Liquid Crystalline Stationary Phases Research Articles

Comparative Thermal and Thermodynamic Study of Homologous series by Chromatography using Liquid Crystalline Stationary Phase

This work consisted in the preparation and the study of a new stationary phase based on liquid crystals for gas chromatography applications. Most of the synthesized compounds that have the character of liquid crystals in their constitution include aromatic molecules. In addition, there is little work that replaces this type of molecules with heterogeneous molecules that have a liquid crystalline character. This study constitutes an approach for the synthesis of a new molecule azoesters involves a 1,3,4-oxadiazole unit instead of aromatic rings, and which exhibits liquid crystalline characters. A liquid crystal has been prepared and characterized. The thermodynamic characteristics of the n-alkanes for the filled column were studied by reverse gas chromatography. The intermediate products, as well as the mesogens obtained, were characterized by various experimental analysis techniques such as infrared spectrometry (IR) and nuclear magnetic resonance (NMR). The thermal properties of synthesized liquid crystal require the use of DSC technique. Thermodynamic parameters such as enthalpies, partial solution molar entropy and infinite dilution, were determined for the liquid crystal-solvent system. The thermodynamic quantities show that these values depend on the number of carbon atoms of the CL phase terminal substituent. The transition temperatures obtained by gas chromatography are in good agreement with those found by DSC.

Separation of thia-arenes and aza-arenes from polycyclic aromatics in snowpack samples from the Athabasca oil sands region by GC×GC/ToF-MS

ABSTRACTComprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC/ToF-MS) was used for the analysis of thia-arenes and aza-arenes in standard mixtures containing 45 polycyclic aromatic compounds (PACs), and in the aromatic fraction of snowpack samples collected from the Athabasca oil sands area of Alberta, Canada. The GC columns used included a shape-selective liquid-crystalline stationary phase (LC-50) and a high-polarity ionic-liquid stationary phase (SLB-IL60), both in the first dimension. A fast diffusion and low-polarity nano-stationary phase (NSP-35) and a mid-polarity stationary phase (Rxi-17), were used in the second dimension, respectively. Both configurations showed good repeatability for retention times in the first and second dimensions, peak areas and peak heights. Instrument detection limits ranged from 0.5 to 10 pg µL−1 for both configurations. In general, the LC-50×NSP-35 configuration favoured the separation of isomeric compounds by using more of the 2D chromatographic space available (>80%), particularly for compounds with molecular mass >160 u. Therefore, LC-50×NSP-35 was recommended for the analysis of thia-arenes and alkylated PAHs in environmental samples collected from the oil sands area. Alternatively, the SLB-IL60×Rxi-17 configuration favoured molecular similarity grouping over isomer separation. This was observed for a group of aza-arenes (i.e. carbazoles, benzo[a]carbazoles and alkylated-derivatives), which were resolved from other PACs and from the sample matrix. The risk of false positives and overestimations in the analysis of thia-arenes, aza-arenes and alkylated PACs in one-dimension GC/MS was explored and further reduced by using GC×GC/ToF-MS with LC-50×NSP-35 and SLB-IL60×Rxi-17.

Evaluation of the Effect of Citronellol Group on Functionalized Mesogenic Materials by Capillary GC

In this paper, the effects of functionalization with terpenes on two new liquid-crystalline stationary phases for gas chromatography (GC) are described. Citronellol was used as the terminal group in the first material, and tetrahydrogeraniol was used with a second material. Inverse GC showed that the new materials have wide liquid-crystalline ranges (mesophases), 371–500 and 395–501 K, respectively. Moreover, they show good thermal stability up to 523 K and good potential as stationary phases for capillary GC. To clarify the effects of the liquid crystal structures and functional groups on retention and separation, the chromatographic behaviors of the two stationary phases were compared by eluting alkylbenzenes, polycyclic aromatic hydrocarbons, aromatic compounds, and terpenoids. The selectivities for a wide range of analytes achieved using the citronellol column were significantly better than those obtained using the tetrahydrogeraniol column. The columns showed different retention behaviors and fine resolutions for some of the main constituents of essential oils. Introduction of the double bond of citronellol greatly improved the polarization interactions involved in the shape recognition of the liquid-crystalline state for isomers. The new citronellol liquid-crystalline stationary phase, therefore, has a high affinity for natural compounds.

