A Comparative Analytical Study of Two New Liquid Crystals Used as Stationary Phases in GC

The investigation of the analytical properties of two new nematic sulphur-containing liquid crystals 5-(4-methoxyphenyl)-azophenyl)-2-butylthio-1,3,4-oxadiazole (Phase I) and 5-(4-(propoxyphenyl)-azophenyl)-2-butyl thio-1,3,4-oxadiazole (Phase II) and which comprise units of 1,3,4-oxadiazole instead of the aromatic cycles, was carried out by gas chromatography using glass capillary columns. For this purpose, many solutes belonging to various families and having different polarities and volatilities were injected. Comparison of the retention data of the studied components has shown that Phase II allowed a better separation than the other phase. The two liquid crystalline materials show a good separation of the studied isomers except for xylene.     

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.     

Evaluation of smectic biphenylcarboxylate ester liquid-crystalline polysiloxane stationary phases for capillary column gas chromatography

Mesomorphic biphenylcarboxylate esters were coupled via flexible aliphatic hydrocarbon spacers to a polysiloxane backbone. The influence of spacer length, percent mesomorphic substitution, and crosslinking of the stationary phase on liquid-crystalline transition temperatures and on chromatographic performance was investigated. Unique selectivity and good efficiency over a wide temperature range for gum and cross-linked liquid-crystalline phases were demonstrated by the separation of various isomeric polycyclic aromatic compounds.     

Synthesis and characterization of benzyl β-cyclodextrins used as GC stationary phases

Summary Heptakis(2,6-di-O-benzyl)-β-CD(I), heptakis(2,6-di-O-benzyl-3-O-pentyl)-β-CD(II), heptakis(2,6-di-O-benzyl-3-O-methyl)-β-CD(III) and heptakis(2,6-di-O-benzyl-3-O-acetyl)-β-CD(IV) derivatives were synthesized and identified. Their thermal stabilities were tested using PyGC. These CD derivatives are stable up to 300°C and suitable for use as gas chromatographic stationary phases. The fused silica capillary columns coated with heptakis(2,6-di-O-benzyl-3-O-methyl)-β-CD and heptakis(2,6-di-O-benzyl-3-O-pentyl)-β-CD showed excellent chromatographic properties in separating positional isomers.     

Thermodynamic properties of solutions by gas chromatography using a liquid crytal compound 4,4-bis(heptyloxy)azoxybenzene as solvent

Summary Thermodynamic properties of 22 solutes at infinite dilution have been determined in the smectic, nematic and isotropic mesophases of 4,4-bis(heptyloxy)azoxybenzone. The thermodynamic properties are discussed in relation to the solute-solvent (liquid crystal) interactions as conditioned by the degree of order in the liquid crystal.     

Gas chromatography in qualitative analysis Part 16. An investigation of the oxidation of reference stationary phases used in column characterization by the Rohrschneider and McReynolds methods

Summary The on-column oxidation of hydrocarbons commonly used as stationary phases in column characterization is investigated. In each case significant shifts of retention are observed, which may be negative or positive depending upon the polarity of the test solutes and the adsorptivity of the support material. In order to reduce the consequent uncertainties in the Rohrschneider [1] and McReynolds [2] polarity values, a new method of column characterization is proposed. The applicability of the new method is illustrated by a consideration of a range of silicone liquid phases.     

A study of chromatographic properties of liquid crystalline polyacrylates using WCOT columns in gas chromatography

Liquid crystalline polyacrylates (LCPAs) are evaluated as stationary phases for capillary gas chromatography (CGC). Aromatic hydrocarbons as well as polar isomeric compounds are used as test solutes to compare the polarity of the LCPAs with a conventional polysiloxane column and a Carbowax 20M column. The relative polarity of the columns with respect to change in the lateral and terminal substituents is also discussed. Lateral substitution, polarity of the terminal group, together with the solute size, affect the LCPA selectivity.     

