Mass spectrometry approaches to natural and synthetic melanin characterization

Characterization of synthetic and natural melanin nanoparticles (MNPs) has been challenging due to the variability of properties and chemical bonding. Although eumelanin was the first to be discovered, four additional subcategories of melanin now exist including pheomelanin, neuromelanin, allomelanin and pyomelanin. Mass spectrometry (MS) methods have provided crucial insights into the bonding, structure, and mechanism regarding MNP formation, but inconsistent findings in the literature have caused further confusion in the analysis and structure of MNPs. This mini-review will focus on the types of MS experiments that have been used to analyze MNPs and what kind of information can be gained from different ionization sources, collisional activation experiments, and reaction monitoring quantitative methods when analyzing different subcategories of MNPs. Additionally, future directions regarding alternative applications of MNPs in MS experiments, mass defect analyses of complex mixtures, and a standardized nomenclature for isomeric redox states will all be addressed. (c) 2023 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry. Examples of different ionization sources commonly used for synthetic/natural melanin nanoparticles (MNPs) and different structural information that has been reported from each technique are shown. The yellow boxes represents sources that are under vacuum while the green box represents sources that are at or near atmospheric pressure. image