Copolymer of Di (methacryloyloxymethyl) naphthalene and divinylbenzene as a column packing for high-performance liquid chromatography

Summary A new porous polyaromatic ester packing was synthetized for high performance liquid chromatography. The relationship between retention and chain length of the members of homologous series of alkylbenzenes, N-alkylanilines, alkylarylethers, alkylbenzoates and alkylarylketones on this new stationary phase using different eluents was investigated. Using the alkylarylketone scale the retention indices of the homologues and test compounds were calculated. The results were compared with those obtained for poly (styrene-divinylbenzene) polymers. For both types of packing the first members of each homologous series gave non-linear behaviour. The methylene group index increments are different for the studied homologous series; thus there is no simple additivity of the retention indices. The efficiency of the porous polymeric columns is a function of the capacity factor of the solute and the organic component of the eluent.     

Study of RP HPLC Retention Behaviours in Analysis of Carotenoids

For determination of selected carotenoids, various types of columns for high-performance liquid chromatography (HPLC) with different properties have been used. The characteristics of the laboratory-used packing material containing monomeric alkyl-bonded phases (C₁₈, C₃₀) and phenyl as well as phenyl-hexyl stationary phases were studied. The retention data of the examined compounds were used to determine the hydrophobicity and silanol activity of stationary phases applied in the study. The presence of the polar and carboxyl groups in the structure of the bonded ligand strongly influences the polarity of the stationary phase. Columns were compared according to methylene selectivity using a series of benzene homologues. The measurements were done using a methanol–water mobile phase. Knowledge of the properties of the applied stationary phase provided the possibility to predict the RP HPLC retention behaviours in analysis of carotenoids including lutein, lycopene and β-carotene. The composition of the mobile phase, the addition of triethylamine and the type of stationary phase had been taken into account in designing the method of carotenoid identification. Also a monolithic column characterised by low hydrodynamic resistance, high porosity and high permeability was applied. The presented results show that the coverage density of the bonded ligands on silica gel packings and length of the linkage strongly influence the carotenoid retention behaviours. In our study, the highest retention parameters for lutein, lycopene and β-carotene were observed for C₃₀ and C₁₈ stationary phase. This effect corresponds with pore size of column packing greater than 100 Å and carbon content higher than 11 %.     

Comparison of a C30 Bonded Silica Column and Columns with Shorter Bonded Ligands in Reversed-Phase LC

The Carotenoid S is a new C₃₀ bonded silica stationary phase, intended for reversed-phase chromatographic applications, which is more hydrophobic and consequently shows stronger retention in comparison to conventionally used C₁₈ stationary phases. We compared the non-polar selectivities of the columns for homologous alkylbenzenes in acetonitrile—water and methanol–water mobile phases and polar reversed-phase selectivities employing the interaction indices and the Linear Free Energy Relationship models. Further, we investigated possibilities of separations of structurally closely related compounds in the groups of phenolic acids, flavones, phthalic acids and related compounds and of acylglycerols on the new C₃₀ column and with different types of columns for reversed-phase chromatography, including shorter alkyl C₄, C₈, C₁₈ and phenyl bonded stationary phases. The C₃₀ column has in some aspects properties similar to the non-endcapped Nova-Pak column for separation of some acylglycerols with equal equivalent carbon numbers, but enables separations of longer chain triacylglycerols in a single gradient run.

     

Isomeric structure of styrene-acrylonitrile and styrene-methylcrylate copolymer pyrolysis products

Summary Gas chromatographic retention indices of pyrolysis products of styrene-acrylonitrile and styrene-methylacrylate copolymers were measured on two columns prepared with different stationary phases. A retention index against retention index diagram proved to be useful to define the composition of product molecules, as the points of homologous series form straight lines. We identified the isomeric structure of dimers and trimers consisting of two kind of monomer units by comparing their retention indices. This comparison was based on the determination of the retention index increments of cyano, methoxy-carbonyl and phenyl groups bounded to primary, secondary or olefinic carbon atom. The isomeric structures of styrene-acrylonitrile dimers and trimers were confirmed by mass spectroscopy.     

