Diffusive gradients in thin films

Environmental chemistry technique

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,[1] sediments[2] and soils.[3] It is well suited to in situ detection of bioavailable toxic trace metal contaminants.[4][5][6] 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.

According to DGT theory, the concentration of an analyte, [C], tends toward 0 (µg/L, ng/L, etc.) as the analyte approaches the binding layer, passing through the diffusive boundary layer (DBL, ẟ) and the DGT device's diffusive gel (thickness of Δg). No reverse diffusion of the analyte back into the solution is assumed to occur.

## Contents

* 1 History
* 2 The DGT device
* 3 Principles of operation
* 3.1 Deployment
* 3.2 Analysis of binding gels and chemical imaging
* 4 The DGT equation
* 5 See also
* 6 References
* 7 External links

## History[edit]

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.[7] 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.[8] Since then, the DGT technique has been modified and expanded to include a significant number of elements and compounds, including cationic metals,[7] nitrate,[9] phosphate and other oxyanions (V, CrVI, As, Se, Mo, Sb, W),[2][10][11][12][13][14] antibiotics,[15] bisphenols,[16] and nanoparticles,[17] and has even been modified for the geochemical exploration of gold.[18] DGT has also been developed and calibrated for the measure of radionuclides, including for the analysis of actinides such as U, Pu, Am and Cm, both in the environment[19] and even in cooling pools for spent nuclear fuel rods.[20]

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[edit]

A photo of a disassembled DGT device, showing piston and cap. The device in this picture has been fitted with activated carbon for assimilating gold and/or bisphenols.

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[edit]

### Deployment[edit]

DGT devices being deployed into groundwater in the Tanami Desert, Australia.

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[edit]

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).[4]: 4.2.1 NaOH can be used to elute most oxyanions from Zr-Oxide(Ding et al., 2010, 2011,2016; Sun et al.,2014).The eluent can then be quantitatively analysed via a range of analytical techniques, including but not limited to: ICP-MS, GFAAS[4]: 4.2.1 ICP-OES, AAS,[22] UV-Vis spectroscopy or computer imaging densitometry.[27] For chemical imaging and to obtain 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.[12][28][29][30][31]

## The DGT equation[edit]

DGT is based on the application of Fick's law.[22] Once the mass of an analyte has been determined, the time-averaged concentration of the analyte in the bulk,  C D G T {\displaystyle C_{DGT}} , can be determined by application of the following equation:

C D G T = M Δ g D t A {\displaystyle C_{DGT}={\frac {M\Delta g}{DtA}}\ }

where  M {\displaystyle M} is the mass of the analyte on the resin,  Δ g {\displaystyle \Delta g} is the thickness of the diffusive layer and filter membrane together,  D {\displaystyle D} is the diffusion coefficient of the analyte,  t {\displaystyle t} is the deployment time, and  A {\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[edit]

