## 4 Conclusions  
This study presents the results obtained from the analysis of nineteen pigments by infrared spectroscopy comparing Fourier Transform Infrared-Spectroscopy in Attenuated Total Reflection (ATR-FTIR) and Diffuse Reflectance (DRIFT) modalities. Prior characterization using X-ray diffraction (XRD) played a crucial role in identifying characteristic bands assigned to impurities and/or additives, highlighting discrepancies between the actual composition of the pigments and the information provided by manufacturer. Furthermore, additional mineral phases were detected through their infrared characteristic bands in both ATR-FTIR and DRIFT modalities, which were not identified by XRD--likely due to their low concentration--falling below the detection threshold (ca. 3 wt.%) of the XRD equipment. With regards to FTIR modalities, the DRIFT modality proved to be highly effective at differentiating pigments with similar chemical natures such as silicates, oxides, carbonates, sulphides and acetates. Also, DRIFT enabled the distinction between natural and synthetic analogues by detecting impurities in natural pigments, as well as manufacturing-related compounds or additives, mainly quartz and calcite, in synthetic ones. These features generally appeared at higher wavenumbers and with higher intensity compared to the spectra obtained by ATR-FTIR modality. DRIFT modality offers notable advantages, particularly its enhanced sensitivity in detecting organic compounds. Its non-destructive nature allowed for the identification of spectral bands associated with organic substances, which may be linked to the levigation process. These organic-related bands were observed in all analysed pigments except for Egyptian blue, synthetic blue ultramarine, chrysocolla, synthetic hematite and verdigris. However, DRIFT also presents certain limitations, including the presence of spectral interferences from atmospheric moisture (broad bands between 4000 and 3100 cm\({}^{-1}\)) and carbon dioxide (doublet band between 2400 and 2300 cm\({}^{-1}\)). These external contributions can complicate spectral interpretation and affect analytical precision. In all, this study sets the foundation for addressing the uncertainty related to the interpretation of the spectra obtained by DRIFT modality. The results presented are considered a starting point for the identification of pigments in pictorial artworks (whatever their nature--mural or easel--or the painting technique) for in situ characterization, without the need for sampling, thus respecting the integrity of the original work. However, given the complexity of heritage materials, it is common practice in heritage science to integrate multiple analytical techniques. Therefore, combining DRIFT spectroscopy with complementary methods such as Raman spectroscopy enhances the depth and accuracy of pigment characterization, providing more comprehensive information. Finally, future studies involving the analysis of real pictorial works should consider the results obtained here in order to provide veracity and establish differences that will help the scientific community. Conceptualization, D.J.-D. and J.S.P.-A. ; methodology, D.J.-D. and J.S.P.-A. ; software, D.J.-D. and J.S.P.-A. ; validation, D.J.-D. and J.S.P.-A. ; formal analysis, D.J.-D. and J.S.P.-A. ; investigation, D.J.-D. and J.S.P.-A. ; resources, J.S.P.-A. ; data curation, [PERSON]-D.; writing--original draft preparation, D.J.-D.; writing--review and editing, [PERSON]-A.; visualization, [PERSON]-D. and [PERSON]-A.; supervision, [PERSON]-A.; project administration, [PERSON]-A.; funding acquisition, J.S.P.-A. All authors have read and agreed to the published version of the manuscript. This research was funded by the PID2021-123395 OA-100 research project funded by MICIU/AEI/10.13039/501100011033, by \"ERDF A way of making Europe\" and, by the Xunta de Galicia ED431F 2022/07 project.