Interpretation of diffusion gradients in thin films (DGT) measurements: a systematic approach

Abstract

Environmental context Dynamic speciation of an element in a natural medium is essential for understanding its availability. The technique of diffusion gradients in thin films (DGT) has become a widely used tool for in situ environmental studies, being applied to determine fluxes of metal cations, anions, organics and nanoparticles. The interpretation of the measurements with suitable physicochemical models gives valuable insights into the behaviour of the system. Abstract Gaining insight into the physicochemical processes integrated in a DGT (diffusion gradients in thin films) measurement and combining them in a model can assist in retrieving fundamental information, both qualitative and quantitative, on the probed system. New experiments (such as varying the thicknesses of the gel or the resin layer) and their mathematical treatment to extract meaningful parameters have been suggested from theoretical considerations. The concept of lability degree is useful in describing an interpretation of the DGT concentration as the summation of the free metal concentration plus the labile fraction of all complexes multiplied by a ratio of diffusion coefficients. In some cases, the lability degree can be directly estimated with specific measurements and a very simple expression. We review the current status of these interpretations, including numerical simulations, with special focus on analytical expressions, because they can be more accessible to the standard DGT practitioner. Present limitations and challenges for future work in DGT interpretation are also discussed.

Influence of the settling of the resin beads on diffusion gradients in thin films measurements

“…Recently, it has been highlighted that the knowledge of the metal distribution in the front and back resin discs can be used to fit the kinetic dissociation constant of the complex in the resin domain [18,20]. For all these reasons, it is of interest to assess the influence of the inhomogeneity of the binding sites within the resin disc on the accumulation, lability degree of the complex, distribution of the accumulated mass (between front and back resin discs) and the retrieval of the kinetic dissociation constant from this distribution.…”

“…Experimentally, in systems with negligible flux of the free metal in comparison with that of the complex, the lability degree can be determined as [20] j % n M en noÀligand M (11) where n M is the accumulation in presence of ligand while n noÀligand M is the accumulation in absence of ligand, at the same total metal concentration. There is an important influence of…”

“…Metal accumulation in the back resin disc will indicate the presence of partially labile complexes that penetrate into the resin by diffusion. The percentage of the total mass that is accumulated in the back resin disc (for 2 disc stacks of R devices) [20] will be:…”

Lability of nanoparticulate metal complexes in electrochemical speciation analysis

“…the extent to which complexes dissociate to release free electroactive species on the timescale of their diffusion towards the sensor surface, e.g. an electrode or a binding phase [5,10]. Electrochemical techniques are particularly attractive for On the occasion of the retirement of M. Lovrić and S. Komorsky-Lovrić these studies because a wide range of operational timescales is accessible [11], and other processes such as possible adsorption of NP-bound electroactive species onto the reactive interface can be probed [12].…”

“…Non-equilibrium speciation techniques, such as various electrochemical methods [4], diffusive gradients in thin film (DGT) [5], and dynamic solid phase microextraction (SPME) [6], provide a measure of the fluxes and reactivities of species [7][8][9]. The pertaining knowledge is operationally formulated in terms of the lability of the complex species, i.e.…”