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This sandbox is in the article namespace. Either move this page into your userspace, or remove the {{User sandbox}} template. The diffusive gradients in thin films (DGT) technique is an environmental chemistry technique for the detection of elements and compounds in aqueous environments, including natural waters, sediments and soils.^[1] 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 bulk solution concentration of the element or compound via a simple equation.

History

The DGT technique was developed in 1994 by Hao Zhang and William Davison at 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.^[2] 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.^[3] Since then, the DGT technique has been modified and expanded to include a significant number of elements and compounds, including phosphorous, antibiotics, and nanoparticles, and has even been modified for the geochemical exploration of gold.^[4]

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 (usually pre-cut into discs that are between 0.13-0.80 mm thick, although this can be varied) 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.

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 face should be perpendicular to the direction of flow, in order to ensure the diffusive boundary layer (DBL) is not increased 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.^[5] Modifications to the diffusive gel (e.g. increasing or decreasing the thickness) can also be undertaken to ensure low detection limits.^[6]

Analysis of binding gels

After the DGT devices have been retrieved, the binding gels can be eluted using methods that depend on the target analyte and the DGT binding gel (for example, 1 mol/L 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, ICP-OES, AAS or UV-Vis spectroscopy.

The DGT equation

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

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

where $M$ is the concentration of the analyte, $\Delta g$ 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.

References

^ Zhang, H.; Davison, D. (1994). "In situ speciation measurements of trace components in natural waters using thin-film gels". Nature. 367 (6463): 546–8. doi:10.1038/367546a0. {{cite journal}}: Cite has empty unknown parameter: |1= (help)
^ Zhang, H.; Davison, D. (1995). "Performance characteristics of diffusion gradients in thin films for the in situ measurement of trace metals in aqueous solution". Analytical Chemistry. 67 (19): 3391–3400. doi:10.1021/ac00115a005.
^ Harper, M.; Davison, D.; Zhang, H.; Wlodek, W. (1998). "Kinetics of metal exchange between solids and solutions in sediments and soils interpreted from DGT measured fluxes". Geochimica et Cosmochimica Acta. 62 (16): 2757–2770. doi:10.1016/S0016-7037(98)00186-0.
^ Lucas, A.; Rate, A.; Zhang, H.; Salmon, U.; Radford, N. (2012). "Development of the diffusive gradients in thin films technique for the measurement of labile gold in natural waters". Analytical Chemistry. 84: 6994–7000. doi:10.1021/ac301003g.
^ Warnken, K.; Zhang, H.; Davison, W. (2006). "Accuracy of the diffusive gradients in thin-films technique: diffusive boundary layer and effective sampling area considerations". Analytical Chemistry. 78 (11): 3780–3787. doi:10.1021/ac060139d.
^ Lucas, A.; Reid, N.; Salmon, U.; Rate, A. (2014). "Quantitative Assessment of the Distribution of Dissolved Au, As and Sb in Groundwater Using the Diffusive Gradients in Thin Films Technique". Environmental Science & Technology. 48 (20): 12141–12149. doi:10.1021/es502468d.

External links

http://www.dgtresearch.com

[pmid6353577-1] Zhang, H.; Davison, D. (1994). "In situ speciation measurements of trace components in natural waters using thin-film gels". Nature. 367 (6463): 546–8. doi:10.1038/367546a0. {{cite journal}}: Cite has empty unknown parameter: |1= (help)

[2] Zhang, H.; Davison, D. (1995). "Performance characteristics of diffusion gradients in thin films for the in situ measurement of trace metals in aqueous solution". Analytical Chemistry. 67 (19): 3391–3400. doi:10.1021/ac00115a005.

[3] Harper, M.; Davison, D.; Zhang, H.; Wlodek, W. (1998). "Kinetics of metal exchange between solids and solutions in sediments and soils interpreted from DGT measured fluxes". Geochimica et Cosmochimica Acta. 62 (16): 2757–2770. doi:10.1016/S0016-7037(98)00186-0.

[4] Lucas, A.; Rate, A.; Zhang, H.; Salmon, U.; Radford, N. (2012). "Development of the diffusive gradients in thin films technique for the measurement of labile gold in natural waters". Analytical Chemistry. 84: 6994–7000. doi:10.1021/ac301003g.

[5] Warnken, K.; Zhang, H.; Davison, W. (2006). "Accuracy of the diffusive gradients in thin-films technique: diffusive boundary layer and effective sampling area considerations". Analytical Chemistry. 78 (11): 3780–3787. doi:10.1021/ac060139d.

[6] Lucas, A.; Reid, N.; Salmon, U.; Rate, A. (2014). "Quantitative Assessment of the Distribution of Dissolved Au, As and Sb in Groundwater Using the Diffusive Gradients in Thin Films Technique". Environmental Science & Technology. 48 (20): 12141–12149. doi:10.1021/es502468d.

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 <!-- EDIT BELOW THIS LINE -->
 '''The diffusive gradients in thin films (DGT) technique''' is an [[environmental chemistry]] technique for the detection of [[elements]] and [[compounds]] in aqueous [[Environment|environments]], including [[Hydrosphere|natural waters]], [[Sediment|sediments]] and [[soils]].<ref name="pmid6353577">{{cite journal |doi=10.1038/367546a0| |title=In situ speciation measurements of trace components in natural waters using thin-film gels |year=1994 |last1=Zhang |first1=H. |last2=Davison|first2=D.|journal=Nature |volume=367 |issue=6463 |pages=546-8 }}</ref> The technique involves using a specially-designed [[Environmental_monitoring#Passive_sampling|passive sampler]] that houses a binding [[gel]], [[Diffusion|diffusive]] gel and [[Membrane technology|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 bulk solution [[concentration]] of the element or compound via a simple equation.
-[[File:DGT_theory.png|thumb|right|300 px|The theoretical behaviour of the DGT technique. The concentration of an analyte (C), passing through the diffusive boundary layer (DBL, ẟ) and the DGT device's diffusive gel (thickness of <math>{\Delta|</math>g, tends toward 0 units (e.g. ug/L, ng/L, etc.) as the analyte approaches the binding gel (assuming this resin completely adsorbs the analyte). In order for the DGT equation to be valid, no reverse diffusion back into the solution can take place.]]
+[[File:DGT_theory.png|thumb|right|300 px|The theoretical behaviour of the DGT technique. The concentration of an analyte (C), passing through the diffusive boundary layer (DBL, ẟ) and the DGT device's diffusive gel (thickness of <math>{\Delta}</math>g, tends toward 0 units (e.g. ug/L, ng/L, etc.) as the analyte approaches the binding gel (assuming this resin completely adsorbs the analyte). In order for the DGT equation to be valid, no reverse diffusion back into the solution can take place.]]
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