Accurate and precise determination of stable Cr isotope compositions in carbonates by double spike MC-ICP-MS

Abstract

Techniques for the separation of small quantities of Cr from carbonate material and for the analysis of stable Cr isotopes in carbonates by MC-ICP-MS are presented in this study. In comparison with previously published methods, we have developed a one-step Cr separation procedure that is relatively simple, and has a low blank (0.12-0.20 ng). Moreover, careful optimisation of the desolvating sample introduction system allows a significant increase in the sensitivity of our MC-ICP-MS technique compared to previous studies. Instrumental mass bias effects and fractionation of Cr isotopes during Cr separation are corrected using a carefully optimised 50 Cr-54 Cr double-spike method. Novel numerical simulations demonstrate that the effects of potential isobaric interferences from Ti, Fe and V are negligible, even if they are isotopically fractionated. Small deviations in the d 53 Cr value of the NBS 979 standard between different analytical sessions are due to small deviations from exponential mass fractionation behaviour. The long-term reproducibility of d 53 Cr for a spiked NBS 979 Cr isotope reference material is AE0.031& (2 S.D., n ¼ 147). Analyses of carbonates reveal that they have d 53 Cr values of 0.747 to 1.994&, distinctly heavier than continental crust and the terrestrial mantle. The carbonates record Cr isotopic fractionation that may be used to understand redox reactions in the oceans. Although this study focuses on carbonate samples, our mass spectrometry technique can be applied to the analysis of any samples with low levels of Cr, including river waters and seawater.

The marine biogeochemistry of chromium isotopes

“…The average Cr recovery of our method (65%) is on par with that of (Chris Holmden, personal communication). We were unable to ascertain the total recovery of the other Cr seawater isotope method from the literature , but it appears to be < 80% (Bonnand et al, 2011). The total procedural Cr blank of our method (0.017 nmol) is of the same magnitude than that of and significantly lower than that of (up to 0.93 nmol).…”

A practical guide for the design and implementation of the double-spike technique for precise determination of molybdenum isotope compositions of environmental samples

“…Of these three methods, the double-spike approach provides the most robust correction for instrumental and laboratory mass fractionation [4], and protocols are now in place for the isotopic analysis of several elements such as Ba, Ca, Cd, Hg, Ge, Mo, Ni, Pb, Pt, Se, U, and Zn [e.g., [6][7][8][9][10][11][12][13][14][15][16][17][18]. However, a more widespread adoption of double-spike protocols has been hindered by perceived difficulties in proper double-spike calibration, mathematical inversion [19], and fear of memory effects [20].…”

A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

“…This solution was added to the extraction columns loaded with 2 mL of pre-cleaned Dowex AG50W-X8 cation resin (200-400 mesh). The extraction procedure principally adhered to that published by Bonnand et al (2011) and Trinquier et al (2008) with only small modifications. The final Cr-bearing liquid cut was dried down at 130 • C, ready to be loaded for Cr isotopic analysis on the thermal ionization mass spectrometer.…”