Optical Photothermal Infrared Spectroscopy – a new reference method for microplastic identification

, Böke Julia Sophie, Krafft Christoph.

Material identification is key to assessing the concentration and abundance of microplastic particles in a sample. So far, which method is best suited to probe microplastic particles is still under debate. In 2016 Käppler et al. (Anal Bioanal Chem 408, 8377–8391) raised the question of whether MP analysis is performed best with FTIR or Raman spectroscopy. IR and Raman-based approaches are attractive candidates due to their label-free fingerprint capability. However, limitations include sensitivity, specificity, identification rate, reproducibility, throughput and particle size. Their conclusion that a combined approach would lead to the highest accuracy seemed unfeasible due to the time and cost of dual-analysis, making routine analysis less probable. In response, we present an inter-system comparison of data from nine reference polymers that quantifies the spectral reproducibility between standalone FTIR and Raman instruments with the quantum cascade laser-based optical photothermal infrared (O-PTIR) spectroscopy, which simultaneously enables acquiring IR and Raman spectra from the same location. As a measure for spectral matching, we introduce the two-dimensional hit quality index (2D-HQI) using both Raman and IR spectra for material identification.

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