Gritti Fabrice, Guiochon Georges
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
J Chromatogr A. 2006 Jan 20;1103(1):43-56. doi: 10.1016/j.chroma.2005.10.010. Epub 2005 Dec 7.
A new chromatographic method estimating the degree of heterogeneity of RPLC packing materials is based on the results of systematic measurements of the adsorption data in a wide concentration range for selected probe compounds. These data are acquired by frontal analysis (FA), modeled, and used for the calculation of the adsorption energy distribution (AED). Four compounds were used, two neutral compounds of different molecular sizes (caffeine and phenol) and two ionizable compounds of opposite charges, 2-naphthalene sulfonate, an anion, and propranololium, a cation. This work was done on a C30-bonded silica stationary phase (Prontosil-C30), using the same aqueous mobile phase (30% methanol, v/v) for all compounds, except that sodium chloride (25 mM) was added to elute the ionizable compounds. All four adsorption isotherms have Langmuirian behavior. The AEDs are tri-modal for phenol, quadri-modal for caffeine. The total saturation capacity of the stationary phase is four-fold lower for caffeine than for phenol, due in part to its larger molecular size. The equilibrium constants on the low-energy sites of types 1 and 2 are eight-fold larger. These two types of sites characterize the heterogeneity of the bonded layer itself. The density of the high-energy sites of types 3 and 4 is higher for caffeine, suggesting that caffeine molecules can be accommodated in some hydrophobic cages into which smaller molecules like phenol cannot. These high-energy types of sites characterize the heterogeneity of the whole stationary phase (silica support included). The ionizable compounds have larger molecules than the neutral ones and, accordingly, a lower relative density of sites of type 2 to sites of type 1. A tri-modal and a quadri-modal energy distributions were observed for the 2-naphthalene sulfonate anion and the propranololium cation, respectively. The fourth types of sites measured and its unusually high equilibrium constant are most probably due to ion-exchange interactions between the non-endcapped ionized silanols and the propranololium ion. No such strong interactions are observed with the anionic compound.
一种估算反相液相色谱（RPLC）填充材料非均一性程度的新色谱方法，是基于对选定探针化合物在宽浓度范围内吸附数据的系统测量结果。这些数据通过前沿分析（FA）获取、建模，并用于计算吸附能分布（AED）。使用了四种化合物，两种不同分子大小的中性化合物（咖啡因和苯酚）以及两种带相反电荷的可电离化合物，阴离子2-萘磺酸盐和阳离子普萘洛尔。这项工作是在C30键合硅胶固定相（Prontosil-C30）上进行的，所有化合物均使用相同的水相流动相（30%甲醇，v/v），只是添加了氯化钠（25 mM）以洗脱可电离化合物。所有四种吸附等温线均呈现朗缪尔行为。苯酚的AED为三峰，咖啡因的为四峰。固定相的总饱和容量，咖啡因比苯酚低四倍，部分原因是其分子尺寸较大。1型和2型低能位点的平衡常数大八倍。这两种类型的位点表征了键合层本身的非均一性。3型和4型高能位点的密度对于咖啡因更高，这表明咖啡因分子可以容纳在一些疏水性笼中，而像苯酚这样的较小分子则无法进入。这些高能类型的位点表征了整个固定相（包括硅胶载体）的非均一性。可电离化合物的分子比中性化合物大，因此，2型位点与1型位点的相对密度较低。分别观察到2-萘磺酸盐阴离子和普萘洛尔阳离子的三峰和四峰能量分布。测量的第四种类型的位点及其异常高的平衡常数很可能是由于未封端的电离硅醇与普萘洛尔离子之间的离子交换相互作用。对于阴离子化合物未观察到这种强相互作用。
