Liquid Crystalline Stationary Phases Research Articles

Determination and theoretical analysis of supercritical fluid chromatographic retention of polycyclic aromatic hydrocarbons in a polymeric smectic phase

A molecular theory of chromatography based on a mean-field lattice model has recently been developed to describe the partitioning or blocklike molecules between an isotropic mobile phase and an anisotropic stationary phase. A carefully designed capillary SFC experiment mimickung the theoretical model has been carried out. The retention behavior for PAH solutes with a smectic liquid-crystalline stationary phase and a supercritical fluid CO 2 mobile phase has been determined over a wide range or temperature and density.

Blended liquid crystalline stationary phases in capillary gas chromatography

Neat and blended liquid crystalline and silicone phases were investigated as stationary phases in glass capillary columns and by use of light and scanning electron microscopes. The columns with blended liquid crystals showed better efficiency and permit better film homogeneity.

Analysis of some synthetic insect pheromones by gas—liquid chromatography

The isomeric composition of insect pheromones belonging to primary olefinic alcohols and their derivatives was analysed by gas—liquid chromatography. The chromatographic behaviour of geometric isomers on liquid crystalline stationary phases was studied. The effectiveness and selectivity of the chromatographic columns and the Kováts retention indices of ( E,Z)- and ( Z,Z)-7,9-dodecadienyl acetates were determined and their dependence on temperature was investigated. Geometric and positional isomers of primary aliphatic alcohols and acetates with one and two double bonds can be separated on capillary columns with 1,2,3-tris-(β-cyanethoxy)propane, diethylene glycol succinate and cholesteryl p-methoxybenzoate.

Long-term use of liquid-crystalline stationary phase for separation and determination of polynuclear aromatic hydrocarbons in carbochemical products : Comparison of results obtained by different methods

Modern technology for the isolation of pure anthracene and carbazole from anthracene oil1 requires appropriate analytical techniques. Gas chromatography (GC) has been used for the determinaton of the components of such samples’, particularly with the use of a liquid-crystalline phase 3,4. The method5 has been improved by modification of the support. In this work, the precision and accuracy of this method were evaluated by comparing the results with those obtained with polarography and UV spectrophotometry6-8.

Thermodynamic study of solute retention on a smectic liquid-crystalline phase in supercritical fluid chromatography

Capillary supercritical fluid chromatography was used to obtain the values for entropy and enthalpy of solute transfer as a function of density for isomeric polycyclics on a smectic liquid-crystalline stationary phase. The values were compared with those obtained from a structurally similar nonmesomorphic stationary phase. Two different fluids were investigated, and it was determined that carbon dioxide has a greater affinity for both stationary phases, than does nitrous oxide

Analysis of coal tar polycyclic aromatic hydrocarbon LC‐fractions by capillary SFC on a liquid crystalline stationary phase

AbstractSupercritical fluid chromatography (SFC) on a capillary column coated with a smectic mesomorphic crystalline phase is shown to exhibit a typical turnover effect (retention versus column temperature) for polycyclic aromatic hydrocarbons (PAHs) at lower temperatures than are found on a methylpolysiloxane phase. Liquid chromatography is used to separate various fractions from a coal tar, which are analyzed by high resolution capillary SFC. Different density and temperature programs were investigated to optimize the separations. Simultaneous density and temperature programs gave the best results, and this is thought to be due to increased solute diffusion coefficients which yield highly efficient separations for the high molecular weight polycyclic aromatic hydrocarbons. The separation mechanism is based on the shape of the liquid crystalline phase, solubility, volatility, and molecular geometry of the PAHs.

Determination of bioconcentration factors of isomeric polycyclic aromatic hydrocarbons in polychaete worms by HRGC on selective liquid crystalline stationary phases

AbstractThe bioconcentration factors (Kb) of parent and alkylated PAHs isolated from polychaete worms collected in coastal sediments were determined for the first time. Capillary gas chromatography using a polymeric liquid crystalline (MPMS) and SE‐54 stationary phases coupled to FID, MS and concurrent elution with authentic standards permitted positive identifications of parent PAHs. The Kb values of 18 compounds were accurately determined on the liquid crystalline column and the those of mono‐ and dialkyl‐phenathrene, ‐pyrene and ‐chrysene were determined by GC‐MS on a SE‐54 coated column. The experimental Kb values exhibited a moderate correlation (r=0.7–0.8) with the reported octanol‐water partition coefficients (Kow) for these compounds. Consequently, kinetic considerations, i.e. biouptake‐release and enzymatic activity seem to be relevant, in addition of biota‐media equilibrium. Therefore, integrated bioconcentration models are needed.

