Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry Tandem Mass Spectrometry and Computational Strategies. 3. MALDI Spectra of 2-Ring Isomers

摘要

Characterization of methylenedianiline (MDA) 2-ring isomers (2,2′-, 2,4′- and 4,4′-MDA) is reported using matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS), a common technique used for characterizing synthetic polymers. MDA is a precursor to methylene diphenyl diisocyanate (MDI) a hard block component in polyurethane (PUR) synthesis. This manuscript focuses on comparing MALDI results to those of our previous electrospray ionization-mass spectrometry (ESI-MS) studies. In ESI, 2-ring MDA isomers formed single unique [M+H]⁺ (199 Da) parent ions, whereas in MALDI each isomer shows significant formation of three precursor ions: [M−H]⁺ = 197 Da, [M·]⁺ = 198 Da, and [M+H]⁺ = 199 Da. Structures and schemes are proposed for the MALDI fragment ions associated with each precursor ion. Ion mobility-mass spectrometry (IM-MS), tandem mass spectrometry (MS/MS), and computational methods were all critical in determining the structures for both precursor and fragment ions as well as the fragmentation mechanisms. The present study indicates that the [M−H]⁺ and [M·]⁺ ions are formed by the MALDI process, explaining why they were not observed with ESI.