Determination of Cr in certified reference material HISS-1, marine sediment, by cold plasma isotope dilution ICP-MS and INAA: comparison of microwave versus closed (Carius) tube digestion

Abstract

The recovery of Cr from National Research Council Canada (NRCC) certified reference material HISS-1 sediment was investigated by non-destructive instrumental neutron activation analysis (INAA) and isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) after two different chemical treatments: (1) microwave dissolution using HNO₃, HF, and HClO₄, followed by reflux with concentrated HClO₄, and (2) a closed system method using sealed Carius tube digestion with HNO₃ and HCl at high temperature (240 °C) and high pressure (1 × 10⁷ Pa, ∼100 atm). The first chemical treatment gave recoveries of only ∼50% in agreement with observations of Yang et al. of NRCC. Samples digested in Carius tubes gave recoveries that ranged from 79 to 110% which may reflect heterogeneity in the samples rather than differing recoveries. This is supported by the excellent agreement of the mean value determined by INAA of 31.4 ± 4.5 µg g⁻¹, and the mean value of 28.1 ± 3.9 µg g⁻¹ by Carius tube digestion and the observation that these standard deviations are essentially equal and much larger than the measurement precisions. The results from this study are also in excellent agreement with the certified value of 30.0 ± 6.8 µg g⁻¹. This preliminary study demonstrates that chemical treatment of this sediment with HNO₃ and HCl acids in closed system Carius tubes is an efficient and capable preparation method for the determination of Cr in silicate materials which typically contain refractory Cr-bearing minerals. This study also demonstrates the unique advantage of using two independent methods for the determination of Cr in a complex matrix which has a high potential for low chemical recovery.