This document specifies a method to determine thorium 232 (²³²Th) by inductively coupled plasma mass spectrometry (ICP-MS). The mass concentrations obtained can be converted into activity concentrations.

The method described in this document is applicable to test samples of drinking water, rainwater, surface and ground water, marine water, as well as cooling water, industrial water, domestic and industrial wastewater after proper sampling and handling and test sample preparation.

The limit of detection depends on the sample volume, the instrument used, the background count rate, the detection efficiency and the chemical yield. In this document, the limit of detection of the method using currently available apparatus is approximately 2 mBq·l⁻¹ (or mBq·kg⁻¹), which is lower than the WHO criteria for safe consumption of drinking water (1 Bq·l⁻¹)^[4].

The method described in this document covers the measurement of ²³²Th in water at activity concentrations between 2 mBq·l⁻¹ and 5 Bq·l⁻¹. Samples with higher activity concentrations than 5 Bq·l⁻¹ can be measured if a dilution is performed.

The method described in this document is applicable in the event of an emergency situation.

Filtration of the test sample is necessary for the method described in this document. The analysis of ²³²Th 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.