Application of MARS-5 and UltraWAVE microwave systems to the digestion of silicate rocks followed by ICP-MS analysis

Abstract

Two microwave systems MARS-5 and UltraWAVE are compared in the efficiency with regard to the digestion of silicate rocks with subsequent determination of 32 elements (Sc, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Nb, Ta, Cs, Ba, 14 REE, Hf, Ta, Th, U) by inductively coupled plasma mass spectrometry (ICP-MS). The development of digestion methods and validation of the obtained results has been carried out using international reference materials — basalts BHVO-2 and BCR-2, serpentinite UB-N and peridotite JP-1. Microwave digestion included a two-stage treatment of samples with a mixture of concentrated acids HF, HNO3, HCl in MARS-5 (T = 190°C, P = 20 bar) and UltraWAVE (T = 240°C, P = 80 bar) with distillation of excess fluorides in the form of SiF4 between microwave digestion stages. The determination of concentrations in the obtained solutions was carried out on a high-resolution mass spectrometer ELEMENT in low and medium resolution according to external calibration with the internal standard (In), taking into account the acid composition of the analyzed solutions. The detection limits of the analytes after acid digestion in MARS-5 and UltraWAVE are comparable and provide the determination of all specified elements, except for Ta in JP-1. The use of the developed sample preparation procedure in MARS-5 ensures complete decomposition of BHVO-2, BCR-2, UB-N followed by ICP-MS determination of 32 specified elements in the obtained solutions without additional preconcentration steps. The relative standard deviations for the determined elements are 2 – 9% for the reference materials BHVO-2, BCR-2, 3 – 12% for UB-N with an increase to 16 – 25% (Nb, Ta) due to the approach to the detection limit. The more efficient microwave digestion in UltraWAVE compared to MARS-5 was proved by the complete decomposition of JP-1 with the transfer of all the elements, including Cr, to the solution.