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WARNING — Persons using this document should be familiar with normal laboratory practices. This document does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and to determine the applicability of any other restrictions. IMPORTANT — It is absolutely essential that tests conducted according to this document be carried out by suitably trained staff. This document specifies methods to determine 99Tc by liquid scintillation counting (LSC) in water supplies, drinking water, rainwater, surface and ground water, marine water, as well as cooling water, industrial water, domestic, and industrial wastewater after proper sampling, handling, and test sample preparation. The detection limit depends on the sample volume, the instrument used, the background count rate, the detection efficiency, the counting time, and the chemical yield. The minimum detectable activity of the methods described in this document, using currently available LSC apparatus, is approximately 5 Bq·l−1 to 20 Bq·l−1, which is lower than the WHO criteria for safe consumption of drinking water (100 Bq·l-1).[4] These values can be achieved with a counting time of 60 min for a sample volume varying between 14 ml to 40 ml. The method presented in this document is not intended for the determination of ultra-trace activity concentrations of 99Tc. The method described in this document is applicable in the event of an emergency situation, but not if 99mTc is present at quantities that could cause interference and not if 99mTc is used as a recovery tracer. Filtration of the test sample is necessary for the methods described in this document if suspended solids are present as the methods presented in this document can only be used to determine soluble 99Tc. The analysis of 99Tc adsorbed to suspended matter is not covered by this method. The analysis of the insoluble fraction requires a mineralization step that is not covered by this document. In this case, the measurement is made on the different phases obtained. The final activity is the sum of all the measured activity concentrations. It is the user’s responsibility to ensure the validity of this test method for the water samples tested.