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Automatic sampler using passive DGT ® devices for water analysis: technical features

DGT* Research

DGT® is a simple commercially available device for measuring pollutants, nutrients and other components in water

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  • Overview

    THOË ® is a patented automatic and autonomous sampler using DGT® devices for the analysis of both dissolved metals and organic compounds in seawaters and freshwaters. Fully programmable, THOË® makes it possible to provide on a monitoring site concentrations time series recorded over several months.


    • Multiple deployment options available.
    • Possibility of exposing 12 DGT ® sequentially.
    • Programming time sequences from few hours to several weeks.
    • Built-in temperature sensor for accurate concentration calculation.
    • Titanium rotation axis to direct the DGT ® towards the current.
    • Maximum immersion depth of 1000 m.
    • Structure, carrousel and seals in chemically inert components (PEEK, PETP, PTFE and Silicon).
    • Autonomy in immersion mode of more than one year.

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  • User stories

    THOË sampler deployed to investigate phosphate pollution from an agricultural catchment

    User stories
      Agricultural activities have the potential to increase dramatically the concentrations of nutrients into water courses. A recent study by Luode Consulting demonstrated the importance of continuous monitoring of phosphate concentrations using our THOË sampler containing DGT devices with an iron oxide binding layer (LSNP-NP) in a small creek downstream of farmland. The THOË automatic sampler was programmed to exposed DGT devices over twelve 24-hour periods. To demonstrate this innovative technique provided an improvement over traditional sampling, several spot samples were collected and analyzed using classical colorimetric determination by CFA (Continuous Flow Analysis). THOË’s integrated temperature logger was used to… Read more

    Monitoring a marine effluent outfall in New Caledonia

    User stories
    While mining is an important part of the new Caledonian economy, being the third top nickel producer in the world, New Caledonia is also home to the third largest barrier reef in the world. This barrier reef and its associated lagoons were listed as a world heritage site in 2008. Given the biological diversity present in these ecosystems, it is essential to ensure these industrial activities do not have any detrimental effects on the marine environment, by performing adequate monitoring. One mining operator has their marine outfall located within one of the world heritages zones and uses THOË water samplers… Read more

    Hexavalent chromium and dissolved metals in rivers

    User stories
    New Caledonia’s ores are not only rich in Nickel but also contain a wealth of other elements including chromium. While chromium is not currently exploited, historical exploitation combined present day mining for nickel ores causes the release of chromium into creeks and rivers. Under oxic conditions the toxic hexavalent form exists and potentially presents a public health problem if not monitored correctly. Over the last few years DGT Research has developed a new DGT device containing a binding layer specifically designed to retain hexavalent chromium. Combing this new DGT with our THOË sampler provides a perfect solution at remote monitoring… Read more

    Monitoring of dissolved metals in a port

    User stories
    Shipping activities have the potential to release undesirable chemicals into the environment. Chronic exposure to these chemicals may result in deleterious effects occurring on the surrounding ecosystems. Passive sampling using the DGT technique allows time averaged concentrations to be determined providing perhaps a more representative data of the condition’s organism are exposed to. THOE samplerswere used to investigate the fluctuations in dissolved metals a port. Each DGT device was exposed for a three-day period. During each DGT exposure period one grab sample was collected. Despite the development of heavy biofouling on the shell of the THOË sampler during the month-long… Read more

    Tonga: Investigating shallow hydrothermal sources of trace elements

    User stories
    TONGA is ainternational multidisciplinary project dedicated to the study of the control of ocean productivity and carbon export driven by micronutrients from hydrothermal origin. It is based both on a 37-day oceanographic cruise in the Western Tropical South Pacific and modelling work and involves hydrothermal geochemists, physical oceanographers, trace element chemists (ocean and atmosphere), biogeochemists, biologists and modelers.   TONGA is a multidisciplinary project dedicated to the study of the control of ocean productivity and carbon export driven by micronutrients from hydrothermal origin. It is based both on a 37-day oceanographic cruise in the Western Tropical South Pacific and modelling… Read more
  • Accessories

    Hard Plastic case


    This case holds a THOE sampler

  • FAQ
    What does THOË measure?

    The THOË autosampler does not directly measure contaminants, but uses commercially available passive samplers (DGT research) that are inserted into the autosampler. These passive samplers (DGT) are used to measure a variety of chemical contaminants. The range of DGT devices available can be found on DGT research’s website (

    What are passive samplers or DGT devices?

    Passive sampling is an environmental monitoring technique involving the use of a collecting device (DGTs) to accumulate chemical pollutants in the environment over time. This is in contrast to grab or spot sampling, which involves taking a sample directly from the water of interest at one point in time. In passive sampling, average chemical concentrations are calculated over a device's exposure time, which avoids the need to visit a sampling site multiple times to collect multiple representative samples.
    Diffusive gradients in thin films (DGT) devices passively sample chemical pollutants. They are composed of a plastic piston and cap, with a window that exposes a binding gel, diffusive gel, and filter membrane to the water being sampled. In most cases, the range of DGT devices can be used in both freshwater and marine environments. Once the mass of accumulated contaminants on the DGT sampler is known, the time averaged water concentration of the chemical pollutant can be calculated.

