J. H. Mason Knox, Jr

Highly ordered mesoporous silicas such as, mobile composition of matter, MCM-41, MCM-48, and the SBA-types of materials have helped to a large extent to understand the formation mechanisms of the pore structure of adsorbents and to improve the methods of pore structural characterization. It still remains an open question whether the high order, the regularity of the pore system, and the narrow pore size distribution of the materials will lead to a substantial benefit when these materials are employed in liquid phase separation processes. MCM-41 type 10 microm beads are synthesized following the route of pseudomorphic transformation of highly porous amorphous silicas. Highly porous silicas and the pseudomorphically transformed derivatives are characterized by nitrogen sorption at 77 K and by inverse size-exclusion chromatography (ISEC) employing polystyrene standards. Applying the network model developed by Grimes, we calculated the pore connectivity n(T) of the materials. The value of n(T) varies between the percolation threshold of the lattice and values of n(T) > 10, the latter being the limiting value above which the material can be considered to be almost infinitely connected such that the ISEC behavior of the material calculated with the pore network model is the same when calculated with a parallel pore model which assumes an infinite connectivity. One should expect that the pore connectivity is reflected in the column performance, when these native and unmodified materials are packed into columns and tested with low molecular weight analytes in the Normal Phase LC mode. As found in a previous study on monolithic silicas and highly porous silicas, the slope of the plate height (HETP) - linear velocity (u) curve decreased significantly with enhanced pore connectivity of the materials. First results on the pseudomorphically transformed MCM-41 type silicas and their highly porous amorphous precursors showed that (i) the transformation did not change the pore connectivity (within the limits detectable by ISEC) from the starting material to the final product and (ii) the slope of the HETP versus u curve for dibutylphtalate did not change significantly after the pseudomorphic transformation.

Section: Assessment Of the Chromatographic Performance (Theoretical Pmentioning

confidence: 99%

Pore structural characteristics, size exclusion properties and column performance of two mesoporous amorphous silicas and their pseudomorphically transformed MCM‐41 type derivatives

Bayram-Hahn¹,

Grimes

Lind

et al. 2007

“…All the columns were packed inhouse by the slurry pressure packing technique, using 50:50 hexane/acetone v/v as the solvent mixture. Analytical columns packed with 3 lm packing materials were obtained from Phenomenex (Luna C 18 (2) 4.66150 mm, Torrance, CA) and YMC (Pro-Pack C 18 4.66150 mm, distributed by Waters Corporation, Milford, MA).…”

Section: Experimental 21 Materialsmentioning

confidence: 99%

Ultrahigh‐pressure liquid chromatography using a 1‐mm id column packed with 1.5‐μm porous particles

Anspach

Maloney

Colón³

2007

The evolution of chromatography has led to the reduction in the size of the packing materials used to fabricate HPLC columns. The increase in the backpressure required has led to this technique being referred to as ultrahigh-pressure liquid chromatography (UHPLC) when the column backpressure exceeds 10000 psi (approximately 700 bar). Until recently, columns packed with sub-2-microm materials have generally fitted into two classes; either short (less than 5 cm) columns designed for use on traditional HPLC systems at pressures less than 5000 psi (350 bar), or capillary columns (inner diameters less than 100 microm). By using packing materials with diameters <2 microm to fabricate UHPLC columns, there is an increase in efficiency and a decrease in the analysis time that are directly proportional to the size of the packing material. In order to realize and exploit the increase in efficiency, however, the columns must maintain lengths typically associated with analytical columns (15-25 cm). We have packed 1 mm diameter, 150 mm in length columns with 1.5 microm packing material, and evaluated their performance in UHPLC. The pressure required to achieve optimum linear velocities in plots of plate height versus linear velocity was in the vicinity of 1104 bar (16000 psi). The 1.5 microm particle-packed column was compared with the more traditional 150 mm long analytical columns packed with 3 microm materials. This column showed an efficiency that was approximately twice that observed with the 3 microm packed column and a concomitant reduction in the analysis time, theoretically predicted.

“…The elution order of the enantiomers was determined by injection of the pure enantiomers as well as by spiking. The void volume for the determination of chromatographic data was measured using water or buffer [26]. To ensure the reproducibility of the results, all measurements were carried out at least in triplicate.…”

Section: Enantiomer Separationsmentioning

confidence: 99%

A comparative evaluation of random and site‐specific immobilization techniques for the preparation of antibody‐based chiral stationary phases

Franco

Hofstetter

Hofstetter³

2006

In this study, one random and four site-directed conjugation strategies were applied to immobilize an mAb, which stereoselectively binds to L-amino acids, onto silica particles. The resulting chiral stationary phases (CSPs) were used for enantiomer separation of the model-analyte D,L-phenylalanine and further examined in frontal affinity chromatography. Although random immobilization of the antibody onto discuccinimidyl carbonate-activated silica resulted in a CSP that enabled baseline separation of the enantiomers of D,L-phenylalanine, the amount of available binding sites was considerably lower compared to the CSPs prepared by site-directed strategies. Immobilization of antibody via its carbohydrate chains, either directly via hydrazone bonds between the support and the protein or indirectly via binding carbohydrate-biotinylated antibody to streptavidin-derivatized silica, resulted in medium column efficiencies. Higher amounts of available active sites were obtained by immobilizing the antibody indirectly through the "crystallizable fragment (Fc)" receptor protein A/G. The best results with regard to amount of available binding sites and column efficiency were obtained by first biotinylating the antibody specifically at its C-termini using carboxypeptidase Y and immobilizing the biotinylated antibody on streptavidin-derivatized silica.