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Southampton Marine and Maritime Institute

University of Southampton Micro-plastics research highlighted by the Optical Society

Published: 25 June 2020
Microscopy plastics

An international team of researchers including Engineering and Southampton Marine & Maritime Institute member Blair Thornton and Institute for Life Science members Tomoko Takahashi and Sumeet Mahajan recently published an article that was highlighted by the Optical Society. The paper, published in the Journal Applied Optics, reports a new method to identify different types of microplastic particles in the ocean.

If you took a litre of seawater from depths of more than a few hundred metres in the open ocean, you would only expect to find a few micrometer sized particles in it. Even so, the cumulative effect of these particles on various chemical cycles in the ocean is thought to be globally significant.

Unfortunately particles form a blind spot for most of the chemical sensing methods that are available for ocean monitoring. To understand their distribution, scientists typically need to recover particles using filters or traps that accumulate particles from seawater over a long period of time. These are then studied back in a laboratory using optical methods like microscopic imaging and spectroscopy to identify the abundance of different particle types. Although this provides a detailed snapshot of marine particles at a particular location over a specific time window, the approach requires expensive research vessels to be deployed for each observation and this limits the resolution of studies that can be performed. It is also not ideal for measuring material that decomposes with time or changes in temperature, pH and pressure from on site conditions. A large proportion of particles in the ocean are also alive, exhibiting behaviours like predation that can only be observed in situ.

The approach developed in this paper describes how instead of bringing particles to the laboratory, we can instead bring the laboratory to the particles. By combining both holographic imaging and Raman spectroscopy, both the shape and composition of particles just a few micrometers in size can be measured on site, out in the open ocean. A key feature of the method is its ability to scan a large volume at high resolution. The setup demonstrated in the paper has a maximum throughput of ~2L/s of water in which it can detect the presence of a particle. Once a particle is detected, the method allows for imaging at a resolution of 5 micrometres (1/20th of the diameter of a strand of hair) anywhere in a 20cm long channel and Raman spectroscopy measurements to determine what the particle looks like and is made out of. The paper demonstrates how the method could be used to identify different types of major plastic particle now found in the ocean. The method does not need any sample preparation and can generate results in realtime. The setup is also compact enough to build a sensor for oceanic gliders and floats, which could allow for long-term ship-free monitoring of particles in the ocean in the future.

The paper was selected as the Editor's pick, and was featured up by the Optical Society with a highlight media article linked to the work.

The paper (open access) can be seen here.

The associated media article with video clip can be seen here.

The research project (RamaCam, PI Blair Thornton, University of Southampton, SMMI member, IRIS FEPS Centre of Excellence co-director) was jointly funded by the UK and Japanese governments under the NERC-JST SICORP Marine Sensor Proof of Concept program. More information about the project can be found here.

More information about intelligent sensing in extreme environments can be found here.

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