Comparison of dopants for charge exchange ionization of nonpolar polycyclic aromatic hydrocarbons with reversed-phase LC-APPI-MS

Abstract

Atmospheric pressure photoionization (APPI) is capable of ionizing nonpolar compounds in LC/MS, through charge exchange reactions following photoionization of a dopant. Recently, several novel dopants-chlorobenzene, bromobenzene, 2,4-difluoroanisole, and 3-(trifluoromethyl)anisole-have been identified as having properties making them wellsuited to serve as dopants for charge exchange ionization under reversed-phase LC conditions. Here, we report the results of experiments comparing their effectiveness to that of established dopants-toluene, anisole, and a toluene/anisole mixture, for the charge exchange ionization of model nonpolar compounds-the 16 polycyclic aromatic hydrocarbons (PAHs) identified by the US EPA as priority pollutants-when using a conventional reversed-phase LC method. Chloro-and bromobenzene were found to be much more effective than toluene for all the PAHs, due to the relatively low reactivity of their photoions with the solvent. Their overall performance was also better than that of anisole, due to anisole's ineffectiveness toward higher-IE compounds. Further, the experiments revealed that anisole's performance for higher-IE compounds can be dramatically improved by introducing it as a dilute solution in toluene, rather than neat. The two fluoroanisoles provided the highest overall sensitivity, by a slim margin, when introduced as dilute solutions in either chloro-or bromobenzene. With APPI, analyte ionization is mostly due to ionmolecule reactions following photoionization of a primary reagent, typically a dopant [4]. Analyte ionization can occur in positive mode through either proton transfer or charge exchange (electron-transfer) reaction pathways. This article regards the ionization of nonpolar, low proton affinity compounds via charge exchange with dopant radical cations.In APPI, for charge exchange ionization to occur the dopant's ionization energy (IE) must be greater than that of the analyte and to be efficient its radical cations must not be consumed through reactions with the solvent, its own neutrals, or impurities. Toluene was the first dopant to be used for charge exchange ionization in APPI [1], and it has a relatively high IE (8.83 eV), making it suitable for a wide range of analytes (all IE values are from reference [5]). In practice, however, toluene is only an efficient charge exchange dopant under normal-phase [6,7] and/or low-flow conditions[8] because its radical cations are rapidly consumed in reactions with methanol and acetonitrile at conventional LC flow rates [6, 9, 10]. Some other dopant is then required for efficient charge exchange ionization with reversed-phase LC methods. The usual alternative to toluene for promoting charge exchange ionization is anisole, whose photoions are stable in the presence of methanol and acetonitrile [11]. Anisole, however, has a relatively low IE (8.20 eV), restricting its applicability. Mixtures of anisole and toluene have been utilized by Itoh et al. as dopants to promote charge exchange ionization under reversed-phase conditions, i...

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

“…One of the most successful charge exchange dopants has been chlorobenzene [12,13], which has IE of 9.1 eV [14]. The chlorobenzene radical cation does not go through proton transfer reaction with solvent clusters (Scheme 2, Reaction 3) in RPLC-APPI-MS conditions [12], and is therefore not depleted in the ion source in the presence of LC solvents but is, instead, available for charge exchange with the analytes.…”

Evaluation of Quantitative Sulfur Speciation in Gas Oils by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Validation by Comprehensive Two-Dimensional Gas Chromatography

“…APPI in conjunction with FT-ICR MS has been widely used for the characterization of sulfur species [19,20] because of its applicability to non-polar aromatic compounds. The role of the solvent as dopant for proton transfer in APPI has been investigated [21,22]. The formation of protonated molecules in case of petroleum samples was attributed to proton transfer from components of the petroleum matrix rather than from the dopant [23].…”

Extractive Atmospheric Pressure Photoionization (EAPPI) Mass Spectrometry: Rapid Analysis of Chemicals in Complex Matrices

“…After the solvent volatilized at room temperature completely, the soil powder was put in 4 mL of methanol/water (v:v = 1:1) in the nebulization cell for subsequent mass analysis. When detecting PAHs in soil, toluene/anisole (v:v = 1:1) was used as the dopant because of its high ionization efficiency [20]. The soil samples were treated with UNE for 0.5 min before analysis.…”