Comparison of dopants for charge exchange ionization of nonpolar polycyclic aromatic hydrocarbons with reversed-phase LC-APPI-MS

Atmospheric pressure photoionization (APPI) is capable of ionizing nonpolar compounds in LC/MS, through charge exchange reactions following photoionization of a dopant. Recently, several novel dopants-chlorobenzene, bromobenzene, 2,4-difluoroanisole, and 3-(trifluoromethyl)anisole-have been identified as having properties making them well-suited to serve as dopants for charge exchange ionization under reversed-phase LC conditions. Here, we report the results of experiments comparing their effectiveness to that of established dopants-toluene, anisole, and a toluene/anisole mixture, for the charge exchange ionization of model nonpolar compounds-the 16 polycyclic aromatic hydrocarbons (PAHs) identified by the US EPA as priority pollutants-when using a conventional reversed-phase LC method. Chloro- and bromobenzene were found to be much more effective than toluene for all the PAHs, due to the relatively low reactivity of their photoions with the solvent. Their overall performance was also better than that of anisole, due to anisole's ineffectiveness toward higher-IE compounds. Further, the experiments revealed that anisole's performance for higher-IE compounds can be dramatically improved by introducing it as a dilute solution in toluene, rather than neat. The two fluoroanisoles provided the highest overall sensitivity, by a slim margin, when introduced as dilute solutions in either chloro- or bromobenzene.