bisporus_ have recently been approved as a source of vitamin D in various food products, including breakfast cereals, vegetable juices, calcium-fortified fruit juices, grain products, and plant- or soy-based products marketed as milk, cheese, or yogurt alternatives [8]. The photolysis of ergosterol by UV light synthesizes not only previtamin D\({}_{2}\) but also tachysterol and lumisterol. According to [PERSON] et al. [9], 85% of ergosterol is converted to pre-ergocalciferol, while the remainder forms over-irradiation products. [PERSON] et al. [5] identified these over-irradiation products as irreversibly formed suprasterols derived from vitamin D\({}_{2}\) and toxisterols derived from previtamin D\({}_{2}\). [PERSON] et al. [9] further stated that prolonged irradiation of previtamin D results in a photochemical conversion to tachysterol and lumisterol, which remain in equilibrium with previtamin D. Vitamin D production can also be thermally initiated from previtamin D [6]. Studies have explored these equilibrium reactions and have sought optimal conditions for vitamin D\({}_{2}\) production. Factors influencing vitamin D\({}_{2}\) conversion in mushrooms include the wavelength of UV light, irradiation time, distance from the  
Figure 1: Formation of vitamin D. Numbers indicate the conventional numbering of carbon atoms in the organic molecules. light source, temperature, and the processing conditions and methods applied to the mushrooms [10; 11; 12]. UV light is categorized into three subregions: UV-C (190-290 nm), UV-B (290-320 nm), and UV-A (320-400 nm). Several studies have utilized all three types of UV light in mushroom irradiation, yet no consensus has been reached on which subregion is most effective for previtamin D\({}_{2}\) conversion. [PERSON] et al. [13] found that the yield of vitamin D\({}_{2}\) was highest with UV-B irradiation, but similar results were achieved with UV-C light. They identified UV-A radiation as the least effective. Contradictory findings have also been reported [14; 15]. [PERSON] et al. [5] discovered that 295 nm (in the UV-B region) was the optimal wavelength for previtamin D\({}_{2}\) conversion, aligning with the conclusions of [PERSON] and [PERSON] [16; 17]. Regarding the yields of tachysterol and lumisterol, [PERSON] et al. [14] reported that shorter wavelengths promoted tachysterol production, while longer wavelengths favored lumisterol formation. Despite these ambiguous results, UV-B light appears to produce the greatest amount of previtamin D\({}_{2}\) with the least formation of byproducts when applied during mushroom irradiation. Since vitamin D\({}_{2}\) synthesis from previtamin D\({}_{2}\) is thermally initiated (Figure 1), higher conversion rates can be achieved by increasing the temperature [13; 14; 15; 16]. Although prolonged irradiation up to 2 h can increase vitamin D\({}_{2}\) production, studies suggest that extended exposure may lead to photodegradation of vitamin D [15]. Optimizing the preparation of mushroom samples prior to UV irradiation is therefore critical. [PERSON] and [PERSON] [15] investigated the effect of moisture content on vitamin D\({}_{2}\) conversion in shitiake mushrooms and found that the most efficient conversion occurs at a moisture content of 78-80%. Their study also highlighted that the surface exposure of the sample plays a significant role in conversion efficiency. There is a demand from both academic and industrial sectors for an accurate and simple method for the analysis of vitamin D\({}_{2}\) and its provitamin. The typical approach for the quantitation of vitamin D involves high-performance liquid chromatography with mass spectrometric detection (HPLC-MS), following alkaline saponification and cleanup steps prior to chromatography [18; 19]. Given the complexity and cost of LC-MS instrumentation, HPLC-UV methods have also been developed recently for the rapid analysis of vitamin D\({}_{2}\)[5; 18; 20].