A practical guide for the design and implementation of the double-spike technique for precise determination of molybdenum isotope compositions of environmental samples
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- Skierszkan, E.K., Amini, M. & Weis, D. Anal Bioanal Chem (2015) 407: 1925. doi:10.1007/s00216-014-8448-6
The isotopic double-spike method allows for the determination of stable isotope ratios by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) with accuracy and precision in the range of ∼0.02 ‰ amu−1, but its adoption has been hindered by the perceived difficulties in double-spike calibration and implementation. To facilitate the implementation of the double-spike approach, an explanation of the calibration and validation of a 97Mo-100Mo double-spike protocol is given in more detail than has been presented elsewhere. The long-term external standard reproducibility is 0.05 ‰ on δ98/95Mo measurements of standards. δ98/95Mo values for seawater and U.S. Geological Survey (USGS) reference materials SDO-1 and BCR-2 measured in this study are 2.13 ± 0.04 ‰ (2 SD, n = 3), 0.79 ± 0.05 ‰ (2 SD, n = 11), and −0.04 ± 0.10 ‰ (2 SD, n = 3) relative to the NIST-SRM-3134. The double-spike method corrects for laboratory and instrumental fractionation which are not accounted for using other mass bias correction methods. Spike/sample molar ratios between 0.4 and 0.8 provide accurate isotope measurements; outside of this range, isotope measurements are inaccurate but corrections are possible when standards and samples are spiked at a similar ratio.