- Store the units in a refrigerator (4oC). The DGT units provided are kept in the sealed clean plastic bags containing a few drops of 0.01M NaNO3 (or 0.01M NaCl) solution. Do not open them until immediately prior to deployment.
- Check the units about once a week to make sure they are under moist conditions. Add a few more drops of trace metal clean 0.01M NaNO3 (or 0.01M NaCl) solution if it is necessary.
INSTRUCTIONS FOR DEPLOYING DGT IN FIELD
(Handling, Deployment and Retrieval)
- Store DGT units in a refrigerator prior to use.
- Do not remove from the sealed plastic bag until immediately (minutes) prior to deployment.
- Only get hold of the DGT unit with clean hands.
- Do not touch the white filter at the face of the unit and do not let it come into contact with anything else.
- Having placed the DGT unit in its holder or attached it to any deployment device through the hole on the unit using any fishing line, deploy the unit immediately (minutes).
- Ensure the unit is deployed in flowing (or moving) water, but avoid excessive turbulence, particularly bubbles.
- Ensure that the white face of the DGT unit is fully immersed during the deployment period.
- Provide an accurate record to the nearest minute of the deployment time and the temperature of the water during the deployment time. If the variation is within + 2oC a mean (or start and end temperature) will suffice. If the variation is greater, ideally the mean temperature should be obtained from an integrated record of temperature (data logger or chart recorder).
- On retrieval of the holder remove the DGT unit immediately (minutes), taking care not to touch the face filter.
- Rinse the DGT unit with a wash bottle stream of distilled/deionised water and shake off obvious surface water (do not dry it).
- Place in the clean plastic bag provided and seal with minimum air space. Mark on the bag. Store it in a refrigerator.
PROCEDURES FOR ANALYSING DGT SAMPLES
Sample Treatment and Analysis
- To retrieve the resin gel after deployment insert a screw driver into the grove in the cap and twist it. The cap will be broken at the weak point. Remove the broken cap and then peel off the filter and diffusive gel layer to reveal the bottom resin-gel layer.
- Place the resin gel in a clean sample tube and add 1 ml of 1M HNO3 solution (if 1.5 ml centrifuge tubes are used). Make sure the resin gel is fully immersed in the HNO3 solution. Leave it 24 hours at least before analysis.
- Pipette an aliquot from the sample tube and dilute it at least 5 times with Miili-Q (or deionised water) prior to analysis by AAS or ICP-MS.
Calculation of the DGT Measured Concentration
- First calculate the mass of metal accumulated in the resin gel layer (M) using equation (1)
M = Ce (VHNO3 + Vgel)/fe (1)
where Ce is the concentration of metals in the 1M HNO3 elution solution (in *g/l), VHNO3 is the volume of HNO3 added to the resin gel, Vgel is the volume of the resin gel, typically 0.15 ml, and fe is the elution factor for each metal, typically 0.8.
- The concentration of metal measured by DGT (CDGT) can be calculated using Equation (2).
CDGT = MΔg/(DtA) (2)
where Δg is the thickness of the diffusive gel (0.078cm) plus the thickness of the filter membrane (0.014 cm), D is the diffusion coefficient of metal in the gel (see Table 1 for open pore gel. Please note the numbers need to time E-6 and the unit should be cm2/sec), t is deployment time (in sec) and A is the exposure area (A=3.14 cm2).
If the DGT units are accidentally dried out after long time storage, they need to be revived before deployment.
Procedures for reviving the DGT units:
- Prepare an acid washed plastic box to accommodate the units.
- Prepare superpure 0.01M NaNO3 (or 0.01M NaCl) solution with MQ water in an acid washed clean container.
- If you don’t have superpure NaNO3, add Chelex-100 into the above NaNO3 (or NaCl) solution (about 5 to 10 grams into 1 litre) and stir overnight to clean the solution.
- Soak the units in MQ water (use the acid washed plastic box) overnight.
- Pour the MQ water away and fill the box with the trace metal clean NaNO3 solution from step (2) or 3 (Decant the NaNO3 solution out carefully from the Chelex-100 beads). Soak the units for a day.
- Deploy the units immediately after they have been revived.
Prepared by Hao Zhang
DGT Research Ltd, Skelmorlie, Quernmore,Lancaster LA2 0QJ, UK
tel : 44-1524-751451, fax: 44-1524-751826 e-mail: email@example.com