4.6.2. Ultraviolet-Visible Spectrophotometry (UV-Vis Spec)

Spectroscopy is a branch of science which deals with the study of electromagnetic radiation and its interaction with matter. The UV-visible spectrophotometry specifically is one of the most often used analytical techniques in the pharmaceutical industry [143,144]. The technique is used to evaluate the concentration of a given organic or biological compound [145,146]. This process is achieved using UV or visible radiations which are absorbed by a substance in solution [143–145]. The ratio or function of the ratio of the intensity of two beams of light in the UV-vis region is measured by instruments known as UV spectrophotometers [144]. Absorption of radiation of a specific wavelength, λ, is an indication of the presence of one or more chromophores [147]. A chromophore is a molecular group that has a pi, π, bond which when inserted into a saturated hydrocarbon, produces a compound with absorption between 185 nm and 1000 nm [148]. Temperature and pH may also cause changes in both the intensity and the λ of the absorbance maxima [149,150].

Analysis of compounds using spectroscopy is governed by the Beer–Lambert law [144]. Beer's law states that the intensity of a beam of parallel monochromatic radiation decreases exponentially with the number of absorbing molecules [144,151–153]. Thus, according to Beer, the concentration of a compound is proportional to the absorbance produced [144,150]. Lambert's law states that the intensity of a beam of parallel monochromatic radiation decreases exponentially as it passes through a medium of homogenous thickness [152,154]. As both laws deal with the intensity of monochromatic radiation, a combination of the two laws has been used to yield the Beer–Lambert law [144,151–154].

Different NCM manufacturing methods and conditions have an influence on the solid state form while environmental conditions affect the thermodynamic stability of the polymorphic form [155]. PXRD, DSC, SEM, FTIR, and RS are the most commonly used methods to establish and monitor the solid-state forms of NCM. The use of microscopy techniques is the mainstay of PS, particle shape, and morphology determination. A summary of techniques and their applications is depicted in Table 1.


**Table 1.** Characterizations associated with NCM (Adapted from [86]).

\* Physical state of samples: DLS, ZP, TEM, UV-Vis, NMR, and HPLC; liquid state. All other techniques; solid state.
