AbstractThe atmospheric pressure chemical ionization of the dinitrotoluene isomers in ambient air was studied with a quadrupole mass spectrometer operating in the negative mode. The isomers can be grouped on the basis of the product ions: 2,5‐, and 2,6‐ and 3,5‐dinitrotoluene give the molecular anion with little fragmentation; 2,3‐ and the 3,4‐dinitrotoluene behave similarly but with more extensive fragmentation; 2,4‐dinitrotoluene gives the quasimolecular [DNT − H]−ion with little fragmentation. The results are discussed in terms of the molecular structure