IMPROVED MEASUREMENT OF STABLE-ISOTOPE RATIOS IN GAS-CHROMATOGRAPHY MASS-SPECTROMETRY USING THE MICROWAVE-POWERED CHEMICAL-REACTION INTERFACE FOR MASS-SPECTROMETRY

The microwave-powered chemical reaction interface for mass spectrometry (CRIMS) has been successfully used for selective detection of analytes labeled with C-13, N-15 and D following capillary gas chromatography separation with good analytical characteristics in biological applications. In this study we evaluated how an advanced data system coupled to a quadrupole mass analyzer could improve precision and sensitivity of stable isotope ratio measurements for C-13 and N-15. The enrichments of C-13 and N-15 are determined by monitoring CO2 (m/z 44 and 45) and NO (m/z 30 and 31). These small molecules are produced from the analyte in the chemical reaction interface in the presence of SO2 as a reactant gas. Using caffeine and its C-13(1), N-15(2)-labeled analog for these quantitative studies, we have found that the Vector 2 system improves overall precision (RSD = 0.6% for both carbon and nitrogen) and sensitivity of stable isotope measurements by at least a factor of two compared to the Vector 1 system, and by more than an order of magnitude compared to our older results. With the optimum system we are now able to measure an atom% enrichment of 0.0044 for N-15 and 0.015 for C-13 in caffeine in the presence of 300 ng of unlabeled material. This is more than half way between isotopic detection limits of conventional gas chromatography/mass spectrometry and the state-of-the-art, which is a gas chromatograph coupled to a chemical combustor and a dual-collector isotope ratio mass spectrometer.