MEMBRANE FILTERS FOR SOLID-PHASE EXTRACTION AND DETERMINATION OF TRACE-ELEMENTS

Membrane filters were proposed as media for solid-phase extraction and determination of trace elements in water. The membrane filter of material with a strong affinity for hydrophobic species in water is useful to retain the species by filtration. The component to be determined is converted into a hydrophobic ion with an appropriate organic reagent. The ion is collected on the membrane filter made of cellulose nitrate, polyethersulfone or polytetrafluoroethylene as an ion associate in the presence of hydrophobic counter ions having the opposite charge to the converted ion. The collection is performed very quickly by filtration under suction with an aspirator. The ion associate collected on the filter is eluted or dissolved together with the membrane in a small amount of organic solvent. The species in the concentrated solution is determined by several instrumental methods, such as, spectrophotometry, ICP-AES and AAS. Reflective spectrophotometry is also useful to determine the species retained on the filter without elution or dissolution. Various factors which affect the collection and determination, such as, the chemical form of the species to be retained, materials and pore size of the membrane, solvent for elution or dissolution of the species retained on the filter, are described in a general survey of the technique. Some typical applications of the technique to trace analysis in environmental waters are summarized. The most attractive features of this technique are the simplicity and rapidity of the procedure, an easily attainable high concentration factor and determination with high-precision. The potential of this technique for trace analysis is described. The distributions of some ion pairs between membrane and water phases are also studied. An ion-pair solid-phase extraction constant was proposed to evaluate the extraction power of the materials of the membrane filter, and extractability of some chemical species. The extraction constant was defined as: K-ex = [C+. A(-)](MF)/([C+][A(-)]), where, [C+. A(-)](MF) is the concentration of the ion pair in the membrane filter phase expressed by the mole numbers per unit volume (dm(3)) or B.E.T. surface area (m(2)) of the membrane, and [C+], [A(-)] are the concentrations of cation and anion in the aqueous phase. It is clear that the cellulose nitrate membrane filter is the most powerful of all the filters tested for the extraction of the ion pairs. It is highly probable that an electrostatic interaction between the polarized nitro-group and the ion pair plays an important role in the extraction. The contribution of each additional methylene group (-CH2-) to the value of log K-ex for solid-phase extraction was determined to be about 0.4; the value for solvent extraction is 0.6.