Departament de Química Analítica, Universitat de València, c/Dr. Moliner 50, Burjassot, Spain.
J Chromatogr A. 2012 Apr 6;1232:166-75. doi: 10.1016/j.chroma.2011.11.054. Epub 2011 Dec 4.
Residual silanols on C18 columns yield undesirable slow-kinetics ion-exchange interactions with positively charged basic compounds that result in asymmetrical peaks, low efficiencies and long retention times. The purity of the silica employed as supporting material, and the technique used to form the bonded phase, which varies with the brand and manufacturer, give rise to different amounts of residual silanols in the packings, and consequently, different chromatographic performance. One of the most efficient and widespread strategies to reduce or even eliminate the different performance among columns is the addition of a reagent to the mobile phase to block the silanol sites. However, the intrinsic nature of both stationary phase and additive leads to particular results. In this work, a group of basic compounds were analysed using six C18 stationary phases (Zorbax SB-C18, X-Terra MS C18, Kromasil, Lichrospher, Nucleosil, and Spherisorb) and acetonitrile-water mixtures. Two ionic liquids (ILs), 1-butyl- and 1-hexyl-3-methyl-imidazolium tetrafluoroborates, were added to the mobile phases to evaluate their silanol suppressing potency, based on the decreased retention of the basic compounds when the silanols are blocked (described by the Horváth equation), and the improvement in peak profile (described by the plots of the peak half-widths at diverse retention times). The suppressing potency based on the retention can be misleading when the adsorption of the IL anion is not negligible, since the anion attracts the cationic basic compounds increasing the retention. However, the accessibility of basic compounds to the silanols is prevented by both IL cation and anion, improving the peak profiles for all stationary phases. This was especially remarkable for Spherisorb, which in the absence of additive yielded by far the worst performance. 1-Hexyl-3-methyl-imidazolium tetrafluoroborate was the best additive in terms of retention and peak profile (width and asymmetry).
C18 柱上残留的硅醇与带正电荷的碱性化合物发生不理想的缓慢动力学离子交换相互作用，导致峰形不对称、效率低和保留时间长。作为支撑材料使用的硅胶的纯度以及形成键合相的技术（因品牌和制造商而异）导致填料中残留硅醇的数量不同，从而导致色谱性能不同。减少甚至消除柱间不同性能的最有效和广泛的策略之一是在流动相中添加一种试剂来封闭硅醇位点。然而，固定相和添加剂的固有性质会导致特定的结果。在这项工作中，使用六种 C18 固定相（Zorbax SB-C18、X-Terra MS C18、Kromasil、Lichrospher、Nucleosil 和 Spherisorb）和乙腈-水混合物分析了一组碱性化合物。两种离子液体（ILs），1-丁基-和 1-己基-3-甲基-咪唑四氟硼酸盐，被添加到流动相中，以评估它们抑制硅醇的能力，这是基于碱性化合物的保留减少来衡量的（通过 Horváth 方程描述），以及峰形的改善（通过不同保留时间的峰半宽图描述）。当 IL 阴离子的吸附不可忽略时，基于保留的抑制能力可能会产生误导，因为阴离子会吸引带正电荷的碱性化合物，从而增加保留。然而，IL 阳离子和阴离子都可以阻止碱性化合物与硅醇的接触，从而改善所有固定相的峰形。对于 Spherisorb 来说，这一点尤其明显，因为在没有添加剂的情况下，它的性能远差于其他柱。1-己基-3-甲基-咪唑四氟硼酸盐在保留和峰形（宽度和不对称性）方面是最好的添加剂。
