Size fractionation techniques combined with INAA for speciation purposes

Abstract

In natural waters trace elements, especially trace metals may be present in a variety of physicochemical forms. They may be associated with forms ranging from simple ions and molecules via hydrolysis products and colloids, pseudocolloids and organic or inorganic particles. The transition between categories is gradual. The presence of species differing in size, charge and density will influence on the transport, mobility and bioavailability of the trace element in question. Fractionation techniques which do not influence the distribution patterns are therefore required for speciation purposes. In the present work dialysis in situ and large membrane (hollow fibers) ultrafiltration are used for fractionation of low molecular weight species, colloids, pseudocolloids and particles. Due to the presence of foreign components transformation processes influence the distribution patterns of trace elements of interest. Sorption to “foreign” surfaces, complexation with agents present and aggregation of colloids (e.g., increasing ionic strength) result in a shift towards higher dimensions while desorption and dispersion processes mobilize the trace elements. Information on several components is therefore needed in speciation studies and a multielemental method of analysis having low determination limits must be applied. Instrumental neutron activation is appropriate to this kind of study because of its high sensitivity for simultaneous determination of a great-number of elements. Size fractionation techniques combined with INAA for the characterization of trace element species in natural waters will be discussed.