Identification of microplastics by FTIR and Raman microscopy: a novel silicon filter substrate opens the important spectral range below 1300 cm−1 for FTIR transmission measurements

Abstract

The presence of microplastics in aquatic ecosystems is a topical problem and leads to the need of appropriate and reliable analytical methods to distinctly identify and to quantify these particles in environmental samples. As an example transmission, Fourier transform infrared (FTIR) imaging can be used to analyze samples directly on filters without any visual presorting, when the environmental sample was afore extracted, purified, and filtered. However, this analytical approach is strongly restricted by the limited IR transparency of conventional filter materials. Within this study, we describe a novel silicon (Si) filter substrate produced by photolithographic microstructuring, which guarantees sufficient transparency for the broad mid-infrared region of 4000-600 cm(-1). This filter type features holes with a diameter of 10 μm and exhibits adequate mechanical stability. Furthermore, it will be shown that our Si filter substrate allows a distinct identification of the most common microplastics, polyethylene (PE), and polypropylene (PP), in the characteristic fingerprint region (1400-600 cm(-1)). Moreover, using the Si filter substrate, a differentiation of microparticles of polyesters having quite similar chemical structure, like polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), is now possible, which facilitates a visualization of their distribution within a microplastic sample by FTIR imaging. Finally, this Si filter can also be used as substrate for Raman microscopy-a second complementary spectroscopic technique-to identify microplastic samples.

Sea Water Contamination in the Vicinity of the Italian Minor Islands Caused by Microplastic Pollution

“…As highlighted by other studies [8,9,31,45,46], a quite high spatial variability was detected in polymer distribution and plastic concentration. The samples were also analyzed to determine PCB and OC contamination, both derived from natural and anthropogenic sources.…”

“…The polymer identification was carried out by comparing the acquired spectra with reference ones from both commercial and specifically developed spectral libraries, and by checking each result using the spectrum's peak by peak identification. This analytical approach was possible because almost all the single microparticles were in the range of 5000-300 µ m, therefore allowing visual sorting and manipulation [24,31]. Some polymers (e.g., polyethylene-terephthalate and polyester) were grouped in the μFT-IR results, as well as other natural/man-made materials (cellulose, wood, rayon).…”

Microplastic in Aquatic Ecosystems

“…Fors maller particles,t he combination of FTIR with an optical microscope has to be applied, so-called micro-FTIR, which can be used in all three modes.S ince single-particle analysis is not feasible,t he MP particles are normally collected on af ilter.I nt ransmission mode,t he light passes through the sample and is collected afterwards.Consequently, only MP samples of acertain thickness can be evaluated and the filter has to be IR transparent (e.g.,a luminum oxide or specially fabricated silicon filters [33,41] ). Thes ample is placed on an ATRc rystal, and the surface is irradiated with an evanescent wave.This enables an FTIR analysis of larger MP with fast and precise identification.…”

Microplastics in Sumba waters, East Nusa Tenggara

“…We analyzed microplastics with sizes more than 250µm using Fourier Transform Infrared (FT-IR). FT-IR can be used to analyze microplastics samples directly [29]. Polymer analysis was done using Nicolet™ iS5 FT-IR Spectrometer, equipped with a laminated diamond crystal Thermo Scientific™ iD5 attenuated total reflectance (ATR) accessory, and corrected using Omnic tm Software.…”