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The ionization mechanisms in direct and dopant-assisted atmospheric pressure photoionization and atmospheric pressure laser ionization.

Journal of the American Society for Mass Spectrometry (2014-09-25)
Tiina J Kauppila, Hendrik Kersten, Thorsten Benter
ABSTRACT

A novel, gas-tight API interface for gas chromatography-mass spectrometry was used to study the ionization mechanism in direct and dopant-assisted atmospheric pressure photoionization (APPI) and atmospheric pressure laser ionization (APLI). Eight analytes (ethylbenzene, bromobenzene, naphthalene, anthracene, benzaldehyde, pyridine, quinolone, and acridine) with varying ionization energies (IEs) and proton affinities (PAs), and four common APPI dopants (toluene, acetone, anisole, and chlorobenzene) were chosen. All the studied compounds were ionized by direct APPI, forming mainly molecular ions. Addition of dopants suppressed the signal of the analytes with IEs above the IE of the dopant. For compounds with suitable IEs or Pas, the dopants increased the ionization efficiency as the analytes could be ionized through dopant-mediated gas-phase reactions, such as charge exchange, proton transfer, and other rather unexpected reactions, such as formation of [M + 77](+) in the presence of chlorobenzene. Experiments with deuterated toluene as the dopant verified that in case of proton transfer, the proton originated from the dopant instead of proton-bound solvent clusters, as in conventional open or non-tight APPI sources. In direct APLI using a 266 nm laser, a narrower range of compounds was ionized than in direct APPI, because of exceedingly high IEs or unfavorable two-photon absorption cross-sections. Introduction of dopants in the APLI system changed the ionization mechanism to similar dopant-mediated gas-phase reactions with the dopant as in APPI, which produced mainly ions of the same form as in APPI, and ionized a wider range of analytes than direct APLI.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Acridine, BioReagent, suitable for fluorescence, ≥97.0% (HPLC)
Supelco
Pyridine, analytical standard
Supelco
Anisole, analytical standard
Sigma-Aldrich
Hexane, anhydrous, 95%
Sigma-Aldrich
Toluene-d8, 99 atom % D, contains 1 % (v/v) TMS
Sigma-Aldrich
Pyridine, anhydrous, 99.8%
Sigma-Aldrich
Anisole, anhydrous, 99.7%
Sigma-Aldrich
Toluene-d8, anhydrous, 99.6 atom % D
Sigma-Aldrich
Quinoline, reagent grade, 98%
Sigma-Aldrich
Toluene, anhydrous, 99.8%
Sigma-Aldrich
Toluene-d8, 99 atom % D
Sigma-Aldrich
Toluene-d8, 99 atom % D, contains 0.03 % (v/v) TMS
Supelco
Hexane, analytical standard
Supelco
Ethylbenzene, analytical standard
Sigma-Aldrich
Toluene-d8, "100%", 99.96 atom % D
Supelco
Toluene, analytical standard
Sigma-Aldrich
Ethylbenzene, anhydrous, 99.8%
Sigma-Aldrich
Anisole, ≥99%, FCC, FG
Sigma-Aldrich
Toluene-d8, "100%", 99.96 atom % D
Sigma-Aldrich
Pyridine, ≥99%
Sigma-Aldrich
Toluene-d8, 99.6 atom % D
Supelco
Acetone solution, certified reference material, 2000 μg/mL in methanol: water (9:1)
Sigma-Aldrich
Acridine, 97%
Sigma-Aldrich
Toluene, ACS reagent, ≥99.5%
Supelco
Quinoline, analytical standard
Supelco
Pyridine, Pharmaceutical Secondary Standard; Certified Reference Material
Supelco
Toluene, Pharmaceutical Secondary Standard; Certified Reference Material
Supelco
Ethylbenzene, Pharmaceutical Secondary Standard; Certified Reference Material
Supelco
Anisole, Pharmaceutical Secondary Standard; Certified Reference Material
Sigma-Aldrich
Hexane, HPLC Plus, for HPLC, GC, and residue analysis, ≥95%