Membrane Techniques | Liquid Membranes☆

Abstract

In most separation processes in chemical analysis based on liquid–liquid or solid-phase extraction, the analyte is first extracted from an aqueous medium into a condensed immiscible phase (solid or liquid). This is followed by analyte back-extraction into another aqueous phase for further processing. Separation based on the use of liquid membranes allows these two processes to be conducted simultaneously on the corresponding sides of the membrane thus simplifying the overall separation process and making it amenable to automation. At the same time liquid membrane-based separation processes provide similar or even better selectivity and enrichment factors compared to conventional extraction techniques. This article outlines briefly the characteristics of the main types of liquid membranes (i.e., bulk liquid membranes, emulsion liquid membranes, supported liquid membranes, and polymer inclusion membranes) and discusses their suitability for analyte separation and preconcentration in chemical analysis. This discussion explains why supported liquid membranes and polymer inclusion membranes have been used so successfully in both online and offline separation and preconcentration of numerous analytes of environmental, clinical and industrial interest (e.g., heavy metal ions, toxic anions, pesticides, and pharmaceuticals). The article also outlines the membrane configurations most frequently used in chemical analysis (e.g., flat-sheet and tubular membrane configurations).