The most frequently used method to quantify microplastics (MPs) visually by microscope is time consuming and labour intensive, where the method is also hindered by the size limitation at 10 µm or even higher. A method is proposed to perform pre-concentration of MPs by vacuum filtration, hydrogen peroxide wet digestion, fluorescent staining and flow cytometric determination to rapidly detect and quantify small MPs sized from 1–50 µm. The method performance was evaluated by the spiking of seven different types of polymer, including polystyrene (PS), low-density polyethylene (LDPE), polypropylene (PP), poly(methyl methacrylate) (PMMA), polyvinyl chloride (PVC), polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) at different levels (400, 4000, 40,000 particles mL⁻¹), with a satisfactory overall % recoveries (101 ± 19.4%) observed, where in general no significant difference between the two methods was observed. Furthermore, a pre-concentration process by vacuum filtration was introduced to reduce the matrix effect. After pre-concentration, satisfactory % recoveries and accuracy in MP counts resulted from both ultrapure water (94.33 ± 11.16%) and sea water (103.17 ± 9.50%) samples. The validated method using flow cytometry can be used to quantify MPs in environmental water samples that can reduce time and human resources.

When handling plastic materials in laboratory settings, there is a real risk of unintentional contamination from the surrounding environment that can compromise experimental results. Common organic compounds, such as chlorinated organophosphate flame retardants, are known to persist on surfaces, including laboratory benches, and may transfer to plastic materials during routine handling, becoming a source of contamination in ecotoxicological tests. Our study offers a concrete example of this issue. The contamination observed was not part of the original hypothesis and only came to light after the study was completed and further quality control analyses were conducted. The original aim of the study was to assess the effects of polyethylene (PE) films leachates on the red macroalga Ceramium tenuicorne , using a suite of biomarkers including growth and antioxidant responses. The experimental setup involved leachates prepared from virgin and thermally aged PE films, and focused on potential effects of additives being present in the leachates. While TDCPP [tris(1,3-dichloro-2-propyl) phosphate] was not detected in the PE film itself, it was unexpectedly found in the leachates. This result raised concerns and led us to perform additional chemical analyses, including surface swabs from laboratory benches, lab coats, and equipment, which confirmed that TDCPP was widely present throughout the laboratory environment. These findings pointed to contamination occurring during material handling as the likely source of TDCPP in the leachates, and, consequently, the cause of the observed biological effects. Here, we present and discuss the experimental outcomes in light of this finding, highlighting how even subtle contamination can shape biological responses and lead to incorrect interpretations. The study underscores the importance of rigorous chemical verification of both materials and experimental environments in studies involving low-dose exposures. It also serves as a cautionary tale, illustrating how undetected contamination can mislead scientific conclusions despite careful design and execution.