The first application of solid phase extraction (SPE) started approximately five decades ago, and until then development of SPE materials is continued, and it seems to be a never-ending story. One group of SPE materials includes those with macrocyclic ligands immobilized on a silica or polymer support; this type of SPE has been reportedly available for ion-selective separation and pre-concentration of ions, and the technique is commonly known as molecular recognition technology (MRT). In this book, application of different... Lately, research is focused in developing more explicit materials to achieve meticulous separation of ions from solutions containing complex matrices with high concentrations of interfering ions. In MRT, the designed ‘host’ materials possess a high degree of recognition to specific ions or groups of ions called ‘guest’, and the recognition capability remains effective at the very low concentrations of the ‘guest’ ion or when those present in complex matrices.