Diffusive gradients in thin films

The diffusive gradients in thin films (DGT) technique is an

History

The DGT technique was developed in 1994 by Hao Zhang and William Davison at the

DGT Research Ltd. was established in July 1997 by the original developers of the technique, Profs. Davison and Zhang, and sells ready-made DGT® devices for water, soil and sediment deployments to measure different analytes, as well as the component parts for self-assembly. The company holds the original patents for the device and DGT® is a trademark which is registered throughout the world. In 2014 a rival company "EasySensor" was set up by Prof. Shiming Ding and supplies devices that the company claims are analogous to the original DGT® products.^[21]

The DGT device

The most commonly used DGT device is a plastic "piston-type" probe, and comprises a cylindrical polycarbonate base and a tight-fitting, circular cap with an opening (DGT window). A binding gel, diffusive gel and filter membrane are stacked onto the base, and the cap is used to seal the gel and filter layers inside^[4]: 4.2.3 Dimensions of the gel layers vary depending on features of the environment, such as the flow rate of water being sampled;^[4]: 4.2.1 an example is an approximately 2 cm device diameter containing a 1mm gel layer.^[22] Other commonly used probe configurations include those for deploying in sediments (to measure solute mobilisation with depth)^[23] and in planar form for measuring solute dynamics in the plant rhizosphere.^[24]

Principles of operation

Deployment

DGT devices can be directly deployed in aqueous environmental media, including natural waters, sediments, and soils.^[1] 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.^{[4]: 4.2.1 [25][9]} Modifications to the diffusive gel (e.g. increasing or decreasing the thickness) can also be undertaken to ensure low detection limits.^[26]

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,

Once the mass of an analyte has been determined, the time-averaged concentration of the analyte in the bulk, ${\displaystyle C_{DGT}}$, can be determined by application of the following equation:

${\displaystyle C_{DGT}={\frac {M\Delta g}{DtA}}\ }$

where ${\displaystyle M}$ is the mass of the analyte on the resin, ${\displaystyle \Delta g}$ is the thickness of the diffusive layer and filter membrane together, ${\displaystyle D}$ is the diffusion coefficient of the analyte, ${\displaystyle t}$ is the deployment time, and ${\displaystyle A}$ is the area of the DGT window.[4]^: Eq.2 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.^[32]

See also

• List of chemical analysis methods
• Environmental monitoring
• Chemcatcher
• Metrology
• Microanalysis

References