Viability of Rapid Unseparated Rare Earth Element Analyses by Isotope Dilution Multicollector Inductively Coupled Mass Spectrometry (ID-MC-ICP-MS)

Isotope dilution mass spectrometry is the analytical technique that yields the highest precision and accuracy for rare earth element (REE) concentrations. Samples can be analysed using either thermal ionization mass spectrometry (TIMS) or multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) However, both analytical platforms typically require the REEs to be separated into various 'cuts' to be analysed individually, a process that is extremely time consuming and expensive. This is necessary to minimize isobaric interferences from neighbouring REEs and polyatomic interferences from lighter REE oxides, as well as to fit the large mass range of the REEs into analytical runs. In an effort to dramatically increase sample throughput, we recently evaluated the potential to directly analyse unseparated REE samples using the Nu Plasma II MC-ICP-MS, an instrument equipped with 'zoom lens' ion optics and sixteen Faraday detectors that can be configured to measure all REE isotopes in three cycles of a single run (Figure 1). The instrument was tuned to minimize oxide production (~0.05% NdO/Nd with a sensitivity of 0.2 V ppb^-1), and all samples were internally corrected for mass fractionation using the ¹⁵³Eu/¹⁵¹Eu ratio, and online isobaric interference corrections for Ce, Nd and Yb were applied to the La, Ce and Lu values, respectively. Spike/sample isotopic ratios for Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb were measured accurately within 2% of accepted natural values in single analyses on various artificial mixed standards. La and Lu were 2.5% and 5-10% off from expected values due to more demanding interference corrections. This demonstrates the ability to produce REE patterns of comparable quality to traditional methods involving REE separation, with sample preparation and analytical times that are comparable to runs against external standards by single-collector ICP-MS.