Dissociation of small poly(methyl
acrylic acid) (
PMAA)
anions produced by electrospray was characterized by tandem mass spectrometry. Upon collisional activation, singly, and doubly deprotonated
PMAA oligomers were shown to fragment via two major reactions,
dehydration and decarboxylation. The elimination of a water molecule would occur between two consecutive
acid groups in a charged-remote mechanism, giving rise to cyclic
anhydrides, and was shown to proceed as many times as pairs of neutral pendant groups were available. As a result, the number of
dehydration steps, together with the abundance of the fragment
ions produced after the release of all water molecules, revealed the polymerization degree of the molecule in the particular case of doubly charged oligomers. For singly deprotonated molecules, the exact number of MAA units could be reached from the number of
carbon dioxide molecules successively eliminated from the fully dehydrated precursor
ions. In contrast to
dehydration, decarboxylation reactions would proceed via a charge-induced mechanism. The proposed dissociation mechanisms are consistent with results commonly reported in thermal degradation studies of
poly(acrylic acid) resins and were supported by accurate mass measurements. These fragmentation rules were successfully applied to characterize a polymeric impurity detected in the tested
PMAA sample.