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

With recent advancements in analytical methods of Mo isotopes, the delta Mo-98/95 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 (R) 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 (N-2), high sensitivity measurement (Mo-95 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 parts per thousand, 2SD) can be determined at a concentration of 3-10 ng level using a Mo-97-Mo-100 double spike. NIST 3134 (0.00 +/- 0.05 parts per thousand), SGR-1b (0.41 +/- 0.05 parts per thousand), NOD-P-1 (-0.86 +/- 0.03 parts per thousand), IAPSO (2.07 +/- 0.04 parts per thousand), 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 delta Mo-98/95 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 delta Mo-98/95 of CLB-1 (1.25 +/- 0.03 parts per thousand), JDO-1 (0.50 +/- 0.02 parts per thousand), GSV-2 (0.47 +/- 0.02 parts per thousand), and other 38 RMs with relatively higher and lower Mo concentrations are reported for the first time. The total average delta Mo-98/95 ratio of 8 soils and 18 sediments is 0.003 +/- 0.277 parts per thousand (1SD, n = 26), slightly lighter than that of the upper continent crust (0.05-0.15 parts per thousand). The delta Mo-98/95 ratios (0.23-0.79 parts per thousand, 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 delta Mo-98/95 ratios of carbonates are much lower than that in seawater.