An improved chromatographic method for separation of Re and PGE mass fractions in organic-rich geological samples

Abstract

The Re–Os geochronometer has been widely applied for determining the depositional ages of organic-rich rocks and constraining the key time points of petroleum systems, and PGE can provide important information about the presence or absence of extraterrestrial materials in organic-rich sedimentary strata or the hydrocarbon source of a petroleum system. However, due to the extremely low mass fractions, precise determinations of Re–Os–PGE elemental and Os isotopic compositions in organic-rich geological samples (ORGS) remain challenging. In this study, we present an improved two-stage column protocol for the separation and purification of Re–PGE from the sample matrix and interfering elements for ORGS, after Os was separated by CCl₄ solvent extraction. In the first step, cation exchange resin (AG50W-X8, 200–400 mesh) was used to separate Re and PGE collectively from the sample matrix and primarily remove interfering elements (particularly Cd, Zr, and Hf). In the second step, LN resin was used to further remove the remaining interfering elements (e.g., Zr and Hf) and to separate Re from PGE (Ir–Ru–Pt–Pd). After these two steps of purification, the Re and PGE mass fractions were determined using MC-ICP-MS (multi-collector inductively coupled plasma-mass spectrometry) and Q-ICP-MS (quadrupole-inductively coupled plasma-mass spectrometry), respectively. To verify the method, three organic-rich reference materials (RMs) (Shales: USGS SBC-1 and USGS SGR-1b; Crude oil: NIST RM 8505) were determined using the procedure, and the analytical results are in good agreement with the literature data. This method is the first report to use LN resin to separate Re and PGE from each other, such that it is allowed to use MC-ICP-MS to measure Re with high precision for Re–Os dating purposes, while using ICP-MS for rapid measurements of PGE (Ir, Ru, Pt and Pd) simultaneously with in-run monitoring of interfering elements for PGE mass fraction determinations.