Reversible solvent and temperature induced “monomeric-like” —“Polymeric-like” transitions in alkyl bonded silica

Summary The study of behaviour of homologous series (and more precisely, the plots of methylene selectivity versus the carbon number, n, of the alkyl chain of the homologue) on monomeric and polymeric phases in liquid chromatography enabled us to characterize two kinds of retention mechanism. These mechanisms are not independent; changes in the temperature or mobile phase composition allow passage from one to the other. The first, for which the methylene selectivity versus n decreases with a discontinuity, defines a retention mechanism where solutes are inserted between the grafted chains. It occurs for monomeric phases at ambient temperature, for polymeric phases, at high temperature or with a strong solvent like THF, CH₂Cl₂, AcMe or CHCl₃. The second gives a plot of methylene selectivity versus n with a maximum. It is consistent with a partition mechanism with total immersion of the solute in the bed of the stationary phase. It occurs for a polymeric phase at ambient temperature or for a monomeric one at low temperature.     

Micellar Liquid Chromatography Study of Quantitative Retention–Activity Relationships for Antihypertensive Drugs

Use of micellar mobile phases in reversed-phase liquid chromatography (RPLC) results in hydrophobic and electrostatic sites for interaction. Modified stationary phases in micellar liquid chromatography (MLC) are structurally similar to biomembranes. To confirm this we focused on the effects of the type and concentration of surfactant (Brij 35, SDS, and CTAB) and mobile phase pH on the retention of antihypertensive drugs on modified C₁₈ stationary phases. Quantitative retention-activity relationships are proposed for the drugs and the different surfactants and compared with those obtained using aqueous–organic mobile phases. Finally, a correlation was obtained between the logarithm of retention factors (log k) and the toxicity (LD₅₀) of antihypertensive drugs. Revised: 14 September 2005 and 4 April 2006     

Ion‐exchange vs reversed‐phase chromatography for separation and determination of basic psychotropic drugs

Ion exchange chromatography, an alternative to reversed‐phase (RP) chromatography, is described in this paper. We aimed to obtain optimal conditions for the separation of basic drugs because silica‐based RP stationary phases show silanol effect and make the analysis of basic analytes hardly possible. The retention, separation selectivity, symmetry of peaks and system efficiency were examined in different eluent systems containing different types of buffers at acidic pH and with the addition of organic modifiers: methanol and acetonitrile. The obtained results reveal a large influence of the salt cation used for buffer preparation and the type of organic modifier on the retention behavior of the analytes. These results were also compared with those obtained on an XBridge C₁₈ column. The obtained results demonstrated that SCX stationary phases can be successfully used as alternatives to C₁₈ stationary phases in the separation of basic compounds. The most selective and efficient chromatographic systems were applied for the quantification of some psychotropic drugs in fortified human serum samples. Copyright © 2015 John Wiley & Sons, Ltd.     

Chromatographic Behaviour and Lipophilicity of Estradiol Derivatives

The retention behaviour of estradiol derivatives has been studied by HPLC on polar chemically bonded stationary phases: C₃CN, DIOL and C₃NH₂, commercially available columns. The mobile phases used were: methanol-water and acetonitrile-water in various proportions. Reversed-phase chromatography occurred on polar chemically bonded stationary phases. Correlation between the retention constants of estradiol derivatives obtained on polar chemically bonded phases and log P calculated via different methods was examined too.     

Prediction of retention indices. VI: Isothermal and temperature-programmed retention indices,methylene value,functionality constant,electronic and steric effects

The Kováts retention index system with n-alkanes as reference standards has properties not fully explored when single, isolated or stand-alone analytes are analyzed by isothermal gas chromatography. When a homologous series of analytes are analyzed by either linear or non-linear temperature-programmed gas chromatography, the retention data of the entire series can be treated systematically to produce an I vs. Z plot that is linear, thereby giving insight into the relationship between chemical structure and retention index. Dead time t_M is both instrument and temperature dependent. With no dead time t_M adjustment, the retention indices of analytes calculated from experimental retention times by the method of either linear or logarithmic interpolation give statistically identical values. Linear regression analysis of the data shows the slope as methylene value (A) and intercept as functionality constant or group retention factor (GRF) of the homologous series. The A and (GRF) values vary with chemical structures, intermolecular electronic and steric interactions, and polarity of column liquid phases, and can link gas chromatographic retention index to chemical structure. Examples of the influence of molecular electronic effects and steric effects on retention index are given and discussed.     

Effect of the alkyl chain length of the bonded stationary phase on solute retention in reversed-phase high-performance liquid chromatography

Summary A comparison of the chromatographic retention characteristics of nonpolar bonded stationary phases was investigated. The results show that the interactions between the solutes and the stationary phases having C₂, C₈ and C₁₈ alkyl groups are almost similar in the range of the mobile phase investigated. This interaction is considered as the solvophobic effect.     

