Abstract
A near-IR laser based spectrometer for continuous measurement of stable carbon isotopes in CO_2 has been developed. ^<12>CO_2 and ^<13>CO_2 are detected with wavelength modulation spectroscopy and a Herriott-type multi-pass cell with a optical path length of 29.9 m using a distributed feedback laser diode in the 2-μm wavelength region. To measure the isotope ratio precisely, the influence of pressure and temperature variation in d^<13>C was evaluated. The limit of detection for our system was 16 ± 1 ppbv. The precision in the determination of δ^<13>C was 0.1 ‰ for 120 seconds signal integration time at ambient concentration levels. We demonstrate that our system enables automated continuous measurements of δ^<13>C of CO_2 in ambient air without any complex operation by users.
