Using LC Retention Times in Organic Structure Determination: Drug 
Metabolite Identification

BACKGROUND: There is a continued need for improvements in the efficiency of metabolite structure elucidation. OBJECTIVE: We propose to take LC Retention Time (RT) into consideration during the process of structure determination. METHODS: Herein, we develop a simple methodology that employs a Chromatographic Hydrophobicity Index (CHI) framework for standardizing LC conditions and introduce and utilize the concept of a pre-dictable CHI change upon Phase 1 biotransformation (CHIbt). Through the analysis of literature exam-ples, we offer a Quantitative Structure-Retention Relationship (QSRR) for several types of biotransfor-mation (especially hydroxylation) using physicochemical properties (clogP, hydrogen bonding). RESULTS: The CHI system for retention indexing is shown to be practical and simple to implement. A da-tabase of CHIbt values has been created from re-incubation of 3 compounds and from analysis of an addi-tional 17 datasets from the literature. Application of this database is illustrated. CONCLUSION: In our experience, this simple methodology allows complementing the discovery efforts that saves resources for in-depth characterization using NMR.