Historical Development of Liquid Crystalline Stationary Phases: An Overview

Nematic liquid crystals have shown particular selectivity and sensitivity as stationary phases for the separation of isomers having similar volatilities. Because of their unique selectivity towards rigid solute isomers, liquid crystal stationary phases were considered at one time to be a very promising class of materials that give the gas chromatographic separations a great deal of attention. In this overview, we present a brief historical view of the liquid crystal stationary phases development, which were successfully used in gas chromatography for separation of isomers.

Gas Chromatographic Separation of Isomeric Phenols by Schiff`s Base Liquid Crystalline Stationary Phases

The chromatographic behaviour of liquid crystalline compounds 4-(propyloxybenzylidene)-4`p-aminoazobenzene (1) and 4-(butyloxybenz--ylidene)-4`-p-aminoazobenzene (2) as stationary phases for the separation of cresol isomers was investigated. These isomers were analysed on 4(propyloxybenzylidene)-4`-p-aminoazobenzene within a nematic range of 142- 153° C with a temperature interval of 5° C. Better peak resolution was at a column temperature of 147° C. The analysis was repeated on a and 4-(butyloxybenzylidene)-4`-p-aminoazobenzene column at a nematic temperature of 176° C, which represented the end of the nematic range, and gave the optimum peak resolution. It was found that isomer better separation was obtained at 20% loading for both liquid crystal materials. Other columns of different liquid crystalline percentages (15% and 25%) were used. A chromatogram with unoverlapped peaks was only obtained in both cases at 20% column loading. The order of elution isomers under the applied chromatographic conditions is discussed. The separation of these isomers is also discussed on the basis of stereo chemical conformations.

Comparison of Analytical Properties of Two Liquid Crystalline Stationary Phases by Capillary GC

Two related novel liquid crystals used as gas chromatographic stationary phases were evaluated for their analytical properties and separation capabilities, 2-(1-ethyloctyloxy)-4-(4′-(4-butoxybenzoyloxy)-benzoyloxy)-4′-ethyl-azobenzene (LCC) and azo-(4-ethylbenzyl)-3-(2-(1-ethyloctyloxy)-4-(4-(4-butyloxybenzoyloxy)benzoyloxy)pyridino (LCN). The thermal properties of LCC and LCN were established with differential scanning calorimetry and polarizing microscopy. The comparative study of the retention behavior for the both liquid crystals was also reported. The study of the chromatographic performance of a column coated with liquid crystals in the solid, nematic and liquid state was done using a series of appropriate solutes. The mesogenic compounds exhibit interesting separation for some positional and geometrical isomers in aromatic hydrocarbons, phenols, volatile aromatic compounds and polyaromatic hydrocarbons.

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

The preparation and characterization of a novel lithocholic acid (LCA)-based liquid crystalline (LC) stationary phase (SP) suitable for application in packed-column CEC and CEC coupled to MS is described. The extent of bonding reactions of LCA-SP was assessed using 1H-NMR, 13C-NMR and elemental analysis. This characterization is followed by application of the LCA-SP for separation of beta-blockers, phenylethylamines (PEAs), polyaromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). Using the optimum mobile phase operating conditions (pH 3.0-4.5, 10 mM ammonium acetate, 85% v/v ACN), a comparison of the chromatographic ability of the aminopropyl silica phase vs. the LCA-bonded phase was conducted. The results showed improved selectivity for all test analytes using the latter phase. For example, the CEC-MS of beta-blockers demonstrated that the LCA-bonded phase provides separation of six out of seven beta-blockers, whereas the amino silica phase provides four peaks of several co-eluting beta-blockers. For the CEC-MS analysis of PEAs, the LCA-bonded phase showed improved resolution and different selectivity as compared to the aminopropyl phase. An evaluation of the retention trends for PEAs on both phases suggested that the PEAs were retained based on varying degree of hydroxyl substitution on the aromatic ring. In addition, the MS characterization shows several PEAs fragment in the electrospray either by loss of an alkyl group and/or by loss of H2O. Finally, the LCA-bonded phase displayed significantly higher separation selectivity for PAHs and PCBs as compared to the amino silica phase.

