使用谷氨酸表面活性剂涂覆的 C18 固定相的短链羧酸的液相色谱法。
Liquid chromatography of short chain carboxylic acids using a glutamic acid surfactant coated C18 stationary phase.
机构信息
Department of Chemistry, 651 E. High Street, Miami University, Oxford, OH, 45056, USA.
Department of Chemistry, 651 E. High Street, Miami University, Oxford, OH, 45056, USA.
出版信息
Talanta. 2020 Jun 1;213:120807. doi: 10.1016/j.talanta.2020.120807. Epub 2020 Feb 7.
A C18 column was modified with the anionic amino acid surfactant lauroyl-l-glutamate (LLG) to facilitate the separation of ten short-chain aliphatic carboxylic acids (oxalic, tartaric, malic, malonic, lactic, acetic, maleic, citric, fumaric, and succinic). The developed method was proven to be fast, versatile, and environmentally friendly. After the coating of the column using 1% LLG solution and optimizing chromatographic conditions such as pH and temperature, near baseline resolution of the ten carboxylic acids within 4 min with excellent peak shape at pH = 1.8 using 100% HO acidified with sulfuric acid was possible. Although the design of this stationary phase, with the hydrophilic group at the end of the alkyl chain, seems to be in contrast to such columns designed for a totally aqueous mobile phase that have a polar (often amide) group embedded near the silica surface, no evidence of phase collapse was noted. Linear relationships of ln retention factor (k) versus 1/Temperature (T) (van't Hoff plots) were generated for all the acids indicating a single retention mechanism was likely. As the pH of the mobile phase decreased, the analyte retention factors increased due to the increase of the fraction of the analyte with neutral charge (alpha zero). The surfactant amide linkage, being electron donating, increased the pKa of the more acidic carboxyl group of glutamic acid so both carboxyl groups were protonated (neutral) at pH 1.8. The exact nature of the retention mechanism is uncertain but there certainly seems to be a pronounced hydrophobic component due to the large difference in retention of fumaric acid and methyl fumarate at pH 1.8. In addition, eleven beverage samples were analyzed for their aliphatic carboxylic acid contents. The results showed that malic, fumaric, and citric acids were the most common carboxylic acids in natural beverages with concentrations as high as 6432 ppm of malic acid in organic apple juice, 64 ppm of fumaric acid in organic concord juice, and 6543 ppm citric acid in strawberry lemonade juice.
C18 柱经阴离子氨基酸表面活性剂月桂酰-L-谷氨酸（LLG）修饰，有利于分离十种短链脂肪族羧酸（草酸、酒石酸、苹果酸、丙二酸、乳酸、乙酸、马来酸、柠檬酸、富马酸和琥珀酸）。所开发的方法被证明快速、通用且环保。在使用 1%LLG 溶液对柱进行涂层并优化色谱条件（如 pH 和温度）后，在 pH=1.8 时，使用 100%HO 酸化硫酸，十种羧酸可在 4 分钟内实现近乎基线分离，且峰形极佳。尽管这种固定相的设计带有烷基链末端的亲水性基团，似乎与设计用于完全水性流动相的固定相相反，后者在硅胶表面附近嵌入极性（通常为酰胺）基团，但没有注意到相崩溃的证据。所有酸的 ln 保留因子（k）与 1/Temperature（T）（van't Hoff 图）的线性关系表明可能存在单一的保留机制。随着流动相 pH 的降低，由于中性电荷（alpha zero）的分析物分数增加，分析物的保留因子增加。表面活性剂酰胺键是供电子的，增加了谷氨酸更酸性羧酸基团的 pKa，因此在 pH 1.8 时，两个羧酸基团都质子化（中性）。保留机制的确切性质尚不确定，但由于在 pH 1.8 时富马酸和富马酸甲酯的保留差异很大，肯定存在明显的疏水性成分。此外，还对 11 种饮料样品的脂肪族羧酸含量进行了分析。结果表明，在天然饮料中，苹果酸、富马酸和柠檬酸是最常见的羧酸，有机苹果汁中苹果酸的浓度高达 6432ppm，有机康科德汁中富马酸的浓度为 64ppm，草莓柠檬水的柠檬酸浓度为 6543ppm。
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