Robust Workflow for Multiclass Host-Gut Microbial Cometabolite Quantitation in Human Stool via 3-Nitrophenylhydrazine Derivatization and LC-MS/MS: A Validated Analytical Platform for Translational Studies

Host-gut microbial co-metabolites, including short-chain fatty acids (SCFA), bile acids (BA), tryptophan metabolites, and branched-chain amino acids (BCAA), have key immune-metabolic functions affecting human health. Dysbiosis-induced alterations in their levels are implicated in the pathogenesis of diseases such as metabolic dysfunction-associated steatotic liver disease (MASLD). However, simultaneous quantitation of these chemically diverse analytes in stool remains analytically challenging due to their diverse physicochemical properties and wide concentration ranges. Here, we developed and rigorously validated a derivatization and targeted liquid chromatography tandem mass spectrometry workflow for the simultaneous quantitation of host-gut microbial cometabolites in human stool. A 3-nitrophenylhydrazine derivatization protocol was optimized by systematically adjusting reagent concentrations and introducing postreaction quenching to suppress in-line acetic acid derivatization. Chromatographic separation was enhanced by using a novel dual-additive mobile-phase strategy (formic acid and ammonium acetate in aqueous and organic phase, respectively) coupled to a mixed-mode C18-anion-exchange stationary phase, enabling improved resolution and sensitivity across chemically diverse metabolite classes. Our optimized analytical method achieved accurate, sensitive, and efficient quantitation of 38 metabolites (15 SCFA, 16 BA, 4 tryptophan metabolites, 3 BCAA) within 23 min, demonstrating excellent linearity (r2 > 0.99) and precision (CV < 15%), with short- (autosampler, 4 °C) and long-term (freezer, −20 °C) stability. Comparative analysis of healthy controls and MASLD stools revealed distinct metabolic signatures, including reduced SCFA and C6-oxidized BA, and elevated conjugated and secondary BA derivatives in MASLD. Our study establishes an analytically rigorous platform for multiclass host-gut cometabolite quantitation in stool, with demonstrated utility for translational research into gut–liver axis disorders.