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Fluorescence microscopy versus Raman spectroscopy for direct identification of small (< 2 µm) microplastics in soils
Summary
Researchers compared fluorescence microscopy and Raman spectroscopy for detecting very small microplastics (1-2 micrometers) in soil samples ranging from pure sand to complex organic-rich soils. Fluorescence microscopy was faster and cheaper but less specific, while Raman spectroscopy provided reliable polymer identification even in complex matrices. The study provides practical guidance on selecting detection methods based on sample complexity and size range of interest.
Research on microplastics (MP) in soils is much complicated due to the lack of dedicated (extraction) methodologies and the strong matrix interferences for MP detection, and there is almost no research on the dynamics of the smallest MP in soil. Here we compared the possible detection of the smallest MP fraction (1-2 µm) by µ-Raman spectroscopy and fluorescence microscopy in matrices of highly varying complexity. Samples of pure quartz sand, soil with removal of native soil organic matter (SOM), and soil with native SOM still present were amended with fluorescent polystyrene (PS) microparticles (diameter 1.65±0.04 µm) in different concentrations ranging from 0.1 to 0.001%, and after mixing and compaction both the Raman spectra and fluorescence microscopy images were obtained. Characteristic PS Raman peaks (main peak at 1009 cm-1) were visible in quartz sand (all concentrations) and soil without SOM (highest concentration only), but not in the other situations, whereas fluorescence microscopy clearly visualized the MP at all concentrations in all matrices. The possibility of direct and unambiguous fluorescent MP detection in real soil also circumvents the need for lengthy extraction procedures, and opens up new avenues for studying mechanistic aspects of the smallest MP fractions in soil.