Molybdenum isotope enrichment by anion-exchange chromatography

Abstract

Stable enriched isotopes with an abundance as close to 100% as possible are most desirable parent materials for preparing synthetic isotope mixtures for calibration of mass spectrometers. In this study, we have examined the extent of Mo isotope fractionation during anion-exchange chromatography and assessed the potential of this separation technique for further isotope enrichment of commercially available isotopically enriched molybdenum. The 60 cm wide molybdenum adsorption band was eluted from a 1.8 m long (3 times 60 cm) chromatographic column, filled with strongly basic anion-exchange resin. It was observed that heavier Mo isotopes were preferentially eluted from the resin, resulting in an enrichment of heavier Mo isotopes at the front of the adsorption band and a corresponding depletion at the rear. An equilibrium isotope effect between dissolved and resin-bound chemical forms of molybdenum appears to be the cause of the Mo isotope fractionation observed. The height equivalent to a theoretical plate (HETP) was calculated to be 0.25 mm, while the separation factor (α = R_resin/R_solution) was found to be 0.99998 per atomic mass unit. These results show that molybdenum can, in principle, be further enriched isotopically by anion-exchange chromatography in a laboratory environment. However, achieving the highest degree of enrichment was confirmed to be very laborious. It has been estimated that in order to reach an increase in the abundance of ⁹⁸Mo isotope from an original value of 98.2% to 99.9%, the molybdenum adsorption band would need to migrate over a distance of at least 400 m through the anion-exchange resin. Another interesting observation made in this study was that no mass-independent isotope fractionation was observed for molybdenum as a result of chemical exchange reactions on the anion-exchange resin.