## 1 Introduction  
Black cumin (_Nigella sativa_) seeds and black cumin seed oil (BCSO) have various pharmacological effects, including analgesic, antiulcer, anti-inflammatory, antibacterial, antimicrobial, and anticancer properties [1]. BCSO has a diverse chemical composition. It contains proteins, carbohydrates, and fixed and volatile oil. In different varieties of black cumin seeds, a lot of bioactive compounds have been isolated, identified, and described, many of which exhibit strong antioxidant properties [2]. Therefore, BSCO was successfully used, blended with some vegetable oils to improve the oxidative stability [3]. However, the antioxidants' effects are not always visible or it is difficult to clearly attribute the observed effects in the extracts or oil from black cumin seeds to specific compounds. Most of the pharmacological properties of the seeds and BCSO are attributed to the presence of thymoquinone (TQ) (Figure 1), which is the main bioactive component of black cumin seed essential oil. The TQ content in the volatile oil can reach up to almost 50% depending on the origin of the plant and seed storage, and on the method of oil production [2, 4, 5]. It was noted that BCSO was found to contain higher amounts of TQ compared to seeds [6]. TQ naturally occurs in _N. sativa_ seed but is also reported in other herbs of genera like Juniperus, Cupressus, and Tetraclinis [7]. In plants, the biosynthesis of TQ takes place mainly by the terpene biosynthetic pathway, where thymol or carvacrol received from the hydroxylation of p-cymene are the intermediate products [8; 9]. It was reported that these compounds were one of the major compositions of essential oil from black cumin seed, occurring in varying amounts depending on the origin, the genotype of the seeds, or the oil extraction methods [5; 10; 11]. On the other hand, TQ can be synthesized in gram quantities as a result of the oxidation of thymol or carvacrol in a controlled catalyzed reaction [8; 12]. Studies have also been published showing that TQ can be derived from thymol via a biotransformation with _Synechococcus_ sp., although the yield reported for this process was low [13]. TQ is characterized by low stability at elevated temperatures, instability in aqueous environments, and significant sensitivity at light [14; 15]. Under light and thermal stress conditions, it was degraded to several products, among others, thymohydroquinone and dithymoquinone, which were identified by the LC-MS/MS assay [15]. Due to the reasons mentioned above, the medicinal use of clear TQ is limited. Furthermore, since TQ is fat-soluble, BCSO may be an effective therapeutic form derived from _N. sativa_ seeds [4; 16]. In oil during heating, TQ, as the volatile compound, decreases under thermal storage but simultaneously remained more stable compared to most of the other terpen [17]. The loss of TQ that may occur during oil storage can affect the pharmacological properties of BCSO. Thus, determining the TQ content and conditions that limit the decrease in the TQ amount may contribute to the preservation of these properties. Moreover, the oxidation processes occurring during oil storage may impact its overall quality and the observed antioxidant properties [18]. The presence of TQ in BCSO can be confirmed by different techniques, among which chromatographic methods are the most commonly used. Others, like UV spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, or Nuclear Magnetic Resonance (NMR), are less frequently employed [4; 19]. The FTIR assay is a fast and non-destructive technique, and sensitive, and requires no special sample preparation; it is widely used in research laboratories and the food industry to characterize oils with specific bands or regions of the spectrum [20]. Wavelengths of absorption peaks can be linked to the types of bonds in a molecule, providing important information about the functional groups it contains.