Determination of caesium and its isotopic composition in nuclear samples using isotope dilution-ion chromatography-inductively coupled plasma mass spectrometry

As the natural isotopes of Ba give isobaric interferences on the radioactive isotopes of Cs at nominal masses of 134, 135 and 137, a chemical separation of Cs from Ba has been necessary for the determination of the isotopic composition of Cs by mass spectrometric techniques in highly active nuclear wastes (HAW), dissolved spent nuclear fuels or radioactively contaminated environmental samples. Ion chromatography (IC), which allows Cs and Ba to be chemically separated according to their different cationic charge, was coupled to an ICP-MS instrument and the chemical separation was performed on-line and followed directly by mass spectrometry. Three separation schemes were compared with respect to chromatographic resolution, accuracy and precision in irradiated spent fuel samples. The mass discrimination Factors for the radioactive Cs isotopes were calculated by using a solution of natural Ba for the different chromatographic processes. They were found to be less than -2.5%. The results obtained by IC-ICP-MS were compared with those obtained by gamma-spectrometry and with simulation calculations based on the KORIGEN code. The method using a CS5 cation-exchanger column and 1 M HNO3 as eluent gave a detection limit of 16 pg g(-1) for total Cs with a precision of 2.5% at a concentration level of 100 ppb (n = 7). Under the same chromatographic conditions, the accuracy of the ratio Cs-134/Cs-137, calculated considering the gamma-spectrometry measurements, was 2.5%.