Differentiation of Isomeric Substituted Diaryl Ethers by Electron Ionization and Chemical Ionization Mass Spectrometry

A series of isomeric substituted diaryl ethers, i.e., 2- or 4-NO₂, 5-F–C₆H₃–O–C₆H₄ (4-R), where R = H, COCH₃, COOCH₃, NO₂, CHO, OCH₃ etc., which comprise ortho and para isomers with respect to the position of the nitro group are studied under GC-EI-MS and CI-MS conditions. The EI mass spectra of ortho and para isomers show distinct fragment ions, where the [M–OH]⁺ and [M–OH–O]⁺ ions specifically appeared in all spectra of the ortho isomers (I), whereas the para isomers (II) contain [M–O]⁺ and [M–NO]⁺ ions. The [M–OH–CO]⁺ and [M–OH–NO]⁺ ions in I, and [M–NO₂]⁺ ion in II are the other specific fragment ions observed but feasibility of these fragment ions are found to depend on the nature of the substituent (R). The substitution (–R) effect is also clearly reflected in the formation of fragment ions due to σ-cleavage process with or without hydrogen migration. Similar differences in the formation of specific fragment ions are also observed in ortho and para isomers of substituted aryl naphthyl ethers. The methane/CI of isomeric compounds resulted in the same set of fragment ions, but prominent differences are observed in the relative abundance of [MH–NO]⁺, which is relatively higher in para isomers compared with corresponding ortho isomer.