Determination of indium isotopic ratios of geostandards with different matrices by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS)

Abstract

Indium (In) is a critical metal in liquid-crystal displays and solar panels. The Earth's crust has low In concentration in the order of 10 ng g⁻¹, but it may be concentrated in sulphides by up to six orders of magnitude. Although In isotope studies may improve our understanding of the behaviour of In during different geological processes, such studies are limited for terrestrial samples. Here, we report a new method for In purification and high-precision isotope analysis for chemically different standards. A one-column method was used to separate In from the matrix with a recovery of 98% ± 2% (2SD). Indium concentrations and isotopic compositions were determined using the column eluate, and the results show large In isotope fractionation on desorbing In from the resin. Based on the mass balance, no significant In isotope fractionation was observed when In recovery is >93.8%. In terms of precision and accuracy of In isotopic compositions, the Ag-doping method was likely better than the standard–sample bracketing (SSB) method for correction of instrumental fractionation, with In uptake concentration down to 1 ng g⁻¹ during isotope analysis. To ensure the accuracy of isotope analyses, the concentration match between the sample and standard should be within ±30%. An elemental doping study indicated that Na, K, Te, Sb, Pb, and Cu have no impact on the accuracy of In isotope analysis within the designated range; however, Ca/In, Mg/In, Cd/In and Sn/In ratios of >5, >20, >0.001 and >0.1, respectively, in sample solutions have significant impacts on In isotope analyses. Four international standards (BCR-2 basalt; J-Zn-1 sulphide; NIST-2711a soil; and OU-3 granite) and three in-house igneous rocks, purified using the developed method, had negligible isobaric interferences and matrix effect during In isotope analysis, demonstrating that the method can be used for terrestrial samples with different compositions.