Herrera Lisandra Cubero, Grossert J Stuart, Ramaley Louis
Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada.
J Am Soc Mass Spectrom. 2008 Dec;19(12):1926-41. doi: 10.1016/j.jasms.2008.07.016. Epub 2008 Jul 23.
The behavior in atmospheric pressure chemical ionization of selected model polycyclic aromatic compounds, pyrene, dibenzothiophene, carbazole, and fluorenone, was studied in the solvents acetonitrile, methanol, and toluene. Relative ionization efficiency and sensitivity were highest in toluene and lowest in methanol, a mixture of molecular ions and protonated molecules was observed in most instances, and interferences between analytes were detected at higher concentrations. Such interferences were assumed to be caused by a competition among analyte molecules for a limited number of reagent ions in the plasma. The presence of both molecular ions and protonated analyte molecules can be attributed to charge-transfer from solvent radical cations and proton transfer from protonated solvent molecules, respectively. The order of ionization efficiency could be explained by incorporating the effect of solvation in the ionization reactions. Thermodynamic data, both experimental and calculated theoretically, are presented to support the proposed ionization mechanisms. The analytical implications of the results are that using acetonitrile (compared with methanol) as solvent will provide better sensitivity with fewer interferences (at low concentrations), except for analytes having high gas-phase basicities.
在乙腈、甲醇和甲苯等溶剂中，研究了芘、二苯并噻吩、咔唑和芴酮等选定的模型多环芳烃化合物在大气压化学电离中的行为。相对电离效率和灵敏度在甲苯中最高，在甲醇中最低，大多数情况下观察到分子离子和质子化分子的混合物，并且在较高浓度下检测到分析物之间的干扰。这种干扰被认为是由于分析物分子在等离子体中争夺有限数量的试剂离子所致。分子离子和质子化分析物分子的存在可分别归因于溶剂自由基阳离子的电荷转移和质子化溶剂分子的质子转移。电离效率的顺序可以通过在电离反应中纳入溶剂化效应来解释。给出了实验和理论计算的热力学数据，以支持所提出的电离机制。结果的分析意义在于，除了具有高气相碱度的分析物外，使用乙腈（与甲醇相比）作为溶剂将在干扰较少的情况下（低浓度时）提供更好的灵敏度。
