FAQ

/FAQ
FAQ 2018-02-02T10:19:33+00:00
Q. What is the meaning of the product codes? 2018-03-01T10:53:41+00:00

A. The product code provides a fairly full description of the product. It is based on the composition of the device rather than the analyte, as some devices can be used for several, often diverse, analytes.

Take a typical example: LSNM-NP

L signifies loaded, meaning that the plastic housing comes complete with the gel and membrane layers, so that it is ready for deployment.

SN signifies that it uses the plastic housing that is appropriate for deployments in solution. Alternatives are SL (the housing appropriate for deployments in soils) and SP, indicating a sediment probe.

M signifies that it uses a binding layer of Chelex100. The complete list of codes for binding layers is given below. In most cases the choice of letters is not intuitive.

B Spheron thiol
C XAD resin
D HLB
E NMDG
H AgI
M Chelex100 resin
P Ferrihydrite
R Activated charcoal
T Metsorb (TiO2)
V TEVA resin
X Mixture of Chelex and Metsorb
Y Mixture of Chelex and Ferrihydrite
Z Zirconium oxide

N signifies the type of diffusion layer used. N corresponds to the normal APA polyacrylamide gel cross-linked with an agarose derivative. The alternative is A, which corresponds to an agarose gel.

P signifies the type of membrane filter used. P indicates polyethersulphone. Alternatives are G, indicating GHP, N, indicating Nuclepore and T, indicating PTFE.

Q. How long should I deploy DGT? 2018-02-02T10:18:41+00:00

A. Deployments in water

In principle DGT can be deployed for a wide range of times from a few hours to a few months and interpretation of the concentration using the standard simple DGT equation will be valid. However, several factors can limit this range. These are the concentrations of the analytes, the selectivity and capacity of the analyte and binding layer combination, the presence of competitive ions, the extent of complexation in solution, the possibility of analyte adsorption to the diffusive layer and the possibility of biofilm formation.

For simple cationic trace metals and a Chelex binding layer, deployment times between 3 days and 3 weeks should be optimal. If the concentrations of the metals are low (less than a few micrograms per litre) and there is no indication of biofilm growth on the surface of the devices, longer times may be appropriate. The deterrent to using shorter times is that complexation in solution may reduce the time taken to reach steady state accumulation. In waters with low concentrations of organic matter or very high metal concentrations, this effect will be minimal and so deployment times as short as a day can be used.

Similar deployment times can be used when measuring oxyanions, including phosphate using a Metsorb binding agent, but times in excess of two weeks would not be recommended for deployments in waters of high ionic strength such as seawater.

Data for the most appropriate deployment times for the measurement of mercury are sparse. Conservatively it would be sensible to deploy between 3 days and 2 weeks.

Early indications are that times of between 3 days and 3 weeks should be suitable for organic compounds.

Deployments in soils and sediments

After a maximum reached within a few hours, the flux to a DGT device deployed in a soil or sediment will progressively decline as the analyte adjacent to the device is consumed. If the requirement is to calculate concentration, a deployment of 1 day is appropriate. Longer deployment times up to three days may, however, be appropriate in some circumstances. For example, if trace metals are strongly complexed by humic substances, the slower diffusion of these complexes delays the time to maximum flux. Deployment of 3 days will overcome this slower approach to a pseudo steady state. When probes are inserted into soils and sediments there will inevitably be a small temporary disturbance of the spatial distribution of analyte. This spatial structure is rapidly restored through the strong redox buffering mechanisms. However, If there is a desire to measure metals at high spatial resolution in sediments or soils, leaving the probes in place for longer than 1 day will allow better establishment of the structure and minimise the effect of the initial disturbance on the time-averaged measured flux.

Q. Where do I find the diffusion coefficients I need? 2018-02-02T10:18:36+00:00

A. The most comprehensive listing of diffusion coefficients of mainly inorganic substances appropriate for DGT measurements can be found in W. Davison and H. Zhang, Diffusion Layer Properties, Chapter 3, in: Diffusive Gradients in Thin-films (DGT) for Environmental Measurements, Editor: William Davison, Cambridge University Press, 2016. A list of consensus values for metal ions at various temperatures is available (Diffusion Coefficients). Simple algorithms are also provided (adapted from the Appendix of the above DGT book) to enable calculation of values at any temperature from 1 to 35oC.

There are no clear consensus values for mercury, as can be seen from the tables of Chapter 3 in the DGT book. However, Suggested values of diffusion coefficients for mercury deduced from available knowledge and information are available.

As new methods for measuring organics using DGT are developed, diffusion coefficients are being determined, as provided in Information for organic analytes. While theses values are expected to be reliable, it is advisable to recognise that generally there are as yet insufficient data to establish them as consensus values.

