Detection of Sub-20 μm Microplastic Particles by Attenuated Total Reflection Fourier Transform Infrared Spectroscopy and Comparison with Raman Spectroscopy

Microplastics are particulate water contaminants that are raising concerns regarding their environmental and health impacts. Optical spectroscopy is the gold standard for their detection; however, it has severe limitations such as tens of hours of analysis time and spatial resolution of more than 10 mu m, when targeting the production of a 2D map of the microparticle population. In this work, through a single spectrum acquisition, we aim at quickly getting information about the whole population of identical particles, their chemical nature, and their size in a range below 20 mu m. To this end, we built a compact setup enabling both attenuated total reflection Fourier transform infrared (ATR-FTIR) and Raman spectroscopy measurement on the same sample for comparison purposes. We used monodisperse polystyrene and poly(methyl methacrylate) microplastic spheres of sizes ranging between 6 and 20 mu m, also measured collectively using a bench-top FTIR spectrometer in ATR mode. The ATR-FTIR technique appears to be more sensitive for the smallest particles of 6 mu m, while the opposite trend is observed using Raman spectroscopy. We use theoretical modeling to simulate and explain the ripples observed in the measured spectra at the shortest wavelength (higher wavenumber) region, which appears as an indicator of the microparticle dimension. The latter finding opens new perspectives for ATR-FTIR for the identification and classification of populations of nearly identical micro-scale bodies, such as bacteria and other micro-organisms, where the same measured spectrum embeds dual information about the chemical nature and the size.

Automated summary

Microplastics are a type of particulate water contaminants that have raised concerns due to their potential environmental and health effects. This study aims to quickly gather information about the population of identical microplastic particles, including their chemical nature and size below 20 μm, through a single spectrum acquisition. The researchers built a compact setup for attenuated total reflection Fourier transform infrared (ATR-FTIR) and Raman spectroscopy measurements on the same sample, and found that ATR-FTIR was more sensitive to particles of 6 μm in size, while Raman spectroscopy showed the opposite trend. The study also used theoretical modeling to explain the ripples observed in the spectra, which can serve as an indicator of microparticle dimension.