Characterization of polyethylenimine by electrospray ionization and matrix-assisted laser desorption/ionization.

Branched polyethylenimines (PEIs) with lower average molecular weights (600, 1200 and 1800 Da) have been studied by Electrospray Ionization (ESI) and Matrix-Assisted Laser Desorption/Ionization (MALDI) mass spectrometry. In both, ESI and MALDI mass spectra, the main distribution arises from protonated PEI oligomers with NH(2) end groups, [PEI + H](+), which are observed at m/z 43n + 18. A trace of sodium contamination in the PEI samples results in the presence of a series that appears at m/z 43n + 40 [PEI + Na](+). However, only the MALDI mass spectra show a [PEI + K](+) series at m/z 43n + 56, because of matrix contamination with potassium, and a series generated by condensation of the matrix with PEI at m/z 43n + 30. Collisionally activated dissociation tandem mass spectrometry (CAD (MS/MS)) of protonated PEI oligomers is shown to yield three fragment ion series b(n), cn″ and K(n). The experiments have demonstrated the capabilities of these mass spectrometry techniques, along with CAD MS/MS to detect and characterize such polar synthetic polymers.