Determination of extremely low 236U/238U isotope ratios in environmental samples by sector-field inductively coupled plasma mass spectrometry using high-efficiency sample introduction

A method by inductively coupled plasma mass spectrometry (ICP-MS) was developed which allows the measurement of 236U at concentration ranges down to 3 × 10−14 g g−1 and extremely low 236U/238U isotope ratios in soil samples of 10−7. By using the high-efficiency solution introduction system APEX in connection with a sector-field ICP-MS a sensitivity of more than 5000 counts fg−1 uranium was achieved. The use of an aerosol desolvating unit reduced the formation rate of uranium hydride ions UH+/U+ down to a level of 10−6. An abundance sensitivity of 3 × 10−7 was observed for 236U/238U isotope ratio measurements at mass resolution 4000. The detection limit for 236U and the lowest detectable 236U/238U isotope ratio were improved by more than two orders of magnitude compared with corresponding values by alpha spectrometry. Determination of uranium in soil samples collected in the vicinity of Chernobyl nuclear power plant (NPP) resulted in that the 236U/238U isotope ratio is a much more sensitive and accurate marker for environmental contamination by spent uranium in comparison to the 235U/238U isotope ratio. The ICP-MS technique allowed for the first time detection of irradiated uranium in soil samples even at distances more than 200 km to the north of Chernobyl NPP (Mogilev region). The concentration of 236U in the upper 0–10 cm soil layers varied from 2 × 10−9 g g−1 within radioactive spots close to the Chernobyl NPP to 3 × 10−13 g g−1 on a sampling site located by >200 km from Chernobyl.