Diode laser absorption spectrometry for (CO2)-C-13/(CO2)-C-12 isotope ratio analysis:Investigation on precision and accuracy levels

Near-infrared laser spectroscopy is used to measure the C-13/C-12 isotope abundance ratio in gas phase carbon dioxide. The spectrometer, developed expressly for field applications, is based on a 2 mu m distributed feedback diode laser in combination with sensitive wavelength modulation detection. It is characterized by a simplified optical layout, in which a single detector and associated electronics are used to probe absorptions of a pair of (CO2)-C-13 and (CO2)-C-12 lines, simultaneously in a sample, as well as a reference gas. For a careful investigation of the achievable precision and accuracy levels, we carried out a variety of laboratory tests on CO2 samples with different isotopic compositions, calibrated with respect to the international standard material by means of isotope ratio mass spectrometry. The 1-sigma accuracy of the (CO2)-C-13/(CO2)-C-12 determinations, reported in the so-called delta notation, is about 0.5% (including both statistical and systematic errors), for delta-values in the range from -30% to +20%. We show that the major source of systematic errors is a consequence of the non-linearity of the Lambert-Beer absorption law, and can be corrected for to a very high degree of accuracy.