Calcium isotope fractionation in liquid chromatography with crown ether resins

Abstract

Breakthrough mode liquid chromatography was employed to study calcium isotope fractionation. Highly porous silica beads, the inner pores of which were embedded with a benzo-18-crown-6 ether resin or a benzo-15-crown-5 ether resin, were used as column packing material. For both the resins, enrichment of heavier isotopes of calcium was observed in the frontal part of their respective calcium chromatograms. The values of the isotope fractionation coefficient were on the order of 10 73 and 10 74for the benzo-18-crown-6 ether and benzo-15-crown-5 ether resins, respectively. The observed isotope fractionation coefficient was dependent on the concentration of hydrochloric acid in the calcium feed solution; a higher hydrochloric acid concentration resulted in a smaller fractionation coefficient value. The present calcium isotope effects were most probably mass dependent, indicating they came from isotope effects based on molecular vibration. Molecular orbital calculations supported the present experimental results in a qualitative fashion.

Calcium isotope fractionation in liquid chromatography with benzo-18-crown-6 resin in aqueous hydrobromic acid medium

“…As model Ca species in the crown ether phase, we considered clusters consisting of a B18C6 molecule, a Ca 2 + ion and two Br − ions with or without up to two HBr molecules ([B18C6 + Ca 2 + + 2Br − ], [B18C6 + Ca 2 + + 2Br − + HBr] and [B18C6 + Ca 2 + + 2Br − + 2HBr]) and some others in which H 2 O molecules were also included. Inclusion of HBr molecules in the crown ether phase was inferred from the results of our previous paper [19] and from the results of Sr isotope fractionation in a solvent extraction system reported by Shibahara et al [23]. The existence of organic networks of the resin in the crown ether phase was ignored.…”

“…Seven chromatographic experiments in the breakthrough mode were carried out. The experimental conditions and results other than isotopic data are summarized in Table 1 together with the one in the HCl system from our previous paper [19] for comparison. The quantity V 1/2 is the effluent volume at which the Ca 2 + ion concentration becomes equal to a half of its value in the feed solution, c 0 , after the breakthrough point.…”

“…The chromatograms including the one from the previous paper [19] are depicted in Figure 1. The findings obtained from Table 1 and Figure 1 may be summarized as follows:…”

“…The adsorption of Ca 2 + ion from aqueous HBr solution onto the resin is thus expected to be slightly exothermic. (3) Runs CB-4 ( in Figure 1) and Ca18-9 ( ) [19] were carried out under similar experimental conditions except for the kind of acid, HBr or HCl. A comparison of the results of these two experiments reveals that the value of Q/V r is much large in the HBr system than in the HCl system.…”

“…The B18C6 resin, embedded in the inner pores of highly porous silica beads (diameter: 40-60 μm), is a copolymer of phenol and monobenzo-18-crown-6 [17]. In our previous papers [19,20], we examined the Ca isotope effects in breakthrough-mode liquid chromatography with the B18C6 resin in aqueous and methanol hydrochloric acid (HCl) media. We found that (1) the lighter isotopes of Ca are preferentially fractionated to the B18C6 resin phase rather than the solution phase; (2) a higher concentration of HCl resulted in smaller Ca isotope effects; (3) observed isotope effects 642 T. Sato and T. Oi are mass-dependent; and (4) the use of methanol as solvent has little advantage over the aqueous system as far as the magnitude of Ca isotope effects is concerned, but a substantial improvement was observed concerning the adsorption capacity of the resin for Ca 2 + ions.…”