Revisiting the liquid mull technique to derive the infrared optical constants of organic powders from transmission IR spectroscopy

The determination of the refractive index (n) and extinction coefficient (k) of a compound is a key step for the prediction of its optical properties in different morphological states. For a solid, these optical constants can be obtained experimentally by spectroscopic methods such as ellipsometry and single-angle reflectance spectroscopy. However, in the context of sustaining databases for hyperspectral imaging with the n and k values of hazardous or noxious chemicals, these methods are not always conceivable due to the unobtainability of a single crystal or the hazards associated with heating, ball-milling, grinding and/or pressing the compounds. Hence, exploring a complementary preparation technique holds great interest. To this end, this work revisits the mull technique, which is classically used to perform qualitative FTIR transmission spectroscopy of a powder by trapping it in a mineral oil such as Nujol. By adding gravimetric measurements during sample preparation, this study seeks to provide a quantitative method satisfactory for deriving the absorption coefficient (α) and, consequently, the extinction coefficient (k).