A survey on uncertainty in bioassay measurements carried out within the OMINEX Project
Workshop on Internal Dosimetry of Radionuclides - Occupational, Public and Medical Exposure - 9-12 September 2002 - New College, Oxford, United Kingdom
C Hurtgen1, P Bérard2 and C Cossonnet3
1 SCK-CEN, Low Level Radioactivity Department, Boeretang 200, B-2400 Mol, Belgium
2 CEA Saclay, DSP-LABM, 91191 Gif-sur-Yvette, France
3 IRSN, DPHD/SDOS, BP17, 92262 Fontenay-aux-Roses cedex, France
OMINEX (Optimisation of Monitoring of Internal Exposure) is an on-going international project part-funded by the European Commission with the 5th (EURATOM) Framework Programme. One of the topics covered by this project was to look at the bioassay measurements performed in the European laboratories. Questionnaires on bioassay measurements by alpha spectrometry or ICP-MS were sent to European laboratories. The main objective of this work is to collect information from laboratories on analytical procedures and on the uncertainties associated with bioassay results.
Alpha spectrometry is the technique most used for the determination of alpha emitters such as plutonium, americium, uranium and thorium in biological samples (urine and faeces). The different parameters influencing the uncertainties on the results and the minimum detectable amount (MDA) have been investigated. Namely these are the counting efficiency, the chemical yield, the background and sample counting time, the yield tracer used and the background observed in the regions of interest of the isotopes measured. Alpha spectrometry is a sensitive technique for the measurements of actinides in biological samples but the main drawback is the long counting time needed to achieve the sensitivity, about 3 days to reach 0.1 mBq.L-1.
ICP-MS is not yet used for routine measurements of actinides in biological samples. It is a very sensitive technique for the measurement of long lived radionuclides (T1/2 > 10E4 y) and is used by a few laboratories. To reach a concentration level of around 0.1 mBq.L-1 ICP-MS measurements can be performed directly on diluted urine for isotopes such as 238U, 235U and 232Th. For others such as 239Pu, 237Np and 234U, chemical treatment, as for alpha spectrometry, is required before measurement. Laboratories apply different methods for calibration: external calibration, isotopic dilution or spike standard additions. For all these methods, the parameters influencing the uncertainties on the results are mainly the dilution factor, the count numbers, the slope of the calibration curves, the concentration of the standards used and the mass bias correction. One of the main advantages of the ICP-MS technique is the short time (minutes) needed to perform the measurement.
This work is part-funded by the European Commission under Contract Number FIKR-CT-2000-0046.