Used as a stable isotope-labeled internal standard in mass spectrometry for the accurate quantification of sulfapyridine in biological samples such as plasma, serum, and urine. Its primary application is in pharmacokinetic studies and therapeutic drug monitoring, where it helps distinguish the analyte from matrix interferences due to its identical chemical behavior but distinct mass. Also employed in metabolic studies to trace the distribution and transformation of sulfapyridine in vivo without altering the biochemical pathway. Commonly used in research related to antibiotic metabolism and in clinical diagnostics for monitoring sulfonamide levels in patients undergoing treatment for bacterial infections.