Determination of trace elements associated with colloidal material in seawater using INAA and RNAA

As most trace metals in fresh water are to a certain extent associated with colloidal or particulate matter, the present paper demonstrates that the extent of association may also be significant for trace elements in seawater. Soon after sampling, samples from a land-locked Norwegian fjord are fractionated according to particle size and charge. Ultrafiltration is performed using Amicon Concentrator, equipped with hollow fiber cartridges having a nominal molecular weight cut-off of 105, and 104, respectively. The fraction of elements present in electroactive forms is determined using controlled potential electrolysis and subsequent irradiation of Carbon-cathodes. Different INAA and RNAA methods have been tested in order to obtain a procedure capable of determining the very low concentrations of trace elements in ultrafiltrates. The results obtained using the following methods on different sample aliquots (total sample, fractions) are discussed. Based on the results obtained using these methods, a significant fraction (30-90%) of several trace metals (Sc, Ce, Eu, U, Mo, Fe, Co, Ag, Zn) is found to be associated with colloidal matter. For Zn, the fraction of electroactive species also correspond to that present in low molecular (M/sub w/ < 104) forms. The present work illustrates that important information on species can be obtained when fractionation methods are combined with low detection multielement NAA techniques. 14 references, 2 figures, 3 tables