MATRIX EFFECTS IN LIQUID CHROMATOGRAPHY-TANDEM MASS SPECTROMETRY BIOANALYSIS: MECHANISTIC FOUNDATIONS, REGULATORY EXPECTATIONS, AND PRACTICAL MITIGATION STRATEGIES

The effect of matrices remains an important source of variation in LC-MS/MS-based bioanalysis due to the presence of co-eluting endogenous and exogenous compounds which affect the ionization processes of analytes. The current paper critically reviews the physicochemical principles of matrix effects in atmospheric pressure ionization (API) interfaces, emphasizing on the principle of electrospray ionization (ESI). Particular attention will be devoted to droplet formation and desolvation kinetics, surface activity effects, charge partitioning, conductivity effects, and ion-molecule reactions in gas phase that control ion suppression/enhancement. Comparative review of the ESI vs APCI ionization mechanisms will allow identifying distinctive features associated with their resistance to the matrix effect. In contrast to various practical recommendations that can be found in the literature, this paper emphasizes the use of different experimental approaches for assessing ionization variations based on post-column addition, post-extraction matrix addition, matrix factor calculation, and internal standardization. The discussion is informed by harmonized principles presented in international bioanalytical validation guidelines, specifically ICH M10, while acknowledging differences among global regulatory authorities. Notably, matrix effects are considered within the framework of indirect regulatory control, where method validation is established by accuracy and precision performance (±15% for quality control samples and ±20% at the lower limit of quantification), rather than by specified matrix effect threshold values. Mitigation approaches are assessed mechanistically, covering selective sample preparation, chromatographic selectivity, ion source parameter optimization, and the use of stable isotope-labeled internal standards. Emerging trends, such as microflow liquid chromatography, high-resolution mass spectrometry, automated methods, and artificial intelligence-assisted optimization, are critically discussed for their potential to enhance reproducibility in complex biological samples such as plasma, tissues, and dried blood spots. Taken together, this review outlines a framework for systematic assessment and control of matrix effects in contemporary LC-MS/MS bioanalysis that is mechanistic, structured, and aligned with regulatory requirements. In contrast to previous narrative reviews, this review article presents a framework that combines ionization science, validation approach, and regulatory thinking in a single model for matrix effect control.