Chemistry Department, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States.
J Am Soc Mass Spectrom. 2020 Oct 7;31(10):2210-2217. doi: 10.1021/jasms.0c00285. Epub 2020 Sep 14.
The most basic site of 4-aminobenzoic acid in aqueous solution is the amino nitrogen, while the carbonyl oxygen is calculated to be the most basic site in the gas phase. However, the preferred protonation site of 4-aminobenzoic acid upon electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) depends upon the ionization solvent and ion source parameters. The influence of the concentration of the analyte on the manifested protonation sites upon APCI has not been investigated and is reported here. Gas-phase ion-molecule reactions of trimethoxymethylsilane were used to identify the protonation sites of 4-aminobenzoic acid ionized using APCI with methanol or acetonitrile-water as the solvent. The nitrogen-protomer was found to be about twice as abundant as the oxygen-protomer at low analyte concentrations (10-10 M) in methanol solvent. This finding was rationalized on the basis of a previous finding that when the O-protomer is surrounded by more than eight methanol molecules in the gas phase it starts behaving as if it were in an aqueous solution and converts to the N-protomer. At greater analyte concentrations (≥10 M), the amino group was predominantly protonated, which was rationalized based on the formation of a particularly stable proton-bound dimer of 4-aminobenzoic acid that preferentially dissociates to form the N-protomer. The above findings suggest that solution processes are much more important in APCI than commonly assumed, in agreement with recent literature. Indeed, when 1:1 (v/v) acetonitrile-water was used as the solvent system for 4-aminobenzoic acid, the N-protomer was predominantly generated at all analyte concentrations.
4-氨基苯甲酸在水溶液中的最基本的质子化位点是氨基氮，而羰基氧则被计算为气相中的最基本的质子化位点。然而，4-氨基苯甲酸在电喷雾电离（ESI）和大气压化学电离（APCI）中的优先质子化位点取决于电离溶剂和离子源参数。目前还没有研究过分析物浓度对 APCI 中表现出的质子化位点的影响，并在此报告。使用三甲氧基甲基硅烷的气相离子-分子反应，鉴定了使用 APCI 用甲醇或乙腈-水作为溶剂电离的 4-氨基苯甲酸的质子化位点。在甲醇溶剂中，在低分析物浓度（10-10 M）下，氮-质子化体的丰度约为氧-质子化体的两倍。这一发现是基于先前的发现得出的，即在气相中，当 O-质子化体被超过八个甲醇分子包围时，它开始表现得好像在水溶液中，并转化为 N-质子化体。在更高的分析物浓度（≥10 M）下，氨基主要被质子化，这是基于形成特别稳定的 4-氨基苯甲酸质子结合二聚体的结果，该二聚体优先解离形成 N-质子化体。上述发现表明，在 APCI 中，溶液过程比通常假设的更为重要，这与最近的文献一致。事实上，当 1:1（v/v）乙腈-水用作 4-氨基苯甲酸的溶剂系统时，在所有分析物浓度下，主要生成 N-质子化体。
