The effect of increased sample volume and peak efficiency on instrument time when measuring to a pre-determined MDA: Applied to AMP-separation of 137Cs in tap-water samples

Environmental monitoring of e.g. radionuclides are important in order to establish a baseline of radionuclides present in the environment, e.g. in order to detect increases in activites. Often low activities are present which require long measurement times. This work presents improvements with respect to reducing instrument time required when measuring to a pre-determined MDA (Minimum Detectable Activity). The method studied, involved the separation of ¹³⁷Cs in tap water samples using AMP-PAN (ammonium molybdophosphate in polyacetonitrile) combined with IDGS (isotope dilution gamma spectrometry). The results show that significantly lower instrument times can be used when measuring to a pre-determined MDA (considered fit-for-purpose), if the same amount of sample can be measured in a more efficient geometry. Moreover, using more sample volume will also decrease the instrument time significantly. Measurement times will, for increased efficiencies, approximately be shortened by a factor equal to the square of the ratio between the efficiencies. Likewise, instrument time will approximately be reduced by a factor equal to the square of the ratios of sample volumes when sample volume is increased. The same relationship is valid for chemical yield, emphasising the importance of high chemical yields. When implementing AMP-separation of ¹³⁷Cs and doubling the sample volume compared to the original laboratory method (evaporation), the instrument time needed to reach the target MDA (0.60 mBq/L) was reduced by a factor of about eight. Apart from a much shorter measurement time, the time for sample preparation was shortened from a few days to about 2 h.