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This document specifies methods to determine the 239Pu, 240Pu, 241Pu and 237Np by inductively coupled plasma mass spectrometry (ICP-MS). The mass concentrations obtained can be converted into activity concentrations.
Due to its relatively short half-life and 238U isobaric interference, 238Pu can hardly be measured by this method. To quantify 238Pu isotope by mass spectrometry, other techniques can be used (ICP-MS with collision-reaction cell, ICP-MS/MS with collision-reaction cell or chemical separation). Alpha spectrometry measurement, as described in ISO 13167,[8] is currently used[9].
The method is applicable to test samples of supply/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.
A chemical separation of Np and Pu, with quantitative removal of all other elements (including uranium, americium, etc.) is mandatory, given all the potential interferences that may bias the measurement or lead to false positives. Chemical separations can eliminate most of the 238U. However, use of reagents, glassware, and atmosphere for the chemical separation, will also add 238U to the samples and prevent measurement of 238Pu.
The limit of detection 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 method described in this document, using currently available ICP-MS apparatus, is approximately 1 mBq·l-1 for 239Pu, 240Pu, 237Np and 1 Bq·l-1 for 241Pu, which is lower or of the same order of magnitude than the WHO criteria for safe consumption of drinking water (1 mBq·l-1 for 239Pu, 240Pu, 237Np and 10 Bq·l-1 for 241Pu[4]).This method covers the measurement of those radionuclides in water at activity concentrations between approximately[10][11]:
— 1 mBq·l−1 to 5 Bq·l−1 for 239Pu, 240Pu and 237Np;
— 1 Bq·l−1 to 5 Bq·l−1 for 241Pu.
Samples with higher activity concentrations than 5 Bq·l−1 can be measured if a dilution is performed.The higher is the concentration factor, the lower are the LD and LQ expressed in Bq/L of water. A preconcentration is especially useful for the quantification of 241Pu that is usually the less abundant isotope of plutonium.
The method described in this document is applicable in the event of an emergency situation.
Filtration of the test sample is necessary. The analysis of 239Pu, 240Pu, 241Pu and 237Np 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 performed separately on each phase or the total phase.
It is the user’s responsibility to ensure the validity of this test method for the water samples tested.
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