A new two-stage purification method for high-precision indium isotope measurement by MC-ICP-MS

Abstract

Conventional indium (In) separation methods based on anion-exchange resins typically involve multiple evaporation and medium-transfer steps or require large volumes of concentrated acids, leading to prolonged processing times and potential contamination risks. Herein, we present an efficient two-stage column separation method for the separation and purification of In from complex geological matrices using two self-made trioctylamine (N235) and tri-n-octylphosphine oxide (TOPO) resins, enabling high-precision In isotopic analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). In the first column, most matrix elements were efficiently removed in HBr medium, and the In fraction together with trace amounts of Mo and Sn were eluted using 0.2 M HCl, and were ready for a second column purification without any evaporation. Subsequently, the residual interference of Mo and Sn was quantitatively removed from the In fraction using the TOPO resin column. This method not only eliminates hazardous reagents such as hydrofluoric acid (HF) but also significantly reduces resin and acid consumption compared to previous methods, achieving high In recoveries (>95%) and low procedural blanks (<0.2 ng). For In isotopic analysis, a combination of standard-sample bracketing with an Sb external method was employed for instrumental mass bias correction. NIST SRM 3124a In standard solution, for the first time, was adopted as a reference with an external reproducibility of 0.07‰ (2SD, n = 233) over a year. The method was validated using nine diverse geological reference materials (RMs). The In isotopic compositions obtained for three internationally recognized RMs were in good agreement with previously reported literature values. For the six other RMs, In isotope data are reported here for the first time, and they exhibit excellent external reproducibility. Moreover, all purified samples showed negligible isobaric interferences and matrix effects during MC-ICP-MS analysis, confirming the method's suitability for challenging geological samples.