Paper-based diffusive gradients in thin films technique coupled to energy dispersive X-ray fluorescence spectrometry for the determination of labile Mn, Co, Ni, Cu, Zn and Pb in river water

Abstract

The diffusive gradients in thin films (DGT) technique has shown enormous potential for labile metal monitoring in fresh water due to the preconcentration, time-integrated, matrix interference removal and speciation analytical features. In this work, the coupling of energy dispersive X-ray fluorescence (EDXRF) with paper-based DGT devices was evaluated for the direct determination of Mn, Co, Ni, Cu, Zn and Pb in fresh water. The DGT samplers were assembled with cellulose (Whatman 3 MM chromatography paper) as the diffusion layer and a cellulose phosphate ion exchange membrane (Whatman P 81 paper) as the binding agent. The diffusion coefficients of the analytes on 3 MM chromatography paper were calculated by deploying the DGT samplers in synthetic solutions containing 500μgL-1 of Mn, Co, Ni, Cu, Zn and Pb (4L at pH 5.5 and ionic strength at 0.05molL-1). After retrieval, the DGT units were disassembled and the P 81 papers were dried and analysed by EDXRF directly. The 3 MM chromatographic paper diffusion coefficients of the analytes ranged from 1.67 to 1.87×10-6cm2s-1. The metal retention and phosphate group homogeneities on the P 81 membrane was studied by a spot analysis with a diameter of 1mm. The proposed approach (DGT-EDXRF coupling) was applied to determine the analytes at five sampling sites (48h in situ deployment) on the Piracicaba river basin, and the results (labile fraction) were compared with 0.45μm dissolved fractions determined by synchrotron radiation-excited total reflection X-ray fluorescence (SR-TXRF). The limits of detection of DGT-EDXRF coupling for the analytes (from 7.5 to 26μgL-1) were similar to those obtained by the sensitive SR-TXRF technique (3.8 to 9.1μgL-1).