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Warning — Persons using this document should be familiar with normal laboratory practice. 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. Warning — It is absolutely essential that tests conducted in accordance with this document be carried out by suitably qualified staff. This document specifies methods to determine 99Tc by inductively coupled plasma mass spectrometry (ICP MS) in supply water, 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 detection limit of the methods described in this document, using currently available ICP-MS instrumentation, is approximately 0,2 ng·l−1 to 0,5 ng·l−1 (0,1 Bq·l−1 to 0,3 Bq·l−1), which is much lower than the WHO criteria for safe consumption of drinking water (100 Bq·l−1)[4]. The methods presented in this document are not intended for the determination of ultra trace amount of 99Tc. The methods described in this document are applicable in the event of an emergency situation, but not if 99mTc is present at quantities that could cause interference. Filtration of the test sample is necessary for the methods described in this document if suspended solids are present. 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. It is the user’s responsibility to ensure the validity of this test method for the water samples tested.