Analysing for Microplastics
Once samples are pre-treated, they are then processed.
This involves the following steps:
Chemical digestion: Digestion is done to remove organic matter from the sample. This is important because organic matter can interfere with both physical identification (visual) as well as with chemical characterisation of the microplastics. There are several chemical digestion methods, these are typically classified as oxidative, acidic, alkaline/basic and enzymatic. All methods have their pros and cons and in Project IMPACT oxidative digestion was carried out using Fenton’s reagent, a solution of hydrogen peroxide with ferrous iron as a catalyst.
Density separation: Plastics have different densities. This may be due to the polymer type that characterises the plastic, as well as due to the addition of chemicals during manufacturing and modification brought about by the degradation process in the sea. Density separation involves physical separation of the microplastics from the rest of substances in the sample. This is done by using the density differences to separate different types of polymers from organic and inorganic natural particles. The process involves mixing the sample with a dense solution (such as Sodium Chloride, NaCl) and agitating it for a period of time.
Filtration: After agitation, the sample is allowed to settle. During this process the more dense substances sink and the less dense particles float or stay in suspension. Substances that settle at the bottom form a solid residue and are known as the precipitate, whilst the liquid lying above this residue is known as the supernatant. While particles can be extracted directly from the liquid surface, most often the supernatant is then filtered.
After filtration, the samples are visually inspected, followed by chemical analysis using Fourier Transform Infrared (FTIR).