Application of multicollector mass spectrometry and laser ablation in the study of magnesium isotopes fractionation phenomenon during transport through an ion-selective membrane

Abstract

The process of magnesium ions transport through a membrane selective for Mg was studied using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques. A system with an ion-selective membrane containing a magnesium ionophore (ETH 5220), a lipophilic salt (potassium tetrakis(p-chlorophenyl)borate), plasticizers (chloroparaffin, o-nitrophenyl phenyl ether), and a polymeric matrix (polyvinyl chloride), which separates the magnesium salt solution from the solution containing calcium salt, was constructed. Elemental and isotopic analyses of the solutions on both sides of the membrane, as well as the membrane surface itself, were conducted.

Changes in magnesium and calcium concentrations in the solutions bathing both sides of the membrane confirmed that ions of these metals are transported from one side of the membrane to the other. It was hypothesized that, similarly to the kinetic factors in chemical reactions, the lighter isotopes are favoured in transmembrane transport by these factor. Consequently, when magnesium ion passes through the membrane, the solution after passage becomes enriched in the lighter isotope ²⁴Mg. Supporting this assumption, the MC-ICP-MS technique used to measure the Mg isotope ratio at the membrane passage indicated the Mg isotopic fractionation (δ²⁶Mg) up to −1.5 ‰ relative to the initial solution. Obtained results proved the ability of isotope ratio measurements to evaluate the diffusion process of magnesium through this kind of membranes.