Oligomer-Dependent Gas-Phase Dissociation Behavior of 2-Butanone Peroxide (MEKP) Cations.

2-Butanone peroxide is a commonly used cross-linker in the chemical industry and a homemade explosive encountered by the forensic community. In this study, the dissociation behavior of ammonium cationized MEKP oligomers was explored using tandem MS. The shorter ammoniated MEKP oligomer cations n = 3, 4 underwent charge-induced fragmentation pathways, which generated cyclic MEKP and smaller oligoperoxide fragment ions. Control experiments with metal ion adducts of MEKP oligomers n = 3, 4 generated distinct dissociation behavior and supported the initial pathway assignment. Longer ammoniated MEKP oligomers n = 6-9 underwent charge-remote fragmentation pathways, which generated terminally modified alkyl ketone MEKP fragment ions. The dissociation behavior followed that of metalated MEKP oligomer cations, which predominantly dissociate through charge-remote pathways. The change in the fragmentation pathway from shorter to longer oligomer length indicates that ammonium is more strongly bound in the longer MEKP oligomers and does not participate in bond breaking and formation. Additional MS3 analyses demonstrated that the alkyl ketone MEKP fragment ions readily undergo consecutive fragmentation to generate mono- and diterminal modifications. Ammoniated MEKP oligomers n = 5 exhibited a combination of charge-induced and -remote pathways, representing a cross over point between the two pathways. Results here will aid future efforts in organic peroxide detection and characterization, especially with considerations to ion adducts, fragmentation pathways, fragment ion-types, and MS approaches.