Methyl-3-a-acetoxycholinate* 0.7* Mean 1.5* 1.4 ± 0.12 °Prepared by exchange between HTO and H3PO2, followed by neutralization with T10H and evaporation in vacuo to recover solid salt (-5). b Prepared by adding a small amount of HTO to the anhydrous salt.c See (2). d Density not known; estimated from similar compounds. * Approximate. / Calculated assuming no isotope effect on the exchange equilibrium. * Calculated from data in (2) and corrected for different planchet area used by those authors.activity required to produce a given level of activity (as measured in the flow counter) in any hydrogen-containing compound (TR) can be calculated by applying the following principle: Ail solid materials having the same specific tritium activity (counts per unit time per unit weight of solid) will give the same observed activity in a given solid-phase counting apparatus, provided that the thickness of the solid is equivalent to infinite thickness. This observed activity will be independent of the density of the solid and will depend only on its mass absorption coefficient. Since the mass absorption coefficient depends on the ratio of atomic weight to atomic number, it is a slowly varying function of atomic weight, and hence the ratio p observed activity of solid TR specific activity of solid TR will be approximately constant for most hydrogen-containing solids, even though they may vary widely in density.This principle can be verified both theoretically and experimentally. Table III gives the values of P obtained with the first counter for five different hydrogen-containing solids in which some of the hydrogen has been replaced by tritium. It is seen that although the density varies over a tenfold range, the values of P are constant within about 7% and show no trend with increasing density'. Thus from the known value of P, one can calculate the specific activity of the tritiated hydrogen-containing compound necessary' to give a convenient observed activity. Knowing the necessary specific activity, one can then easily calculate the tritiated water activity necessary to give this activity. SUMMARY Exchange and tracer experiments with tritium can be set up and carried out using the windowless flow counter as an analytical tool, without having' to resort to gas-phase counting techniques for tritium assay. It is unfortunate that a more detailed

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