Abstract
We performed highly sensitive measurements of stable carbon isotopes of CO2 using wavelength modulation spectroscopy with a distributed feedback laser diode in the 2-μm wavelength range. Ro-vibrational transitions, which belong to the different combination bands, were selected to measure the 13CO2/12CO2 carbon isotope ratio. The δ13C 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 2ν1+ν3 R(16) line within the laser emitting frequency region. The 10-h long term precision was a δ13C standard deviation of 0.24‰ (1σ) with the best suited line pairs of 12CO2 and 13CO2 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.
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