Gritti Fabrice, Guiochon Georges
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
Department of Chemistry, University of Tennessee, Knoxville, TN 37996-1600, USA.
J Chromatogr A. 2014 Aug 15;1355:164-78. doi: 10.1016/j.chroma.2014.05.076. Epub 2014 Jun 11.
In a previous report, it was reported that columns packed with fully porous 1.9μm Titan-C18 particles provided a minimum reduced plate height as small as 1.7 for the most retained compound (n-octanophenone) under RPLC conditions. These particles are characterized by a relatively narrow size distribution with a relative standard deviation (RSD) of only 10%. A column packed with classical 5μm Symmetry-C18 particles, used as a reference RPLC column, generated a minimum reduced plate height of 2.1 for the same retained compound. This work demonstrates that this was due to an unusually low intra-particle diffusivity across these particles, which leads to a small longitudinal diffusion coefficient along the column. The demonstration is based on the combination of accurate measurements of the height equivalent to a theoretical plate (HETP), inverse size exclusion chromatography (ISEC), peak parking (PP), and minor disturbance method (MDM) experiments. The experimental results show that the reduced eddy dispersion HETP term (A=0.8 for a reduced velocity of 5), the internal particle porosity (ϵp=0.35), and the enrichment of acetonitrile in the pore volume (75% acetonitrile in the bulk, 85% inside the mesoporous volume) are identical on both the Titan-C18 and Symmetry-C18 columns. The difference between the internal structures of these two brands of RPLC-C18 fully porous particles lies in the values of the internal obstruction factor γp, which is 0.42 for the Symmetry-C18 but only 0.26 for the Titan-C18 particles. This is in part related to the diffusion hindrance due to the small average pore size of the Titan-C18 particles, around 59Å versus 77Å for Symmetry-C18 particles. A simple model of constriction along diffusion paths having the shape of a truncated cone suggests that the width of the pore size distribution (RSD of 30% and 20% for Titan-C18 and Symmetry-C18 particles) is mostly responsible for the difference in their obstruction factors.
在之前的一份报告中，据报道，在反相液相色谱（RPLC）条件下，填充完全多孔的1.9μm泰坦-C18颗粒的色谱柱对于保留时间最长的化合物（正辛基苯乙酮）提供了低至1.7的最小折合板高。这些颗粒的特点是粒径分布相对较窄，相对标准偏差（RSD）仅为10%。填充经典5μm对称-C18颗粒的色谱柱用作参考RPLC柱，对于相同的保留化合物产生的最小折合板高为2.1。这项工作表明，这是由于这些颗粒间异常低的颗粒内扩散率，这导致沿色谱柱的纵向扩散系数较小。该论证基于对理论塔板高度（HETP）的精确测量、反相尺寸排阻色谱（ISEC）、峰停驻（PP）和微扰法（MDM）实验的结合。实验结果表明，在折合流速为5时，折合涡流扩散HETP项（A = 0.8）、颗粒内部孔隙率（ϵp = 0.35）以及孔体积中乙腈的富集情况（本体中75%乙腈，中孔体积内85%乙腈）在泰坦-C18和对称-C18色谱柱上是相同的。这两种品牌的RPLC-C18完全多孔颗粒内部结构的差异在于内部阻碍因子γp的值，对称-C18颗粒的γp为0.42，而泰坦-C18颗粒仅为0.26。这部分与泰坦-C18颗粒平均孔径较小导致的扩散阻碍有关，泰坦-C18颗粒平均孔径约为59Å，而对称-C18颗粒为77Å。一个沿具有截头圆锥形状的扩散路径收缩的简单模型表明，孔径分布宽度（泰坦-C18和对称-C18颗粒的RSD分别为30%和20%）主要导致了它们阻碍因子的差异。
