Thermodynamic Study of Ga Extraction for Trace Element Analysis by ICP-MS

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) has a detection limit of sub-parts-per-trillion (ppt) level. However, it was found the detection limit increased to ~10 ppb for the analysis of impurities in ultra-pure Ga of 7 N (99.99999%) and 8 N (99.999999%) by ICP-MS due to matrix-induced interference. By extracting Ga during sample preparation, matrix-induced interference can be reduced and a lower detection limit can be achieved in the ICP-MS measurement. The dextran-based resin Sephadex G-25 can chemically separate the impurity Ge from Ga. The hydroxide complexes of Ga and Ge are adsorbed on Sephadex G-25 and desorbed into acid depending on pH. Viability of Ge separation from Ga by pH change was evaluated by thermodynamic calculation of the hydrolysis reaction of Ga and Ge. The distributions of Ga-hydroxide species and Ge-hydroxide species in 0.64 M HNO3 were derived by numerical calculation for thermodynamic equilibrium. The effect of presence of medium and ionic strength in aqueous solutions on hydrolysis reaction was evaluated. From the derived speciation diagram for Ga and Ge hydrolysis reactions, the optimal pH range to separate impurity Ge from Ga for ICP-MS sample preparation was investigated.