Separation of steroid isomers using a liquid crystalline polysiloxane stationary phase in capillary supercritical fluid chromatography

AbstractA liquid crystalline biphenylcarboxylate ester polysiloxane stationary phase was used to study the effect of solute structure on retention for the separation of isomeric steroids in capillary supercritical fluid chromatography. It was determined that theretention mechanism is related to solute molecular shape rather than to properties such as volatility or polarity. The elution order of isomeric androstanes and estrogens could be correlated with the planarity of molecules: more‐planar molecules were retained longer on the column. The resolution of steroid isomers was improved significantly using the liquid crystalline stationary phase when compared with results obtained using other stationary phases.

Optimization of the separation of multicomponent mixtures by HRCGC

The window diagram technique was applied for one-and two-parameter optimization in capillary gas-liquid chromatography. The factors optimized were isothermal temperature and relative ratio of two stationary phases. Glass capillary columns with OV-101 and a new liquid crystalline stationary phase PBHP connected in series at constant overall length were used for the separation of a multicomponent mixture of alkylbenzenes and n-alkanes with carbon atom number up to 13. Optimal conditions for the separation of this mixture under the condition log α≥0.002 were proposed.

Shape selectivity in liquid and gas chromatography: Polymeric octadecylsilane (C18) and liquid crystalline stationary phases

Octadecylsilane (C18) stationary phases for liquid chromatography (LC), which are prepared by polymeric rather than monomeric phase synthesis, exhibit shape selectivity for isomeric polycyclic aromatic hydrocarbons (PAH) that is similar to the shape selectivity observed for liquid crystalline phases in gas chromatography (GC). Relative retention measurements for several sets of isomeric PAH on a polymeric C18 stationary phase (reversed-phase LC) and liquid-crystalline polysiloxane stationary phase (capillary GC) were compared and correlated with the simple length-to-breadth shape descriptor of the solute. Similar retention behavior was observed for both chromatographic systems. In fact, anomalies in elution order relative to length-to-breadth ratios were generally found to be consistent in both chromatographic systems and could often be ascribed to secondary shape factors (i.e., planarity of the molecule).

New liquid crystalline stationary phase for capillary gas chromatography

In the last few years the use of capillary columns with liquid crystalline stationary phases has increased. In general, liquid crystals are stationary phases of medium polarity, therefore a mixed retention mechanism is expected]. It is known that the surface of the support affects the orientation of the molecules of the liquid crystalline stationary phase and that this effect is greater than with conventional stationary phase+. Recently we have systematically studied the influence of the film thickness and the surface quality on the properties of selected liquid crystalline stationary phases coated on glass capillary columns, and on the retention data of compounds having different structures and polarities3. The aim of this work was the introduction of a new, more polar, liquid crystalline stationary phase and to compare its properties in capillary columns with that of comparable structure.

Isothiocyanate as a liquid crystalline stationary phase in capillary gas chromatography for the separation of polyaromatic hydrocarbons

Isothiocyanate as a liquid crystalline stationary phase in capillary gas chromatography for the separation of polyaromatic hydrocarbons

Prospectives of polymeric liquid crystal stationary phases for capillary column gas chromatographic separations

The liquid crystalline polysiloxane substrate offers unique selectivity when used in capillary column gas chromatography. The separation and identification of priority pollutants along with other compounds is very difficult but important. In particular, the separation of isomeric organic compounds having similar volatilities could not be obtained on capillary columns using conventional stationary phases. The complete separation of ortho, meta, and para isomers of certain disubstituted benzenes as well as polycyclic aromatic compounds was achieved on a capillary column using a liquid crystalline polysiloxane stationary phase. The complete separation of octachlorodibenzodioxin from octachlorodibenzofuran was also obtained on this column which is not possible on columns of identical lengths using conventional stationary phases. The separation of polycyclic aromatic compounds in a complex sample of airborne particulate matter extract on the liquid crystal column is demonstrated.

