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 νi = 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 (df) of 250 nm, over N = 95,000 for df = 320 nm to N = 89,000 for df = 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.
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