Atmospheric pressure photoionization. II. Dual source ionization.

In this paper we describe results based on the combination of atmospheric pressure photoionization (APPI) with atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). The main purpose of combining more than one ionizer is to extend the range of compounds that can be simultaneously analyzed. Three modes of operation are presented; use of either ionizer, simultaneous use of two ionizers, and rapid switching between ionizers during a single chromatographic run. The dual ionizer configurations only minimally affect the performance of either ionizer relative to the standard single-ionizer sources. However, it is observed that the operation of both ionizers together does not typically give the sum signal from either source operating alone. For APCI/APPI the signal can range from less than that of either source alone to the sum of the two individual sources. For ESI/APPI, we observed large suppressions of the ESI multiply-charged signal of proteins when the APPI source was on. These behaviors are presumed to be due to the interaction of the initially formed ions by both sources and attests to the importance of ion-molecule reactions that occur during and after the primary ionization events. We give examples of compounds that are preferentially ionized by either APPI, APCI or ESI and present thermochemical arguments based on molecular structure and functionality to explain this behavior. The dual source is also shown to be able to operate in negative ion mode opening up the potential to conduct wide ranging chemical analyses.