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.