Optimization of a vapour generation method for metal determination using ICP-OES

Abstract

A multivariate optimization of the main parameters which might affect Ag, Au, Cd, Cu, Ni and Zn vapour generation and detection using inductively coupled plasma optical emission spectrometry has been carried out. Sodium tetrahydroborate was used as reducing agent and constant concentrations of 8-hydroxyquinoline and cobalt were added to all the standards in order to increase the efficiency of the reaction. A home-made gas–liquid separator was used and vapour was carried directly to the plasma using an Ar flow. A 2⁶⁻² factorial design plus two additional points in the centre of the experimental domain was run to select influential factors in the determination; afterwards, an orthogonal composite design 2⁴ + star was performed to select optimum values for the factors. Desirability was the parameter used to select optimum conditions to carry out simultaneous determination of all the elements. Optimum values of studied factors were: [Co], 1 mg L⁻¹; [HNO₃], 0.43 M; [8-hydroxyquinoline], 38 mg L⁻¹; [NaBH₄], 2.24%; and Ar nebulizer flow rate (injection Ar flow rate), 0.74 L min⁻¹. Detection limits were under 10 μg L⁻¹ for all the elements except for Ag (silver vapour formation increased using hydrochloric acid). Relative standard deviations were under 10% (n = 11) and an interference study was carried out. Ni and Cu are the most serious interferents that drastically reduce responses for most of the elements. It must be pointed out that using the developed method, vapours of cobalt, chromium, molybdenum and iron were obtained as well. The limit of detection for iron was 7.4 μg L⁻¹ and the RSD (%) was 7.7 (Fe 238.204 nm).