Determination of Molybdenum Isotopic Compositions in Eleven Geological Reference Materials by MC-ICP-MS Combined with BPHA Resin Separation

The 98/95 Mo values relative to NIST SRM 3134 and the molybdenum (Mo) mass fractions were determined for eleven rock and soil reference materials.Among them, GBW07109 (syenite), GBW07110 (trachyte), GBW07111 (granodiorite), GBW07113 (rhyolite), GBW07122 (amphibolite) and three soil reference materials GBW07401a, GBW07405a and GBW07407 were reported for the first time.A modified chemical separation method was employed, and isotopic measurements were conducted using MC-ICP-MS with a 97 Mo-100 Mo double spike for mass bias correction.Long-term reproducibility for NIST SRM 3134 was 0.06 (2SD, n = 790), while the seawater standard (NASS-7) processed alongside samples showed a long-term external reproducibility of 0.05 (2SD, n = 64) over 18 months.The 98/95 Mo values ranged from -0.54 to +0.01 for igneous rocks, -0.040.08for the metamorphic rock (amphibolite), and -0.28 to +0.22 for soil.Mo mass fractions varied from 0.07 to 3.92 g/g, with the relative standard deviation (RSD) of 0.05% to 5.30%, indicating minimal variability suitable for diverse lithologies, especially low Mo content (0.06-0.50 g/g).High organic content complicates purification and measurement, yet studies on Mo isotopic behavior during dry-ashing at varying temperatures remain limited.Therefore, to evaluate the behavior of Mo isotopic compositions and mass fractions by using the dry-ashing digestion method at various temperatures (550 C, 600 C, 650 C, 700 C), three soil reference materials with different total organic carbon (TOC) values and an oil shale SGR-1b (USGS) were selected.Dry-ashing digestion (550 C-700 C) was evaluated against highpressure bomb digestion (190 C), significant Mo loss occurred for SGR-1b (39.51%) and GBW07405a (14.57%) at 700 C, but the others was less than 10% at 550 C-650 C.No significant 98/95 Mo shifts were observed between methods.These results demonstrate that dry ashing at 550 C-650 C effectively removes organic matter without inducing isotopic fractionation, making it suitable for high-organic samples in Mo isotope and mass fraction analysis.