Electrospray ionization mass spectrometry of the non-covalent complexes of ammonium ions with high molar mass polyethers

The formation of low charge state non-covalent complexes of high molar mass polymers with primary and tertiary amines and quaternary ammonium salts (Quats) was studied by electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). The amines studied were of different degree of substitution and size, and were added to the acidic polymer solution prior to MS analysis. A comparison was made between the non-covalent complexes of high molar mass polyethers with various amines and Quats, based on the ability to form low charge state adduct ions, overall MS response, and presence of fragment ions due to up-front fragmentation. Tertiary amines showed the best performance in forming low charge state adduct ions and produced mass spectra with the lowest degree of up-front fragmentation. In both primary and tertiary amines, more simplified mass spectra are produced, since no cluster ions are formed (as in the case of Quats). The comparison of the overall MS response did not reveal a specific trend. An amine and a Quat with an optimal performance for formation of low charge state adduct ions were applied to the mass spectrometric analysis of a poly(lactic acid)-block-poly(ethylene glycol)-block-poly(lactic acid) triblock copolymer, allowing the detection of low charge state adduct ions and revealing information about the molecular weight distribution. The method demonstrated here shows that addition of primary/tertiary amines and quaternary ammonium salts to the spray solvent facilitates the analysis of high molar mass polyethers by ESI-MS.

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