Sensitive measurements of stable carbon isotopes of CO2 with wavelength modulation spectroscopy near 2 μm

We performed highly sensitive measurements of stable carbon isotopes of CO2 using wavelength modulation spectroscopy with a distributed feedback laser diode in the 2-mu m wavelength range. Ro-vibrational transitions, which belong to the different combination bands, were selected to measure the (CO2)-C-13/(CO2)-C-12 carbon isotope ratio. The delta C-13 value was determined through the signals obtained in a Herriott-type multipass cell with an optical path length of 29.9 m. The limit of detection for CO2 using our measurement system was 16 +/- 1 parts per billion by volume at the strongest absorption peak that is assigned to the 2 nu (1)+nu (3) R(16) line within the laser emitting frequency region. The 10-h long term precision was a delta C-13 standard deviation of 0.24aEuro degrees (1 sigma) with the best suited line pairs of (CO2)-C-12 and (CO2)-C-13 and with careful temperature and pressure control in the cell. The 3-min response and high precision of this measurement allows for precise continuous measurements of stable carbon isotopes in ambient CO2.