Comparison of different speciation techniques to measure Zn availability in hydroponic media

Highlights: • Different speciation techniques provide complementary information on the system. • AGNES and PIM measure free Zn concentrations, while LSAV and DGT measure fluxes. • A plot of fluxes vs. free concentrations provides dynamic speciation information. • Lability degrees can be computed combining results of speciation techniques. - Abstract: Four analytical techniques are compared: AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), LASV (Anodic Stripping Voltammetry with Linear stripping), DGT (Diffusive Gradients in Thin films) and PIM (Polymer Inclusion Membranes). These techniques have been designed to provide the free ion concentration or a labile fraction, complementarily contributing to an integrated description of speciation and availability. Their simultaneous application to the determination of free Zn concentrations or labile fluxes in seven solutions of a hydroponic medium reveals characteristics of each technique and correlations between their results. All dynamic results can be interpreted in terms of a general theoretical framework on fluxes. Indeed, in techniques under diffusion-limited conditions in the sample, the flux can be split into the free contribution (linearly proportional to the free fraction), plus the contribution of the complexes (where mobility, lability and abundance of complexation are intertwined). A methodology to compute lability degrees is developed. Measurements with PIM devices confirm that diffusion in the sample solution is not rate limiting, so its flux is proportional to the free metal in the donor solution. A proportionality between the responses of any given two techniques is observed, which suggests that, for the low ligand-to-metal concentration ratios used in the present work, any of these techniques would correlate similarly with uptake, toxic or nutritional measurements.