Direct liquid scintillation measurement applied to environmental water sample

There are two standard recommendations [ISO-9698 and AWWA-APHA-WEF method] that specify a method for determining tritium concentration in water by liquid scintillation counting. Except for some slight differences, the principle of the two chemical treatments of water samples is basically the same. Distillation of the water sample and measurement by liquid scintillation counting are common procedures for both recommendations. The first step toward establishing the routine procedure for our laboratory was to select an appropriate method for assessing counting efficiency. The methods most widely used to check counting efficiency in the quench presence include the Spectral Quench Parameter of external standard method (SQP(E)) and Internal Standard Method. In our study, a low background liquid scintillation system detector, Quantulus 1220, was used to determine tritium activity concentration in seven heavy water samples with different concentration levels from 99.66 D/(H+D)% to 1.65 D/(D+H)%. A comparison between dilution factors, calculated for both D/(D+ H)% concentration and tritium activity and measured by the two methods, allowed us to conclude that the Internal Standard Method provided accurate results especially for lower D/(D+H)% concentration, which is the case of our environmental samples. Existing commercial standards don't meet the requirements for an accurate environmental tritium measurement. The Compton electrons produced by the external standard are energetic beta particles and do not have the same weak beta-particle behavior in the sample itself. Hence, a small amount of quenching may remain undetected in the tritium sample. Once the efficiency problem settled, we focused on the sample preparation. We studied the earlier mentioned standard methods to determine tritium concentrations in different types of water: drinking water, precipitation, surface water, and wastewater. We decided that for our laboratory conditions, i.e. equipment, practice, and materials used routinely for the monitoring programs, ISO method would be appropriate. The uncertainty of the method is high and sometimes makes the measurement useless for hydrological studies of single events, where high precision measurement is required, but liquid scintillation counting still provides an alternative method for rapid screening at a relatively low price. (authors)