Mass-independent Isotope Fractionation of Chromium in Ligand Exchange

Introduction: Recently, the classic theory of stable isotope fractionation of the Bigeleisen-Mayer equation [1] has been expanded by the original author to include the mass-independent term named the nuclear field shift effect [2]. The improved theory successfully explained the observed non-linear isotope effect on U in a chemical exchange system [2]. Since then, researchers have applied the new Bigeleisen theory to explain other unusual isotope effects (see, e.g. [3] and references therein). How such mass independent isotopic fractionation would affect interpretations of isotopic anomalies observed in the primitive meteorites were discussed subsequently by [4,5]. The matter clearly requires further attention. In this context, we conducted experiments to verify the magnitude of the mass-independent isotope fractionation induced by laboratory experiments. We present experimental evidence of the mass-independent isotope fractionation of Cr in a ligand exchange reaction and a redox reaction (electron exchange reaction). We chose Cr as our target element for the following reason: first, because of its magic number neutrons N=28, Cr has the smallest nuclear charge radius [6] among the stable chromium isotopes, Cr, Cr, Cr, and Cr (see Fig. 1). This pronounced nuclear property of Cr makes it a most favorable element in the periodic table for identification of non-linear isotopic effect in chemical systems; second, Cr plays an increasingly important role in modern isotope cosmochemistry and geochemistry. Mn-Cr system (T1/2=3.7 Myr) has been successfully applied as a high resolution chronometer in the early solar system [7-10]. Cr anomalies observed in primitive meteorites and planetary bodies [9,10] are now accepted as widespread nebula wide phenomena. There are some unresolved issues in Cr isotope cosmochemistry, however. For example Cr anomalies were observed by one laboratory [9] but not by the other [7] in the same materials thought to come from Vesta. Whether Cr anomalies reflect heterogeneous distribution of Mn in the solar nebula, Mn/Cr fractionation, or Cr heterogeneity is still to be understood [7]. How chemical mass-independent fractionation a la Bigeleisen may affect Cr isotope systematics is an important question to be addressed. Experimental: Experiment (a) Liquid-liquid extraction with macrocyclic compounds is known to be effective to show larger chemical isotope fractionation [3]. Hydrated chromium trichloride (CrCl3•H2O, 99.99% purity) was dissolved in HCl to produce 0.36 mol/L (M) Cr(III) solutions at various strength of HCl. The organic phase was 0.1 M DC18C6 (dicyclohexano-18-crown-6) in 1,2-dichloroethane. Aqueous and organic solutions were contacted (297±0.5 K) in a glass vial, and after the equilibrium, the two phases were separated by centrifugation. Cr in the organic phase was back extracted into pure H2O and was analyzed by ICP-AES to determine the distribution ratio (D~10). The leftover Cr in the back-extracted water solution was further purified by employing a cationexchange chromatographic separation. A solution containing 400 ppb Cr in 0.1 M HNO3 was prepared for mass spectrometric analysis. Isotopic ratios of Cr/Cr and Cr/Cr were analyzed with a Nu Plasma HR-MC-ICP-MS at UC Davis coupled with a desolvating nebulizer, Nu DSN-100. The instrumental mass bias was controlled by bracketing each sample with the unprocessed Cr solution under identical condition. Isobaric interferences with Ti and V were below the detection limit of the mass spectrometer.