Negative ions generated by reactions with oxygen in the chemical ionization source. II. The use of wall‐catalyzed oxidation reactions for differentiation of polycyclic aromatic hydrocarbons and their methyl derivatives

Polycyclic aromatic hydrocarbons (PAH) undergo wall‐catalyzed oxidation reactions, in conjunction with electron‐and ion‐molecule reactions, when oxygen is present in the chemical ionization source. Under conditions that promote these wall‐catalyzed oxidation reactions (addition of oxygen, elevated ion source temperature) oxidized products are easily detected by measurement of the negative ion chemical ionization (NICI) mass spectra. The NICI mass spectra are dominated by ions that result from electron‐capture ionization of oxidation products such as ketones, quinones and anhydrides. In this study the NICI mass spectra of 30 PAH and 46 methyl‐substituted PAH were measured. These spectra can be used to differentiate certain isomeric PAH based upon differences in molecular structure. Of particular interest are ions that appear in the spectra of methyl‐substituted PAH, which can provide information regarding the position of methyl substituents.