    How do passive samplers work?

    Passive samplers relies on the establishment of a steady concentration gradient through a plane parallel diffusive medium (referred to as the diffusive gel), one face of which is in contact with the water containing the chemical contaminant and the other in contact with a sorbent layer (or resin), which traps any target contaminant that leave the diffusive medium to enter the sorbent layer.As a result, contaminants concentrate on the sorbent layer and time averaged water concentrations can be calculated

    How do the quantification limits of passive samplers compare to traditional sampling techniques?

    Passive samplers (DGT) can in many cases achieve quantification limits lower than traditional techniques. Chemical contaminants accumulate on the sorbent layer of the passive samplertherefore, the longer the sampler is exposed to the water the lower the quantification limit that can be achieved.

    How exactly does THOË work?

    DGT devices are placed in the THOË auto sampler and THOË then sequentially exposes each DGT devices over a period of time, allowing uninterrupted time series to be collected. The sampler can expose up to 12 DGT devices over a single deployment period.

    What’s the difference between deployment time and exposure time?

    When we refer to deployment time it is the time the THOË sampler is submerged, whereas exposure time is the time each DGT devices is programmed to be exposed to the environment.

    Are there any advantages of using THOË over other automatic water samplers?

    Other automatic water samplers usually involved collecting discrete water samples in containers. The advantage of using DGT devices combined with the THOË auto-sampler is that each DGT devices collect chemical contaminants over the exposure period meaning the sample will capture contaminant concentration fluctuations over the whole exposure period. In many cases this reflects more accurately contaminant concentrations that are bioavailable to organisms than other sampling methods. Traditional discrete sampling does not do this, since collected samples only represent a single moment in time and multiple grab samples must be taken to observe variation in contaminant concentrations over time. As a result, THOË has the potential to be a less time-intensive, less expensive and provide more suitable results for monitoring chronic contamination.In addition, once the DGT device has finished being exposed, the sample is isolated and no stabilization of the sample is required, making the recovery date of the THOË less critical.

    Can I reduce the cost of my sampling?

    Cost saving can vary, but large savings can be made if high frequency sampling is required in isolated locations. In offshore environments, only two boat trips are required, one for mooring the auto-sampler and another for recovering the auto sampler. In addition, weather conditions may stop manual sampling whereas THOË will continue to sample in all weather conditions.

    How do I install THOË in the environment I wish to monitor?

    THOË has been design to be used in a range of situations, from shallow rivers to deep marine environments. It can be easily fixed on a mooring line, suspended from a fixed platform or placed on the river or seabed using optional feet.

    Can I use the THOË in any environments and under any conditions?

    THOË has been designed to withstand the harshest environments. The components are made from inert plastics or titanium to avoid corrosion. The motor housing has been designed to withstand pressure of up to 1000meters in depth.

    How reliable is THOË?

    THOË is very reliable and AEL has not only developed THOË for sale, but as an environmental consulting firm, AEL are users of the THOË auto-sampler. Therefore, AEL are fully aware of the difficulties and issues associated with sampling.
    The THOË has been designed with almost all the parts being field replaceable. The electronic and motor parts have been tried and tested for many years and use the same technology as used in TECHNICAP’s sediment traps.
    The batteries in THOË are designed last for up to 1 year. Typical deployment times will vary according to users specific needs. Generally, deployment times will be up to 3 months.

    Can I use THOË straight out of the box?

    Depending on what THOË will be used for, some cleaning may be required. Disassembly is relatively easy and only requiring a simple hex key.

    Why is the DGT exposed downwards?

    The orientation of the exposure window ensures no particles settle on the surface of the DGT that may affect the results. In addition, THOË has been designed to ensure water flows freely over the DGT surface to ensure that the water is in contact with the DGT device is replenished.

    What about regulatory acceptance?

    Passive sampling is being increasingly accepted by regulators. Despite advances in passive sampling, it is common that individual regulators will still need to approve passive sampling on a case-by-case basis.

    To determine time averaged concentrations I need to know the temperature of the water over thedeployment time?

    THOË has an integrated temperature logger that can be programmed to record at different frequencies.

    What other benefits are there in purchasing a THOË auto-sampler?

    THOË has been designed to use the same electronics and motor as the sediment traps and are interchangeable between different THOË samplers and the sediment traps. Often deployments can be staggered to use one motor for two sampling devices.

    Is biofouling or dirt build-up a problem?

    The sampler has been designed so that the opening or exposure window faces downwards to limit particles settling on the DGT’s surface. All the internal parts of the sampler are isolated from the environment within a sealed shell. When submerging the sampler, water enters the interior of the shell to equalize the pressure, but once the pressure has been equalized valves on shell seal.

    Over time biofouling may affect the performance of the DGT device and the user should perform some initial tests to ensure biofilm development does not affect the DGT’s performance. This test should be performed when exposing DGTs manually or in combination with the sampler.

    What do I do with my DGT devices after recovering the THOË sampler?

    AEL can provide an analytical service to determine contaminant concentrations, in which case it is necessary to remove the DGT device from the sampler and seal in a clean plastic bag and ship it to us.