Quantitative Reliability of μ-FTIR-Based Microplastic Analysis: Effects of Filtration, Rinsing, and Software Calibration

Methodological variability remains a major source of uncertainty in environmental microplastics (MPs) analysis, particularly for micro-Fourier transform infrared (μ-FTIR) spectroscopy operated in reflection mode. This study quantitatively evaluates how key analytical procedures influence recovery efficiency and identification performance in μ-FTIR-based MPs analysis. Polystyrene (PS) standard particles (90 μm and 30 μm; density 1.05 g/cm3) were employed in direct titration and standard addition experiments. The effects of filter materials, rinsing suspensions, filtration approaches, and identification protocols were systematically assessed. Under the tested reflection-mode μ-FTIR conditions, silicon and stainless-steel filters provided sufficient spectral readability and microscopic particle visibility for PS particle recognition and were therefore selected for subsequent recovery evaluation. Rinsing with 50% ethanol reduced aggregation and enhanced recovery. Pump-assisted filtration (200 mmHg) achieved high PS recovery (88 ± 7.25%), and standard addition in river samples reached 91 ± 14.5%. Manual calibration using reference standards further improved classification consistency. These findings quantitatively link procedural choices to recovery and identification outcomes, providing practical guidance to improve reliability and inter-study comparability in μ-FTIR-based MPs analysis.