Factorial Design and Doehlert Matrix in Optimization of Flow System for Preconcentration of Copper on Polyurethane Foam Loaded with 4‐(2‐Pyridylazo)‐resorcinol

Abstract

The present paper describes a flow preconcentration system for copper determination by flame atomic absorption spectrometry (FAAS) based on the sorption of Cu(II) ions onto a mini‐column of polyurethane foam (PUF) loaded with a chromogenic reagent [4‐(2‐pyridylazo)‐resorcinol (PAR)]. The variables associated with flow preconcentration system performance, such as pH, buffer concentration (BC), and sampling flow rate (SF), were optimised using a full factorial (2³) design plus a central point and Doehlert matrix. The results obtained, based on the Pareto chart and analysis of variance (ANOVA), demonstrated that pH and BC, as well as the interaction (pH × BC) are statistically significant at the 95% confidence level. By using 20 mL of sampling volume the proposed method, under conditions optimised by the Doehlert matrix (formation of Cu(II)‐PAR (1:2) complex at pH 6.4, 0.017 mol L⁻¹ phosphate BC, and 10 mL min⁻¹ SF), allows to determine Cu(II) ions with a detection limit of 0.35 µg L⁻¹ and precision (n = 8) as relative standard deviation (RSD) of 3.2 and 1.1 for copper solutions containing 10 and 30 µg L⁻¹, respectively. Also, a satisfactory linear range (1.2–40 µg L⁻¹), an enrichment factor of 105 and a sample throughput of 28 hr⁻¹ were achieved. Interference studies showed that the PUF‐PAR mini‐column retains Cu(II) ions in the presence of several transition and alkali metals without presenting interferences. The method was applied for copper determination in water samples (mineral and tap water) and high salt aqueous samples (physiological serum containing 0.9% (m/v) NaCl). The validation of the method was checked by analysing the Cu(II) content in the samples, as well as from analyte addition. The recoveries ranged from 91.0% to 101.1%.

Keywords:

• Doehlert matrix
• Flow preconcentration system
• Polyurethane foam
• 4‐(2‐Pyridylazo)‐resorcinol
• Copper

Acknowledgments

The authors would like to thank Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP, grant number 01/05614‐0) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support. M. A. Z. A and S. L. C. F are grateful to CNPq for fellowships. We are grateful to Prof. Carol H. Collins for language assistance.