title: Photon-independent gas-phase-ion formation in capillary electrophoresis-mass spectrometry using atmospheric pressure photoionization.
abstract: In a previous study on capillary electrophoresis-atmospheric pressure photoionization mass spectrometry (CE-APPI-MS), it was observed that the formation of gas-phase ions does not always proceed through photon-induced mechanisms (Hommerson, P.; Khan, A. M.; De Jong, G. J.; Somsen, G. W. Electrophoresis 2007, 28, 1444-1453). That is, analyte signals were observed when the VUV excitation source was switched off. The aim of the present study was to further explore this photon-independent ionization (PII) process. Parameters such as MS capillary voltage, compound nature, background electrolyte (BGE) composition, and presence of dopants were studied using a CE-APPI-MS setup. Infusion experiments showed a relatively low MS capillary voltage of approximately 600 V to be the main prerequisite for PII. Quaternary ammonium compounds showed strong responses in PII-MS but could not be observed in dopant-assisted APPI. Basic amines could be ionized by both photoionization (PI) and PII, whereas neutral compounds (steroids) could only be observed using PI. Nonvolatile BGEs appeared to cause substantial ionization suppression in PII, while PI signals remained largely unaffected. Selection of the proper interface and MS settings allowed PI and PII to proceed simultaneously, which broadened the range of compounds that could be analyzed in a single CE-APPI-MS run. Based on the observed characteristics, it is concluded that PII most probably occurs by a liquid-phase ionization mechanism, which appears to arise in the APPI source when specific conditions are selected.
authors.affiliation: Department of Biomedical Analysis, Utrecht University, P.O. Box 80082, 3508 TB Utrecht, The Netherlands. P.Hommerson@uu.nl
authors.forename: Paul
authors.initials: P
authors.lastname: Hommerson
authors.forename: Amjad M
authors.initials: AM
authors.lastname: Khan
authors.forename: Tony
authors.initials: T
authors.lastname: Bristow
authors.forename: Wilfried
authors.initials: W
authors.lastname: Niessen
authors.forename: Gerhardus J
authors.initials: GJ
authors.lastname: de Jong
authors.forename: Govert W
authors.initials: GW
authors.lastname: Somsen
mesh_terms: D001274:Atmospheric Pressure; D019075:Electrophoresis, Capillary / Q000379:methods*; D005740:Gases; D007477:Ions / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods*; D010777:Photochemistry; D017785:Photons; D013696:Temperature; D014466:Ultraviolet Rays; D014835:Volatilization
chemical_list: D005740:Gases; D007477:Ions