In spite of the great improvement in selectivity and sensitivity of modern analytical instrumentation, the classical preconcentration and separation techniques, such as liquid-liquid extraction, ion-exchange chromatography, precipitation, etc., are still widely used to improve analytical measurements, especially in connection with trace and ultratrace analysis. Liquid-liquid extraction is among the most frequently used conventional methods for sample pretreatment preceding measurements of the analyte concentratioin. It offers a potential for selectivity and sensitivity improvement in a large number of determinations.