Single particle - MC-ICP-MS for isotopic analysis of uranium particles

Single particle - multi-collector - inductively coupled plasma - mass spectrometry (SP-MC-ICP-MS) was employed to measure a suspension of 1 µm U₃O₈ particles (∼1.3 pg total U/particle) to determine their individual isotopic compositions of ²³⁴U/²³⁸U, ²³⁵U/²³⁸U, and ²³⁶U/²³⁸U. The effects of different detector combinations for ²³⁵U and ²³⁸U, including secondary electron multipliers (SEM) and Faraday detectors (10¹¹ and 10¹³ Ω amplifiers), were explored for accuracy and precision optimization on the observed ²³⁵U/²³⁸U. The minor isotopic ratios (i.e., ²³⁴U/²³⁸U and ²³⁶U/²³⁸U) were analyzed such that the ²³⁴U and ²³⁶U were monitored on SEM detectors and the ²³⁸U was monitored on a Faraday (10¹¹ Ω) detector. Various integration times (5, 10, 25, and 50 ms) were investigated in all detector configurations to gain a better understanding of their impact on sensitivity, accuracy, and precision. For ²³⁵U/²³⁸U ratios of 1 µm U₃O₈ particles, a dual Faraday detector measurement with 10¹¹ Ω was the optimal choice; measurement of 1021 particles yielded an average ²³⁵U/²³⁸U ratio of 0.00170 (14), a -1.8% relative difference (% RD) from the reference value. The minor isotopic compositions were determined to be 0.0000070 (14) and 0.0000758 (48) for the ²³⁴U/²³⁸U and ²³⁶U/²³⁸U, respectively. These measurements correspond to <8% and <1% RD from their reference value for the ²³⁴U/²³⁸U and ²³⁶U/²³⁸U, respectively. SP-MC-ICP-MS was also able to provide insight into measurement sensitivity. In these individual particles, merely 15 and 165 atto-grams (ag) of ²³⁴U and ²³⁶U were present (calculated). Initial limits of detection for SP-MC-ICP-MS were determined to be ∼1.0 ag (when measured via SEM detectors). This valuable approach is applicable to areas including nuclear forensics, nuclear safeguards, and geochemical analysis, which require high-precision measurements of uranium within micron-sized particles.