Department of Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin, Iran.
TOF Tech. Pars Company, Isfahan Science & Technology Town, Isfahan, Iran.
Anal Chem. 2020 Jun 2;92(11):7924-7931. doi: 10.1021/acs.analchem.0c01318. Epub 2020 May 12.
Ion mobility spectra of citric acid (CA) are complex, and several peaks are observed for CA and its fragments in both the positive and negative modes. Using DFT calculations, we found that the fragments are both less acidic and less basic than CA in gas phase. Hence, we used a strong base, NH, in positive mode to produce NH as an alternative reactant ion (RI) and prevent protonation of the fragments. In the presence of NH, only one peak for CA was observed because of its higher proton affinity (873 kJ mol) compared to NH (854 kJ mol). In the negative mode, CHCl, CHBr, and CHI were used as dopant gases to produce Cl, Br, and I as RIs. These halides have less basicity than the common RIs in negative mode (NO, NO, O) and selectively deprotonated CA in the presence of its fragments. Hence, using dopants with appropriate basicity, we could suppress the fragment peaks and obtain a plain IMS spectrum for CA containing only one peak in both the positive and negative modes. Using NH and CHCl dopants, the amount of CA in fresh lemon juice was determined as 39.5-42 g L by direct injection without any purification. The effect of hydration of the reactant and product ions on the ionization mechanism in both negative and positive modes was investigated theoretically.
柠檬酸（CA）的离子迁移谱较为复杂，在正、负离子模式下均观察到 CA 及其碎片有多个峰。通过 DFT 计算，我们发现 CA 的碎片在气相中比 CA 的酸性和碱性都要弱。因此，我们在正模式下使用强碱性的 NH 作为替代反应离子（RI），以防止碎片质子化。在 NH 存在的情况下，由于 CA 的质子亲和能（873 kJ/mol）高于 NH（854 kJ/mol），仅观察到 CA 的一个峰。在负离子模式下，我们使用 CHCl、CHBr 和 CHI 作为掺杂气体，产生 Cl、Br 和 I 作为 RI。这些卤化物的碱性比负离子模式中的常见 RI（NO、NO、O）弱，并且可以在其碎片存在的情况下选择性地去质子化 CA。因此，通过使用具有适当碱性的掺杂剂，我们可以抑制碎片峰，并获得 CA 的纯净 IMS 谱，在正、负离子模式下均只有一个峰。使用 NH 和 CHCl 掺杂剂，通过直接注入，无需任何纯化，就可以确定新鲜柠檬汁中 CA 的含量为 39.5-42 g/L。还从理论上研究了反应物和产物离子的水合作用对正、负离子模式下的电离机制的影响。