Liquid-crystalline stationary phases for gas chromatography

Physico-chemical properties of new liquid-crystalline stationary phases (LCSPs) for gas chromatography are reviewed. The mechanism of chromatographic separation on liquid-crystalline stationary phases is discussed and examples of analyses of complex mixtures of organic compounds using capillary and packed columns are given.

Applications of inverse gas chromatography in the study of liquid crystalline stationary phases

We describe measurements on a number of low-molar-mass and polymeric liquid crystals that contain the same mesogenic groups. Transition temperatures for the mesophases have been measured and the supercooled region of hexyloxycyanobiphenyl studied, revealing some differences from the stable mesophase. Activity coefficients and interaction parameters for a range of probes have been measured and allow us to determine the nature and origin of the thermodynamic interactions in the systems. A discussion of how this fundamental information can be used in designing more efficient stationary phases for analytical gas chromatography is also presented.

Laterally attached liquid-crystalline polymers as stationary phases. Effect of temperature in RP HPLC and comparison with a commercial C18 stationary phase

Specific side-on-fixed liquid-crystalline polymers (SOLCP) have been synthesized for use in silica-modified stationary phases in high-performance liquid chromatography (HPLC). The mesogenic side group of the SOLCP is composed of three benzoate-type phenyl rings with terminal alkoxy chains and is laterally linked to a polysiloxane backbone via an alkyl ester spacer arm. The dependence of the logarithm of the retention factor on the reciprocal temperature showed that the liquid-crystalline anisotropic order was conserved in the small pores (200 A diameter) of the silica gel. The first-order nematic-isotropic transition is lost and probably becomes second-order. Adsorption enthalpies for the liquid-crystalline stationary phases have been measurement for three polycyclic aromatic hydrocarbon isomers (ortho-terphenyl, triphenylene, and chrysene) and compared with those for a commercial C18 phase. The adsorption enthalpies never exceeded 30 kJ mol−1, i.e. ten times the thermal agitation energy,RT. They were always less on the SOLCP stationary phase than on the C18 column, emphasizing the more rigid structure of the liquid crystalline phase and its mechanism based upon adsorption. Better separation of steroids, pesticides and amino acids were obtained with the LCP-coated silica than the commercial bonded C18 column. Four small peptides were successfully separated by using pure water as mobile phase.

Application of liquid crystals in liquid chromatography. From low- to high-molecular-weight liquid crystals

The use of liquid crystals as stationary phases for liquid chromatography is reviewed. The synthetic pathways used for covalent immobilization of low-molecular-weight liquid crystals on the silica-gel surface are discussed. The techniques used to characterize stationary phase structure are also presented. Finally, trend for replacement of low-molecular-weight liquid crystals by liquid-crystalline polymers, either coated or bonded, is described. Examples are given of the separation of specific chemical compounds on liquid-crystalline stationary phases.

Correlation between hydrophobic and molecular shape descriptors and retention data of polycyclic aromatic hydrocarbons in reversed-phase chromatography on non-liquid-crystalline, nematic, and smetic stationary phases

The local anisotropic ordering of side-chain liquid-crystalline polymer (SCLCP) stationary phases has been revealed statistically. For this purpose the RP HPLC separation of polycyclic aromatic hydrocarbons (PAH) on silica coated with three classes (non-liquid-crystalline, nematic, and smectic) of side-chain polymer (SCP) has been compared. The logarithm of the capacity factor (logk) was correlated with three PAH descriptors-the connectivity index (χ) or the hydrophobic fragmental constant (logP), the length-to-breadth ratio (L/B), and a non-planarity term (Np). Statistical results revealed good correlation between the model and experimental data, enabling the different stationary phases to be compared. Recognition of solute size seems similar for each class of polymer but solute non-planarity recognition grows continuously as ordering of the liquid-crystal polymer increases. Recognition of solute lengthening is non-existent for non-liquid-crystalline polymers and suddenly, appears with liquid crystalline polymers. Shape recognition is better for smectic than for nematic SCP. The predictive, ability of this model was tested on highly condensed aromatic compounds. The connectivity index, χ, did not seem appropriate for such systems. its replacement by Rekker's hydrophobic, fragmental constant, logP, was necessary for fitting the retention of these solutes on liquid-crystalline stationary phases.

