US EPA

3425261 

Journal Article 

Chromatographic speciation of Cr(III)-species, inter-species equilibrium isotope fractionation and improved chemical purification strategies for high-precision isotope analysis 

Larsen, KK; Wielandt, D; Schiller, M; Bizzarro, M 

2016 

Yes 

Journal of Chromatography A
ISSN: 0021-9673
EISSN: 1873-3778 

1443 

162-174 

English 

27036208 

10.1016/j.chroma.2016.03.040 

WOS:000374362600020 

Chromatographic purification of chromium (Cr), which is required for high-precision isotope analysis, is complicated by the presence of multiple Cr-species with different effective charges in the acid digested sample aliquots. The differing ion exchange selectivity and sluggish reaction rates of these species can result in incomplete Cr recovery during chromatographic purification. Because of large mass-dependent inter-species isotope fractionation, incomplete recovery can affect the accuracy of high-precision Cr isotope analysis. Here, we demonstrate widely differing cation distribution coefficients of Cr(III)-species (Cr3+, CrCl2+ and CrCl2+) with equilibrium mass-dependent isotope fractionation spanning a range of similar to 1 parts per thousand/amu and consistent with theory. The heaviest isotopes partition into Cr3+, intermediates in CrCl2+ and the lightest in CrCl2+/CrCl3o. Thus, for a typical reported loss of similar to 25% Cr (in the form of Cr3+) through chromatographic purification, this translates into 185 ppm/amu offset in the stable Cr isotope ratio of the residual sample. Depending on the validity of the mass-bias correction during isotope analysis, this further results in artificial mass-independent effects in the mass-bias corrected Cr-53/Cr-52 (mu Cr-53* of 5.2 ppm) and Cr-54/Cr-52 (mu Cr-54* of 13.5 ppm) components used to infer chronometric and nucleosynthetic information in meteorites. To mitigate these fractionation effects, we developed strategic chemical sample pre-treatment procedures that ensure high and reproducible Cr recovery. This is achieved either through 1) effective promotion of Cr3+ by >5 days exposure to HNO3 -H2O2 solutions at room temperature, resulting in >similar to 98% Cr recovery for most types of sample matrices tested using a cationic chromatographic retention strategy, or 2) formation of Cr(III)-Cl complexes through exposure to concentrated HCl at high temperature (>120 degrees C) for several hours, resulting in >97.5% Cr recovery using a chromatographic elution strategy that takes advantage of the slow reaction kinetics of de-chlorination of Cr in dilute HCl at room temperature. These procedures significantly improve cation chromatographic purification of Cr over previous methods and allow for high-purity Cr isotope analysis with a total recovery of >95%. (C) 2016 Elsevier B.V. All rights reserved. 

Chromatographic purification; Isotope fractionation; Chromium speciation; High-precision isotope analysis; Chromium distribution coefficient