Multivariate optimization of solid-phase extraction applied to iron determination in finished waters

In this work, Amberlite XAD-4 resin functionalized with salicylic acid was synthetized, characterized and applied as a new packing material for an on-line system to iron determination in aqueous samples. The detection method is based on the sorption of Fe(III) ions in a minicolumn containing the synthesized resin, followed by a desorption step using an acid solution and measurement of iron by vis-spectrophotometry (CAS method). The optimization of the solid-phase extraction system was performed using factorial design and Doehlert matrix considering six variables: sample percolation rate (0.5–9 ml min−1), sample metal concentration (20–200 μg l−1), flow-through sample volume (0–5 ml) (all three directly linked to the extraction step), elution flow-rate (0.5–9 ml min−1), concentration and volume of eluent (HCl 0.1–0.5 M) (all three directly linked to the elution step). The aim of this study was to obtain a set of operating ranges for the six variables tested in order to obtain – by means of a mathematical function allowing maximisation of each response (desirability function) – at least 90% of iron recovery rates. Using the experimental conditions defined in the optimization, the method allowed iron determination with achieved detection limit of 2.3 μg l−1 and precision (assessed as the relative standard deviation) of 9.3–2.8% for iron solutions of 10.0–150 μg l−1. Real samples (coming from a water treatment unit) were used successfully when evaluating potentialities of the developed SPE procedure coupled to a spectrophotometric determination.