Flowing-afterglow mass spectrometry, FA-MS, is a sensitive and quantitative mass spectrometry analytical approach that offers a route to on-line, real-time deuterium abundance measurements in water vapour in breath and above aqueous liquids, including urine and serum. This method involves the production and flow of thermalised hydrated hydronium cluster ions in inert helium or argon carrier gas along a flow tube following the introduction of a humid air sample. These ions react in multiple collisions with water molecules, their isotopic compositions reach equilibrium and the relative magnitudes of their isotopomers are measured by a quadrupole mass spectrometer located downstream.
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