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

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 mu m were visually sorted and manually analyzed by mu-Raman and attenuated total reflection (ATR)-FTIR spectroscopy. Microplastics <= 500 mu m were concentrated on gold-coated filters and analyzed by automated single-particle exploration coupled to mu-Raman (ASPEx-p-Raman) and FTIR imaging (reflection mode). The number of identified MPs >500 mu m was slightly higher for mu-Raman (+23%) than ATR-FTIR analysis. Concerning MPs <= 500 mu m, ASPEx-mu-Raman quantified two-times higher MP numbers but required a four-times higher analysis time compared to FTIR imaging. Because ASPEx-mu-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 mu m 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-mu-Raman for reliable MP counting down to 1 mu m.