An integrated spectral library and analysis workflow for laboratory identification of microplastic polymers

Abstract Microplastics are an increasing environmental concern, and their reliable identification requires fast, reproducible, and non-destructive analytical approaches. This study presents a reflectance spectral library and a dedicated software tool, the Microplastic Spectral Analysis App version 3.1, for organizing, visualizing, comparing, and classifying microplastic spectra. The library contains reflectance spectra polyethylene terephthalate, high-density polyethylene, polyvinyl chloride, low-density polyethylene, polypropylene, and polystyrene measured across the visible, near-infrared, and short-wave infrared range under controlled laboratory conditions. The samples represented multiple nominal sieve fractions and selected colours. Spectral profiles showed descriptive differences associated with polymer type, nominal sieve fraction, and colour. Principal component analysis was used to visualize polymer-related grouping patterns among transparent samples. In group-level cross-validation, the support vector machine provided the best polymer-type classification, with an out-of-fold accuracy of 0.649, whereas the random forest provided the best nominal sieve-fraction classification, with an out-of-fold accuracy of 0.355. The library and software provide an integrated framework for organizing and reusing laboratory microplastic reflectance spectra.