Quercetin-d5 is primarily used as an internal standard in analytical chemistry for the quantification of quercetin in various samples. Its deuterated form ensures accurate and reliable measurements in mass spectrometry, particularly in pharmacokinetic studies and metabolic research. This compound is essential in tracking the absorption, distribution, metabolism, and excretion of quercetin in biological systems. Additionally, it aids in validating analytical methods and ensuring precision in studies related to dietary flavonoids, antioxidant activity, and their health benefits. Its application is critical in advancing research on natural compounds and their therapeutic potential.