Fractionation of Calcium Isotopes in Cation-Exchange Chromatography

Ion-exchange displacement chromatography of calcium has been carried out successfully for the purpose of observing calcium isotope fractionation effects. Small but definite accumulation of the heavier isotopes has been observed at the front parts of the calcium adsorption bands, which means they are preferentially fractionated into the solution phase. The average values of the single-stage separation factor minus one per unit mass difference between isotopes (ε/Δ M) have been 2.0 × 10 −5 for the calcium chloride system, 5.2 × 10 −5 for the calcium lactate system, and 2.3 × 10 −5 for the calcium acetate system at 25°C. The reduced partition function ratios of the calcium species involved in the present study have been estimated by using separation factor data and available data on calcium hydration under appropriate assumptions. The reduced partition function ratios of the complex species have been found to be larger than that of the simple hydrated calcium ion, which is a cause of the experimental results that the separation factor values of the calcium lactate and acetate systems are larger than that of the calcium chloride system. It has also been found that, for the alkali and alkaline earth metals, the magnitude of isotope effect per unit mass difference between isotopes accompanying pure ion exchange is nearly inversely proportional to the square of the atomic mass.