Open Tubular Capillary Electrochromatography Using Etched Fused-Silica Tubing Modified with Chemically Bonded Liquid Crystals

Abstract

Two liquid crystal compounds, cholesterol-10-undeceneoate and 4-cyano-4‘-pentoxybiphenyl, are attached to the inner wall of a fused-silica capillary that has been etched by ammonium hydrogen fluoride. The bonding process involves formation of a hydride layer on the etched surface via a silanization reaction with triethoxysilane followed by attachment of the liquid crystal using a hydrosilation reaction. The etched surface is characterized by photoelectron spectroscopy (ESCA) and the inner wall after being chemically modified with the liquid crystal is analyzed by diffuse reflectance infrared Fourier transform spectroscopy. The surface properties of these materials are further probed by measuring electroosmotic flow as a function of pH. The performance of the etched chemically modified capillaries as a separation medium is evaluated by electrochromatographic experiments using mixtures of proteins, pyrimidine/purine bases and a nucleoside, benzodiazepines, the synthetic and metabolic compounds of serotonin, and other small basic molecules. In all cases, peak symmetry is good and efficiency is generally considerably higher than in packed capillary CEC. In some instances, there are significant differences in selectivity between the two capillaries indicating that the type of liquid crystal (cholesteric or nematic) is an important factor in the separation mechanism. Finally, during the evaluation period for the capillaries used in this study there was no significant loss in chromatographic performance, indicating that the long-term stability of these materials is good.

Open‐tubular electrochromatographic characterization of synthetic peptides

“…A feature of the etched capillaries used in the present investigation is that they are based on hybrid materials with a new type of substrata (with a surface matrix containing nitrogen and fluorine atoms introduced during the etching process in addition to the chemical modification of the polysiloxyl/silanol backbone of the Type I and II silica). Such chemically modified, surfaceetched capillaries are less adsorptive than naked, fused silica [6]. In addition, during the etching procedure, the surface area of the inner surface can be increased by up to 1000-fold with radial silica protrusions extending from the surface by as much as five mm [7].…”

“…However, the silanization reaction essentially removes about 95% of the silanols [8] so that after addition of the organic moiety (via hydrosilation) few if any Si-OH groups remain for interaction with basic analytes. Another useful characteristic of etched capillaries is that the electroosmotic flow (EOF) can be either anodic or cathodic [6]. Thus, when buffer electrolytes of pH values below about pH 4.5 are used, the EOF is anodic whilst with buffer electrolytes where the pH is .…”

Polar stationary phases for capillary electrochromatography

“…However, open-tubular columns often suffer from low surface coverage, which leads to a low phase ratio, and hence low sample loadability. Pesek et al [49][50][51][52][53] developed a method to prepare the etched capillary to address this problem. Starting with a 20 mm ID capillary and etching with caustic reagent, a variety of surface structures ranging from ripples to coral-like rods can be produced.…”

“…In this way, Pesek et al [49,50] [51]. The performance of the prepared columns was evaluated by electrochromatographic separation of mixtures of peptides, proteins, pyrimidine/purine bases and a nucleoside, benzodiazepines, the synthetic and metabolic compounds of serotonin, and other small molecules.…”

Packed‐column capillary electrochromatography and capillary electrochromatography‐mass spectrometry using a lithocholic acid stationary phase

“…To date, the most popular approach for application of LC-SP in CEC entails etching the inner wall of the capillary followed by chemical modification of the open tubular capillary surface, open tubular CEC [16]. Moreover, the use of LC-SP using cholesterol as the SP in open tubular CEC has been applied for separations of peptides, mixtures of proteins, pyrimidine/purine bases, metabolic compounds, including serotonin, and other basic molecules [17][18][19][20][21]. Although cholesterol has been applied in open tubular CEC, currently there are no reports for the use of this biological ligand in packed column CEC.…”