon dioxide isotope measurements

Simultaneous concentration measurements of the three main stable carbon dioxide isotopologues (16O12C16O, 16O13C16O and 18O12C16O) in ambient air are demonstrated. The instrument employs a quasi-room temperature, pulsed quantum cascade laser emitting at 4.3 μm. Both the laser and the IR detector are thermoelectrically cooled, which greatly facilitates cryogenic-free, continuous operation. The Allan plot of the time series of the retrieved spectroscopic ratio [13CO2]/[12CO2] shows a variance minimum that corresponds to 0.16‰ for δ13C with an averaging time of about 500 s. To demonstrate the feasibility of high precision in situ isotope ratio measurements, the instrument was operated in the field and provided continuous mixing ratio values for all three CO2 isotopologues at 1 s time resolution. The spectroscopically derived isotopic ratios were compared to the standard laboratory-based isotope-ratio mass spectrometer measurements made on field-collected flask samples. At stable laser operation, the two data sets agreed within 0.28‰.