Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization

• Published in: Journal of the American Society for Mass Spectrometry Vol. 27; no. 8; pp. 1291 - 1300
• Main Authors: Vaikkinen, Anu, Kauppila, Tiina J., Kostiainen, Risto
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.08.2016; Springer Nature B.V
• Subjects: Analytical Chemistry; Aromatic compounds; Atmospheric pressure; Bioinformatics; Biotechnology; Cation exchanging; Charge efficiency; Charge exchange; Charge transfer; Chemistry; Chemistry and Materials Science; Dopants; Efficiency; Flow injection analysis; Flow velocity; Ionization; Ions; Mass spectrometry; Organic Chemistry; Photoionization; Polycyclic aromatic hydrocarbons; Proteomics; Protons; Reagents; Research Article; Solvents; Dopant; Atmospheric pressure chemical ionization; Charge exchange; Atmospheric pressure photoionization
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1007/s13361-016-1399-8

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