Q: Which equation should I use to calculate concentration in solution? 2018-02-02T12:52:40+00:00

A: In most situations where DGT is deployed in water that is flowing or subject to convection currents the standard DGT equation is appropriate.          

                                                                                 

cDGT (nmol or ng mL-1) is the concentration of analyte in the deployment medium measured by DGT.

M (nmol or ng) is the mass of analyte accumulated in the binding layer.

Δg (also known as δg) (cm) is the total thickness of the materials that comprise the diffusion layer (gel and filter membrane).

Dmdl (cm2 s-1) is the diffusion coefficient of analyte in the material diffusion layer.

Ap (cm2) is the physical area of the exposed filter membrane.

t (s) is the deployment time.

Δg and Ap are known properties for the supplied DGT device. The time of deployment, t, is known by the operator. DMDL is also available provided the deployment temperature is known. The accumulated mass, M, is measured in the laboratory. Usually it is obtained by eluting the analyte and calculating the mass from the measured concentration in the known volume of eluate. Recommended units to facilitate easy calculation are shown.

This standard DGT equation incorporates an automatic correction for a modest diffusive boundary layer (DBL) at the surface of the device. Such a modest DBL thickness will apply when there is good solution flow over the surface of the DGT device. In this case cDGT is likely to be accurate to ±5%. More generally when DGT devices are deployed in natural waters with unknown hrdrodynamics, because of uncertainties in the thickness of the DBL due to the local hydrodynamics and deployment configuration, cDGT should be regarded as accurate to ±20%.

If greater accuracy is required the DBL thickness should be measured in situ using 3 or more DGT devices with different values of Δg. Then a fuller equation is required. Detailed accounts of the most appropriate calculation procedure for different circumstances are available in W. Davison and H. Zhang, Principles of measurements in simple solutions, Chapter 2, in: Diffusive Gradients in Thin-films (DGT) for Environmental Measurements, Editor: William Davison, Cambridge University Press, 2016 and in W. Davison and H. Zhang, Progress in understanding the use of diffusive gradients in thin-films – back to basics, Environ. Chem. 9: (2012), 1-13. These articles also consider when the necessary requirement that the time taken to reach a steady state transfer of analyte to the DGT device is negligible compared to the deployment time.

The most commonly used equation for multiple devices is

AE is the effective area which has a value of 3.8 cm2 for standard solution devices. δmdl is the combined thickness of diffusive gel and filter membrane and δdbl is the thickness of the diffusive boundary layer. Concentration, in this case denoted by cDGTe, and δdbl can be obtained from linear plots of 1/M versus δmdl.

The above treatments assume that there are no competition effects or limitations to the capacity of the binding layer, which will generally be true. Calculations can still be made if these situations apply, using the approaches and equations provided in M. Jimenez-Piedrahita, A. Altier, J. Cecilia et al., Extending the use of diffusive gradients in thin films (DGT) to solutions where competition, saturation, and kinetic effects are not negligible, Anal. Chem, 89: (2017), 6567-6574.

Q. Can I calculate concentration from deployments in soils or sediment? 2018-02-02T12:59:41+00:00

A: Yes you can. The basic DGT equation can be used.

                                                                                                  

cDGT (nmol or ng mL-1) is the mean concentration of analyte measurable by DGT in the pore water adjacent to the surface of the DGT device, averaged over the total deployment time.

M (nmol or ng) is the mass of analyte accumulated in the binding layer.

Δg (also known as δmbl) (cm) is the total thickness of the materials that comprise the diffusion layer (gel and filter membrane).

Dmdl (cm2 s-1) is the diffusion coefficient of analyte in the material diffusion layer.

Ap (cm2) is the physical area of the exposed filter membrane or the area of sliced gel that is measured.

t (s) is the deployment time.

The key thing here is that because DGT continually removes analyte, its concentration at the surface of the DGT device may be lowered during the course of the deployment. In some soils and sediment the analyte may be continually resupplied to solution from the solid phase. When this effective buffering is substantial the measured cDGT is the same as the cDGT that would be measured directly on the porewaters. Comparison of cDGT with alternative measurements of concentrations in porewaters (or soil solution) can provide information on the dynamics of analyte exchange between porewater and solid phase.

A detailed appreciation with thorough referencing of the principles involved in deploying DGT in soils and sediments can be found in

  1. J, Lehto, Principles and applications in soils and sediments, Chapter 7, in: Diffusive Gradients in Thin-films (DGT) for Environmental Measurements, Editor: William Davison, Cambridge University Press, 2016
Q. How should I store DGT devices and for how long? 2018-02-02T11:20:22+00:00

A: According to our quality tests, most binding gels (except precipitated ferrihydrite gel) and diffusive gels (except bis-acrylamide cross-linked polyacrylamide gel known as BPA) can be stored at room temperature for more than one year in a well-sealed container with 0.01-0.03 M NaNO3 or NaCl solution.