Gas chromatographic properties of some liquid crystals containing dioxyethylene ether terminal chains

Summary Comparative gas chromatographic properties of four rod-like liquid crystals coated on capillary columns are studied. These four liquid crystals contain the same number of atoms in their backbones. They differ by the partial or total replacement of the two terminal heptyloxy chains by a 2-(2-methoxyethoxy)ethoxy (called also dioxyethylene ether) chain. Synthesis and thermal properties of these four liquid crystals are presented. It appears that the introduction of dioxyethylene ether decreases the two transition temperatures as well as the nematic range. With regards to the chromatographic separation abilities, the four liquid crystals are efficient before and after solid-nematic or nematic-liquid transitions; however, the plate numbers are higher in the nematic state. Interesting analytical applications are demonstrated in different fields: isomeric separation of alkanes, aromatics, polyaromatics, volatile aroma compounds, cis and trans isomers and phenols. The increase of the number of oxyethylene units permits to drastically increase the separation efficiency for polar solutes.     

Effect of solute polarity on the performance of porapak type porous polymers

Summary The breakthrough volume of ådsorbent traps made with porous polymer beads Porapak N, P. Q, R, S was evaluated by analysis of polarity reference substances proposed by Rohrschneider (ethanol, methyl ethyl ketone, nitrometane, benzene, pyridine) by McReynolds (butanol, methyl propyl ketone) and of the first term of some homologous series: alcohols, aldehides, carboxilic acids, nitrocompounds, iodoalkanes and nitriles.Adjusted retention times of the compounds were measured at 200°C and correlated with the physical properties and with polarity class according to Ewell.Trapping efficiency of the various Porapak types was evaluated and the theoretical breakthrough volumes at 25°C was calculated for the used test substances.Dedicated to Prof. Dr. A. Liberti on the occasion of his 70th birthday.     

Free radical crosslinking of the gas chromatographic stationary phase containing europium chelates

Summary A gas chromatographic stationary phase containing europium chelates was modified by a free radical crosslinking with 2,2-azobis(2-methylpropionitrile). The stationary phase was characterized by injections of the McReynolds series of standard solutes. A better thermal stability and an enhanced polarity was observed.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

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.     

Effect of chain length distribution on the phase behavior of polyglycerol fatty acid ester in water

The effect of the chain length distribution on the phase behavior, the structure of liquid crystals, and physicochemical properties was investigated in water/ polyglycerol fatty acid ester. Polyglycerol fatty acid esters with sharply distributed polyglycerol (10G*0.7L) and with broadly distributed polyglycerol (10G0.7L) were used. Unreacted polyglycerol in both surfactants was removed. 10G*0.7L forms hexagonal liquid crystals at a higher concentration than 10G0.7L. The effective cross-sectional area of the lipophilic parts in the hexagonal phase of 10G0.7L is smaller than that of 10G*0.7L owing to the difference in the chain length distribution. Evidently, 10G0.7L molecules are tightly packed in aggregates; therefore, 10G0.7L decreased the surface tension more strongly and promoted emulsification. Received: 11 January 2000 Accepted: 8 March 2000     

分离生物大分子的液相色谱固定相

本文介绍了各类用于生物大分子分离的液相色谱填料,并对它们在生物大分子分离及制备中的性能、基本特征作了评述。     

New materials: analytical and environmental applications in ion chromatography

This review details ion chromatography developments and related applications, mainly in the field of environmental analysis, gained by the evolution in the science and technology for new stationary phases. Unconventional approaches to ion chromatographic separations are also outlined.     

Novel stationary phases based on asphaltenes for gas chromatography

We present the results of investigations on the possibility of the application of the asphaltene fraction isolated from the oxidized residue from vacuum distillation of crude oil as a stationary phase for gas chromatography. The results of the investigation revealed that the asphaltene stationary phases can find use for the separation of a wide range of volatile organic compounds. The experimental values of Rohrschneider/McReynolds constants characterize the asphaltenes as stationary phases of medium polarity and selectivity similar to commercially available phases based on alkyl phthalates. Isolation of asphaltenes from the material obtained under controlled process conditions allows the production of a stationary phase having reproducible sorption properties and chromatographic columns having the same selectivity. Unique selectivity and high thermal stability make asphaltenes attractive as a material for stationary phases for gas chromatography. A low production cost from a readily available raw material (oxidized petroleum bitumens) is an important economic factor in case of application of the asphaltene stationary phases for preparative and process separations.     

The effect of hydrocarbon chain length and end-capping on the liquid chromatographic properties of stationary phases containing bonded nitrile groups

Summary Stationary phases containing bonded nitrile groups have been synthesized. The effect of the n-alkyl chain length connecting the nitrile groups to the surface and of the end-capping on the chromatographic properties was studied. The synthesized material was compared with commercial nitrile phases in the separation of isomeric cresols and xylenols.     

