Christopher Siebert

Determination of mass-dependent variations in nickel isotope compositions using double spiking and MC-ICPMS

We present a new technique for the accurate and precise determination of mass-dependent variations in nickel isotope compositions in geological materials. Our method involves an ion-exchange procedure comprising three columns and utilising the ability of Ni to form strong complexes with both ammonia and dimethylglyoxime. The separation procedure is independent of sample pH and works even for samples with large matrix to analyte ratios. Processed Ni solutions are free of matrix elements and direct isobars of Ni, and the yield is normally 85–95%. The purified Ni solutions were analysed using a Nu Plasma, multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), where instrumental mass fractionation—together with potential isotopic fractionation during chemical separation due to incomplete yield—was corrected for by a double-spike technique, where samples were spiked prior to column chemistry. Tests performed on both mixtures of synthetic and natural terrestrial standards demonstrates that the method is accurate. Replicate measurements of USGS reference materials (peridotite PCC-1, basalt BHVO-2, and shale SCo-1) yield a long-term external reproducibility (2 s.d.) of typically ± 0.07‰, ± 0.1‰, and ± 0.14‰ for 60Ni/58Ni, 61Ni/58Ni, and 62Ni/58Ni respectively.

Determination of mass-dependent variations in tungsten stable isotope compositions of geological reference materials by double-spike and MC-ICPMS

We present a double-spike technique for precise determination of mass-dependent fractionation of tungsten (W) stable isotopes. Instrumental mass bias effects and isotope fractionation during W separation are corrected for by means of a 180W–183W double-spike added prior to sample dissolution. The separation of W from the matrix is achieved via three-step anion-exchange chromatography. Tungsten stable isotopic compositions were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Results are expressed relative to the NIST SRM 3136 W standard as the permil difference in 186W/184W (δ186W). The external reproducibility of the NIST SRM 3163 W standard is ±0.05‰ with an average composition of −0.01‰ on 186W/184W (2s.d., n = 171). Tungsten stable isotope compositions for USGS standard reference materials are as follows: AGV-2 (andesite; δ186W = 0.18 ± 0.05‰, 2s.d., n = 6), SDC-1 (mica schist; δ186W = 0.36 ± 0.05‰, 2s.d., n = 6), SDO-1 (shale; δ186W = 0.26 ± 0.06‰, 2s.d., n = 6), and manganese nodules NOD-A-1 (δ186W = 0.07 ± 0.05‰, 2s.d., n = 10) and NOD-P-1 (δ186W = 0.31 ± 0.04‰, 2s.d., n = 7). The long-term external reproducibility (2s.d.) for these rock standards is ±0.05‰.