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

## References[edit]

1. ^ a b Chaudhary M, Quanz M, Williams J, Maltby E, Oakes K, Spooner I, Walker TR (September 2020). "Assessment of metal(loid) concentrations using Diffusive Gradient Thin (DGT) films in marine, freshwater and wetland aquatic ecosystems impacted by industrial effluents". Case Studies in Chemical and Environmental Engineering. 2: 100041. doi:10.1016/j.cscee.2020.100041.
2. ^ a b Zhang H, Davison W, Gadi R, Kobayashi T (August 1998). "In situ measurement of dissolved phosphorus in natural waters using DGT". Analytica Chimica Acta. 370 (1): 29–38. doi:10.1016/S0003-2670(98)00250-5.
3. ^ Wilkins CL (October 1983). "Hyphenated techniques for analysis of complex organic mixtures". Science. 222 (4621): 291–6. Bibcode:1983Sci...222..291W. doi:10.1126/science.6353577. PMID 6353577.
4. ^ a b c d e f g "Diffusive Gradients in Thin-films (DGT): A Technique for Determining Bioavailable Metal Concentrations" (PDF). International Network for Acid Prevention. March 2002. Retrieved 23 April 2015.
5. ^ Strivens J, Hayman N, Johnston R, Rosen G (May 2019). "Effects of Dissolved Organic Carbon on Copper Toxicity to Embryos of Mytilus galloprovincialis as Measured by Diffusive Gradient in Thin Films". Environmental Toxicology and Chemistry. 38 (5): 1029–1034. doi:10.1002/etc.4404. PMID 30840314.
6. ^ Strivens J, Hayman N, Rosen G, Myers-Pigg A (April 2020). "Toward Validation of Toxicological Interpretation of Diffusive Gradients in Thin Films in Marine Waters Impacted by Copper". Environmental Toxicology and Chemistry. 39 (4): 873–881. doi:10.1002/etc.4673. PMID 32004383.
7. ^ a b Zhang H, Davison W (October 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.
8. ^ Harper MP, Davison W, Zhang H, Tych W (August 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. Bibcode:1998GeCoA..62.2757H. doi:10.1016/S0016-7037(98)00186-0.
9. ^ a b Corbett, Thomas D. W.; Dougherty, Hannah; Maxwell, Bryan; Hartland, Adam; Henderson, William; Rys, Gerald J.; Schipper, Louis A. (2020-05-20). "Utility of 'Diffusive Gradients in Thin-Films' for the measurement of nitrate removal performance of denitrifying bioreactors". Science of the Total Environment. 718: 135267. doi:10.1016/j.scitotenv.2019.135267. ISSN 0048-9697.
10. ^ Santner J, Prohaska T, Luo J, Zhang H (September 2010). "Ferrihydrite containing gel for chemical imaging of labile phosphate species in sediments and soils using diffusive gradients in thin films". Analytical Chemistry. 82 (18): 7668–74. doi:10.1021/ac101450j. PMC 3432420. PMID 20735010.
11. ^ Luo J, Zhang H, Santner J, Davison W (November 2010). "Performance characteristics of diffusive gradients in thin films equipped with a binding gel layer containing precipitated ferrihydrite for measuring arsenic(V), selenium(VI), vanadium(V), and antimony(V)". Analytical Chemistry. 82 (21): 8903–9. doi:10.1021/ac101676w. PMID 20936784.
12. ^ a b Guan DX, Williams PN, Luo J, Zheng JL, Xu HC, Cai C, Ma LQ (March 2015). "Novel precipitated zirconia-based DGT technique for high-resolution imaging of oxyanions in waters and sediments". Environmental Science & Technology. 49 (6): 3653–61. Bibcode:2015EnST...49.3653G. doi:10.1021/es505424m. PMID 25655234.
13. ^ Stockdale A, Davison W, Zhang H (April 2010). "2D simultaneous measurement of the oxyanions of P, V, As, Mo, Sb, W and U" (PDF). Journal of Environmental Monitoring. 12 (4): 981–4. doi:10.1039/b925627j. PMID 20383381.
14. ^ Pan Y, Guan DX, Zhao D, Luo J, Zhang H, Davison W, Ma LQ (December 2015). "Novel Speciation Method Based on Diffusive Gradients in Thin-Films for in Situ Measurement of Cr(VI) in Aquatic Systems". Environmental Science & Technology. 49 (24): 14267–73. Bibcode:2015EnST...4914267P. doi:10.1021/acs.est.5b03742. PMID 26535488.
15. ^ Chen CE, Zhang H, Jones KC (May 2012). "A novel passive water sampler for in situ sampling of antibiotics". Journal of Environmental Monitoring. 14 (6): 1523–30. doi:10.1039/c2em30091e. PMID 22538362.
16. ^ Zheng JL, Guan DX, Luo J, Zhang H, Davison W, Cui XY, et al. (January 2015). "Activated charcoal based diffusive gradients in thin films for in situ monitoring of bisphenols in waters". Analytical Chemistry. 87 (1): 801–7. doi:10.1021/ac503814j. PMID 25412473.
