Background and Theory of the DGT Passive Sampler

What is DGT?

DGT stands for diffusive gradients in thin films. Simple, but precision plastic DGT devices accumulate dissolved substances in a controlled way. After analysis, it provides the in situ concentration during the time of deployment. This approach, which DGT pioneered for trace metals, is known as passive sampling.

How does it work?

The simple device uses a layer of binding agent impregnated in a hydrogel to accumulate the analytes. A diffusion layer comprising a diffusive hydrogel and a filter membrane overlies the binding-layer. Ions diffuse through the filter and diffusive gel and bind to the binding-layer. It is the establishment of a constant concentration gradient in the diffusion layer that forms the basis for measuring metal concentrations quantitatively without the need for separate calibration.

In situ?

As DGT can be configured as a simple, robust plastic device, it is readily deployed in situ. The discrimination between species occurs in situ, as DGT accumulates the analyte. The analysis of the separated species is performed under well-controlled laboratory conditions.

What substances can be measured?

Any dissolved species for which there is a selective binding agent. Using a chelating resin that binds metals, this includes Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn. Many more cations are possible, including rare earth elements and Ca, Mg, Ba and Sr. Using alternative binding agents, sulphide and several radionuclides, including Cs and Tc can be measured, as well as oxyanions such as phosphate, As(V), Mo, and V. It can also be used for many polar organic compounds, including antibiotics and some pesticides.

What concentrations can be measured?

DGT can measure a wide range of concentrations, especially for metals when allied to ICP-MS. Use of long deployment times accumulates more metal and lowers the detection limit. For a one-day deployment the detection limit for most metals is < ppt (ng l-1). The maximum concentration that can be measured depends on the capacity of the resin. For a 1-day deployment with a typical DGT device it is 0.5 mmol l-1 (30 to 100 mg l-1).

Has DGT been validated?

There are now more than 1000 scientific publications on DGT. Scientists throughout the world have confirmed that DGT works for a wide range of substances, and there is now a comprehensive, validated theory on exactly what it measures in different media. With this level of research investment, the performance capabilities and strengths and weakness of DGT are very well known.

Use in Water

In its routine form, DGT measures all solution species that are labile (available to biota). This includes all inorganic species in true solution and most organic complexes. It does not measure substances that are incorporated inside mineral particles and are therefore inert or unreactive. These properties make DGT passive samplers an ideal choice for monitoring water quality. In more advanced studies, DGT has been used as a chemical speciation tool and to investigate kinetic process.

Features include:

  • Independent of flow (above a low threshold flow), pH and ionic strength
  • Independent of matrix (not affected by oils or surfactants)
  • Easily deployed in situ, even in sewage effluents and underground waters
  • Monitors continuously for any time (hours to weeks)
  • Simple, robust and low cost
  • Built in calibration and preconcentration
  • Measures speciation and bioavailability

Use in sediments

DGT measures directly the mean flux of labile species to the device during the deployment. This can be interpreted directly as the mean concentration at the interface between the device surface and the sediment, during the deployment. Often this is the same as the concentration in bulk pore-water.

DGT probes can be inserted directly into sediments. On retrieval the binding-gel layer can be sliced as thinly as 1mm and analyte measured in each slice. Measurements have been made at much finer spatial resolution and in two dimensions by measuring the analyte in the binding gel by a laser ablation ICP-MS, or employing simple colorimetric procedures.

The DGT measurement has been shown to predict well the toxic response of some benthic organisms.

Use in Soils

For a given device and deployment time, the interfacial concentration can be related directly to the effective concentration of labile metal, CE. CE represents as a concentration the supply of metal from both solution and solid phase.

Concentrations of metals in plants are very well correlated with CE, measured by DGT. DGT mimics an important mechanism of plant uptake by lowering the concentration locally and inducing diffusive supply and release from the solid phase.

DGT can be used in soils to obtain high resolution images of solute concentrations adjacent to plant roots.

A dynamic, numerical model (DIFS) that simulates the interaction of DGT with saturated soils and sediments can be used to obtain thermodynamic and kinetic constants.