Cl, Br and I measurements in rock samples by ICP-MS/MS using reaction gases

Abstract

Analysis of chlorine (Cl), bromine (Br) and iodine (I) in poorly concentrated geological materials remains a major analytical challenge. Analysis of halogens extracted in solution using single quadrupole ICP-MS is highly sensitive but suffers from numerous mass interferences. Experimental determination of species potentially interfering with halogens when analyzing silicates shows that mass interferences with ³⁵Cl are low (<1 % of the ³⁵Cl signal) but that ³⁷Cl signal is completely hidden by ¹⁹F- oxides and ¹H³⁶Ar signals. Mass interferences with Br isotopes signals may be up to 20 times higher in Br-poor, K-rich or REE-rich samples: they are mainly due to [⁴⁰Ar³⁹K]⁺ and [¹⁵⁸Gd]⁺⁺ for ⁷⁹Br and to [¹⁶²Dy]⁺⁺ for ⁸¹Br. Due to the low abundance of ¹²⁷I, mass interferences are always significant and mainly due to argides and chlorides: [³⁶Ar⁹¹Zr]⁺, [³⁸Ar⁸⁹Y]⁺, [⁴⁰Ar⁸⁷Rb]⁺, [⁴⁰Ar⁸⁷Sr]⁺, [³⁵Cl⁹²Zr]⁺, [³⁷Cl⁹⁰Zr]⁺. In addition, matrix effects combined to solvent molarity (NH₄OH or HNO₃) may significantly affect halogens ionisation leading to possible systematic bias in halogens analysis. The use of a triple quadrupole mass spectrometer (ICP-MS/MS) with H₂ and N₂O reactions/collision gases allows the reduction of all interferences and matrix effects to values lower than analytical errors. ³⁵Cl and ³⁷Cl are measured through the reactant species ClH₂⁺ in H₂ reaction mode, and ⁷⁹Br rand ⁸¹Br are measured through BrO⁺ species in N₂O reaction mode. ¹²⁷I is measured in N₂O collision mode that slightly reduces argides and chlorides species and significantly increases the ¹²⁷I signal by collision focusing.

Sample digestion in NH₄HF₂ and dissolution of the residue in NH₄OH (He et al., 2019) allow a simple and complete recovery of all halogens. Combined with ICP-MS/MS and gas reactions, the technique improves the precision and detection limits of halogen measurements by an order-of-magnitude compared to the currently reported performances of single quadrupole ICP-MS measurements. A set of international standard reference materials (ISRM) that cover a large range of halogen compositions are analysed: basalts BIR-1, BCR-1, BHVO-2, andesites AGV-1 and AGV-2, rhyolites RGM-1, ATHO and JR-1, minerals, marine sediment, soil, copper deposit and NIST-610 and -612 glass standards.