Eective Plastic Isolation
Researchers are presented with a range of techniques for isolating microplastics from biota, including dissection, depuration, digestion and density separation. Determining the appropriate method will largely depend on the research question (e.g., risks of human consumption, total body burden, localized accumulation). Digestion of whole organisms or excised tissues is widely used (Table 8.2), however caution must be given in selecting an appropriate digestive agent because of the potential destruction of contaminants. For example, >50% formic acid, >35% НСЮ4, >40% hydrofluoric acid, >80% H202, >50% HNO;, >70% HC104, >50% KOH and >95% sulfuric acid can be particularly damaging to specific polymers. Some digestive agents, including H202 and НСЮ4, are simply ineffective in breaking down tissues. Our analysis found that some recent studies used high percentages of acids to which several polymers are not resistant. In particular, high percentages of НСЮ4 have been utilized in several studies [16,23,38,51,52,54,60,61,105]. For example, only PET and PVC are resistant to 50% HNO,, whereas, PE and PP are partially resistant and PA, PC and PS are not resistant. Typically, a balance will need to be struck between finding a cost-effective digestive agent with the capacity to effectively break down tissue without losing microplastics. Based on our analyses, the rapid 10% KOH (60°C overnight) [58,135] and enzymatic digestion protocols [12,140] appear, on balance, to be among the most widely tested and effective digestive treatments currently available; in all cases, the costs, strengths and weaknesses, and applicability of each method to the study organism in question, should be carefully considered. As with sampling, steps for mitigating and accounting for external contamination are paramount.
Methods for verifying isolated microplastics vary in complexity and expense. The method used is dependent not only on resources available to the research group, but also the degree of information required by the study. Studies examining the total body burden or the rate of uptake may only require the most cursory identification to confirm that the particles recovered are indeed plastic, whereas research examining the potential origins of plastics or the presence of adsorbed contaminants and additives require more rigorous testing. We concur with the European standard of polymeric identification of a 5%-10% subsample of isolated microplastics. While it has been suggested that this level is insufficient to accurately determine the ability of the researcher to accurately identify plastics, this can be improved when selecting subsamples for verification. Researchers must ensure a representative sample encompassing all categories of recovered microplastics, and particular consideration should be given to commonly misidentified forms, such as fibers and small size fractions. Where possible, researchers should include the weight and number of plastic items, and the masses of sampled organisms should be recorded in cases where animals are grouped prior to analysis. Such standardization will improve comparability between studies.