Szymon Bocian

Solvent excess adsorption on the stationary phases for reversed-phase liquid chromatography with polar functional groups.

The measurement of acetonitrile and methanol adsorption was carried out on stationary phases with specific functionalities. The results were compared with the adsorption of those solvents on alkyl-modified adsorbents. This comparison allows us to describe the effect of polar groups on the adsorption of the organic modifiers. Our results clearly demonstrate how the functional groups modify the chromatographic properties of the homogeneous hydrophobic adsorbents.

New alkyl-phosphate bonded stationary phases for liquid chromatographic separation of biologically active compounds

A new type of bonded stationary phase for liquid chromatography, with the properties of immobilized artificial membranes, has been synthesized. Alkyl-phosphate adsorbents were obtained by modification of aminopropyl silica gel. The structures of the synthesized materials were confirmed by use of instrumental techniques—elemental analysis, infrared spectroscopy (FTIR), and 13 C and 29Si CP/MAS NMR. Analysis revealed that the adsorbents mimic the phospholipids present in natural cell membranes. The new synthesized alkyl-phosphate stationary phases may be used for liquid chromatographic separation of biologically active compounds of different polarity.

Zeta potential determination as a new way of stationary phases characterization for liquid chromatography

A set of seven homemade octadecyl silica-based bonded phases was investigated. Their zeta potential data in methanol and ACN as well as in methanol-water and ACN-water solution were obtained using Zetasizer. The influence of both the coverage density of bonded ligands and the end-capping of the modified surface on these data was investigated. Presented results may give useful information about the accessibility of the residual silanols in different mobile phases during the chromatographic analysis. Those measurements may be useful to choose chemically bonded stationary phases for CEC. The results also confirm the phenomena of anion exclusion from the pores of stationary-bonded phase.

Residual silanols at reversed-phase silica in HPLC – a contribution for a better understanding

As porous silica gel is the most common adsorbent and support for bonded stationary phase synthesis, residual silanol groups are a recurring problem in the field of liquid chromatography and other separation techniques. Residual silanols most often have a negative effect on the separation process by causing peak tailing. Therefore, there was an attempt to remove residual silanols during stationary-bonded phase synthesis. The type and surface concentration of residual silanols were measured using different instrumental techniques such as NMR and infrared spectroscopy, calorimetry, and various chromatographic methods. Residual silanols exhibit acidic characteristic and they can ionize depending on the environment. Thus, they posses cation-exchange properties and cause the zeta potential of silica particle in liquid environment. Presented review discusses the influence of the residual silanol groups on the solvation process and retention of polar compounds. The novel methodology of residual silanols determination is presented as well as the influence of the silanols on the zeta potential of the stationary-bonded phases in chromatographic conditions.

Functionalized polymeric stationary phases for ion chromatography

Synthesis and properties of the multilayered stationary phases, which contain quaternary amine functional groups for the analysis of anions by ion chromatography, are described. The bonded phases were characterized by elemental analysis, solid-state (13)C NMR spectroscopy and chromatographic methods. The surface of 1,4-di(2-hydroxy-3-methacryloyloxypropoxy)phenol (solid support) was coated with polymeric layers formed by condensation polymerization of primary amine with diepoxide. Each layer of the anion exchange stationary phase consists of copolymer of methylamine (MA) and 1,4-butanedioldiglycidyl ether (BDDE). A series of stationary phases with different numbers of polymerized layers were tested. The separation of an inorganic anions sample (F(-), Cl(-), NO(2)(-), Br(-), NO(3)(-), additionally HPO(4)(2-) and SO(4)(2-)) was performed. In the measurement, a hydroxide, carbonate, bicarbonate and their mixture were used as mobile phases.

Study of solvent adsorption on chemically bonded stationary phases by microcalorimetry and liquid chromatography.

