## 1 Introduction
However, in order to apply ATR (with benchtop or portable equipment), it is necessary to exert pressure with the equipment plunger against the FTIR crystal (made up of diamond, germanium or zinc selenide), and is therefore destructive. 3. Lastly, reflectance modality can also be used and is recommended when samples cannot be taken from the artwork and in situ measurements are needed. Here, measurements can be performed in three different manners as follows: (1) reflection-absorption where the IR light passes through a very thin sample and reflects on a reflective substrate, which is useful for analysing thin tissues or coatings; (2) specular reflection, where the IR light is bounced off a reflective surface, which is useful for examining samples like polymers, gemstones, metals, thin films, glass, etc. ; and (3) diffuse reflection, where the light is scattered off a sample surface (Diffuse Reflectance Infrared Fourier Transform spectroscopy, DRIFT), which is commonly applied to characterise rough and matte surfaces such as stones and paintings. Regarding results, ATR-FTIR spectra are like those acquired in transmission modality, as stated by [PERSON] et al. [9]. However, [PERSON] et al. [10] compared the transmission and DRIFT spectra of several blue pigments and observed significant differences. It was concluded from their findings that DRIFT modality was more effective in the identification of pigments whether they were pure or mixed with rabbit glue. Moreover, [PERSON] et al. [11] also observed great differences when comparing the spectra of different materials (nitrates, sulphates, carbonates, oxides and oxalates) obtained with a FTIR benchtop equipment (in transmission and ATR modalities) and with a handheld device in DRIFT modality. Overall, handheld DRIFT devices can yield interesting results, though this technique has been scarcely used in cultural heritage and is vaguely reported in scientific literature (e.g., [9; 10; 12; 13; 14; 15]) as opposed to the other FTIR modalities (transmission and ATR). Beyond the evident advantages of portability and non-invasiveness in handheld DRIFT devices, this IR spectroscopy technique is particularly beneficial due to its ability to simultaneously identify both organic and inorganic materials [16]. However, DRIFT modality may encounter challenges related to spectral interferences which can arise from low material quantities, surface roughness and distortions in the spectra [16]. In particular, spectral distortion can occur because the diffusion and specular reflection components cannot be optically separated, leading to signal distortion [17]. This occurs since the refractive index (\(n\)) and absorption index (\(k\)) of materials influence the specular reflectance [11]. Thus, when a sample contains a compound with \(k>1\), _Reststrahlen_ bands appear (German word meaning'residual rays' [6; 8]), and this can happen with many inorganic materials (carbonates, sulphates, silicates, etc.) [18; 19]. These bands basically refer to specific wavelengths of light that are strongly reflected rather than transmitted or absorbed by the material, making it difficult to interpret the spectra [6]. This is more pronounced when several compounds are present in a sample, as is the case of pigments where impurities or additives are usually present. In this study, an ATR-FTIR and DRIFT comparative study has been carried out for the analysis of nineteen pigments. This comparison aimed to enhance the identification of characteristic functional groups, addressing the ongoing challenges and uncertainties in interpreting spectra obtained through DRIFT modality. Regarding the selection of pigments, there are various ways of classifying pigments in the literature [2; 20; 21; 22; 23; 24]: origin of manufacture (natural or synthetic), composition (organic and inorganic), colour (blue, red, green, etc. ), among others.