Dušan Berek

Survey and trends in the preparation of chemically bonded silica phases for liquid chromatographic analysis

In order to increase chromatographic selectivity and to extend the analytical capability of reversed phase liquid chromatography (RP HPLC) many investigators have concentrated on the preparation of silica based column packings with chemically bonded phases (CBP). These phases have also been successfully used in sample preparation techniques, mainly in solid phase extraction (SPE). Although alkyl bonded phases (e.g., C2, C8, and C18) are the most widely used packings in RP HPLC and SPE, various specific applications require CBPs with polar functional groups (e.g., -NH2, -NO2, -CN, and/or -OH). The solution of problems with separation of complicated chiral compounds was attempted by applying stationary phases with chiral selectors (e.g., cyclodextrins, Pirkle phases, crown ethers, etc.). On the other hand, packings with pseudo-membrane or liquid crystal properties have been utilized for the separation of various substances of natural origin. Porous silica is commonly used as a support in the preparation of CBPs. Its physico-chemical characteristics, such as: type and structure of siliceous matrix, porosity, type and concentration of silanol groups, as well as surface purity, strongly influence the density and structure of chemically bonded phases. Recognition of these properties is helpful in optimizing separation processes based on RP HPLC elution and/or extraction of substances with polar character.

Polymers immobilized on silica gels as stationary phases for liquid chromatography

SummaryA variety of stationary phases for HPLC can be prepared by immobilization of organic polymers on silica gels. Such silica-polymer adsorbents combine the strength of an inorganic matrix with the selectivity and chemical inertness of organic resins. Moreover, effective shielding of residual silanols and the increase of column packing stability can also be achieved by modification of silica gels with organic polymers. There are two main approaches to the preparation of such composite sorbents. The first includes various routes for coating the silica surface with a polymer layer and the second is by filling the silica pores with a polymer network. A survey of polymers immobilized on silica gels and of various experimental procedures which can be used for composite sorbent synthesis is given in this review.

Size exclusion chromatography – A blessing and a curse of science and technology of synthetic polymers

Size exclusion chromatography, SEC is one of the most popular methods for the separation of different kinds of macromolecules. This critical review gives concise information about macromolecules and their behavior in solution, basic understanding about principles, instrumentation, and application possibilities of SEC, and more in detail discusses drawbacks and pitfalls of the method with the emphasis on synthetic polymers. Selected practical advices are included to help enhance the quality of SEC results.

Liquid chromatography of macromolecules at the critical adsorption point. II. Role of column packing : Bare silica gel

Liquid chromatography of macromolecules at the critical adsorption point ( LC CAP ) presents a potentially very powerful method for molecular characterization of complex polymers. However, LC CAP applicability is limited due to various experimental problems. The pore sizes and surface chemistry of the column packings belong to the most important weak points of the method. The LC CAP behavior of poly(methyl methacrylate)s was investigated using bare silica gels of 6, 12, and 100 nm pore sizes and with various amounts of surface silanols. Tetrahydrofuran as the adsorption suppressing liquid and toluene as the adsorption promoting liquid were mixed to form the nearly critical eluents. Both pore size and surface chemistry of silica were found to strongly influence the retentive characteristics of the system in the critical adsorption area. Macromolecules that were large enough to be excluded from the packing pores hardly followed the LC CAP rules: their retention volumes changed irregularly with the polymer molar mass and their recovery dropped sharply. The narrow pore silica gel-packed column governed the elution patterns of the whole column set composed of silica gels with different pore sizes. This makes the conventional LC CAP characterization of common polymers with broader molar mass distribution impractical and even not feasible. A hybrid column system was proposed containing narrow pore nonadsorptive column added in series to the meso- and macroporous LC CAP silica gels. This narrow pore column would allow separation of gas, impurities, and system peaks from the polymer peaks. The possible successive changes of the surface of silica gel, e.g., due to formation of silanols by hydrolysis or due to irreversible adsorption of some admixtures from the sample or eluent may make the LC CAP irrepeatable. Pronounced peak broadening was observed in the critical adsorption area and this effect increased strongly with the polymer molar mass.

