Combining ATR far- and mid-infrared spectroscopy to distinguish native Australian plant exudates for cultural heritage analysis

Native Australian plant exudates are an important material for a variety of cultural uses including hafting materials and pigment binders. Spectroscopic analysis of exudates informs on their composition, properties, use and conservation status. However, native Australian exudates are not as well characterized as European species, although there are often chemical parallels between the two. While mid-IR has been demonstrated as an effective and standard method to distinguish some key exudate species, the fingerprint-region characterisation can become challenging for a particular group of exudates due to spectral similarities or overlaps, and therefore discrimination is not easily achievable. Therefore, the complementary data on molecular interactions observed in the far-IR region can provide additional means to characterize and discriminate between genera. In this study, samples from European artist materials and native Australian exudates were studied by both laboratory-based mid-IR and synchrotron based far-IR. Results from this work include establishing a novel non-destructive far-IR method for plant exudates such as resins, gums and kinos on the molecular level, as well as multivariate statistical analysis to differentiate at both the genera and species level. These outcomes identify an innovative approach to understanding these complex molecular chemistries of plant exudates as well as a method to characterize resins, kinos and gums in important archaeological and cultural heritage materials in Australia and worldwide.