Gritti Fabrice, Guiochon Georges
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
J Chromatogr A. 2014 Dec 12;1372C:42-54. doi: 10.1016/j.chroma.2014.10.003. Epub 2014 Nov 6.
The effects of the surface concentration of positive charges attached to the surface of research BEH-C hybrid particles on the overloaded band profiles and the adsorption isotherms of a neutral (caffeine) and a positively charged (nortryptilinium hydrochloride) compounds were measured and investigated. The inverse method (IM) of chromatography was used to determine the isotherm parameters. Three columns were packed with endcapped BEH-C particles doped with three different charge densities on their surfaces (LOW, MEDIUM and HIGH). Two other columns packed with unbonded, non-endcapped, and endcapped BEH-C particles served as standard reference materials. Minor disturbance method (MDM) experiments were conducted with acetonitrile/water mixtures in order to assess qualitatively the surface densities of the fixed positive charges. A more quantitative approach based on the solution of the linearized Poisson-Boltzmann equation and the decrease of the experimental Henry constant was also applied. The results show that the surface concentrations of the fixed charges in the LOW, MEDIUM and HIGH columns were 0.029, 0.050, and 0.064μmol/m, e.g., close to two orders of magnitude smaller than the surface density of bonded C chains (2.1μmol/m). The adsorption isotherm of the ionizable compound nortryptilinium onto the BEH-C columns is consistent with a two-sites adsorption model. The density of the high energy sites correlates directly to the total amount of the fixed charges and isolated silanols amidst the C-bonded chains. The amount of low energy sites reflects the specific surface area of the adsorbent. The binding constants on the high- and low-energy adsorption sites are respectively ten and two times lower on the HIGH column than on the reference endcapped column. The active sites are closer to the adsorbent surface than the weak adsorption sites. Finally, a higher production rate of ionizable compounds can be achieved in preparative chromatography with the charge doped than with the reference RPLC-C columns. For the same analysis time, larger recovery yields are observed despite a slight loss in selectivity due to non-selective repulsive electrostatic interactions.
研究了附着在BEH-C杂化颗粒表面的正电荷表面浓度对中性化合物（咖啡因）和带正电荷化合物（盐酸去甲替林）的过载谱带轮廓及吸附等温线的影响。采用色谱反演法（IM）确定等温线参数。用表面掺杂三种不同电荷密度（低、中、高）的封端BEH-C颗粒填充三根色谱柱。另外两根填充未键合、未封端和封端BEH-C颗粒的色谱柱用作标准参考材料。用乙腈/水混合物进行微扰法（MDM）实验，以定性评估固定正电荷的表面密度。还应用了一种基于线性化泊松-玻尔兹曼方程解和实验亨利常数降低的更定量方法。结果表明，低、中、高色谱柱中固定电荷的表面浓度分别为0.029、0.050和0.064μmol/m，例如，比键合C链的表面密度（2.1μmol/m）小近两个数量级。可电离化合物盐酸去甲替林在BEH-C色谱柱上的吸附等温线符合双位点吸附模型。高能位点的密度与固定电荷总量以及C键合链之间孤立的硅醇直接相关。低能位点的数量反映了吸附剂的比表面积。高、低能吸附位点上的结合常数在高电荷色谱柱上分别比参考封端色谱柱低10倍和2倍。活性位点比弱吸附位点更靠近吸附剂表面。最后，与参考反相液相色谱-C色谱柱相比，在制备色谱中使用电荷掺杂的色谱柱可实现更高的可电离化合物产率。在相同的分析时间内，尽管由于非选择性排斥静电相互作用导致选择性略有损失，但仍观察到更高的回收率。