17. ^ Pouran HM, Martin FL, Zhang H (June 2014). "Measurement of ZnO nanoparticles using diffusive gradients in thin films: binding and diffusional characteristics". Analytical Chemistry. 86 (12): 5906–13. doi:10.1021/ac500730s. hdl:2436/621804. PMID 24831848.
18. ^ Lucas A, Rate A, Zhang H, Salmon SU, Radford N (August 2012). "Development of the diffusive gradients in thin films technique for the measurement of labile gold in natural waters". Analytical Chemistry. 84 (16): 6994–7000. doi:10.1021/ac301003g. PMID 22812590.
19. ^ Chaplin J, Warwick P, Cundy A, Bochud F, Froidevaux P (25 August 2021). "Novel DGT Configurations for the Assessment of Bioavailable Plutonium, Americium, and Uranium in Marine and Freshwater Environments". Analytical Chemistry. 93 (35): 11937–11945. doi:10.1021/acs.analchem.1c01342. PMID 34432435.
20. ^ Chaplin J, Christl M, Straub M, Bochud F, Froidevaux P (2 June 2022). "Passive Sampling Tool for Actinides in Spent Nuclear Fuel Pools". ACS Omega. 7 (23): 20053−20058. doi:10.1021/acsomega.2c01884.
21. ^ "global-easysensor.com". Archived from the original on 2020-08-08.
22. ^ a b c Thomas P (1 July 2009). "Metals pollution tracing in the sewerage network using the diffusive gradients in thin films technique". Water Science and Technology. 60 (1): 65–70. doi:10.2166/wst.2009.287. PMID 19587403.
23. ^ Zhang H, Davison W, Miller S, Tych W (1995-10-01). "In situ high resolution measurements of fluxes of Ni, Cu, Fe, and Mn and concentrations of Zn and Cd in porewaters by DGT". Geochimica et Cosmochimica Acta. 59 (20): 4181–4192. Bibcode:1995GeCoA..59.4181Z. doi:10.1016/0016-7037(95)00293-9. ISSN 0016-7037.
24. ^ Williams PN, Santner J, Larsen M, Lehto NJ, Oburger E, Wenzel W, et al. (2014-08-05). "Localized flux maxima of arsenic, lead, and iron around root apices in flooded lowland rice". Environmental Science & Technology. 48 (15): 8498–506. Bibcode:2014EnST...48.8498W. doi:10.1021/es501127k. PMC 4124062. PMID 24967508.
25. ^ Warnken KW, Zhang H, Davison W (June 2006). "Accuracy of the diffusive gradients in thin-films technique: diffusive boundary layer and effective sampling area considerations". Analytical Chemistry. 78 (11): 3780–7. doi:10.1021/ac060139d. PMID 16737237.
26. ^ Lucas AR, Reid N, Salmon SU, Rate AW (October 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): 12141–9. Bibcode:2014EnST...4812141L. doi:10.1021/es502468d. PMID 25252140.
27. ^ McGifford RW, Seen AJ, Haddad PR (March 2010). "Direct colorimetric detection of copper(II) ions in sampling using diffusive gradients in thin-films". Analytica Chimica Acta. 662 (1): 44–50. doi:10.1016/j.aca.2009.12.041. PMID 20152264.
28. ^ Davison W, Fones GR, Grime GW (June 1997). "Dissolved metals in surface sediment and a microbial mat at 100-μm resolution". Nature. 387 (6636): 885–888. Bibcode:1997Natur.387..885D. doi:10.1038/43147. S2CID 4261454.
29. ^ Warnken KW, Zhang H, Davison W (October 2004). "Analysis of polyacrylamide gels for trace metals using diffusive gradients in thin films and laser ablation inductively coupled plasma mass spectrometry". Analytical Chemistry. 76 (20): 6077–84. doi:10.1021/ac0400358. PMID 15481956.
30. ^ Williams PN, Santner J, Larsen M, Lehto NJ, Oburger E, Wenzel W, et al. (5 August 2014). "Localized flux maxima of arsenic, lead, and iron around root apices in flooded lowland rice". Environmental Science & Technology. 48 (15): 8498–506. Bibcode:2014EnST...48.8498W. doi:10.1021/es501127k. PMC 4124062. PMID 24967508.
31. ^ Hoefer C, Santner J, Puschenreiter M, Wenzel WW (April 2015). "Localized metal solubilization in the rhizosphere of Salix smithiana upon sulfur application". Environmental Science & Technology. 49 (7): 4522–9. Bibcode:2015EnST...49.4522H. doi:10.1021/es505758j. PMC 4394708. PMID 25782052.
32. ^ Yabuki LN, Colaço CD, Menegário AA, Domingos RN, Kiang CH, Pascoaloto D (February 2014). "Evaluation of diffusive gradients in thin films technique (DGT) for measuring Al, Cd, Co, Cu, Mn, Ni, and Zn in Amazonian rivers". Environmental Monitoring and Assessment. 186 (2): 961–9. doi:10.1007/s10661-013-3430-x. PMID 24052239. S2CID 9781883.

## External links[edit]

* "DGT Research".
* Global-EasySensor.com