Two-dimensional liquid chromatography of synthetic polymers

AbstractTwo-dimensional liquid chromatography, 2D-LC of synthetic polymers is critically assessed. Similarities and differences of 2D-LC of low-molecular-mass and polymeric substances are reviewed. The rationale of application of 2D-LC to macromolecular substances is discussed. Basic information on retention mechanisms in liquid chromatography of synthetic polymers is furnished. The principles, reasons, and significance of coupling of retention mechanisms are explained. The resulting separation processes are elucidated, and the technical concepts of the corresponding experimental arrangements are described. The benefits of 2D-LC are demonstrated together with numerous problems and shortcomings of the method. FigureSchematic diagram of contour plot of the results of 2D-LC separation. A complex polymer system contains three constituents, each with both composition and molar mass distribution. Sample concentration is visualized by use of color, with concentration increasing from blue to yellow.

On the possibility of estimating polymer compatibility from viscosity measurements of ternary systems polymer-polymer-solvent☆

A study was made of the possibility of estimating the compatibility of polymer pairs by means of viscosity measurements of the corresponding ternary systems polymer—polymer—solvent. It was established that there are discrepancies between the intrinsic viscosities of ternary systems and those calculated from results on the appropriate binary systems. Deviations from additivity of intrinsic viscosity values in binary systems and corresponding ternary systems were established. The viscosity interaction parameters (b23) for polymer—polymer, according to the model of Krigbaum and Wall, are dependent on the mol. wt. as well as on the natures of the polymers and solvent used. A simple empirical relationship was found between the viscosity interaction parameter (b23) and the Flory—Huggins interaction parameter polymer—polymer (α23).

Liquid chromatography of macromolecules at the critical adsorption point: behaviour of a polymer chain inside pores

Abstract The structure of macromolecules inside pores at the critical adsorption point (the point of exclusion-adsorption transition) is studied. The critical adsorption point is of importance in the liquid chromatography of polymers: chromatography under these special conditions represents an intermediate mode between size-exclusion and adsorption modes. The specific feature of this mode is the independence of elution volume on the molecular weight of the polymer (the distribution coefficient being unity for any molecular and pore sizes). The polymer segment concentration distributions for various distances of a macromolecule from the pore walls are calculated at the critical adsorption point. A simple picture of a polymer molecule inside a pore at the point of exclusion-adsorption transition is presented, based on the Debye-Bueche model.

Separation of poly(methyl methacrylate)s according to their tacticity

SummaryThe unconventional liquid chromatography of polymers under limiting conditions of their solubility was applied to poly(methyl methacrylate)s differing in their tacticity. The polymers were dissolved in thermodynamically good solvents and injected into an appropriate size exclusion chromatographic (SEC) column flushed with matched, moderately strong nonsolvents. The set of (limiting) conditions was found under which less soluble syndiotactic poly(methyl, methacrylate) moved along the column together with the zone of its initial solvent while better soluble isotactic poly(methyl methacrylate) was eluted according to the SEC mechanism. This indicates the potential of liquid chromatography under limiting conditions to characterize the tacticity of poly(methyl methacrylate)s.

Silica gel and carbon column packings for use in high-performance liquid chromatography

SummaryThe basic properties of the most important groups of HPLC column packings are briefly compared. The physical properties of totally porous silica and carbon-based sorbents, especially the pecularities of their pore structure, are discussed in more detail. The advantages of the sponge-like sorbent structure over a corpuscular structure are elucidated. The unconventional concept of pellicular sorbents with a thick porous layer and, especially, the concept ofsuperficially crazed sorbents is presented. Besides pores of the sizes that are necessary for the particular application in HPLC, the latter material also contains large and relatively deep cracks with widths two to five times larger than the specific mean pore diameters. An experimental crazed silica gel is presented and some of its chromatographic properties are shown.

The Effect of the Reaction Medium on the Coverage Density of C18 Chemically Bonded Phase

Abstract A wide-pore silica for HPLC (SBET = 360 m2 g−1, D = 20 nm, Vp = 2.1 cm3 g−1) was silanized with n-octadecyl dimethylchlorosilane in the presence of several base activators. It has been found that the density of bonded octadecyl ligands is not only a function of reaction time and temperature, but also depends upon the structure of the organic base used as reaction activator. The reproducible synthesis of silica bearing more than 4/umol m–2 of octadecyl ligands is described.