Discriminating alkylbenzene isomers with tandem mass spectrometry using a dielectric barrier discharge ionization...

Soft ambient ionization sources generate reactive species that interact with analyte molecules to form intact molecular ions, which allows rapid, sensitive, and direct identification of the molecular mass. We used a dielectric barrier discharge ionization (DBDI) source with nitrogen at atmospheric pressure to detect alkylated aromatic hydrocarbon isomers (C₈ H₁₀ or C₉ H₁₂ ). Intact molecular ions [M]^•+ were detected at 2.4 kV_pp , but at increased voltage (3.4 kV_pp ), [M + N]⁺ ions were formed, which could be used to differentiate regioisomers by collision-induced dissociation (CID). At 2.4 kV_pp , alkylbenzene isomers with different alkyl-substituents could be identified by additional product ions: ethylbenzene and -toluene formed [M-2H]⁺ , isopropylbenzene formed abundant [M-H]⁺ , and propylbenzene formed abundant C₇ H₇ ⁺ . At an operating voltage of 3.4 kV_pp , fragmentation of [M + N]⁺ by CID led to neutral loss of HCN and CH₃ CN, which corresponded to steric hindrance for excited state N-atoms approaching the aromatic ring (C-H). The ratio of HCN to CH₃ N loss (interday relative standard deviation [RSD] < 20%) was distinct for ethylbenzene and ethyltoluene isomers. The greater the number of alkyl-substituents (C-CH₃ ) and the more sterically hindered (meta > para > ortho) the aromatic core, the greater the loss of CH₃ CN relative to HCN was.