A detailed, molecular-level description of the sorption mechanism in reversed-phase liquid chromatography is of great interest to analytical chemists. For this purpose, solvent adsorption in the octadecyl stationary bonded phase was investigated. Preferential adsorption of solvents from an acetonitrile-water and methanol-water mobile phase was measured on a series of non-end-capped octadecyl bonded phases with different coverage densities of bonded ligands using the minor disturbance method. For a comparison, a microcalorimetric study of organic solvent adsorption on the stationary phase was executed. The results from the excess isotherm measurement agree well with the experimental measurement of the heat of immersion of the bonded stationary phases by the test solvents. The microcalorimetric measurement is another method for determination of solvation processes of the stationary phases. Changes of the heat of immersion provide information about the surface accessibility for interaction with solvent molecules. The increase of the stationary phase coverage density reduces the free space between bonded chains and penetration of solvent between organic chains.

Study of the retention and selectivity of cholesterol bonded phases with different linkage spacers

The chromatographic properties of four cholesterol bonded phases with different structures were studied. The columns used were packed with a stationary phase containing a cholesterol molecule attached to the silica surface using different types of linkage molecules. As a basic characteristic of the bonded phases the hydrophobicity and silanol activity (polarity) were investigated. The presence of the polar amino and carboxyl groups in the structure of the bonded ligand strongly influences the polarity of the bonded phase. Columns were compared according to methylene selectivity using a series of benzene homologues and according to their shape and size selectivity using polycyclic aromatic hydrocarbons (PAHs). The measurements were done using MeOH-water and ACN-water mobile phases. The presented results show that the coverage density of the bonded ligands and length of the linkage strongly influence the retention and selectivity of cholesterol bonded phases.

Functionalized anion exchange stationary phase for separation of anionic compounds.

Synthesis of the multilayered stationary phases containing quaternary ammonium functional groups on the silica support was described. Bonded phases were characterized by elemental analysis, solid state (13)C NMR spectroscopy and chromatographic methods. The surface of silica support was coated with different number of polymeric layers formed by condensation polymerization of primary amine (methylamine) with diepoxide (1,4-butanedioldiglycidyl ether). A series of stationary phases with different number of polymerized layers were tested. Separation of an inorganic anions sample (F(-), Cl(-), NO2(-), Br(-), NO3(-)) and nucleotides was performed.

Study of solvation processes on cholesterol bonded phases.

Four cholesterol bonded phases with different structures were investigated. The columns studied were packed with stationary phase containing cholesterol attached to the silica surface using different types of linkage molecules. The presence of the polar amino and carboxyl groups in the structure of the bonded ligand strongly influence on the solvation process. The possibility of hydrogen bonding, dipole-dipole and π-π electron interactions lead to preferential solvation of bonded ligands. The coverage density of bonded ligands and length of the linkage strongly influence the adsorption of solvent from the mobile phase. The removal of residual silanols during the hydrosilation procedure significantly influences solvation of the bonded phase. Excess isotherms of the commonly used solvents in RP HPLC (methanol and acetonitrile) were obtained using the minor disturbance method. For comparison of the stationary phases prepared on different silica gels the excess adsorbed amounts were calculated per volume of the stationary phase in the column. The hydrosilated UDC Cholesterol bonded phase is preferentially solvated by methanol whereas the highest coverage Cosmosil Cholester phase exhibit high adsorption of acetonitrile. Polar groups in the Amino-cholesterol type bonded phase are solvated with both solvent but the mechanisms of these processes are different.

Modeling solvation on the chemically modified silica surfaces

A detailed molecular-level description of the retention mechanism in RP-HPLC is a point of great interest to analytical chemists. For this purpose, the solvent adsorption on the octadecyl stationary-bonded phase was investigated. Preferential adsorption of solvents from the acetonitrile-water mobile phase was modeled on the silica surface with one, two, three and four organic ligands, which represents a series of non-end-capped-bonded phases with different coverage density of bonded ligands. As a result of the computer simulation, the increase of adsorbed acetonitrile around bonded ligands is observed. The number of water molecules near the modeled surface is observed as well. The results are in agreement with experimental measurement of acetonitrile excess adsorption isotherms on the series of in-house made stationary-bonded phase.