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

Abstract

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.

Structural Characterization of Methylenedianiline Regioisomers by Ion Mobility-Mass Spectrometry and Tandem Mass Spectrometry. 4. 3-Ring and 4-Ring Isomers

“…The [M-H] + species is the most stable precursor compared to [M+H] + and [M.] + (see Figure 5). The increase in stability is due to the fluorene-backbone, as seen in the 2-ring [M-H] + species versus the [M+H] + species, discussed previously 3. In Figure 3c, the MS/MS spectrum shows three major fragment ions deriving from the 302 Da precursor ion at 20 eV: 285 ([M-17] + ), 209 ([M-93] + ), and 106 Da ([M-196] + ).…”

“…Due to the numerous fragment ions formed uniquely for each precursor, this manuscript will focus on the interpretation of major low energy fragment ions formed by the MALDI process and will be introduced by schemes proposing fragment ion pathways and collision-induced dissociation ion breakdown curves (CID curves). Additional MS 3 data found in Supporting Information (Figures S5 and S6) was also acquired and used to support the interpretation of unique fragment ion pathways.…”

“…Here we observe [M+H] + as the least stable precursor amongst [M.] + and [M-H] + species, forming more fragment ions at lower collision energy (Figure 3d-f); this is consistent with our previous 2-ring studies. 3 The MALDI MS/MS spectra for the 4-ring MDAs show major fragment ion peaks at low collision energy for each precursor species: [M+H] + , [M.] + , and [M-H] + . It is interesting to note that an increase in fragment ions is observed in Figure 3d for the [M+H] + species (15 eV) compared to [M.] + and [M-H] + in Figure 3e-f (20 eV).…”

“…From those studies, the m/z resolving quadrupole was set to 18 resolving power for the MS/MS experiments providing the best separation of the three precursor ions, without significant loss of signal. 3 To perform MS 3 experiments, the ion of interest is selected by the quadrupole and is fragmented in the “Trap” region located prior to the TWIM drift cell. The first generation of product ions are then separated by the TWIM and subjected to a second stage of CID in the “Transfer” region to produce 2nd generation and 1st generation product ions (i.e., MS/IM/MS experiments).…”

“…MDA is formed from the reaction between aniline and formaldehyde, generating the 2-ring MDA regioisomers 2,2′-MDA, 2,4′-MDA, and 4,4′-MDA, as well as larger multimeric species including 3-ring and 4-ring MDAs. 1,2 In our previous work, 2-ring MDAs (2,2′-MDA, 2,4′-MDA, and 4,4′-MDA) were characterized using electrospray ionization-ion mobility-mass spectrometry (ESI-IM − MS) 1 and matrix-assisted laser desorption/ionization-ion mobility-mass spectrometry (MALDI-IM − MS), 3 two MS techniques commonly used to analyze PUR polymers. 4−8 In those studies, one unique parent ion ([M + H] + = 199 Da) was observed for each isomer using ESI-MS; however, when the isomers were studied by MALDI-MS, we observed three distinct precursor ions, [M − H] + = 197 Da and [M • ] + = 198 Da, in addition to the [M + H] + = 199 Da species.…”