Separation of polycyclic aromatic hydrocarbons with a C60 bonded silica phase in microcolumn liquid chromatography

A chemically bonded C₆₀ silica phase was synthesized as a stationary phase for liquid chromatography (LC) and its retention behavior evaluated for various polycyclic aromatic hydrocarbons (PAHs) using microcolumn LC. The results indicate that the C₆₀ bonded phase offers selectivity different from that of octadecylsilica (ODS) bonded phases in the separation of isomeric PAHs. With the C₆₀ phase, PAH molecules having a partial structure similar to that of the C₆₀ molecule, e.g. triphenylene and perylene, were retained longer than with ordinary ODS stationary phases. The results also show that good correlation exists between the retention data with this C₆₀ bonded phase and with C₆₀ itself as the stationary phase.     

Retention behavior of a homologous series and positional isomers of aliphatic amino acids in hydrophilic interaction chromatography

The retention behavior of several series of free α‐ and ω‐amino acids and positional isomers of amino pentanoic acid in the hydrophilic interaction chromatography mode (HILIC) was studied. The study was carried out on three stationary phases followed by post‐column derivatization with fluorescence detection in order to describe the retention mechanism of the tested amino acids. The effect of chromatographic conditions including acetonitrile content in the mobile phase, mobile phase pH (ranging from 3.5 to 6.5) and concentration of buffer in the mobile phase was investigated. The effect of the number of carbon atoms (n_C) in aliphatic chains of the individual homologue of α‐ and ω‐amino acids and the logarithm of the partition coefficient (logD) on retention was also a part of the presented study. A good correlation (r > 0.98) between the logk and logD values of amino acids or n_C, respectively, was observed. The described linear relationships were subsequently applied to predict the retention behavior of individual members of the homologous series of amino acids and to optimize the mobile phase composition in HILIC. The obtained results confirmed that the retention mechanism of α‐amino acids, ω‐amino acids and positional isomers of amino acids was based on the logD values and the number of carbon atoms in the aliphatic chains of amino acids. The elution order of ω‐amino acids and positional isomers of amino pentanoic acid was strongly dependent on the mobile phase pH in the investigated range whereas the retention factors of all α‐amino acids remained essentially unchanged on all tested stationary phases.     

Correlation between retention and the C18 silica surface coverage in reversed-phase liquid chromatography

Summary Members of three homologous series and five non-homologous solutes with various functional groups were chromatographed on a series of well-characterized C₁₈ reversed stationary phases with a range of methanol-water mobile phases. Measured capacity factors of solutes were correlated with the concentration of C₁₈ ligands on the stationary phase. A linear relationship with the slope depending on a solute molecular structure and the volume fraction of methanol in the mobile phase was obtained. A method for the evaluation of phase ratio is also proposed. Presented at the 17th International Symposium on Chromatography, September 25–30, 1988, Vienna, Austria.     

Evaluation of the influence of residual silanol groups on the total retention in reversed-phase chromatography

It was demonstrated that the characteristics of stationary phases for reversed-phase chromatography can be compared by analyzing the retention of several compounds belonging to a homologous series during elution with one and the same composition of mobile phase or the retention of a nonpolar compound during elution with several mobile phases of different compositions. The slope of the corresponding linear correlations can be used to characterize the hydrophobicity of the columns (stationary phases) under study. To analyze the polar interactions between the sorbate and the stationary phase it is advantageous to study the retention of a polar substance in several (at least in two) mobile phases of different compositions. The degree of displacement of the linear correlation relative to that for nonpolar substances can be considered a measure of the influence of residual silanol groups on the total retention characteristics.     

π-Selective stationary phases: (III) Influence of the propyl phenyl ligand density on the aromatic and methylene selectivity of aromatic compounds in reversed phase liquid chromatography

The retention characteristics of phenyl type stationary phases for reversed phase high performance liquid chromatography are still largely unknown. This paper explores the retention process of these types of stationary phases by examining the retention behaviour of linear PAHs and n-alkylbenzenes on a series of propyl phenyl stationary phases that have changes in their ligand density (1.23, 1.31, 1.97, 2.50 μmol m⁻²). The aromatic and methylene selectivities increased with increasing ligand density until a point where a plateau was observed, overall the propyl phenyl phases had a higher degree of aromatic selectivity than methylene selectivity indicating that these columns are suitable for separations involving aromatic compounds. Also, retention characteristics relating to the size of the solute molecule were observed to be influenced by the ligand density. It is likely that the changing retention characteristics are caused by the different topologies of the stationary phases at different ligand densities. At high ligand densities, the partition coefficient became constant.     

