Diffusive gradients in thin films


The diffusive gradients in thin films technique is an environmental chemistry technique for the detection of elements and compounds in aqueous environments, including natural waters, sediments and soils. It is well suited to in situ detection of bioavailable toxic trace metal contaminants. The technique involves using a specially-designed passive sampler that houses a binding gel, diffusive gel and membrane filter. The element or compound passes through the membrane filter and diffusive gel and is assimilated by the binding gel in a rate-controlled manner. Post-deployment analysis of the binding gel can be used to determine the time-weighted-average bulk solution concentration of the element or compound via a simple equation.
File:DGT theory.png|thumb|right|300 px|According to DGT theory, the concentration of an analyte, , tends toward 0 as the analyte approaches the binding layer, passing through the diffusive boundary layer and the DGT device's diffusive gel. No reverse diffusion of the analyte back into the solution is assumed to occur.

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 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, 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. 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.

Principles of operation

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 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 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. 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 obtain two-dimensional 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,, can be determined by application of the following equation:
where is the mass of the analyte on the resin, is the thickness of the diffusive layer and filter membrane together, is the diffusion coefficient of the analyte, is the deployment time, and 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.