The influence of column temperature on selectivity in reversed-phase liquid chromatography for shape-constrained solutes

Relationships between column temperature and retention behavior are reviewed for separations in reversed-phase liquid chromatography on hydrocarbonaceous bonded phases. Changes in selectivity are observed to occur with changes in temperature for a wide variety of stationary phase types. The largest temperature effects are observed for long alkyl chain length stationary phases and liquid crystalline stationary phases. Changes in stationary phase conformational structure are characterized by spectroscopic approaches, and correlations are made with retention behavior. In general, improved separations of isomers are usually possible at reduced column temperature. Applications are presented for classes of compounds that exhibit well-defined (constrained) molecular structure.

Simulation Studies of Retention in Isotropic or Oriented Liquidn-Octadecane

In gas−liquid chromatography with liquid-crystalline stationary phases and in liquid−liquid chromatography with polymeric bonded phases, the retention selectivity has been linked to the orientation of the retentive phase. In an effort to explore the nature of this effect, molecular simulations utilizing the configurational-bias Monte Carlo technique in the Gibbs ensemble were performed. Through this simulation technique, the partition coefficients of benzene, naphthalene, n-heptane, and n-dodecane were calculated when partitioning takes place between an oriented or an isotropic liquid n-octadecane phase of equal density and a helium gas phase. The calculated partition coefficients demonstrate that the n-alkane solutes prefer the oriented n-octadecane phase over the isotropic one, while the opposite behavior (but to a smaller extent) is observed for benzene and naphthalene solutes. The n-alkane solute transfer into the oriented n-octadecane phase is favored by a smaller entropic penalty and a minor enthalp...

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography: V. Study of retention mechanism using linear solvation energy relationships

A linear solvation energy relationship model was used to characterize the retention behavior of a stationary phase based upon a nematic side-on liquid crystalline polymer (SOLCP) in reversed-phase liquid chromatography. The set of solutes was constituted of a high variety of compounds whose molecular sizes were considerably smaller than the mesogenic unit size. The results showed good statistical fits for these retention data in 65:35, 75:25 and 85:15 (v/v) methanol–water mobile phases. Both the cavity term and excess molar refraction are the most important favorable retention-governing parameters, whereas the solute hydrogen bond acceptor basicity is the most unfavorable retention parameter. Hydrophobicity and π–π interactions decrease strongly when the percentage of methanol increases, leading to an important retention decrease despite the fact that the hydrogen bond interaction weakens as the organic solvent is added. The shape recognition ability of this side-on liquid crystalline stationary phase on polycyclic aromatic hydrocarbon solutes is partly explained by the solutes’ high polarizability due to the presence of π-electrons. However, the solute polarizability is not sufficient and a stationary phase’s “structure effect” must to be taken into account for the shape discrimination observed. The strong interaction between liquid crystal molecules caused likely a adsorption retention mechanism rather than a partition mechanism.

Synergistic Effect of Heptakis(2,3,6-tri-o-pentyl)- -Cyclodextrin and o-Methyl-p-Phenylene-bis-(p-Heptoxy Benzoate) Mixed Stationary Phase in Capillary Gas Chromatography

The separations for benzene-cyclohexane, cyclohexane-hexane, halogen hydrocarbons, aromatics, phenol, and alcohol on heptakis(2,3,6-tri-o-pentyl)-β-cyclodextrin and o-methyl-p-phenylene-bis-(p-heptoxy benzoate) liquid crystalline mixed stationary phase in capillary gas chromatography are carried out. It is observed that the chromatographic retention properties for most of the pairs tested on the mixed stationary phase are not the simple addition of 2 pure phases. These facts show that there exists a synergistic effect in the mixed stationary phase. This effect depends on the mixing ratio of mixed stationary phases and the linear velocity of the carrier gas.

