3.3 - Lower Limit of Detection
If only the measuring statistic is considered, the Lower Limit of Detection (LLD) is calculated according to EN 25482 and ISO 11929 as follows:
To determine the minimum detectable activity (MDA), other parameters like chemical yield η, counting efficiency e and amount of sample analyzed (Vs) should be taken into account in the calculation
MDA = LLD / ε * η * Vs
The background plays an important role for the calculation of the LLD. The following background count rates have been considered for the calculation of the LLD (Triathler, HIDEX for PSD α-channel):
in the α-channel:
organic phase (mini-vial: 8 ml or less) = 0.5 counts/hour (0.00091 s-1/ sample)
organic phase (20 ml vial) = 2 counts/hour (0.0018 s-1/ sample)
aqueous phase (gel) = 40 counts/hour (0.0082 s-1/ sample)
aqueous phase with severe quenching (EDTA, oxalic acid) = 120 counts/hour (0.014 s-1/ sample)
and in the β-channel:
aqueous phase (Low Level b-counter) = 300 counts/hour (0.022 s-1/ sample)
aqueous phase (Triathler) = 150 counts/minute (0.123 s-1/ sample)
Data for the detection limits of the analytical procedures in the text are preferably stated as MDA, if analysis parameters like sample volume are constant and known. Otherwise the Lower Limit of Detection LLD is given as activity per sample, without considering any preconcentration step.
For all calculations, one hour measuring time both for sample and background count rate have been considered. In this sense background repeatability should be assessed, in order to confirm that background uncertainty can only be attributed to measuring statistics (i.e. Poisson distribution).
