Diffusive gradients in thin films

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[1] and soils.[2] It is well suited to in situ detection of bioavailable toxic trace metal contaminants.[3]:i-v 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.

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.

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.[4] 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.[5] Since then, the DGT technique has been modified and expanded to include a significant number of elements and compounds, including cationic metals,[6] phosphate and other oxyanions (V, CrVI, As, Se, Mo, Sb, W),[7][8][9][10][11][12] antibiotics,[13] bisphenols,[14] and nanoparticles,[15] and has even been modified for the geochemical exploration of gold.[16]

The DGT device

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 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 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.[3]:4.2.3 Dimensions of the layers vary depending on features of the environment, such as the flow rate of water being sampled;[3]:4.2.1 an example is an approximately 40mm device diameter containing a 1mm gel layer.[17]

Principles of operation

Deployment

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. 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.[3]:4.2.1[18] Modifications to the diffusive gel (e.g. increasing or decreasing the thickness) can also be undertaken to ensure low detection limits.[19]

Analysis of binding gels

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).[3]:4.2.1 The eluent can then be quantitatively analysed via a range of analytical techniques, including but not limited to: ICP-MS, GFAAS[3]:4.2.1 ICP-OES, AAS,[17] UV-Vis spectroscopy or computer imaging densitometry.[20] To get the 2D sub-mm high resolution distribution of analytes in heterogenous environments, such as sediments and rhizosphere, the retrieved gel strips can be analyzed by PIXE or LA-ICP-MS after gel drying.[10][21][22][23]

The DGT equation

DGT is based on the application of Fick's law.[17] Once the mass of an analyte has been determined, the time-averaged concentration of the analyte in the bulk, C_{DGT}, can be determined by application of the following equation:

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

where M is the mass of the analyte on the resin, \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.[3]: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.[24]

See also

References

  1. ↑ Zhang, Hao; Davison, William; Gadi, Ranu; Kobayashi, Takahiro (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.
  2. ↑ 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.
  3. 1 2 3 4 5 6 7 "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.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ Zhang, Hao.; Davison, William. (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.
  7. ↑ Zhang, Hao; Davison, William; Gadi, Ranu; Kobayashi, Takahiro (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.
  8. ↑ Santner, Jakob; Prohaska, Thomas; Luo, Jun; Zhang, Hao (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): 7668–7674. doi:10.1021/ac101450j. PMC 3432420. PMID 20735010.
  9. ↑ Luo, Jun; Zhang, Hao; Santner, Jakob; Davison, William (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): 8903–8909. doi:10.1021/ac101676w.
  10. 1 2 Guan, Dong-Xing; Williams, Paul N.; Luo, Jun; Zheng, Jian-Lun; Xu, Hua-Cheng; Cai, Chao; Ma, Lena Q. (2015). "Novel Precipitated Zirconia-Based DGT Technique for High-Resolution Imaging of Oxyanions in Waters and Sediments". Environmental Science & Technology 49 (6): 3653–3661. doi:10.1021/es505424m.
  11. ↑ Stockdale, Anthony; Davison, William; Zhang, Hao (2010). "2D simultaneous measurement of the oxyanions of P, V, As, Mo, Sb, W and U". Journal of Environmental Monitoring 12 (4). doi:10.1039/b925627j.
  12. ↑ Pan, Yue; Guan, Dong-Xing; Zhao, Di; Luo, Jun; Zhang, Hao; Davison, William; Ma, Lena Q. (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): 14267–14273. doi:10.1021/acs.est.5b03742.
  13. ↑ Chen, Chang-Er; Zhang, Hao; Jones, Kevin C. (2012). "A novel passive water sampler for in situ sampling of antibiotics". Journal of Environmental Monitoring 14 (6). doi:10.1039/c2em30091e.
  14. ↑ Zheng, Jian-Lun; Guan, Dong-Xing; Luo, Jun; Zhang, Hao; Davison, William; Cui, Xin-Yi; Wang, Lian-Hong; Ma, Lena Q. (2015). "Activated Charcoal Based Diffusive Gradients in Thin Films for in Situ Monitoring of Bisphenols in Waters". Analytical Chemistry 87 (1): 801–807. doi:10.1021/ac503814j.
  15. ↑ Pouran, Hamid M.; Martin, Francis L.; Zhang, Hao (2014). "Measurement of ZnO Nanoparticles Using Diffusive Gradients in Thin Films: Binding and Diffusional Characteristics". Analytical Chemistry 86 (12): 5906–5913. doi:10.1021/ac500730s.
  16. ↑ 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.
  17. 1 2 3 Thomas, P. (2008). "Metals Pollution Tracing in the Sewerage Network using the Diffusive Gradients in Thin Films Technique" (PDF). 11th International Conference on Urban Drainage. Retrieved 23 April 2015.
  18. ↑ 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.
  19. ↑ 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.
  20. ↑ McGifford, RW; Seen, AJ; Haddad, PR (3 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.
  21. ↑ Davison, W.; Fones, G. R.; Grime, G. W. (1997). "Dissolved metals in surface sediment and a microbial mat at 100-μm resolution". Nature 387 (6636): 885–888. doi:10.1038/43147.
  22. ↑ Warnken, Kent W.; Zhang, Hao; Davison, William (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): 6077–6084. doi:10.1021/ac0400358.
  23. ↑ Williams, Paul N.; Santner, Jakob; Larsen, Morten; Lehto, Niklas J.; Oburger, Eva; Wenzel, Walter; Glud, Ronnie N.; Davison, William; Zhang, Hao (2014). "Localized Flux Maxima of Arsenic, Lead, and Iron around Root Apices in Flooded Lowland Rice". Environmental Science & Technology 48 (15): 8498–8506. doi:10.1021/es501127k. PMC 4124062. PMID 24967508.
  24. ↑ Yabuki, Lauren Nozomi Marques; Colaço, Camila Destro; Menegário, Amauri Antonio; Domingos, Roberto Naves; Kiang, Chang Hung; Pascoaloto, Domitila (20 September 2013). "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): 961–969. doi:10.1007/s10661-013-3430-x.

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