INTERACTION OF SAMPLE, COCKTAIL, AND HEADSPACE VOLUME WHEN MEASURING AQUEOUS RN IN SMALL VOLUME SAMPLES

Techniques for measuring aqueous 222Rn (Rn) by liquid scintillation methods (LSC) are well established, including those incorporating alpha counting by pulse shape discrimination or analysis (PSD or PSA). The most widely used method involves injection of a 10 ml aqueous sample directly into a standard LSC vial preloaded with 10 ml of LSC cocktail. When desirable, the Rn can be pre-concentrated by direct extraction of the Rn from the water sample. This allows Rn measurement from sample volumes of up to 1.0 L, and is particularly useful when very low Rn concentrations are anticipated. There are instances, however, when sample volumes of even 10 ml are not feasible. This is often encountered in bench-scale work where the physical size of the experimental apparatus is limited, or where large sample volumes would disrupt other analytical measurements. In those instances where sample volume is limited, it is important that the measurement protocol makes the most efficient use of the Rn within the small sample volume. With respect to LSC methods, Rn is unique in that it distributes itself between the aqueous, organic (cocktail) and gas (headspace) phases within the LSC vial. When small sample sizes are involved ( £ 5 ml), serious consideration must be given to the volume of cocktail used and th