A “six-point S-shaped” sampling strategy based on micro-Raman spectroscopy enabling the rapid and accurate detection of small-sized microplastics in soil

Microplastic contamination in agricultural soils is of increasing concern, yet non-destructive detection of small-sized microplastics remains challenging due to the resolution limitations of FTIR spectroscopy and the time-consuming nature of traditional Raman imaging. Herein, we proposed an extrapolation method based on a "six-point S-shaped" sub-region sampling strategy for the high precision and ultra-efficient micro-Raman detection of microplastics. An optimized sample preparation protocol using 300 mg/L sodium dodecyl sulfate effectively facilitated the uniform dispersion of microplastics on excellent filter membranes. Six representative sub-regions were designed to reflect the actual particle distribution, including the center, transition region, and the edge of the membrane. Impressively, this strategy achieved over 91 % accuracy while reducing detection time by over 90 % and 70.6 % as compared to full-membrane detection and the fastest existing extrapolation method, respectively. Adjustments to image acquisition origin and spectral scanning parameters enabled the first rapid detection of soil microplastics as small as 1 μm. Furthermore, field application revealed that the microplastics in soil predominantly originate from polyethylene mulching residue, with smaller-sized particles being the most prevalent. All these results highlight the high efficiency and sensitivity of microplastic detection in soils, offering solid support for ecological risk assessment and management of agricultural environments.