AbstractThe electron impact and methane chemical ionization mass spectra of 2‐fluoro‐5‐nitroaniline and 4‐fluoro‐3‐nitroaniline have been investigated. Similar fragment ions were observed for both compounds under electron impact conditions, but the difference in the relative ion intensities of two key fragment ions allowed the positive identification of these two isomers. The difference can be attributed to anorthoeffect involving fluorine and the amino substituent. A fragmentation pathway consistent with this observation i