Sample preparation for quantitation of tritium by accelerator mass spectrometry

The capability to prepare samples accurately and reproducibly for analysis of tritium (H-3) content by accelerator mass spectrometry (AMS) greatly facilitates isotopic tracer studies in which attomole levels of H-3 can be measured in milligram-sized samples. A method has been developed to convert the hydrogen of organic samples to a solid, titanium hydride, which can be analyzed by AMS. Using a two-step process, the sample is first oxidized to carbon dioxide and water. In the second step, the water is transferred within a heated manifold into a quartz tube, reduced to hydrogen gas using zinc, and reacted with titanium powder. The H-3/H-1 ratio of the titanium hydride is measured by AMS and normalized to standards whose ratios were determined by decay counting to calculate the amount of H-3 in the original sample. Water, organic compounds, and biological samples with H-3 activities measured by liquid scintillation counting were utilized to develop and validate the method. The H-3/H-1 ratios were quantified in samples that spanned 5 orders of magnitude, from 10(-10) to 10(-15), with a detection limit of 3.0 x 10(-15), which is equivalent to 0.02 dpm tritium/mg of material. Samples smaller than 2 mg were analyzed following addition of 2 mg of a tritium-free-hydrogen carrier. Preparation of organic standards containing both C-14 and H-3 in 2-mg organic samples demonstrated that this sample preparation methodology can also be applied to quantify both of these isotopes from a single sample.