Diffusive gradients in thin films

History

The DGT technique was developed in 1994 by Hao Zhang and William Davison at the Lancaster Environment Centre of Lancaster University in the United Kingdom. The technique was first used to detect metal cations in marine environments using Chelex 100 as the binding agent. Further characterisation of DGT, including the results of field deployments in the Menai Strait and the North Atlantic Ocean, was published in 1995. The technique was first tested in soils in 1998, with results demonstrating that kinetics of dissociation of labile species in the porewater (soil solution) could be determined via DGT. Since then, the DGT technique has been modified and expanded to include a significant number of elements and compounds, including cationic metals, phosphate and other oxyanions (V, Cr^VI, As, Se, Mo, Sb, W), antibiotics, bisphenols, and nanoparticles, and has even been modified for the geochemical exploration of gold.

The DGT device

The DGT device is made of plastic, and comprises a piston and a tight-fitting, circular cap with an opening (DGT window). A binding gel, diffusive gel and filter membrane are stacked onto the piston, and the cap is placed over the assembly. The dimensions of the device normally ensure that the two gels and filter membrane are well-sealed when the cap is put on. Dimensions of the layers vary depending on features of the environment, such as the flow rate of water being sampled; an example is an approximately 2 cm device diameter containing a 1mm gel layer.

Deployment

DGT devices can be directly deployed in aqueous environmental media, including natural waters, sediments, and soils. In fast-flowing waters, the DGT device's face should be perpendicular to the direction of flow, in order to ensure the diffusive boundary layer (DBL) is not affected by laminar flow. In slow-flowing or stagnant waters such as in ponds or groundwater, deployment of DGT devices with different thicknesses of diffusive gel can allow for the determination of the DBL and a more accurate determination of bulk concentration. Modifications to the diffusive gel (e.g. increasing or decreasing the thickness) can also be undertaken to ensure low detection limits.

Analysis of binding gels and chemical imaging

After the DGT devices/probes have been retrieved, the binding gels can be eluted using methods that depend on the target analyte and the DGT binding gel (for example, nitric acid can be used to elute most metal cations from Chelex-100 gels). The eluent can then be quantitatively analysed via a range of analytical techniques, including but not limited to: ICP-MS, GFAAS ICP-OES, AAS, UV-Vis spectroscopy or computer imaging densitometry. For chemical imaging and to optain two-dimensional (2D) sub-mm high resolution distribution of analytes in heterogenous environments, such as sediments and the rhizosphere, the retrieved gel strips can be analyzed by PIXE or LA-ICP-MS after gel drying.

The DGT equation

DGT is based on the application of Fick's law. Once the mass of an analyte has been determined, the time-averaged concentration of the analyte in the bulk, C D G T , can be determined by application of the following equation:

where M is the mass of the analyte on the resin, Δ g is the thickness of the diffusive layer and filter membrane together, D is the diffusion coefficient of the analyte, t is the deployment time, and A is the area of the DGT window. More elaborate analysis techniques may be required in cases where the ionic strength of the water is low and where significant organic matter is present.