A comparative study of the gas chromatographic behavior of liquid crystals and polyester stationary phases using paraffinic hydrocarbons as standards

Two new categories of stationary phase have been studied: liquid crystals and polyesters. The liquid crystals were p,p′-azoxyphenetol, p(n-hexyloxy)phenyl-p′-methoxy benzoate, and p-pentyloxy-p′-ethoxyazoxybenzene; and the polyester phases 1,4-butanediol succinate (LAC-860), ethylene glycol succinate (LAC-886), diethylene glycol succinate (LAC-738), diethylene glycol adipate (LAC-296), and neopentyl glycol adipate (LAC-769). Abnormal chromatographic behavior, has been observed: an increase in specific retention volumes and an improvement in the separation of normal, branched, and cycloparaffins, depending on their structures, at high temperatures within a specified range. This phenomenon is the opposite of the behavior of traditional stationary phases. From the chromatograms obtained, plots of log v against 1/T, and differential thermal analysis it is concluded that this abnormal behavior may be attributed to the penetration of solutes through the ordered structure of the mesophase and the rod-like molecules of the polyesters. Furthermore, the chromatographic behavior of some of the polyester (LAC-series) stationary phases in respect of temperature changes is similar to that of the nematic mesophases.     

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

Summary 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.Dedicated to Professor J. F. K. Huber on the occasion of the his 60th birthday.     

Solute retention in reversed-phase chromatography as a function of stationary phase properties: Effect of n-alkyl chain length and ligand density

Summary Two series of bonded phases were synthesized employing LiChrospher Si 100, 10 m and n-alkyldimethylmonochlorosilanes as silanizing reagents. In series A the n-alkyl chain length, n, of the bonded phase was varied between 1 and 20 at a constant ligand density of 3.5±0.2mol·m^–2. In series B the ligand density, d, was gradually changed from 0 to 4.1mol·m^–2 on the C₁, C₄, C₆, C₈ and C₁₈ bonded phases, respectively.The capacity factors of benzoic acid esters and anilines as solutes were found to increase linearly with the n-alkyl chain length of packings at constant eluent composition (series A) up to a so-called critical chain length, n_crit, where the capacity factor remained constant. n_crit was in the range from 11 to 14. The same pattern was observed when plotting the solute capacity factor against the ligand density of the packing at constant n and constant eluent composition (series B). The critical ligand density, _crit, varied between 2.3 and 3.2 mol·m^–2 depending on n and the solute. Furthermore, solute retention was slightly higher on RP packings with an even number of carbon atoms in the alkyl chain than on those with an odd number.These findings are consistent with the results of Berendsen and de Galan (J. Chromatogr., 196, 21 (1980)), Dill (J. Phys. Chem., 91, 1980 (1987)) and Simpson and Lau (to be published). The observed phenomena reflect the dynamic structure of RP silicas which are related to the mobility of solvated n-alkyl chains. Due to the lack of a precise conformational analysis of the surface of RP silicas, a semiquantitative model was applied to interprete the described dependencies.     

Characterization of hexacationic imidazolium ionic liquids as effective and highly stable gas chromatography stationary phases

Polycationic ionic liquids (ILs) are an attractive class of ILs with great potential applicability as gas chromatography stationary phases. A family of hexacationic imidazolium ILs derived from the cycloalkanol family was chemically first prepared in a straightforward manner and then applied for analytical separation purposes. Four tuneable engineering vectors, namely cation ring size structure, anion nature, spatial disposition of cycloalkanol substituents and O‐substitution, were considered as experimental parameters for the design of the desired ionic liquids. A total number of five new phases based on a common benzene core respectively exhibited column efficiencies around to 2500 plates/m, broad operating temperature ranges and also, even more importantly, good thermal stabilities (bleeding temperature between 260 and 365°C), finding variations in the selectivity and analytes elution orders depending on the IL structures. Their solvation characteristics were evaluated using the Abraham solvation parameter model, establishing clear correlations between their cation structure and retention capability with respect to certain analytes. The study of relationships between the ILs structure and solvation parameters gives us an idea of the IL stationary phase to be used for specific separations.