title: Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization.
abstract: The efficiencies of charge exchange reaction in dopant-assisted atmospheric pressure chemical ionization (DA-APCI) and dopant-assisted atmospheric pressure photoionization (DA-APPI) mass spectrometry (MS) were compared by flow injection analysis. Fourteen individual compounds and a commercial mixture of 16 polycyclic aromatic hydrocarbons were chosen as model analytes to cover a wide range of polarities, gas-phase ionization energies, and proton affinities. Chlorobenzene was used as the dopant, and methanol/water (80/20) as the solvent. In both techniques, analytes formed the same ions (radical cations, protonated molecules, and/or fragments). However, in DA-APCI, the relative efficiency of charge exchange versus proton transfer was lower than in DA-APPI. This is suggested to be because in DA-APCI both dopant and solvent clusters can be ionized, and the formed reagent ions can react with the analytes via competing charge exchange and proton transfer reactions. In DA-APPI, on the other hand, the main reagents are dopant-derived radical cations, which favor ionization of analytes via charge exchange. The efficiency of charge exchange in both DA-APPI and DA-APCI was shown to depend heavily on the solvent flow rate, with best efficiency seen at lowest flow rates studied (0.05 and 0.1 mL/min). Both DA-APCI and DA-APPI showed the radical cation of chlorobenzene at 0.05-0.1 mL/min flow rate, but at increasing flow rate, the abundance of chlorobenzene M(+.) decreased and reagent ion populations deriving from different gas-phase chemistry were recorded. The formation of these reagent ions explains the decreasing ionization efficiency and the differences in charge exchange between the techniques. Graphical Abstract ᅟ.
authors.affiliation: Faculty of Pharmacy, University of Helsinki, P.O. Box 56 , (Viikinkaari 5 E), 00014, Helsinki, Finland.
authors.forename: Anu
authors.initials: A
authors.lastname: Vaikkinen
authors.affiliation: Faculty of Pharmacy, University of Helsinki, P.O. Box 56 , (Viikinkaari 5 E), 00014, Helsinki, Finland.
authors.forename: Tiina J
authors.initials: TJ
authors.lastname: Kauppila
authors.affiliation: Faculty of Pharmacy, University of Helsinki, P.O. Box 56 , (Viikinkaari 5 E), 00014, Helsinki, Finland. risto.kostiainen@helsinki.fi.
authors.forename: Risto
authors.initials: R
authors.lastname: Kostiainen
keywords: Atmospheric pressure chemical ionization; Atmospheric pressure photoionization; Charge exchange; Dopant