Accurate mass determination of melamine–formaldehyde synthetic polymers after separation on preparative HPLC

• Structures of 12 fractions from commercially available melamine–formaldehyde resin were studied.
• Significant in-source fragmentation due to thermal degradation above 70 °C was found.
• NMR results showed different conformers.
• Conformation change process is reversible.

Separation of 57 species from solutions of commercial melamines and analysis by high resolution mass spectrometry using electrospray ionisation and atmospheric pressure chemical ionisation is reported. Structures for melamine-based crosslinked polymers were also confirmed with two orthogonal techniques for characterisation of polymers: Fourier Transform Infrared Spectrometry (FTIR) and Nuclear Magnetic Resonance spectroscopy (NMR), respectively.Accurate mass measurements of melamine–formaldehyde resin structures were achieved in the ppm range, using an internal calibration standard reserpine. Results were within 5 ppm of calculated formulae. Relative isotopic abundances of [M+H]+ ions were evaluated for different fractions. Results were independent of the mobile phase used. NMR studies also confirmed structures of fractions and their thermal stability up to 70 °C. Differences in NMR spectra were explained by change of conformation and it was confirmed that this is a reversible process by reheating samples. Stability at 70 °C was confirmed by MS, since when the temperature of the ion transfer capillary in MS was set above this temperature decomposition occurred.

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