Characteristics of ionization behaviors of aminonitrotoluene isomers produced by atmospheric pressure chemical ionization

Rationale: Six of the isomers of aminonitrotoluene (ANT) are 2-amino-3-nitrotoluene (2A3NT), 2A4NT, 2A5NT, 2A6NT, 4A2NT, and 4A3NT. Some of them can be identified by chromatography and spectroscopy. Biochemical transformation of 2,4,6-trinitrotoluene (TNT) and dinitrotoluenes (DNTs) is very complex and ANTs are decomposition products of TNT and DNTs. Methods: Each isomer in acetone was ionized using atmospheric pressure chemical ionization in positive and negative ion modes, and kinds and abundances of the product ions were analyzed. Energy-minimized structures of the product ions and their energies were calculated to explain the analysis results. Results: The [M + H]⁺, [M + H + Acetone − H₂O]⁺, and [M + H + Acetone]⁺ ions as positive product ions were detected, while [M − H]⁻, M^•−, and [M + O₂]^•− ions as negative ones were observed. The order of the ionization efficiencies for the positive product ions was 4A3NT > 4A2NT > 2A4NT > 2A6NT > 2A5NT > 2A3NT, while that of the negative ones was 2A5NT > 2A3NT > 4A3NT > 2A4NT > 2A6NT > 4A2NT. Ion abundance ratios for 2A3NT and 2A5NT showed very similar trends, while those of 2A6NT and 4A2NT also showed similar trends. Differences in the ionization behaviors were explained using the heats of reaction. Conclusions: The product ions were produced by ion–molecule reactions with the reactant ions of [2Acetone + H]⁺ and [Acetone + O₂]^•−. The [M + H + Acetone]⁺ ion was fragmented to produce [M + H]⁺ and [M + H + Acetone − H₂O]⁺, while the [M + O₂]^•− ion was fragmented to generate the [M − H]⁻ and M^•− ions. Differences in the ionization behaviors of the ANTs can be used for their differentiation.