Application of three-variables Doehlert matrix for optimisation of an on-line pre-concentration system for zinc determination in natural water samples by flame atomic absorption spectrometry

This article describes an on-line pre-concentration system for zinc determination in environmental samples by flame atomic absorption spectrometry (FAAS). It was based on the sorption of zinc(II) ions in a minicolumn of polyurethane foam loaded with 4-(2-pyridylazo)-resorcinol (PAR). The optimisation step was carried out using two-level full factorial and a Doehlert design. Three variables (sampling flow rate, buffer concentration and pH) were regarded as factors in the optimisation. Results of the two-level full factorial design 2³ for 8 runs (in duplicate) based on the Analysis of Variance (ANOVA) demonstrated that all the factors in the tested levels are statistically significant. Besides, the interaction (sampling flow rate×buffer concentration) was also statistically significant. A three-variables Doehlert design was applied in order to determine the best condition for pre-concentration and determination of zinc. The validation process was assessed as: parameters of the analytical curve, precision, effect of other ions in the proposed system, robustness test and accuracy. The proposed system allowed determination of zinc with detection limit (3σ/S) of 0.28 μg l⁻¹, and a precision (reproducibility), calculated as relative standard deviation (R.S.D.) of 10.0 and 3.7% for zinc concentration of 1.0 and 5.0 μg l⁻¹, respectively. The achieved pre-concentration factor was 91.23 and the sampling frequency was 48 samples per hour. The achieved recovery for zinc determination in presence of several cations demonstrated that this procedure could be applied for analysis of water samples. The accuracy was confirmed by analysis of three certified reference materials. This procedure was applied for zinc determination in several kinds of water samples including saline aqueous waste from oil Refinery.