Evaluation of the optimization space for atmospheric pressure photoionization (APPI) in comparison with APCI

The usefulness of atmospheric pressure photoionization (APPI) is difficult to evaluate for unknowns due to the fragmented literature. Specifically, the variation of dopants with a wide set of compounds or the use of APPI in the negative mode have rarely been explored. Thirty compounds were selected that were not suitable for ESI with a wide variety of functional groups and investigated with atmospheric pressure chemical ionization (APCI) and APPI in the positive and negative ion modes. The influence of the mobile phase (eluents containing acetonitrile or methanol) and--for APPI--four different dopants (acetone, chlorobenzene, toluene, and toluene/anisole) were explored. Stepwise variation of the organic mobile phase allowed to elucidate the ionization mechanism. Atmospheric pressure photoionization was especially useful for compounds, where the M(●+) and not the [M + H](+) was formed. The dopants chlorobenzene and anisole promoted the formation of molecular ions M(●+) for about half of the compounds, and its formation was also positively influenced by the use of mobile phases containing methanol. In the negative ion mode, APPI offered no advantage toward APCI. Best results were generally achieved with the dopant chlorobenzene, establishing that this dopant is suitable for a wide set of compounds. For one quarter of the compounds, significantly better results were achieved with mobile phases containing methanol for both APPI and APCI than those with acetonitrile, but only in the positive mode. With either of the methods--APPI or APCI--about 10% of the compounds were not detected. Strategies to get results quickly with difficult unknowns will be discussed.