Institute of Organic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany.
Anal Chem. 2009 Dec 15;81(24):10136-42. doi: 10.1021/ac901911w.
The relationship between alkyl phase structure and chromatographic performance is investigated for a series of octadecyl (C(18))-modified silica surfaces with defined spacing of the alkyl surface by a "pre-end-capping" technique. Stationary phases were prepared by a two step process with (1) reaction with less than stochiometric amounts of a small monofunctional silane, followed by (2) solution or surface polymerization with octadecyltrichlorosilane. The results of solid-state and suspension nuclear magnetic resonance (NMR) spectroscopy are correlated with the chromatographic behavior regarding shape selective separations. Two sets of six different stationary phases were prepared by solution and surface polymerization approaches, yielding materials with surface coverages from 2.7 to 5.6 micromol/m(2). (13)C cross-polarization magic angle spinning (CP/MAS) NMR spectra show a predominance of trans conformations for the set of surface polymerized phases with a C(18) coverage greater than 4.5 micromol/m(2). For the solution polymerized phases, no predominance for the trans conformation was observed, even for surface coverages greater than 5.1 micromol/m(2). Proton spectra in suspension indicate the trend that a higher coverage for the surface polymerized materials correlates with a more rigid alkyl chain conformation. The set of solution polymerized stationary phases confirms this tendency but minor deviations are observed for high coverages. These structural abnormalities are confirmed by differences in the (29)Si CP/MAS spectra. Furthermore, the (29)Si CP/MAS spectra indicate a lower amount of cross-linking for the materials with the highest amount of placeholder (spacer). The use of the different spectroscopic and chromatographic methods provides a wealth of information on the surface morphology of the systematically prepared C(18) materials and extends the understanding of surface morphology of alkyl modified silica and its influences of the molecular recognition process in liquid chromatography.
采用“预封端”技术，通过控制烷基表面的空间位阻，研究了一系列十八烷基（C(18)）改性硅胶表面的烷基相结构与色谱性能之间的关系。采用两步法制备固定相：（1）与少量单官能硅烷反应，然后（2）用十八烷基三氯硅烷进行溶液或表面聚合。固态和悬浮核磁共振（NMR）光谱的结果与关于形状选择性分离的色谱行为相关联。通过溶液和表面聚合方法制备了两组六种不同的固定相，得到了表面覆盖率为 2.7 至 5.6 μmol/m(2)的材料。（13）C 交叉极化魔角旋转（CP/MAS）NMR 谱表明，对于表面聚合的 C(18)覆盖率大于 4.5 μmol/m(2)的一组固定相，反式构象占主导地位。对于溶液聚合的相，即使在表面覆盖率大于 5.1 μmol/m(2)的情况下，也没有观察到反式构象的优势。悬浮质子谱表明，表面聚合材料的更高覆盖率与烷基链构象更刚性的趋势相关。溶液聚合的固定相系列证实了这一趋势，但在高覆盖率时观察到较小的偏差。这些结构异常通过（29）Si CP/MAS 谱的差异得到证实。此外，（29）Si CP/MAS 谱表明，具有最高占位剂（间隔物）量的材料的交联程度较低。不同光谱和色谱方法的使用为系统制备的 C(18)材料的表面形态提供了丰富的信息，并扩展了对烷基改性硅胶的表面形态及其对液相色谱中分子识别过程的影响的理解。
