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 automatic on-line extraction preconcentration system coupled to a graphite furnace atomic absorption spectrometer is presented. Two operational fractions with environmental meaning (able and not able to be accumulated by aquatic organisms) were determined through kinetic discrimination; a Chelex-100 labile 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 retention capacity, pre-concentration efficiency, ease of elution, type of eluant (i.e., compatible with the graphite furnace operation) and the sample–resin time of contact. Special attention is given to the characterization of humic acids and their Cd complexes and to their sorption and elution capabilities from different extracting materials.