Selected ion flow tube mass spectrometry analyses of stable isotopes in water:: Isotopic composition of H3O+ and H3O+(H2O)3 ions in exchange reactions with water vapor

A new method has been developed for the determination of the isotope abundance ratios of deuterium, D, and oxygen-18, O-18, in water vapor (and water) using selected ion flow tube mass spectrometry (SIFT-MS). H3O+ ions are injected into the helium carrier gas where they associate with the H2O and HDO molecules in a sample of water introduced into the carrier gas. The D and O-18 contents of the product cluster ions H8DO4+ and (H9OO3+)-O-18 at m/e = 74 and 75, respectively, are determined by reference to the majority cluster ion H9O4+ at m/e = 73. Allowance is made for the contribution of the (H8OO3+)-O-17 ions to the m/z = 74 ions. Absolute isotopic ratios are measured within seconds without the need for precalibration of the SIFT-MS instrument, currently to an accuracy of better than 2%. (J Am Soc Mass Spectrom 2000, 11, 866-875) (C) 2000 American Society for Mass Spectrometry.