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Pre-treatment of Microplastic Samples

During sampling a manta net is used. The manta net is made up of fine pores (very small holes), which means that whilst it is being dragged in the sea, water passes through it but most substances are trapped. Microplastics are trapped and subsequently collected, however, other substances can also be collected. Such substances include macro plastics, pieces of debris and anything else that is on or close to the surface of the water.

It is very difficult to remove these items on-site, especially since one might risk removing microplastics which could have adhered to these “unwanted” items. Therefore, once samples are taken to the lab, they are pre-treated and eventually undergo a series of steps to ensure that these substances are removed without affecting the microplastics. It is important that microplastics are not affected in any way (ex. removed by mistake, contaminated, etc.), because this would have implications on the results.

Pre-treatment involves passing the sample through a stack of sieves. Given that microplastics are typically defined as plastics which have a diameter of less than 5mm, a 5mm sieve is placed on top of another sieve which has a finer mesh/pore size. This ensures that the analysis is done on microplastics. Substances that are bigger than 5mm are not discarded, but are rinsed (to remove any microplastics that might have stuck to them) and eventually stored. Once the fraction of interest is obtained, the sample is dried. The sample is now ready to be processed and eventually analysed by physical and chemical characterisation.

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