Design of simple and sensitive methods for trace analysis by means of concentration with a membrane filter

Novel techniques of membrane filtration for the enrichment and determination of trace components in water are reviewed from an aspect of downsizing in analytical chemistry. One method is based on the conversion of a component in water to a hydrophobic species by a chemical reaction, and its retention on a membrane by a hydrophobic interaction between the species and the membrane material. The species retained on the membrane are eluted or dissolved together with the membrane in a small volume of solvent and determined by an appropriate instrumental method, such as spectrophotometry, GF-AAS or ICP-AES. More than a 100-fold enrichment has been easily attainable by this method. Other enrichment/determination approaches without elution and dissolution, by means of reflective spectrophotometry, tristimulus colorimetry and using a transparent membrane filter have exhibited new potentials of this method from the view point of zero emission-chemistry, in addition to a simple and rapid procedure. Some typical applications of this technique to the preconcentration and determination of trace components in environmental water samples were described. The retention behavior of ion-pairs on a membrane filter was interpreted by ion-pair solid-phase extraction equilibrium. The effects of functional groups in the ions and solid-phase materials, the structure of the ions and the organic solvent added to the aqueous phase on the extraction behavior are described.