Division of Metabolomics, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
Vrije Universiteit Brussel, Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussels, Belgium.
J Chromatogr A. 2018 Dec 14;1580:63-71. doi: 10.1016/j.chroma.2018.10.023. Epub 2018 Oct 16.
We report on possibility to enhance the hydrophobicity of octadecylsilylated silica-based porous layered open tubular (PLOT) columns with an inner diameter (i.d.) of 5 μm by applying hybrid tetramethoxysilane (TMOS)/methyltrimethoxysilane (MTMS) layers with inserted methyl groups. Due to this higher hydrophobicity, thinner porous layers suffice to achieve similar retention factor (k) as in octadecylsilylated silica-based PLOT columns synthesized using TMOS only. Since thinner layers have a lower intra-layer mass transfer resistance, this in turn allows to obtain superior column efficiencies in comparison with separations carried out with TMOS-based PLOT columns at the same retention. Since layer thickness contributes to the C-term type of band broadening, this is most pronounced at high velocities. Typical gains in column efficiency at a reduced velocity of ν = 30 are on the order of 15%. Preparing the hybrid PLOT columns in 5 μm i.d.-capillaries with a length of 0.4 m using different TMOS/MTMS preparation mixtures leads to different layer thickness in the capillaries. It is shown that column efficiencies for the most retained compound (k = 0.9-1.5) went from N = 101,000 for PLOT columns with a layer thickness (d) of 250 nm, over N = 95,000 for d = 320 nm to N = 89,000 for d = 400 nm, corresponding to plate heights (H) in the order of 3.5-3.9 μm (reduced plate heights (h = 0.8-1.0)). By applying the same preparation mixtures for much longer capillaries of 1.3 m, a high repeatability of the volumetric phase ratio (m) (difference <1%) and the k-values (difference <5%) was observed between the 0.4 m and 1.3 m PLOT columns. In addition, also a very similar band broadening was obtained, as the h-values in the longer columns coincided well (order of a few % difference) with the reduced plate height curves measured in the shorter columns. The effect of the retention factor and layer thickness on these reduced plate height curves furthermore fits well with the Golay-Aris theory. Depending on the layer thickness, plate numbers in the longer capillary columns were varying from N = 282,000 to N = 379,000 for the most retained compound.
我们报告了一种可能性，即在十八烷基硅烷化的基于二氧化硅的多孔层开管（PLOT）柱内直径（i.d.）为 5μm 的情况下，通过应用混合的四甲氧基硅烷（TMOS）/甲基三甲氧基硅烷（MTMS）层并用插入的甲基进行增强其疏水性。由于这种更高的疏水性，较薄的多孔层足以获得与仅使用 TMOS 合成的十八烷基硅烷化的基于二氧化硅的 PLOT 柱相似的保留因子（k）。由于较薄的层具有较低的层内传质阻力，这反过来又允许与在相同保留下使用基于 TMOS 的 PLOT 柱进行的分离相比获得更高的柱效率。由于层厚度会导致 C-term 类型的带宽展宽，因此在高流速下最为明显。在降低的流速 ν = 30 下，柱效率的典型增益约为 15%。使用不同的 TMOS/MTMS 制备混合物在 5μm i.d.的毛细管中制备混合 PLOT 柱，长度为 0.4m，会导致毛细管中不同的层厚度。结果表明，对于最保留的化合物（k = 0.9-1.5），从层厚度（d）为 250nm 的 PLOT 柱的 N = 101,000，到 d = 320nm 的 N = 95,000，再到 d = 400nm 的 N = 89,000，相应的板高（H）约为 3.5-3.9μm（降板高度（h = 0.8-1.0））。通过对长度为 1.3m 的更长毛细管应用相同的制备混合物，可以在 0.4m 和 1.3m PLOT 柱之间观察到体积相比（m）（差异<1%）和 k 值（差异<5%）的高度重复性。此外，还获得了非常相似的带宽展宽，因为较长柱中的 h 值与在较短柱中测量的降板高度曲线非常吻合（差异约为几个百分点）。保留因子和层厚度对这些降板高度曲线的影响也与 Golay-Aris 理论很好地吻合。根据层厚度的不同，对于最保留的化合物，在较长毛细管柱中的板数从 N = 282,000 变化到 N = 379,000。
