Precise determination of Sm, Nd concentrations and Nd isotopic compositions at the nanogram level in geological samples by thermal ionization mass spectrometry

Geology, Isotope analysis, Sensitivity, Sm, Nd.

In this paper, a high sensitivity method for measurements of Nd isotopes as NdO⁺, on a TIMS using a single W filament with TaF₅ as an ion emitter is presented. Although analyzing Nd isotopes as oxides (NdO⁺) is a well known technique, this is the first report to analyze Nd isotopic compositions as oxides using W filaments and the TaF₅ emitter. When 0.5-1 ng loads of a Neodymium isotopic reference reagent, JNdi-1, were measured using this method, the ion yields were found to be mostly in excess of 15% and could be as high as 32%. Internal precision on ¹⁴³Nd/¹⁴⁴Nd could be better than 10 ppm (2SE) for 1 ng JNdi-1 loads and better than 15 ppm (2SE) for 0.5 ng JNdi-1 loads; thirteen replicates of 0.5-1 ng JNdi-1 loads yielded a ¹⁴³Nd/¹⁴⁴Nd value of 0.512112 ± 0.000028 (2SD). Compared with the previously reported NdO⁺ measurement method using the Re (or W) filaments + Silica-gel + H₃PO₄ loading techniques, this method has advantages including higher sensitivity, a more stable ion beam, and no need for oxygen gas to be bled into the ion source chamber. Sm isotopes were analyzed as Sm⁺ using the W filaments and the TaF₅ emitter, and high sensitivity and good ion beam stability were also obtained. Several international rock reference materials, including an ultramafic rock reference material USGS PCC-1 that contains very low amounts of Sm and Nd, were analyzed with full column chemistry and the TaF₅ method, and the results of Sm, Nd concentrations and Nd isotopic data are in good agreement with the reported values. Combined with a highly efficient and low-blank column chemistry to separate Nd from Sm, Ce, and Pr, this method holds potential to analyze Sm, Nd concentrations and Nd isotopic compositions of highly depleted peridotites; very small aliquots of minerals such as garnets; extra-terrestrial materials of limited sample size; and environmental samples that contain very low quantities of Sm and Nd.