Hot or not: systematic review and laboratory evaluation of the hot needle test for microplastic identification

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Microplastics are small (< 5 mm) synthetic polymers that are a contaminant of emerging concern and can be difficult to identify due to their diversity in size, shape and composition. The hot needle test, or hot point test, helps researchers identify suspected microplastics under optical microscopy by probing their physical melt or deformation behavior, and is a low-cost and practical method for widespread use. However, to our knowledge the accuracy of this test has not been fully evaluated. We noted that articles commonly referenced by researchers for the hot needle method do not have a detailed description nor evaluation of the method accuracy. To address this knowledge gap, we took a mixed methods approach to describe the conditions under which the hot point test performs accurately, including a systematic literature review, reporting of the response of known fibers to a hot point, and evaluation of method performance by researchers in both controlled and environmental samples. In a single-blind trial of researchers applying different hot point conditions to a set of synthetic, semi-synthetic and natural fibers, synthetic and some natural fibers were correctly identified > 70% of the time. While cotton and semi-synthetic fiber results were less consistently identified (< 65% correct), this was improved (82–100% correct) in a second trial when clearer, updated guidance was given regarding the difference between a “pass” and “fail” response, showing the potential for the hot needle test to help analysts avoid false positives. Cellulose acetate from cigarette filters was the most challenging to identify because although this material may melt, response of individual fibers to heat varies and can be difficult to observe for smaller microfibers. Reported confirmation rates by spectroscopy of suspected microplastics that pass the hot needle test vary widely in the literature. Using detailed hot needle test criteria, > 90% of microplastics that we selected from environmental samples (water, sediment) were confirmed by Raman microscopy. It is recommended that researchers assess their hot needle test methods against known standards of both target microplastics and background materials like natural fibers, report the response criteria used in their studies and optimally include spectroscopic verification of results for higher confidence.