Ionization behaviors of nitrotoluenes and dinitrotoluenes by reactions with acetone-related reactant ion

Dinitrotoluenes (DNTs) and nitrotoluenes (NTs) are found in the environment as metabolites of trinitrotoluene (TNT). When acetone is used as the solvent/eluent in atmospheric pressure chemical ionization-mass spectrometry (APCI-MS), the reactant ion is [2Acetone + O₂ ]^•- for the negative ion mode. The reactant ion reacts with an analyte to produce M^•- and/or [M - H]^- under atmospheric pressure. In this study, ionization behaviors of NT (2-, 3-, and 4-NTs) and DNT isomers (2,3-, 2,4-, and 2,6-DNTs) by reactions with [2Acetone + O₂ ]^•- were investigated. The energy-minimized structures of the product ions and their energies were calculated to explain the differences in the ionization behaviors. Typical NT- and DNT-related ions were produced by reactions with [2Acetone + O₂ ]^•- ; NT^•- , [NT - H]^- , DNT^•- , [DNT - H]^- , and [DNT - NO]^- ions. The ionization efficiencies of NT- and DNT-related ions increased by increasing the source fragmentor voltage, and those of DNT-related ions were higher than those of the NT-related ions owing to the presence of an additional nitro group. The ionization efficiency of 3-NT^•- was higher than that of [NT - H]^- , while that of [DNT - H]^- was higher than those of DNT^•- and [DNT - NO]^- . The ionization efficiency order of NT^•- was 3-NT > 4-NT > 2-NT, while that of [DNT - H]^- was 2,4-DNT > 2,6-DNT > 2,3-DNT. The [NT - H]^- and [DNT - H]^- ions were stabilized by resonance structures containing nitro groups. The [DNT - NO]^- ions were formed through the transition state.