Synthesis and characterization of novel polar-embedded silica stationary phases for use in reversed-phase high-performance liquid chromatography

We have developed a series of new C₁₀ dipeptide stationary phases via a simple and effective synthetic method. The preparation of the new phases involves the synthesis of silanes and the surface modification of silica. Chromatographic evaluations of these columns were performed using the Engelhardt, Tanaka, and Neue test mixtures. The applicability of these new stationary phases was also evaluated using a series of diagnostic probes including acids, bases or neutral compounds and several generic applications. These new C₁₀ dipeptide stationary phases showed excellent hydrolytic stability over a wide pH range. Like other existing amide-embedded columns, these new stationary phases exhibit higher retention for polar and hydrophilic compounds and different selectivity as compared to conventional C₁₈ columns. These new phases are compatible with 100% aqueous mobile phases, and also provide high column efficiency and good peak shapes for both acidic and basic compounds.     

Cyclam complexes of Cu(II) and Co(II) as stationary phases for gas chromatography

New macrocyclic stationary chemically bonded phases were synthesized and tested in gas chromatography conditions. The complexes of 1,4,8,11-tetraazacyclotetradecane with Cu(II) and Co(II) were bonded to the silica support through the (3-chloropropyl)triethoxysilane reactant. The packings obtained were analyzed by diffuse-reflectance ultraviolet–visible spectroscopy (DRUV–Vis), differential thermal gravimetry (DTG), porosimetry, and elementary analysis. Preliminary study of the novel silica gas chromatography (GC) stationary phases containing cyclam complexes was carried out using packed 1/8 in. i.d. columns. The study was conducted on: cyclic, linear and branched olefins, aromatic hydrocarbons and ethers. Characterization of interactions between the compounds mentioned and new stationary phases was based upon analysis of Kováts retention indices (I), difference between retention indices for two phases (ΔI), and molecular retention indices (ΔM_e). Results have shown that the new stationary phases interact sufficiently strongly with molecules of high electron density and can be applied in capillary gas chromatography for the analysis of light hydrocarbons.     

Mathematical models of solute retention in gas chromatography as sources of thermodynamic data. Part 1. Methyl n-alkyl ketones as the test analytes

Although gas chromatography (GC) is mainly used for analytical and preparative separation of complicated mixtures and then identification of the separated components, it can also be used as a relatively simple tool for the determination of physicochemical (including thermodynamic) properties. In this study, we discuss three different retention models devised specially for GC. They are tested for a homologous series of methyl n-alkyl ketones chromatographed on stationary phases of low and medium polarity at several different isothermal temperatures. The statistical performance of each model is excellent, which makes them a convenient tool for the evaluation of analyte retention under the given working conditions. They can also be used for the determination of the numerical values of thermodynamic properties (e.g., the enthalpy of vaporization of the analytes and the standard molar chemical potential of the partitioning of one methylene group (-CH2-) between the stationary and mobile phases).     

Results from solvophobic theory applied to methylene selectivity in reversed-phase HPLC

A significant amount of work has been previously dedicated to the understanding of methylene selectivity parameter. The conventional theory applied for this understanding was mostly based on the assumption that the difference in the Gibbs free energy of transfer from the mobile phase to the stationary phase is a constant for any two compounds in a homologous series that differ by a CH₂ group. In the present study, it is shown based on solvophobic theory that this assumption is indeed correct, but it provides a theoretical justification for it. Exemplification of the results of theory was obtained using the values for methylene selectivity (α(CH₂)) measured experimentally for seven different C18 chromatographic columns including two core–shell columns and using water and either methanol or acetonitrile as an organic component. Four different homologous series of compounds were used for evaluation. The study proved the theoretical prediction that the values for α(CH₂) obtained using different homologous series of compounds are only slightly different from those obtained using the toluene–butylbenzene series. Even using different homologous series, the same type of information regarding the columns comparison, and the changes in log α(CH₂) with the solvent composition was obtained.     

New Fullerene-Based Stationary Phases for Gas Chromatography

Regularities were revealed for the retention of different classes of organic compounds on stationary phases based on C₆₀ and carbon nanotubes and on phases of mixed composition (C₆₀ and dibenzo-24-crown-8, C₆₀ and dibenzo-30-crown-10, and C₆₀ and -cyclodextrin) in gas chromatography. A synergistic effect was observed for the stationary phases of mixed composition. Quantitative estimation of the dispersion potential demonstrated that the C₆₀-based stationary phase differs significantly from hydrocarbon adsorbents and more closely resembles nonpolar liquid stationary phases. Under the conditions of solid-phase microextraction, an adsorbing element based on fullerene C₆₀ can be used for the preconcentration of aromatic hydrocarbons.