From “Undetectable” to “Sensitive Detection”: Advances in Derivatization Techniques for LC-MS Analysis of Genotoxic Impurities

Many genotoxic impurities (GTIs) remain “invisible” to conventional LC-MS due to poor ionization or chemical instability under electrospray ionization, yet their sub-ppm acceptable intake limits under ICH M7(R2) demand exceptional analytical sensitivity. Derivatization—the chemical introduction of ionizable moieties, stable tags, or MS/MS information carriers—offers a powerful strategy to overcome this limitation. This review provides a critical systematic overview of derivatization techniques for LC-MS analysis of GTIs over the past decade (2015–2025, based on a literature search across PubMed, Web of Science, and Scopus). We construct a functional-group-based strategic framework covering alkyl halides, nitroaromatics, sulfonyl chlorides, hydroxylamine, alcohols, aldehydes, carboxylic acids, and amines, while placing specific emphasis on typical impurities within these classes such as methyl iodide, methyl chloride, nitrobenzene, and benzenesulfonyl chloride, and discuss the evolution of reagents from simple “reaction tags” to “MS/MS information carriers” that provide characteristic neutral losses or product ions for enhanced selectivity. Quantitative analysis reveals that derivatization typically enhances ESI response by 2–3 orders of magnitude, consistently achieving LODs below 1 ppm—the ICH M7(R2) threshold. Key analytical trade-offs are critically evaluated, including the balance between derivatization efficiency and reaction time, by-product management, and the fundamental kinetic and chromatographic constraints that render post-column derivatization impractical for most GTIs. We conclude with perspectives on high-throughput automation, smart multifunctional reagents, online integration, and green chemistry, aiming to provide a practical roadmap for developing robust, sensitive, and regulatory-compliant LC-MS methods for GTI control.