Automatic on-line ultratrace determination of Cd species of environmental significance in natural waters by FI-ETAAS

The ultratrace determination of Cd species in natural waters by using an au tomatic on-line extraction preconcentration system coupled to a graphite fu rnace atomic absorption spectrometer is presented. Two operational fraction s with environmental meaning (able and not able to be accumulated by aquati c organisms) were determined through kinetic discrimination; a Chelex-100 l abile fraction consisting of Cd ions and weak Cd complexes and a silica C18 hydrophobic inert fraction consisting of stable cadmium-organic complexes. The figures of merit for the determination of both fractions are given and the obtained values are fully discussed. The aptitude of different resins for the extraction of each fraction is evaluated taking into account retent ion capacity, pre-concentration efficiency, ease of elution, type of eluant (i.e., compatible with the graphite furnace operation) and the sample-resi n time of contact. Special attention is given to the characterization of hu mic acids and their Cd complexes and to their sorption and elution capabili ties from different extracting materials.