Using DGT passive samplers and MC-ICPMS to determine Pb and Zn isotopic signature of natural water
Increasing numbers of studies use zinc (Zn) and lead (Pb) isotopic compositions to track anthropogenic pollutions in surface water and groundwater. However, given the low content in Pb and Zn in natural waters, we are often restricted for analysis by MC-ICPMS. One solution is to use passive Diffusive Gradients in Thin films (DGT) samplers for in-situ preconcentration of these metals. The technique of DGT is a useful tool for in situ measurements of metals concentration in surface waters. In these devices, labile fractions of metals are fixed in a layer of chelex resin after diffusion through a polyacrylamide gel. The goal of this study is to validate the use of DGT samplers to determine the isotopic signature of dissolved metals. We focused our work on Zn and Pb isotopic systems. First, this development was tested under laboratory conditions using mineral water (Volvic) spiked in Zn and Pb. The second experiment was performed in 3 PVC columns simulating piezometer systems with 3 different water flow rates (1, 2 and 4 meters/day). Two protocols were used to extract metals from chelex resin: the “classical” protocol in one step and a more recent protocol in three steeps. For Zn isotopes, a fractionation of 0.06‰ between dissolved and adsorbed metal is measured , likely related to diffusion processes. However, in agreement with systematics of diffusion-driven fractionation in solution, this Zn fractionation is systematic, and can therefore be corrected. Despite a recovery yield of 80% the extraction protocol 1, commonly used, does not induce isotopic fractionation. Moreover, this protocol provides a weak amount of metal pollution compared to protocol 2. In the case of Pb, no measurable fractionation could be observed within the reported precision of MC-ICPMS measurements. The tests performed in an experimental system simulating a piezometer suggest that DGT samplers are suitable for studies of the Zn and Pb isotopic composition in water with a low water flow rate.
Copyright © 2015 The Authors. Published by Elsevier B.V.
(Anne-Marie Desaulty1, Céline Bodard1, Thierry Laurioux1, Catherine Guerrot1, Romain Millot1 and Catherine Berho1)
Fig. 1. Results for tests performed in cleanroom, DGT were immerged in 4.5L of mineral water (Volvic) spiked with Zn and Pb
monoelementary solution: (a) G66Zn/64Zn for immersion solution at day=0 and 1 and Zn monoelementary solution (blue dots), DGT
immerged during 1 day extracted with protocol 1 and 2 (orange squares). The corrected values are calculated with the equation (1); (b) 206Pb/204Pb ratios for immersion solution at day=0, 1, 5 and 7, and Pb monoelementary solution (blue dots), DGT immerged during 1, 5 and 7
days and extracted with protocol 2 (orange squares).