Used as an internal standard in mass spectrometry for the quantitative analysis of 4’-hydroxy diclofenac in biological samples. Enables accurate measurement of drug metabolism and pharmacokinetics in research settings. Commonly applied in studies involving cytochrome P450 enzyme activity, particularly CYP2C9, due to its role as a major metabolite of diclofenac. Its deuterated structure allows clear differentiation from non-labeled compounds during analytical testing, improving precision in metabolic profiling and bioanalytical assays.