Chromatographic Separation of Calcium Isotopes using Benzo-18-Crown-6-Ether Resin and Acetic Acid Solution

Research Article

Austin Chromatogr. 2016; 3(1): 1040.

Chromatographic Separation of Calcium Isotopes using Benzo-18-Crown-6-Ether Resin and Acetic Acid Solution

Umehara S¹, Okumura S², Kishimoto T³, Fujii Y³*, Nomura M², Kaneshiki T² and Ozawa M²

¹Osaka University, Research Center for Nuclear Physics, Japan

²Tokyo Institute of Technology, Research Laboratory for Nuclear Reactors, Japan

³Osaka University, Graduate School of Science, Japan

*Corresponding author:Yasuhiko Fujii, Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguroku, Tokyo, 152-8550, Japan

Received: January 18, 2016; Accepted: March 08, 2016; Published: March 14, 2016


Chromatographic separation of 48Ca isotope was studied using an acetic acid solution as an eluent and a column packed with benzo-18-crown-6- ether resin. Prior to the chromatographic experiment, adsorption of calcium ions from acetic acid solutions was examined in batch mode experiments at different concentrations of acetic acid. The distribution coefficient Kd of calcium ions was observed to increase at highly concentrated acetic acid solutions. Chromatographic separation experiment, performed based on the batch mode adsorption test, showed that the heavy isotope 48Ca was enriched in the front boundary region. Separation coefficientε and HETP (height equivalent to a theoretical plate) were calculated from the experimental data. The observed ε of the calcium acetate experiment was 4.6 x10-3 and HETP was 2.2cm. These results suggest that separation coefficient ε is relatively larger than that of HCl solution system previously reported. The value of HETP of the present system was found to be very large, if compared with the HETPs of the HCl solution systems. The large ε is advantageous, but the large HETP is disadvantageous from the viewpoint of production of 48Ca enriched calcium. The large value of HETP is presumably due to the slow isotopic exchange between the solution phase and the resin phase.

Keywords: Benzo-18-Crown-6-Ether; Acetic acid; Isotope effect; Isotope separation; HETP; 48Ca


HETP: Height Equivalent to a Theoretical Plate; B18C6E: Benzo- 18-Crown-6-Ether: NMR: Nuclear Magnetic Resonance


Natural calcium consists of 40Ca, 42Ca, 43Ca, 44Ca, 46Ca, and 48Ca. Among these isotopes, 43Ca has the nuclear spin of 7/2 and is used as an NMR detectable calcium isotope. The radioisotope 47Ca, produced by (n, 2n) reaction of 48Ca or by (n,γ) reaction of 46Ca, can be used for medical purposes. In addition, 48Ca is regarded as a stable isotope but actually a double beta decay nuclide; two β rays are emitted simultaneously. The β decay is closely related to the characteristics of neutrino. The double β decay is considered to be a key to understand the physics of neutrino. For the neutrino research, the enrichment of 48Ca is anticipated, since its natural abundance is very low, only 0.187%.

Isotope separation processes based on the chemical exchange have been developed for light elements, such as hydrogen, lithium, boron, carbon, oxygen, and nitrogen. The enriched isotopes of D, 6Li, 7Li, 10B, 13C, 17O, 18O and 15N are commercially available. The principle of the isotope separation of these elements is based on the quantum effects in molecular vibration. Vibration energy remains even at zero degree K. Since the vibrational frequency depends on the mass of the isotope in the molecule, the vibrational energy shows the mass dependence and affects the thermodynamic properties of molecules, such as complex formation constants.

Calcium isotope separation has been studied using various chemical exchange processes. By using strongly acidic cation exchange resin, Aaltonen reported calcium isotope separation coefficient ε (48Ca/40Ca) as 8.7 x 10-4 [1]. By using strongly acidic cation exchange resin and EDTA eluent, Klinskii et al. reported ε (48Ca/40Ca) as -1.7 x 10-4. The minus value of ε shows the heavier isotope is enriched in the cation exchange resin [1].

Heumann et al. [1] studied calcium isotope fractionation in the systems of ion exchange resin, crown ether resin and criptand resin. Among the studied systems, criptand resin showed the largest isotope fractionation, or the largest isotope separation coefficient ε (48Ca/40Ca), as 12.6x10-3. Jepson et al. [2] studied 44Ca isotope separation by breakthrough chromatography with synthesized crown ether resin. The maximum value of ε (44Ca/40Ca) reported by Jepson is 2.7x10-3. From this value, ε (48Ca/40Ca) is theoretically expected to be 4.6 x10-3, which is 1.7 times as large as experimentally observed ε (44Ca/40Ca) Authors of the present work have developed crown ether resin embedded in fine porous silica beads. Using this type of resin, Hayasaka et al. [3], Fujii et al. [4], Oi et al. [5], Umehara et al. [6], and Okumura et al. [7], reported research work on calcium isotope separation.

In the case of crown ether resin, the isotope fractionation takes place in the isotopic exchange process between the calcium ions in the crown-ether resin and those in the outside solution. The exchange process of calcium ions is depicted in Figure 1. The solvents used by Heumann et al. and Jepson et al are chloroform/methanol mixed solutions. The present authors have used concentrated HCl solution and HCl/ethanol mixed solution for Ca isotope separation [3,4,6,7]. The typical values of observed ε are 3.6x10-3, and 3.0 x10-3 and 3.8x10-3 are listed in Table 1.