Development of a simple and efficient two-step microwave-assisted digestion method for the determination of REEs, HFSEs and other elements in granite samples by ICP-OES

Abstract

An efficient two-step microwave-assisted digestion (MWD) method using a dilute acid mixture for the total dissolution of granite materials for the quantitative determination of rare earth elements (REEs) and other elements including high field strength elements (HFSEs) by inductively coupled plasma optical emission spectrometry (ICP-OES) was developed. In the first step, 8 mL of an acid mixture containing 20% (v/v) HF + 25% (v/v) HCl + 10% (v/v) HNO₃ was added to the ∼0.25 g of granite sample and the MWD procedure was carried out. After completion of the first step of the MWD process (MWD-1), the sample mixture was transferred to a graphite bottom Teflon beaker and evaporated on a hot plate at 150 °C to incipient dryness for the removal of the silica matrix (evaporation step (ES)). The residual sample matrix was re-dissolved in 8 mL of an acid mixture containing 20% (v/v) HCl + 20% (v/v) HNO₃ and further subjected to the second step of the MWD process (MWD-2) for its total decomposition. The complete sample treatment protocol [MWD-1 (1.5 h) + ES (1 h) + MWD-2 (1.5 h)] requires only about 5 h, including cooling steps. The final sample digests were very clear with no fluoride precipitates indicating complete decomposition of different phases present in the granite material. After suitable dilution, the final and clear sample digests were analysed for REEs (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb, Sc and Y), HFSEs (Ti, Zr, Nb, Hf and Th) and other elements (Al, Ca, Fe, Sr and Ba) by ICP-OES. Critical experimental parameters related to MWD processes were optimized for achieving quantitative recovery (>95%) for most of the considered analytes. The accuracy of the method was validated through the analysis of two certified reference materials SARM-1 and NCS DC 73376. The optimized approach (MWD-1 + ES + MWD-2) was subsequently applied to multiple granite samples collected from different geological origins. These investigations clearly demonstrate that the proposed sample preparation method using a dilute acid mixture in conjunction with ICP-OES has great potential for the quantitative determination of REEs, HFSEs and other elements in large batches of granite materials, generated in mineral exploration studies.