Evaluation of filter substrates for concurrent optical photothermal infrared and Raman spectroscopy in microplastic...

Optical photothermal infrared (O-PTIR) spectroscopy is an emerging technique to analyze submicron mid-infrared absorption. It can be combined with concurrent, co-located Raman spectroscopy on the same instrument platform, offering considerable potential for microplastic analysis. However, since measurements are typically performed directly on collection filters, substrate selection becomes a critical yet un-examined factor for these concurrent modalities. Here, we characterized nine commercially available filter substrates, namely Anodisc, silicon, gold-coated PET, gold-coated PC, silver, glass and quartz microfiber, PC, and cellulose, for O-PTIR and Raman analysis using 14 µm and 7 µm PMMA microplastic beads. Spectral fidelity was quantified via hit quality indices against a CaF₂ reference. Our results show that substrate suitability is governed by modality-specific requirements. O-PTIR demands mid-infrared spectral neutrality and is additionally sensitive to the substrate surface, whereas the quality of Raman spectra primarily depends on the absence of substrate Raman bands. Beyond spectral match quality, the reproducibility of PMMA spectra is consistently higher in the Raman channel than in the O-PTIR channel. This difference is attributed to O-PTIR's sensitivity to local thermal contact and surface morphology, which introduce measurement variability absent in direct Raman scattering. Of the tested substrates, Anodisc and gold-coated PET performed the best in both modalities and with both particle sizes. These findings demonstrate that recommendations for substrates established for conventional FTIR or Raman microscopy cannot be transferred directly to O-PTIR. They also provide an evidence-based framework for selecting substrates in multimodal microplastic analysis workflows.