The even-electron rule in electrospray mass spectra of pesticides

A study of the fragmentation and ion formation of three major families of pesticides (including herbicides, insecticides, and fungicides) by liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS) and liquid chromatography/quadrupole time-of-flight mass spectrometry (LC/Q-TOF-MS) was carried out using positive electrospray ionization (ESI) and the results compared with those by gas chromatography (GC)/TOF-MS with electron ionization (EI) in order to test the validity of the even-electron rule in electrospray ionization. First, the majority of the fragmentations by positive ion ESI were even electron (EE) ions (93% of the fragment ions). Secondly, the formation of odd-electron (OE) fragment ions was greater with EI, where the fragment ions were present in a ratio of approximately 1:2 (35% OE ions and 65% EE ions). Thirdly, in-source collision-induced dissociation (CID) fragmentation by LC/TOF-MS and CID fragmentation in the collision cell by LC/Q-TOF-MS/MS resulted in 95% of the fragment ions being identical between the two types of fragmentation. As ESI in the positive ion mode yields an EE precursor ion (normally a protonated molecule), this commonly leads to EE fragment ions by elimination of molecules - a process called the even-electron rule. Neutral radical losses were less common in ESI but were common in the EI spectra of the same compounds. The structures that did lead to OE ions in ESI (exceptions to the even-electron rule approximately 7% of all ESI ions) favored electronegative radical losses in approximately the following order: .SO(2)CH(3), .NO(2), .CH(3), .Cl, .SCH(3), .CH(2)CH, and .OH.