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

Abstract

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 (Cr(3+), CrCl(2+) and CrCl2(+)) with equilibrium mass-dependent isotope fractionation spanning a range of ∼1‰/amu and consistent with theory. The heaviest isotopes partition into Cr(3+), intermediates in CrCl(2+) and the lightest in CrCl2(+)/CrCl3°. Thus, for a typical reported loss of ∼25% Cr (in the form of Cr(3+)) 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 (53)Cr/(52)Cr (μ(53)Cr* of 5.2 ppm) and (54)Cr/(52)Cr (μ(54)Cr* 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 Cr(3+) by >5 days exposure to HNO3H2O2 solutions at room temperature, resulting in >∼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 °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%.

High‐Precision Measurement of Stable Cr Isotopes in Geological Reference Materials by a Double‐Spike TIMS Method

“…Prior to launching the Cr column, the samples were diluted with HPW to a total volume of 1.2 ml 1 mol l −1 HCl solution and then loaded carefully onto the resin beds. Chromium in the samples formed as Cr(III)‐Cl complexes in concentrated HCl at 130 °C and was further eluted with 3.5 ml 1 mol l −1 HCl (Larsen et al ). Then, sub‐purified Cr samples were re‐digested with 20 μl concentrated HNO 3 and diluted with 2 ml HPW (~ 0.16 mol l −1 HNO 3 ) before loading on column 2.…”

Multi-element ion-exchange chromatography and high-precision MC-ICP-MS isotope analysis of Mg and Ti from sub-mm-sized meteorite inclusions

“…1. It takes advantage of novel sample pretreatment procedures to ensure a high Cr recovery (>95%) 22 and is further rened from a Mg purication protocol for larger samples ($100 mg Mg), known to provide high-purity Mg for high-precision isotope analysis by MC-ICP-MS. 23 Although applicable to most types of inorganic sample matrices, the purication method described here is specically tailored for samples with highly anomalous isotope compositions whose measured isotope composition may be compromised by minute analyte impurity contributions. The protocol is well suited for sample matrices with particularly high amounts of Mg, Al, Ca and Fe, such as CAIs and chondrules.…”

“…The extremely sluggish kinetics of Cr speciation in acidic media at room temperature and the large differences in the selectivity of a cation exchanger for various Cr(III) species are known to be problematic for high Cr recovery through chromatographic Cr purication. 22 Thus, to speciate >97% of the Cr into the Cr(H 2 O) 6 3+ form characterized by a high affinity for the cation resin, we used a sample pre-treatment procedure involving dissolution in 2 M HNO 3 followed by dilution and equilibration in 0. Fig.…”

“…This time we employed a sample pretreatment procedure which promotes the formation of neutral, mono-and divalent Cr(III)-Cl complexes with low cation exchanger selectivities, thereby allowing for efficient elution of Cr (>97.5%) from the column. 22 This speciation was ensured by sample exposure to 10 M HCl on a hotplate at 130 C for $1 day.…”

Heterogeneity and incorporation of chromium isotopes in recent marine molluscs (Mytilus)

“…Here, it is important to note that complexation with organic matter (OM) may stabilise Cr(III) in solution (Sander & Koschinsky, ; Sander and Koschinsky, ) and can contribute up to 90% of total dissolved Cr in sea water (Connelly, Statham, & Knap, ). Also, in contrast to the typical redox‐dependent Cr isotope fractionation, redox‐independent Cr isotope fractionation occurs during complexation of Cr with ligands typically present in natural systems, such as organic acids, siderophores or Cl ‐ (Larsen, Wielandt, Schiller, & Bizzarro, ; Saad, Wang, Planavsky, Reinhard, & Tang, ). Large amounts of OM favour the reduction of Cr(VI) to Cr(III), for example, as Cr(OH) 2 + , which is the stable Cr(III) species at a slightly lower pH‐Eh range of sea water (Connelly et al., ; Cranston & Murray, ).…”