Factorial design for Fe, Cu, Zn, Se and Pb preconcentration optimization with APDC and analysis with a portable X-ray fluorescence system.

Preconcentration of heavy metals in water with ammonium pyrrolydine dithiocarbamate (APDC) is a common practice in analytical chemistry. A literature review on this topic showed that several authors use this precipitation agent, but in different preconcentration conditions, conducting to divergent results. The objective of this work is to use factorial design to optimize the factors involved in the preconcentration process of heavy metals using APDC. Five factors were studied: sample volume, solution pH, APDC concentration, APDC volume and stirring time. The assays were performed by energy dispersive X-ray fluorescence (EDXRF). The values for detection limits within 95% confidence level, in microg L(-1), were: Fe (6.0+/-0.1), Cu (4.0+/-0.1), Zn (2.0+/-0.1), Se (4.0+/-0.1) and Pb (5.0+/-0.1). The value for quantification limit for the five elements was 20 microg L(-1), with 3% deviation. Multi-element standard solutions were prepared. Precipitation procedure was applied in the spiked solutions and the samples were filtered in cellulose ester membrane for quantification measurements. The optimum values obtained were 300 mL of sample solution, pH 4, 1 mL of 2% APDC and 10 min of stirring time. The concentration results obtained for the validation measurements were satisfactory for in situ survey employing a portable instrument.