Preconcentration of traces of cobalt, nickel, copper and lead in water by thermoresponsive polymer-mediated extraction for tungsten filament electrothermal vaporization-inductively coupled plasma mass spectrometry

Abstract

A simple and rapid preconcentration method was developed for the determination of trace metals (Co, Ni, Cu, Pb) in water samples by electrothermal vaporization (ETV) inductively coupled plasma mass spectrometry (ICP-MS). The method is based on the thermoresponsive precipitation of a water-soluble polymer, poly(N-isopropylacrylamide) [PNIPAAm], from the aqueous sample solution and the simultaneous incorporation of hydrophobic metal chelates into the precipitate. To a 100 ml sample solution (pH 3) were added 20 mg of ammonium pyrrolidinedithiocarbamate (APDC), 60 mg of PNIPAAm, and 850 mg of NaNO₃. When the solution was warmed at 50 °C, PNIPAAm precipitated to form a gum-like aggregate (polymer phase). Hydrophobic metal–APDC chelates were quantitatively incorporated into the polymer phase. The polymer phase was easily taken up and dissolved with N,N-dimethylformamide to prepare 1 ml of solution. A 5 µl aliquot of the resulting solution, containing palladium as a modifier, was placed onto a doubly coiled tungsten filament in an ETV system. Four metals (Co, Ni, Cu, and Pb) were simultaneously determined with ETV-ICP-MS, whereas Cd gave very small signals probably due to the volatile property of the Cd–APDC chelate. Because of the highly efficient concentration (100-fold), the determination of these heavy metal ions at pg ml⁻¹ to ng ml⁻¹ levels was successfully achieved. The precision and accuracy of the proposed method were evaluated by analyzing a certified water sample.