Infrared characterisation of fossils down to the nanoscale: Insights from an insect in amber

• O-PTIR and AFM-IR techniques achieve micrometric and nanometric lateral resolutions. • An extremely flat surface is necessary for accurate O-PTIR and AFM-IR analyses. • Only AFM-IR records fossil spectra with no to little contribution from amber. • These techniques can resolve micrometric chemical heterogeneity in ancient material. IR spectroscopy is a powerful tool for characterising the chemistry and chemical variability of organic remains in fossils. Optical-Photothermal InfraRed (O-PTIR) and Atomic Force Microscopy-InfraRed (AFM-IR) are novel IR spectroscopy techniques based on the photothermal effect that are able to bypass the diffraction limit imposed to classical IR techniques to reach micrometric and nanometric lateral resolutions respectively. Here we introduce a novel methodology to characterise, across multiple spatial scales, the heterogeneity of a fossil-amber interface. By employing an ultramicrotomy-based sample preparation, we demonstrate the potential of such an approach to collect both IR spectra and maps, revealing key information on the chemical heterogeneity within the amber. This includes insights into the potential hydrolysis of the amber filling the inclusion, as well as the infrared signature of the fossilised cuticle, demonstrating the inherent complementarity of each technique. Based on the case study of the long-term interactions of insect cuticles fossilisation and amber, we evaluate the strengths and limitations of each technique, both technically and experimentally, to assist future users in assessing their suitability for the study of ancient materials.