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 (CH4)-C-13/(CH 4)-C-12 (similar to1/100) and (NNO)-N-15-N-14-O-16/(NNO)-N-14-N-15-O-16/(NN O)-N-14-N-14-O-16 (similar to0.4/0.4/100). We combined wavelength-stabilize d near-infrared semiconductor lasers with a multipass absorption cell in wh ich laser beams passed different times to compensate the large abundance di fference and balance the signal levels. Then we could choose strong absorpt ion Lines whose absorption coefficients have almost the same temperature de pendencies so as to minimize the effect of the changes in gas temperature o n the ratio measurements. Normalization of the second-harmonic absorption s ignal by the de signal restored a good proportionality between the signal a mplitude and the absorbance. An intercomparison between absorption spectros copy and mass spectrometry for pure natural-abundance methane sample's prov ed that the precision of the present method is +/-3 x 10(-4) (+/-0.3%)) and the relative accuracy is about the same. The demonstrated high-precision i ndicates that the laser-spectroscopic method can complement mass spectromet ry, especially for isotopic molecular species of a small mass difference, a nd is potentially a useful means for precise analysis of the biogeochemical cycles of the substances. (C) 2001 Elsevier Science B.V. All rights reserv ed.