An indirect method for the mass spectrometric detection of hydrogen isotopes is described. The method involves the detection of protonated reagent gas ions rather than the direct detection of •Hi'1" and 12Hi+ at masses that are below the scan limits of most quadrupole GC/MS systems. Several reagent gases were investigated, viz., helium, neon, argon, and nitrogen.Argon was selected as the reagent gas because (i) there are no isotopes heavier than the most common, ^Ar, to interfere with the detection of the protonated species, (ii) the stability of HAr"1" is comparable to that of the protonated ions of other reagent gases, and (iii) argon is inexpensive and readily available. The mass-selective detection system permits the analysis of mixtures of hydrogen isotopes in the form of •HAr'*' and 2HAr+ without prior separation. The protium and deuterium spin isomers can be separated by gas/solid chromatography at 77 K. The indirect detection system allows the

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