Understanding Mixed-Mode Retention Mechanisms in Liquid Chromatography with Hydrophobic Stationary Phases

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• 作者：Alberto Cavazzini ; Nicola Marchetti ; Roberta Guzzinati ; Luisa Pasti ; Alessia Ciogli ; Francesco Gasparrini ; Aldo Lagan脿
• 刊名：Analytical Chemistry
• 出版年：2014
• 出版时间：May 20, 2014
• 年：2014
• 卷：86
• 期：10
• 页码：4919-4926
• 全文大小：302K
• 年卷期：v.86,no.10(May 20, 2014)
• ISSN：1520-6882

文摘

The chromatographic retention mechanisms of two hydrophobic bonded phases, octadecyl ethyl-bridged organic/inorganic (BEH-C₁₈) and straight-chain perfluorohexylpropyl silica (C₆F₁₃), have been investigated by using a homologous series of alkyl-benzenes and perfluoroalkyl acids as test compounds in a variety of acetonitrile/water mobile phases and at different temperatures. On both columns, polar compounds exhibited a characteristic U-shape retention behavior in function of acetonitrile amount in the eluent, whereas retention of neutral molecules decreased continuously, following an increase of organic modifier, over the entire mobile phase range. The dependence of perfluoromethylene selectivity upon eluent composition explains the typical reversed-phase behavior (decreasing in retention following an increase of acetonitrile in mobile phase) initially exhibited by perfluoroalkyl acids, but alone it cannot justify their increasing of retention at organic-rich mobile phases (approximately >90% v/v for acetonitrile with the C₆F₁₃ column and acetonitrile >80% v/v for the BEH-C₁₈ one). It actually predicts an opposite trend, indicating thus the presence of mixed-mode retention mechanisms. Indeed it was found that, at organic-rich mobile phases, the transfer from the mobile to the stationary phase of the polar moiety of molecules drives retention. This finding has been correlated to the excess adsorption isotherm of acetonitrile/water binary mixtures and thus to the composition of the stationary phase. At organic-rich mobile phases, in fact, stationary phases are characterized by a positive excess of adsorbed water that creates an 鈥渆nvironment鈥?suitable to the transfer herein of polar groups.