Norfloxacin-d5 (ethyl-d5) is primarily used as an internal standard in analytical chemistry for the quantification of norfloxacin in various samples. Its deuterated structure ensures minimal interference during mass spectrometry, enhancing accuracy and precision in drug metabolism studies. This compound is particularly valuable in pharmacokinetic research, where it helps track the absorption, distribution, and excretion of norfloxacin in biological systems. Additionally, it plays a critical role in environmental analysis, aiding in the detection and measurement of norfloxacin residues in water and soil samples. Its application is essential for ensuring compliance with regulatory standards and assessing the environmental impact of pharmaceutical contaminants.