Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
J Chromatogr A. 2012 Feb 17;1225:79-90. doi: 10.1016/j.chroma.2011.12.055. Epub 2011 Dec 27.
We investigated the mass transfer mechanism in four research prototypes of silica monolithic columns of the second generation provided by their manufacturer (Merck KGaA, Darmstadt, Germany). The heights equivalent to a theoretical plate (HETP) of these columns were measured. The different contributions to the total HETP (longitudinal diffusion term B/u(S), skeleton/eluent mass transfer resistance term Cu(S), and eddy diffusion term A) were determined experimentally for a non-retained (uracil) and for a retained (naphthalene) compound. We used the peak parking method to determine the longitudinal diffusion term, a recently developed accurate model of effective diffusion in silica monolithic structures to determine the skeleton/eluent mass transfer resistance term, and an accurate method of measurement of the total column HETP to determine the eddy diffusion term. The results show that the minimum plate heights of these monolithic column prototypes range between 6 and 7 μm for retained analytes, three times lower than those observed for monolithic columns of the first generation. A detailed analysis of the eddy diffusion term demonstrates that the improvement observed in the column efficiency is explained in part by the 40% reduction of the domain size (which provides thinner half-height peak width) but mostly by a two-fold decrease of the radial velocity bias across the silica rods (which provides more symmetrical peaks). Yet, the rods in these columns exhibit a residual radial heterogeneity leading to a minimum HETP of only 10 μm for non-retained compounds.
我们研究了由制造商（德国达姆施塔特默克公司）提供的第二代硅胶整体柱的四个研究原型的传质机制。测量了这些柱子的理论塔板高度等效值（HETP）。对于非保留（尿嘧啶）和保留（萘）化合物，我们通过实验确定了总 HETP 的不同贡献（纵向扩散项 B/u(S)、骨架/洗脱液传质阻力项 Cu(S)和涡流扩散项 A）。我们使用峰停车法来确定纵向扩散项，使用最近开发的用于确定骨架/洗脱液传质阻力项的硅胶整体结构有效扩散的精确模型，以及用于测量总柱 HETP 的精确方法来确定涡流扩散项。结果表明，这些整体柱原型的最小板高对于保留的分析物在 6 到 7μm 之间，比第一代整体柱的观察值低三倍。对涡流扩散项的详细分析表明，柱效率的提高部分归因于域尺寸减少 40%（提供更薄的半高峰宽），但主要归因于硅胶棒径向速度偏差减少两倍（提供更对称的峰）。然而，这些柱子中的棒仍然存在残余的径向不均匀性，导致非保留化合物的最小 HETP 仅为 10μm。
