Lost in Overlap: Mapping Local Composition Landscapes to Predict the Upper Measurable Mass Limit of Copolymers and Terpolymers by MALDI-TOF MS

Abstract Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is widely used for polymer characterization. However, the compositional analysis of copolymers becomes increasingly challenging at higher molecular weights because multiple polymer compositions and their isotopic distributions yield overlapping peaks. Here, we present a predictive framework that facilitates the interpretation of complex copolymer mass spectra by revealing how individual copolymer species contribute to the observed mass spectral features and by identifying the possible compositions associated with each spectral peak. The framework further determines the upper measurable mass limit (UMML), defined as the molecular-weight limit below which neighboring peaks remain distinguishable under specified instrumental conditions. Starting from a specified main polymer composition, the algorithm identifies neighboring and overlapping feasible compositions within the corresponding local m/z region for both copolymers and terpolymers. Their relative contributions are estimated using a statistical description of monomer incorporation, while their isotope distributions are calculated using m/z-dependent peak profiles determined experimentally. The results demonstrate that the UMML is governed by the system-specific mathematical structure and density of feasible compositions, isotope distributions, and instrumental resolving power. Although the numerical UMML values are instrument- and criterion-specific, the framework is transferable to other mass spectrometric platforms by introducing the corresponding resolving-power or peak-width vs. m/z function. The method, therefore, provides both a detailed map of the local composition landscape and a priori estimate of the molecular-weight range over which meaningful compositional information can be obtained.