Isotope analysis of environmental substances by a new laser-spectroscopic method utilizing different pathlengths

We demonstrate a new method of absorption spectroscopy for precise isotopic composition measurements of environmental molecules such as 13CH4/12CH4 (∼1/100) and 15N14N16O/14N15N16O/14N14N16O (∼0.4/0.4/100). We combined wavelength-stabilized near-infrared semiconductor lasers with a multipass absorption cell in which laser beams passed different times to compensate the large abundance difference and balance the signal levels. Then we could choose strong absorption lines whose absorption coefficients have almost the same temperature dependencies so as to minimize the effect of the changes in gas temperature on the ratio measurements. Normalization of the second-harmonic absorption signal by the dc signal restored a good proportionality between the signal amplitude and the absorbance. An intercomparison between absorption spectroscopy and mass spectrometry for pure natural-abundance methane samples proved that the precision of the present method is ±3×10−4 (±0.3‰) and the relative accuracy is about the same. The demonstrated high-precision indicates that the laser-spectroscopic method can complement mass spectrometry, especially for isotopic molecular species of a small mass difference, and is potentially a useful means for precise analysis of the biogeochemical cycles of the substances.