Innovative Chromatography Group, Irish Separation Science Cluster (ISSC), Department of Chemistry, and the Analytical and Biological Chemistry Research Facility (ABCRF), University College Cork, Cork, Ireland.
J Chromatogr A. 2013 Jul 12;1298:86-94. doi: 10.1016/j.chroma.2013.05.024. Epub 2013 May 16.
This research uses solid-state nuclear magnetic resonance (NMR) spectroscopy to characterise the nature and amount of different surface species, and chromatography to evaluate phase properties of a pentafluorophenylpropyl (PFPP) bonded silica phase prepared and end-capped using supercritical carbon dioxide (sc-CO2) as a reaction solvent. Under sc-CO2 reaction conditions (at temperature of 100 °C and pressure of 414 bar), a PFPP silica phase was prepared using 3-[(pentafluorophenyl)propyldimethylchlorosilane] within 1h. The bonded PFPP phase was subsequently end-capped with bis-N,O-trimethylsilylacetamide (BSA), hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) within 1h under the same sc-CO2 reaction conditions (100 °C/4141 bar). Elemental microanalysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to provide support data to solid-state NMR and chromatographic evaluation. Results revealed a surface coverage of 2.2 μmol/m(2) for the non-end-capped PFPP silica phase while the PFPP phase end-capped with BSA gave a higher surface coverage (3.9 μmol/m(2)) compared to HMDS (2.9 μmol/m(2)) and TMCS (2.8 μmol/m(2)). (29)Si CP/MAS NMR analysis of the PFPP end-capped with BSA shows a significant decrease in the amount of Q(3) (free silanols) and Q(4) (siloxane groups) species, coupled with the absence of the most reactive Q(2) (geminal silanols) in addition to increased amount of a single resonance peak centred at +13 ppm (MH) corresponding to -Si-O-*Si-CH3 bond. (13)C CP/MAS NMR shows the resonance corresponding to the propyl linkage (CH3CH2CH2-) and methyl groups (Si(CH3)n) confirming successful silanisation and endcapping reactions in sc-CO2. Chromatographic evaluation of the BSA end-capped PFPP phase with Neue text mixture revealed improved chromatographic separation as evidenced in the enhanced retention of hydrophobic markers and decreased retention for basic solutes. Moreover, chromatography revealed a change in column selectivity for the BSA end-capped PFPP phase with dipropylphthalate eluting before naphthalene, indicating decreased silanol groups and increased hydrophobicity. The extend of BSA end-capping as measured by the increase in column efficiency (67,260 N/m vs. 60,480 N/m) on a 2.1 i.d.×50 mm column, methylene group selectivity (α(CH(2)) = 2.27 vs. 2.14) and decreased silanophilic interactions (S=3.7 vs. 4.10) indicate that the increase in carbon loading (3.9 μmol/m(2) vs. 2.2 μmol/m(2)) and improvement in chromatography in good peak shape and symmetry is attributed to end-capping with trimethylsilyl groups.
本研究采用固态核磁共振（NMR）光谱法来描述不同表面物种的性质和数量，并采用色谱法来评估使用超临界二氧化碳（sc-CO2）作为反应溶剂制备和端封的五氟丙基丙基（PFPP）键合硅胶相的相性质。在 sc-CO2 反应条件下（温度为 100°C，压力为 414 巴），在 1 小时内使用 3-[(五氟丙基)丙基二甲基氯硅烷]制备了 PFPP 硅胶相。随后，在相同的 sc-CO2 反应条件（100°C/4141 bar）下，用双 N,O-三甲基硅基乙酰胺（BSA）、六甲基二硅氮烷（HMDS）和三甲基氯硅烷（TMCS）对键合的 PFPP 相进行了端封，时间为 1 小时。元素微量分析、热重分析（TGA）和扫描电子显微镜（SEM）用于为固态 NMR 和色谱评估提供支持数据。结果表明，未端封的 PFPP 硅胶相的表面覆盖率为 2.2 μmol/m2，而用 BSA 端封的 PFPP 相的表面覆盖率（3.9 μmol/m2）高于 HMDS（2.9 μmol/m2）和 TMCS（2.8 μmol/m2）。用 BSA 端封的 PFPP 的 29Si CP/MAS NMR 分析表明，Q(3)（游离硅醇）和 Q(4)（硅氧烷基团）的数量显著减少，同时最具反应性的 Q(2)（偕二硅醇）也不存在，并且单一共振峰的数量增加，中心位于+13 ppm（MH）处，对应于-Si-O-*Si-CH3 键。13C CP/MAS NMR 显示与丙基键（CH3CH2CH2-）和甲基（Si(CH3)n）对应的共振，证实了在 sc-CO2 中成功的硅烷化和端封反应。用 Neue text mixture 对 BSA 端封的 PFPP 相进行色谱评估表明，疏水性标记物的保留增强，碱性溶质的保留减少，从而改善了色谱分离。此外，色谱分析表明，BSA 端封的 PFPP 相的柱选择性发生了变化，邻苯二甲酸二丙酯先于萘洗脱，表明硅醇基团减少，疏水性增加。在 2.1 id×50mm 柱上，柱效率（67,260 N/m 对 60,480 N/m）、亚甲基基团选择性（α(CH(2))=2.27 对 2.14）和硅烷亲合性相互作用的降低（S=3.7 对 4.10）的增加表明，碳载量（3.9 μmol/m2 对 2.2 μmol/m2）的增加和色谱性能的改善（良好的峰形和对称性）归因于三甲基硅基的端封。
