On-line preconcentration system for flame atomic absorption spectrometry using unloaded polyurethane foam: determination of zinc in waters and biological materials

An on-line procedure for the preconcentration and determination of zinc in waters and biological materials was developed. Zinc is preconcentrated from acidic medium (pH 3.0) as its thiocyanate complex onto a polyurethane foam mini-column placed in the loop of a four-way valve. The elution step is performed with a stream of 30% acetone in 2% HNO3 and the zinc displaced is introduced directly into the nebuliser of a flame atomic absorption spectrometer. The system was operated under two different preconcentration times since the concentration of zinc in the two types of samples was different. For biological samples, 1 min was used and a detection limit of 3.0 µg L–1 was achieved with a throughput of 40 samples per hour. At a concentration of 20 µg L–1, an RSD of 2.5% was obtained. For the determination of zinc in natural water, a preconcentration time of 3 min was employed and a detection limit of 0.85 µg L–1 was obtained. In this case, the system was slower with a throughput of 17 samples per hour. The RSD at 10 µg L–1 was 6.0%. The continuous flow system was applied to the analysis of several biological reference materials and natural water samples.