Endcapped Stationary Phase Research Articles

Preparation of core–shell silica particles with tunable pore size by phenylethanol as swelling agent for chromatographic separation

The core–shell particles with tunable pore size were synthesized by a bi-phase reaction successfully. By adjusting the amount of phenylethanol, fibrous shells with tunable pore sizes ranging from 4 to 19 nm could be obtained in one pot. The obtained silica microspheres were used for chromatographic separation after modification with octyltrichlorosilane and end-capping with N-(trimethoxysilylpropyl)imidazole. Better resolutions of nitroimidazoles and anilines were obtained on the end-capped stationary phase. This particle could serve as an HPLC stationary phase with good reproducibility and particular selectivity.

Development of a polymer‐coated stationary phase with improved chemical stability in alkaline mobile phases

An endcapped stationary phase is prepared by thermal immobilization of poly(methyltetradecylsiloxane) (PMTDS) onto a doubly zirconized silica support followed by endcapping using a mixture of hexamethyldisilazane and trimethylchlorosilane. The preparation of the Si-Zr(PMTDS)ec phase shows good repeatability with RSD <3.0% for carbon loadings and column efficiency. This new stationary phase has a lower density of residual hydroxyl groups, according to spectroscopic methods while basic compounds from the Tanaka and Engelhardt test mixtures are eluted with essentially symmetric peaks. Furthermore, the stability of the Si-Zr(PMTDS)ec stationary phase, measured using an accelerated aging test, is twice as great as the stability of a similar nonendcapped phase. The new phase shows promise for the separation of basic pharmaceuticals.

High speed gradient elution reversed phase liquid chromatography of bases in buffered eluents: Part II. Full equilibrium

In this work we determined when the state of thermodynamic (full) equilibrium, i.e. time-invariate solute retention, was achieved in gradient elution reversed-phase chromatography. We investigated the effects of flow rate, temperature, organic modifier, buffer type/concentration, stationary phase type, n-butanol as eluent additive, and pore size. We also measured how selectivity varied with reequilibration time. Stationary phase wetting and the ability of the stationary phase to resist changes in pH strongly affect the time needed to reach full equilibrium. For example, full equilibrium is realized with many endcapped stationary phases after flushing with only two column volumes of acetonitrile–water containing 1% (v/v) n-butanol and 0.1% (v/v) trifluoroacetic acid. Trends in retention time (<0.010 min) and selectivity become quite small after only five column volumes of reequilibration. We give practical guidelines that provide fast full equilibrium for basic compounds when chromatographed in buffered eluents.

Effect of the endcapping of reversed-phase high-performance liquid chromatography adsorbents on the adsorption isotherm

The retention mechanisms of n-propylbenzoate, 4- t ert-butylphenol, and caffeine on the endcapped Symmetry-C 18 and the non-endcapped Resolve-C 18 are compared. The adsorption isotherms were measured by frontal analysis (FA), using as the mobile phase mixtures of methanol or acetonitrile and water of various compositions. The isotherm data were modeled and the adsorption energy distributions calculated. The surface heterogeneity increases faster with decreasing methanol concentration on the non-endcapped than on the endcapped adsorbent. For instance, for methanol concentrations exceeding 30% (v/v), the adsorption of caffeine is accounted for by assuming three and two different types of adsorption sites on Resolve-C 18 and Symmetry-C 18, respectively. This is explained by the effect of the mobile phase composition on the structure of the C 18-bonded layer. The bare surface of bonded silica appears more accessible to solute molecules at high water contents in the mobile phase. On the other hand, replacing methanol by a stronger organic modifier like acetonitrile dampens the differences between non-endcapped and endcapped stationary phase and decreases the degree of surface heterogeneity of the adsorbent. For instance, at acetonitrile concentrations exceeding 20%, the surface appears nearly homogeneous for the adsorption of caffeine.