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 (10-9-10-6 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 (≥10-4 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.

中文翻译：

---

气相离子分子反应表明，大气压化学电离时，分析物浓度对4-氨基苯甲酸质子化位点的影响。

水溶液中4-氨基苯甲酸的最碱性位点是氨基氮，而羰基氧被认为是气相中最碱性的位点。但是，在电喷雾电离（ESI）和大气压化学电离（APCI）时，4-氨基苯甲酸的首选质子化位置取决于电离溶剂和离子源参数。尚未研究过分析物浓度对APCI上明显质子化位点的影响，并在此处进行了报道。使用三甲氧基甲基硅烷的气相离子分子反应来确定使用APCI以甲醇或乙腈-水为溶剂离子化的4-氨基苯甲酸的质子化位点。在甲醇溶剂中低分析物浓度（10-9-10-6 M）时，发现氮富集剂的含量约为氧富集剂的两倍。该发现是基于先前的发现而合理化的，即当O-protomer在气相中被8个以上的甲醇分子包围时，它开始表现为好像在水溶液中一样，并转化为N-protomer。在更高的分析物浓度（≥10-4M）下，氨基主要被质子化，这是根据形成的特别稳定的质子结合的4-氨基苯甲酸二聚体合理化的，该二聚体优先解离形成N-质子。上述发现表明，与最近的文献一致，APCI中的解决方法比通常假定的要重要得多。确实，当1：