Liquid-Vapor Isotope Fractionation Measurements

Airborne may be collected in any of the forms discussed in Section 5.3.2. Samples may arrive with several forms in the same collector for separation at the laboratory, or the forms may already be separated at collection. Most commonly, tritiated water vapor is brought to the laboratory as a water sample. When collected on silica gel or molecular sieve, tritiated water vapor is removed by heating the collector material in a distillation flask above the boiling point of water. The vapor is transferred to a separate flask and condensed. A measured amount of the condensed water is analyzed for tritium in a liquid scintillation (LS) counter in terms of becquerel per gram water. If the collector is silica gel, the result must be corrected for dilution by a small amount of water originally in the silica gel (Rosson et al. 2000). In addition, a small isotope effect, mentioned in Section 4.8, results in distilling a greater fraction of H2O than HTO unless the entire sample is distilled. 

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