Comparison of Raman and Fourier Transform Infrared Spectroscopy for the Quantification of Microplastics in the Aquatic Environment

Abstract

Microplastics (MPs, <5 mm) have been reported as emerging environmental contaminants, but reliable data are still lacking. We compared the two most promising techniques for MP analysis, namely, Raman and Fourier transform infrared (FTIR) spectroscopy, by analyzing MPs extracted from North Sea surface waters. Microplastics >500 μm were visually sorted and manually analyzed by μ-Raman and attenuated total reflection (ATR)-FTIR spectroscopy. Microplastics ≤500 μm were concentrated on gold-coated filters and analyzed by automated single-particle exploration coupled to μ-Raman (ASPEx-μ-Raman) and FTIR imaging (reflection mode). The number of identified MPs >500 μm was slightly higher for μ-Raman (+23%) than ATR-FTIR analysis. Concerning MPs ≤500 μm, ASPEx-μ-Raman quantified two-times higher MP numbers but required a four-times higher analysis time compared to FTIR imaging. Because ASPEx-μ-Raman revealed far higher MP concentrations (38–2621 particles m–3) compared to the results of previous water studies (0–559 particles m–3), the environmental concentration of MPs ≤500 μm may have been underestimated until now. This may be attributed to the exceptional increase in concentration with decreasing MP size found in this work. Our results demonstrate the need for further research to enable time-efficient routine application of ASPEx-μ-Raman for reliable MP counting down to 1 μm.

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

“…These issues are pronounced in the detection of smaller sized MP (<100 mm) that may not be distinguished with a light microscope and are hard to manually select for further analysis. 41 Several solutions are currently available to aid with these limitations like particle finder mechanisms 40,48,49 or imaging systems, [50][51][52][53] but increase the cost for the spectroscopic instruments. As an alternative, the use of MP dyes or stains act as a cost and time effective method that has the potential to reduce researcher bias and include smaller MP 54 in count data.…”

“…Due to the diffraction limit, 238 the LOD of FT-IR is currently discussed between 20 and 10 mm, while the determined size distribution was confirmed by comparison with Raman 40 down to 10 mm. Particles < 10 mm are challenging to measure 40,185 with FT-IR and need to be identified by additional or combined techniques. Further, measurements cannot be performed in presence of water as its spectrum will overlay the target spectra.…”

Critical Review of Processing and Classification Techniques for Images and Spectra in Microplastic Research

“…Moreover, there is no mention of automated spectral matching strategies for EDS spectra, in contrast to IR, Raman, and Py-GC-MS spectra, for which extended libraries and tools exist. 46,74,104 Good Practices for Developing In-House Reference Library and Matching Tools Since most of the reviewed studies developed their own reference libraries and matching tools but did not expand upon the considerations involved, we highlight some of the most important aspects in developing a spectral reference library: (i) Keep thorough metadata records. Spectra can differ between labs dependent on their techniques for obtaining clear spectra, and their individual machinery.…”

“…Here, we focus on QA/QC of analysis procedures, which has been conducted in some microplastic studies. 9,46,55,60,[134][135][136] The most commonly used analysis validation techniques for visual and spectral procedures were blank 110,137,138 and positive controls. [139][140][141] Visual QA/QC techniques are currently more qualitative and manual than the quantitative and automated QA/QC procedures used in spectral matching.…”

“…Two publications shared in-house spectral libraries. 9,46 The most common type of data that was shared in publications were images of plastics that had been visually identified. 73,145,146 Opportunities for Data Storage and Sharing…”