Determination of Polycyclic Aromatic Sulfur Heterocycles in Fossil Fuel-Related Samples

An analytical method is described for the separation, identification, and quantification of a number of polycyclic aromatic sulfur heterocycles (PASHs) in three fossil fuel-related samples including two Standard Reference Materials (SRMs), SRM 1597 (coal tar) and SRM 1582 (petroleum crude oil), and a decant oil. The compounds measured include the 3 possible naphtho[b]thiophenes; dibenzothiophene and selected methyl-, ethyl-, dimethyl-, and trimethyl-substituted isomers; the 3 possible benzo[b]naphthothiophenes; and the 30 methylbenzo[b]naphthothiophenes isomers. Because of the occurrence of polycyclic aromatic hydrocarbons and PASHs together with their large number of possible alkyl-substituted isomers, the analytical method described requires a number of prerequisites: effective sample cleanup, selective stationary phases, and selective methods of detection. The sample cleanup involves solid-phase extraction using aminopropylsilane cartridges with different solvent mixtures followed by normal-phase liquid chromatographic isolation of the PASHs based on the number of aromatic carbons. These aromatic ring fractions are then separated by capillary gas chromatography using two stationary phases with different selectivities, 5% phenyl-substituted methylpolysiloxane stationary phase and 50% phenyl-substituted methylpolysiloxane stationary phase, and analyzed with mass-selective detection and atomic emission detection. A liquid crystalline stationary phase was also used to separate the methylbenzo[b]naphthothiophene isomers in the crude oil sample. Advantages and limitations of each chromatographic and detection technique are discussed.

Separation of important PCBs and PAHs on a prototype smectic liquid-crystalline polysiloxane stationary phase capillary column

A prototype smectic liquid-crystalline polysiloxane GC column was tested as stationary phase for the separation of 7 BCR PCBs and 16 EPA PAH compounds. The separation efficiency on the smectic phase was much higher for both groups compared to a non-polar stationary phase column of the DB-5 type. Moreover non-ortho substituted (coplanar) PCBs were well separated from the mono-ortho and diortho PCBs and appeared as a late eluting group. Finally the smectic column is very useful for qualitative analysis of PAHs containing extracts.

Gas—liquid chromatographic study of thermodynamics of solution of some alkanes on liquid crystal stationary phases

A gas—liquid chromatographic study of the interaction and elution characteristics of C 5-C 9 normal, branched and cyclic alkanes using liquid crystalline stationary phases and at different column temperatures of 60–100°C for p-( n-hexyloxy)phenyl p′-methoxybenzoate and 90–150°C for p-pentyloxyphenyl p′-ethoxyazoxybenzoate as stationary phases is reported. Negligible surface effects at the liquid crystal— solid support interface were ensured using a high loading (20%, w/w) of liquid crystal stationary phase on 80–100- mesh Chromosorb W AW DMCS. The results are interpreted in terms of activity coefficients and related thermodynamic quantities.

Gas chromatographic separation of cis-trans isomers of alkylcyclohexylbenzenes on a capillary column with a liquid crystalline stationary phase

Liquid crystal derivatives of cyclohexylbenzene with an isothiocyanate group have interesting properties with respect to application in displays. However, only trans isomers of these compounds are useful for this purpose, hence separation of the isomers during synthesis and the determination of the amount of the waste cis isomer are necessary. The cis-trans isomers can readily be separated and analysed using short (15 m) glass capillary columns coated with a liquid crystalline stationary phase. A liquid crystal derivative with an isothiocyanate group has been applied. For comparison, some chromatographic separations on Hewlett-Packard Ultra-2 column were performed. Because of the specific retention mechanism of the liquid crystalline phase it was possible to separate isomers with better selectivity and in a shorter time than on the Ultra-2 column. However, as a result of its high efficiency, the Ultra-2 column is better for the chromatographic separation of the other by-products of the synthesis.