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

Abstract

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.

Utilizing ion mobility spectrometry and mass spectrometry for the analysis of polycyclic aromatic hydrocarbons, polychlorinated biphenyls, polybrominated diphenyl ethers and their metabolites

“…However, their hydroxylated metabolites, OH-PAHs, OH-PCBs and OH-PBDEs, were ionized effectively by all three techniques. PAHs were observed to form both protonated and radical ions in the positive ionization mode during APCI and APPI, which is consistent with previous studies [35,36]. APCI was also found to generate a higher fraction of protonated species, while APPI mainly produced radical species and much less of the protonated versions.…”

The influence of different types of reactant ions on the ionization behavior of polycyclic aromatic hydrocarbons in corona discharge ion mobility spectrometry

“…In APCI‐MS PAHs are usually detected as molecular ions (M •+ ) rather than protonated molecules ([M + H] + ). Because anisole and halobenzenes generate mainly molecular ions by corona discharge, 24,26 these molecular ions (SM •+ ) should react with PAH molecules to produce PAH molecular ions (PAH •+ ) by charge transfer reactions as follows: The IMS ionization region will contain air, moisture, solvent molecules, and PAH molecules. Because the boiling points of the anisole, fluorobenzene, chlorobenzene, and bromobenzene solvents are 154°C, 85°C, 13°C, and 156°C, respectively, much lower than those of the PAHs, the solvent is evaporated first.…”

“…Young et al analyzed PAHs on borosilicate glass using laser desorption/ionization and IMS, 45 whereas Eiceman and Vandiver used photoionization‐IMS, 46 and other workers used IMS‐MS 47,48 . The analysis of PAHs using corona discharge MS has been reported, 24,26,49 but there have been no reports of their analysis using IMS coupled with corona discharge ionization except for our previous work 35 . In the present work, the influence of the types of RIs on the ionization behavior of PAHs was investigated by introducing the sample solution spotted on a smear matrix into the sample inlet region before evaporation of the solvent.…”

“…A flame ionization detector and electron ionization mass spectrometry (EI‐MS) are typically used as detectors of PAHs in GC 19–21 . In liquid chromatography (LC), a UV detector and atmospheric pressure ionization mass spectrometry (API‐MS) are used for the detection of PAHs 22–24 . In API‐MS, dopants such as anisole, chlorobenzene, bromobenzene, and toluene have been used to improve the ionization efficiency of PAHs 24–26 …”

Mass Spectrometry as a Workhorse for Preclinical Drug Discovery: Special Emphasis on Drug Metabolism and Pharmacokinetics

“…When compared to APCI, APPI offers lesser matrix effects and minimal source contamination. Success of APPI as an ionization technique was reported in the analysis of steroids and quinones [31][32][33].…”