**6. Other Characterization Techniques**

In addition to the above-mentioned techniques, several classical characterization techniques have also been widely used in the field of pharmaceuticals for several decades such as FT-IR, Raman spectroscopy, powder X-ray diffraction (PXRD), differential scanning calorimeter (DSC), etc. [92–95]. These classical characterization techniques have found new applications and they are integrated with other techniques [96,97]. For instance, Purohit et al. investigated the miscibility of itraconazole ASDs by the atomic force microscopy technique coupled with nanoscale IR spectroscopy and nanothermal analysis [97]. These combined analytical techniques are proposed to be promising for investigating the phase behaviors of ASDs with high resolution [97]. In the case of PXRD technique, recent studies revealed that it could monitor the extremely low levels of crystallization with enhanced sensitivity by using the synchrotron X-ray source [71].

Moreover, several emerging approaches are also developed for characterizing amorphous pharmaceutical solids including terahertz spectroscopy, X-ray photoelectron spectroscopy, fluorescence lifetime imaging microscopy, etc. [98–100]. Chen et al. systemically investigated the surface enrichment or depletion of components in ASDs by XPS technique [100]. For these spray dried ADSs, surface composition is quite different from those in the bulk [100]. In addition, enrichment or depletion of the drug on the surface of ASDs was found to be strongly dependent on the drug–polymer combination as well as the molecular weight of polymer [100]. In a recent study, an advanced surface characterization platform was developed by combining XPS and time-of-flight secondary ion mass spectrometry [101]. This platform could provide the quantitative measurement of surface composition with high sensitivity and spatial resolution [101].
