Effect of sample presentation on the identification of polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS) using a portable near infrared instrument

The quality of both the data and the sampling method plays an essential role in model development in near infrared (NIR) spectroscopy. Among the different factors, sampling presentation is key during the development of a NIR application, particularly when portable instruments are used to analyse microplastics (MP). The objective of this study was to evaluate the effect of sample presentation mode (e.g. ceramic, aluminium foil and glass vial) on the classification of different MPs, such as polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS) using a portable NIR instrument. The results demonstrated that NIR spectroscopy could classify PP, PET, and PS microplastics, although classification accuracy varied depending on sample presentation. The PCA score plots showed differences between the sampling modes used to collect the spectra. Furthermore, the PLS models indicated that their performance depends on the sampling mode used where the coefficient of determination in cross-validation (R²_cv) ranged between 0.79 and 0.90. The results also showed that NIR spectroscopy was able to classify PP, PET, and PS with accuracies ranging from 73 to 100 %, depending on the sample presentation mode utilised. The development and implementation of rapid analytical methods, such as NIR spectroscopy, provides a non-destructive and green tool to identify the type of MP analysed.