Validating the high-precision measurement of Mo isotopes at the 5 ng level using double spike MC-ICP-MS

Abstract

With recent advancements in analytical methods of Mo isotopes, the δ^98/95Mo ratios of most geological and environmental samples can be determined. Still, it remains a challenge to obtain high-precision Mo isotope data for low-Mo samples with complex matrices such as igneous and plant samples. Here, we present an improved Mo purification and cleaning resin scheme for reducing the total procedure blank to ≤0.16 ng using common Muromac®1X8(AG1-X8) anion and AG50-X8 cation resins. By an improved Aridus II with ice chamber in sample introduction system (SIS) and adding nitrogen (N₂), high sensitivity measurement (⁹⁵Mo signal intensity: 200–330 V ppm⁻¹) of Mo isotopes was achieved on Neptune Plus MC-ICP-MS. Thus, the sample size containing 30–60 ng Mo is sufficient to be purified, and isotope measurement with high-precision (≤0.06‰, 2SD) can be determined at a concentration of 3–10 ng level using a ⁹⁷Mo–¹⁰⁰Mo double spike. NIST 3134 (0.00 ± 0.05‰), SGR-1b (0.41 ± 0.05‰), NOD-P-1 (−0.86 ± 0.03‰), IAPSO (2.07 ± 0.04‰), and other international reference materials (RMs) were analyzed at 3, 5, 10, and 20 ng mL⁻¹ levels to be in excellent agreement with the published δ^98/95Mo values, demonstrating that good accuracy and precision of Mo isotope analysis can be achieved with an injecting sample size as small as 5 ng Mo. Our improved method can be applied to various geological and environmental samples. The δ^98/95Mo of CLB-1 (1.25 ± 0.03‰), JDO-1 (0.50 ± 0.02‰), GSV-2 (0.47 ± 0.02‰), and other 38 RMs with relatively higher and lower Mo concentrations are reported for the first time. The total average δ^98/95Mo ratio of 8 soils and 18 sediments is 0.003 ± 0.277‰ (1SD, n = 26), slightly lighter than that of the upper continent crust (0.05–0.15‰). The δ^98/95Mo ratios (0.23–0.79‰, n = 8) of plant and animal origins from the land show they are enriched in heavy isotopes relative to the bulk silicate earth (BSE). The δ^98/95Mo ratios of carbonates are much lower than that in seawater.