Improved abundance sensitivity in MC-ICP-MS for determination of 236U/238U isotope ratios in the 10−7 to 10−8 range

Abstract

A multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS)—Nu Plasma HR—equipped with an ion deceleration filter (‘high abundance sensitivity channel’) was optimized for the determination of extremely low uranium isotope ratios and applied for the first time for the analysis of ²³⁶U/²³⁸U isotope ratios in the 10⁻⁸ to 10⁻⁷ range in isotopic reference material IRMM-184, as well as in two unknown samples obtained in the frame of a round-robin exercise. The application of the ion deceleration lens system in the Nu Plasma ICP-MS allowed a reduction of peak tailing from ²³⁸U⁺ ions at m/z = 236 down to 3 × 10⁻⁹, whereas the absolute sensitivity for uranium was reduced by only about 30%. Thus, abundance sensitivity was improved by almost two orders of magnitude and the minimum determinable ²³⁶U/²³⁸U ratio was improved by more than one order of magnitude compared with conventional sector-field ICP-MS or TIMS. However, interference by ²³⁵U¹H⁺ ions deteriorated the accuracy and increased the measurement uncertainty up to 48%, in particular in the case of samples enriched in ²³⁵U. Deviation of the measured value from the certified ones varied from −24% to 17.7% but was within the measurement uncertainty.