Cavity ring-down spectroscopy versus high-temperature conversion isotope ratio mass spectrometry; a case study on d(2)H and d(18)O of pure water samples and alcohol/water mixtures

The analysis of the stable isotope abundance in water and other natural materials has hitherto been the domain of stable isotope ratio mass spectrometry. To reliably quantify the small variability of the deuterium amount in natural samples an analytical precision of better than 0.15 ppm (61ppt in delta notation(a)) is necessary. Due to the low natural abundance and to the smaller isotopic fractionation the required precision for (18)O is even smaller ( < 0.02ppm or < 0.1ppt in delta notation). To analyse these fine variations using mass spectrometry, a chemical conversion from the original material, mostly water, into a light gas suitable for mass spectrometry (H(2), CO(2), CO, O(2)) is necessary. No reliable high-precision mass spectrometric method has been devised that would enable isotopic analysis using water directly. The conversion step often turns out to limit the achievable precision. Recently, a new technique making use of photo absorption by water molecules has succeeded in reaching the required precision level. This Cavity Ring-Down Spectroscope (CRDS) technique employs high finesse reflectivity mirrors that keep the laser beam inside a cavity for a large number of reflections, achieving (mean absorption path lengths of up to 10 km.