Liquid crystals : VI. Comparative study of the supercooling effect of some liquid crystalline stationary phases using gas—liquid chromatography

Liquid crystals : VI. Comparative study of the supercooling effect of some liquid crystalline stationary phases using gas—liquid chromatography

Synthesis of liquid crystalline polyacrylates and their use in capillary gas chromatography

AbstractNew enantiotropic liquid crystalline compounds based on 4‐(p‐substituted phenylazo)phenol and m‐cresol esters containing an acryloyloxy group were synthesized. Polyacrylates were prepared from these monomers by radical polymerization. The resulting polymers exhibit nematic enantioropic properties. The polyacrylates have broader nematic ranges and higher nematic to isotropic transition temperatures as compared to their corresponding monomers. The nematic range is dependent on both the lateral substituents and the terminal substituents. The laterally substituted polyacrylates gave a narrower nematic range than unsubstituted compounds. Increase in carbon number in the terminal alkyl group leads to a broader nematic range. The liquid crystalline polyacrylates could be coated uniformly on the inside of a glass capillary tubing. The coated capillary columns show an excellent selectivity in gas chromatography for the separation of isomeric compounds. The liquid crystalline stationary phases are highly stable up to 230°C without column bleeding.

Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase

New polyacrylate liquid crystalline compounds were coated onto glass or fused-silica capillary columns as stationary phases and applied to supercritical fluid chromatography. These stationary phases, were very stable: no bleeding was observed at 200°C and up to 200kg/cm2 pressures of carbon dioxide mobile phase. The wide working range of the capillary column was extended below the g-n transition temperature. Isomeric compounds such as α- and β-methoxynaphthalene, anthracene and phenanthrene and several phenolic compounds were separated.

Thermochemical investigations of gas liquid chromatography : III. Retention behavior of alkane solutes on binary liquid crystalline solvent mixtures

Retention volumes have been determined for n-heptane, n-octane, and n-nonane on binary butyl p-( p-ethoxyphenoxycarbonyl)phenyl carbonate and p-( p-ethyoxyphenylazo)phenyl undecylenate liquid crystalline stationary phases from 60 to 90°C. Results of these measurements are used to calculate enthalpies, entropies, and free energies of solution at the six solvent compositions studied. The three enthalpy and entropy of solution curves show pronounced minimum values near an equimolar mixture of the two stationary phases. The Gibbs free energies of solution, in comparison, are linear or nearly linear with solvent composition.

The dependence of the properties of liquid crystals on the film thickness and surface quality of capillary columns

Glass capillary columns were prepared from non-deactivated and deactivated glass and coated with (2-methyl-4-[trans-4-n-propyl-cyclohexylcarbonyloxy]-benzoic acid-[4-n-heptyloxy-phenylester]) as the liqud phase, in different film thicknesses. The columns were tested using substances of different structures and polarities. It was verified that the capacity factors, retention indices and selectivity significantly depend on the thickness of the liquid crystalline stationary phase film and the quality of the tube, particularly in the case of columns with thin films. Trasition temperatures (melting and clearing point) of the liquid crystal are also dependent on these two factors.

Correlations between retention on liquid crystalline stationary phases and chemical structure : I. Dimethylnaphthalenes

The chromatographic behaviour of isomeric dimethylnaphthalenes on a capillary column with a liquid crystalline stationary phase was studied. The liquid crystal used was 4-ethyl-4′-( p-methylbenzoyloxy)azobenzene, the mesophase of which is stable at temperatures ranging from 119 to 232°C. A simple interpretation of retention data based on the concept of shape parameter and the connectivity parameter does not lead to a satisfactory description of the retention mechanism. A model of retention is proposed that explains the behaviour of dimethylnaphthalenes and that may also be applicable to other systems involving alkyl-substituted aromatic rings.

Ligand-exchange gas chromatography of dialkyl sulphides

The separation and retention behaviour of dialkyl sulphides in ligand-exchange gas chromatography have been studied using a copper(II) stearate stationary phase and an ammonia-containing mobile phase. Short retention times and sharp peaks were obtained when ammonia was introduced into the mobile phase. On a solid copper(II) stearate column a linear relationship was found between the capacity factors and the reciprocal of the ammonia concentration, and some equilibrium constants have been calculated. At higher temperatures, copper(II) stearate behaved as a liquid crystalline stationary phase. A good separation of sulphides was achieved by appropriate adjustment of the ammonia concentration and the column temperature. Factors affecting the retention and sample selectivity are discussed.