High precision measurements of gallium isotopic compositions in geological materials by MC-ICP-MS

Abstract

An analytical protocol for the accurate and precise determination of the gallium isotope ratio in geological materials is presented for the first time by using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Separation of Ga from natural sample matrices was achieved by using a three-column ion-exchange chromatograph with one anion exchange AG MP-1M column and two cation exchange AG 50W-X8 columns. This approach provides an efficient purification with a low blank and high yield of Ga from an excess amount of Fe existing in geological samples. The instrument mass bias was monitored and corrected by using a model of standard-sample bracketing with internal normalization, and copper was used as an internal standard which was added to both the sample and standard solutions. The long-term external reproducibility of δ^71/69Ga obtained is 0.04‰ (2SD). Ga isotopic compositions of geological reference materials including basalt, andesite, rhyolite, granodiorite, soil, sediment, carbonatite and shale were measured using the proposed approach. The ⁷¹Ga/⁶⁹Ga ratio in these geological reference materials spanned over a rather narrow range between 0.74‰ and 0.90‰, relative to NIST SRM 994.