Impurity gradients in solution-grown ice and MgSO4 · 12H ₂O crystals measured by cryo-laser ablation and high-resolution- induced-coupled plasma mass spectrograph

During the eutectic freeze crystallization (EFC) of an industrial aqueous MgSO4 solution, ice and magnesium sulfate duodecahydrate (MgSO4·12H2O(s)) were crystallized simultaneously near the eutectic point. It was found that the crystallization was highly selective: although the industrial feed solution contained appreciable levels of inorganic impurities (typically: 320mg/LCl-, 410mg/LCa2, 40mg/LMn2+, 70mg/LNa+ and 50mg/LK+), the formed ice and salt crystals contained lower levels of impurities (typically: 17mg/LCl-, 8mg/LCa2+, 17mg/LMn2+, and 5mg/LNa). Also the ice was pure: only traces (typically: 20mg/LSO42- and 5mg/LMg2+) of magnesium and sulfate were found in the ice crystals. In this work the spatial distribution of the impurities in the crystals is investigated. Gradients of composition in solids are measured by laser ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS). A special cryogenic sampling cell (<-80°C) for laser ablation has been constructed. The focused (5-10μm width) laser shoots at the frozen sample, scanning its surface. The vapor is fed to the mass spectrometer. In this way, the impurity concentration as a function of position in the crystal can be measured. The results of this method with MgSO4·12H2O and ice are presented.