Characterization of C8H10 alkylbenzenes by negative ion mass spectrometry

The O−˙ chemical ionization mass spectrri of the C8H10 alkylbenzenes, o-, m-. andp -xylene and ethylbenzene, show formation of [M − H + O]−, [M − H]−, [M − H2]−˙ and, for the xylenes, [M − CH3 + O]− as primary reaction products; the relative importance of these products depends on the isomer. However, [OH]− is a primary product from reaction of O−˙ with both the C8H10 isomers and hydrogen-containing impurities; [OH]− reacts further with the alkylbenzenes to produce [M − H]− with the result that the chemical ionization mass spectra depend on experimental conditions such as sample size and the presence of impurities. The collision-induced charge inversion mass spectra of the [M − H + O]− and [M − H]− products allow only distinction of ethylbenzene from the xylenes. However, the collision-induced charge inversion mass spectra of the [M − H2]−˙ ions show differences which allow identification of each isomer.