For loaded DGT devices packed in a plastic bag, evaporative leakage could induce dehydration of diffusive gels and affect DGT performance.  To eliminate this impact, we normally add 0.4 mL of NaNO3 or NaCl solution to each plastic bag for long term storage. There was no significant influence on measurements using DGT devices stored in these bags at room temperature for one month and at 4oC for six months. The small amount of solution remaining in the plastic bag should not present  a problem. The DGT devices can be deployed directly on removal from the bag.

Q. Should I treat and analyze my sample directly after deployments? 2018-02-02T11:21:03+00:00

A: The concentrations and speciation of analytes should not change after they bind to the binding gels. Thus the DGT devices can be stored in well controlled conditions until it is convenient to analyse them.

We strongly recommend rinsing your devices with MQ/deionized water immediately after retrieval. This treatment is important to get rid of any solid particles or microorganisms attached to the device’s surface, as there is the potential for these solid phases to leak further analyte during transportation or storage.

If the storage time prior to analysis is longer than two weeks  dehydration may be a problem. It can make the separation of binding and diffusive gels difficult. We recommend that, for times longer than 2 weeks, DGT devices are packed in individual clean plastic bags with a small volume of MQ/deionized water.

Q. What should I do if the binding gel cannot be separated from the diffusive gel? 2018-02-02T11:23:09+00:00

A: In most situations, this problem can be solved by adding 100 – 150 μL of MQ/deionized water to the gel surface and waiting for 10 – 20 mins. If the problem still exists, we suggest that the diffusive and binding gels are eluted together. Vgel (mL) here is close to the volume of water used for rehydration provided that all water is absorbed by the gels. M (nmol or ng) is the mass of analyte accumulated in the binding layer. X is the dilution time. fe is the elution factor. Ve (mL) is the volume of eluent. Cicp (nmol/mL or ng/mL) is the concentration measured by ICP-MS or other relevant instruments.

  

Q. Which DGT is best for measuring cations and oxyanions together? 2018-02-02T13:17:17+00:00

A: Generally we would recommend a device with a mixed binding layer of Metsorb (TiO2) and Chelex. This is the most robust combination which generally performs well in both freshwater and seawater.

For deployments in all types of waters you will need device LSNX-NP

For deployment in soilsA you will need device LSLX-NP

For deployments in sedimentsB you will need LSPX-NP

Details of the performance of this binding layer can be found in:

  1. G. Panther, W. W. Bennett, D. T. Welsh and P. R. Teasdale, Simultaneous measurement of trace metal and oxyanion concentrations in water using diffusive gradients in thin films with a chelex-metsorb mixed binding layer, Anal. Chem., 86: (2014), 427-434.

Although a binding layer that uses ferrihydrite instead of Metsorb has some performance limitations, especially for deployments longer than a few days in seawater, it does have advantages in some situations. Devices with a mixed Chelex and ferrihydrite binding layer are preferred where analysis is by total digestion of the binding layer, elution is restricted to a single acid step, or for phosphate where the final analysis is colorimetric.

For deployments in all types of waters you will need device LSNY-NP

For deployment in soilsA you will need device LSLY-NP

For deployments in sedimentsB you will need LSPY-NP

Q. Should I deoxygenate sediment probes? 2018-02-02T11:26:50+00:00

A: The simple answer is ideally yes, but there are other considerations. Because the DGT assembly is thin it does not contain a lot of oxygen and, within a short period of time, (say 30 minutes) the oxygen within the DGT gels will be consumed by the oxygen diffusing out into the sediment and electron donors diffusing into the gels. Clearly this will compromise the DGT measurement of redox sensitive and secondary affected components (e. g. trace metals) within this 30 minute time period of initial accumulation. However, DGT is usually deployed for times in excess of a day. Therefore, this initial period where the presence of oxygen can have an effect on the DGT accumulation will only represent about 2% of the total accumulation time. Consequently the effect on the DGT measurement is likely to be negligible.  There is an urgent need for work to be done to verify experimentally the effect of preliminary deoxygenation on DGT measurements.

Even deoxygenation of the DGT probes is not so straightforward because it will only be effective if the transfer of the DGT probes from their deoxygenation solution to the sediment can be done within no more than a few 10s of seconds. Exposure of the probe to the air for longer times will quickly replenish the oxygen, as demonstrated in Davison, W., Zhang, H. & Grime, G.W. (1994).  Performance characteristics of gel probes used for measuring the chemistry of pore waters. Envi. Sci. Technol. 28, 1623-1632.