Development and Validation of a Highly Sensitive Isotope-Coded Equivalent Reporter Ion Assay for the Semi-Quantification of Isocoumarins in Complex Matrices

The accurate quantification of multicomponents using LC-MS is pivotal for ensuring the quality control of herbal medicine, as well as the investigation of their analysis of biological tissue distribution. However, two significant challenges persist: the scarcity of authentic standards and the selection of appropriate internal standards. In this study, we present a highly sensitive isotope-coded equivalent reporter ion assay (iERIA) that combines equivalently quantitative ion and isotope-coded derivatization strategies. This method offers triple functionality: enabling the semidetermination of multiple components using a single standard, introducing stable isotope-labeled internal standards, and enhancing MS detection signals. Using four isocoumarins as a model, namely, 5-carboxylmellein, 5-hydroxymethylmellein, 5-methylmellein, and 5-hydroxymellein, we successfully quantified these compounds across various matrices, including herbal extracts, plasma, urine, and liver tissue. Reporter ions at m/z 170 and 234 generated by the dansulfonyl derivatives of isocoumarins, were subsequently detected for calculating the concentrations of samples based on the equivalent ion method. It is very beneficial for trace detection in biological samples free of any concentration steps, with an increased LOD of 50 times after dansyl chloride derivatization. Additionally, the introduction of stable isotope-labeled internal standards using d₆-dansyl chloride mitigated matrix effects and instrument drift, ensuring the accuracy and precision of the semiquantification. This practical UPLC-MS/MS strategy significantly expands the applicability of multicomponent determination, with promising implications in diverse domains such as herbal medicine active ingredient analysis, food function and safety assessment, and metabolomics research.