Development of an optimal filter substrate for the identification of small microplastic particles in food by micro-Raman spectroscopy

Abstract

When analysing microplastics in food, due to toxicological reasons it is important to achieve clear identification of particles down to a size of at least 1 μm. One reliable, optical analytical technique allowing this is micro-Raman spectroscopy. After isolation of particles via filtration, analysis is typically performed directly on the filter surface. In order to obtain high qualitative Raman spectra, the material of the membrane filters should not show any interference in terms of background and Raman signals during spectrum acquisition. To facilitate the usage of automatic particle detection, membrane filters should also show specific optical properties. In this work, beside eight different, commercially available membrane filters, three newly designed metal-coated polycarbonate membrane filters were tested to fulfil these requirements. We found that aluminium-coated polycarbonate membrane filters had ideal characteristics as a substrate for micro-Raman spectroscopy. Its spectrum shows no or minimal interference with particle spectra, depending on the laser wavelength. Furthermore, automatic particle detection can be applied when analysing the filter surface under dark-field illumination. With this new membrane filter, analytics free of interference of microplastics down to a size of 1 μm becomes possible. Thus, an important size class of these contaminants can now be visualized and spectrally identified. Graphical abstract A newly developed aluminium coated polycarbonate membrane filter enables automatic particle detection and generation of high qualitative Raman spectra allowing identification of small microplastics.

Optimization, performance, and application of a pyrolysis-GC/MS method for the identification of microplastics

“…Furthermore, the use of imaging techniques coupled to spectroscopic approaches allows automatization of MP identification [16][17][18]. In addition to spectroscopic methods, another type of chemical identification is thermal analysis [12].…”

TUM-ParticleTyper: A detection and quantification tool for automated analysis of (Microplastic) particles and fibers

“…The general scheme for single particle analysis of MP is a workup step for the extraction and purification of MP [10] after which all remaining particles-microplastic as well as residual environmental colloids-are deposited on a smooth filter surface. The smoothness of the filter is of high importance, as any subsequent measurement, be it Fourier-transform infrared spectroscopy (FTIR) or confocal Raman microspectroscopy, will depend on a flat surface to enable optimal focus on the particles [11,12]. This is especially true if automated routines are used, where particles are first identified by acquiring images for a morphological assessment, including the determination of the particle centers for the subsequent measurement.…”

Critical Assessment of Analytical Methods for the Harmonized and Cost-Efficient Analysis of Microplastics

“…For larger particles (>300 mm), it is advised to adhere particles to a flat base material such as double-sided tape on transparency paper, 244 or an aluminum sheet to minimize background fluorescence. 245 Particles should be clearly characterized (e.g., circled and numbered) to aid particle identification using the Raman microscope. For smaller particles (<300 mm) that cannot be manually picked, particles should be analyzed directly from a filter to avoid contamination or particle loss.…”

“…The favored filter materials are metal covered PC membranes ($US$20-50), e.g., gold coated 40,49,242 or aluminum coated. 245 Käppler et al 159,185 suggested a specially fabricated silicon membrane ($US$10-30) as a possible filter material for FPA-FT-IR and RM analysis, which was limited in pore size to 10 mm. In previous studies, aluminum oxide (e.g., Anodisc) membranes ($US$5-20) or GF filters were recommended (for particles > 100 mm).…”