13CO2/12CO2 isotopic ratio measurements with a continuous-wave quantum cascade laser in exhaled breath

A laser spectrometer based on a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser at ∼2308 cm−1 has been evaluated for measurement of 13CO2/12CO2 isotopic ratio (δ13C) changes in exhaled breath samples and in CO2 gas flows in the concentration range 1–5%. Mid-infrared CO2 absorption spectra were measured in a 54.2-cm long optical cell using balanced detection whereby the beam passing through the cell was ratioed against a reference beam split-off from the main beam before the cell. Signal-to-noise ratios (SNR) were estimated for CO2 concentration measurements determined from either absorption peak amplitude or absorption peak area. The highest SNR were achieved in the measurements based upon a fitted absorption peak area. Typical short-term δ13C precisions of 1.10/00 (1-s integration time) and 0.50/00 (8–12-s integration time) were estimated from the two-sample (Allan) variance plots of data recorded in the optical cell at a pressure of 20 Torr and with no active temperature stabilization of the cell and gas flow. The best precision of 0.120/00 was achieved for averaging 80 successive 1-s integration time measurements.