Selected ion flow tube mass spectrometry analyses of stable isotopes in water: isotopic composition of H 3O + and H 3O +(H 2O) 3 ions in exchange reactions with water vapor

Abstract

A new method has been developed for the determination of the isotope abundance ratios of deuterium, D, and oxygen-18, 18O, in water vapor (and water) using selected ion flow tube mass spectrometry (SIFT-MS). H 3O + ions are injected into the helium carrier gas where they associate with the H 2O and HDO molecules in a sample of water introduced into the carrier gas. The D and 18O contents of the product cluster ions H 8DO 4 + and H 9 18OO 3 + at m/e = 74 and 75, respectively, are determined by reference to the majority cluster ion H 9O 4 + at m/e = 73. Allowance is made for the contribution of the H 8 17OO 3 + 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%.