Influence of structure and polarity of polysiloxanes on retention of N-TFA butyl esters of amino acids

In this work, we have investigated the influence of the polarities and structures of 13 polysiloxanes on the retention of some N-TFA N-butyl esters of amino acids. Gehrke [1] was the first to examine the possible relationship between polarity and retention data of amino acid derivatives for 9 stationary liquid phases. He concluded that the molecular structure of the stationary liquid phase is an important factor determining retention.     

Influence of the lateral alkoxy chain length on the gas chromatographic properties of five liquid crystals

The gas Chromatographic properties of five laterally substituted liquid crystals are the subject of a comparative study. These liquid crystals belong to the homologous series: 2-alkoxy-3-methyl-4-(4-ethoxybenzoyl-oxy)-4′-(4-trans-n-pentyclcyclohexanecarboxyloxy) azobenzene, referred to as ALn, where n, the carbon number in the lateral alkoxy chain, is equal to 4, 6, 8, 10, or 12 carbon atoms. Their thermal properties were established by differential scanning calorimetry (DSC). It appears that the introduction of a lateral alkoxy chain affects the nematic temperature range which decreases when the carbon number increases. The chromatographic separation abilities of AL4, AL6, AL8, AL10, and AL12 were studied using capillary glass columns. The five liquid cystals are efficient before and after solid-nematic or nematic-liquid transitions; however, the plate numbers are higher in the nematic state. Interesting analytical properties were noted in various fields: iisomeric separation of alkanes, aromatics, polyaromatics, volatile aroma compounds, and cis and trans isomers. However, the chain length does not exert any great influence on the separation abilities.     

Comparison of two liquid crystals as stationary phases in capillary gas chromatography

Summary The comparative gas chromatographic behavior has been investigated for two new liquid-crystal stationary phases, 2-(3-chlorobenzyloxy)-3-hexanoxy-4-(4-chlorobenzoyloxy)-4′-(4-methylbenzoyloxy)azobenzene (denoted 3-Cl) and 2-(3-methylbenzyloxy)-3-hexanoxy-4-(4-chlorobenzoyloxy)-4′-(4-methylbenzoyloxy)azobenzene (denoted 3-CH₃). The structures of the main core of the two liquid crystals are the same but they differ by lateral aromatic branch— 3-chlorobenzyloxy and 3-methylbenzoyloxy. Their thermal properties were established by use of differential scanning calorimetry (DSC) and their chromatographic separation properties by use of glass capillary columns. Interesting analytical performance was obtained in the separation of isomeric aromatic compounds, polyaromatic compounds, volatile aroma compounds, andcis andtrans isomers. Separation efficiency was higher for 3-Cl, especially for volatile aroma compounds, light aromatic compounds, and polyaromatic compounds, although the phases were similar in otherways.     

Evaluation of Thermal and Analytical Properties of Two Liquid Crystals in Capillary GC

Comparative gas chromatographic applications of two new liquid crystals were investigated. The characterization of the two mesogenic compounds was performed with ¹H NMR and mass spectrometry. The thermal properties were established by differential scanning calorimetry. The optical characterization of the liquid crystal was made by using polarizing optical microscopy. The chromatographic separation abilities of the mesogenic compounds were studied using fused silica capillary columns. Interesting analytical performances were obtained notably in the separation of volatile aroma compounds and derivatives phenols.     

Effect of the chain length on the thermal and analytical properties of laterally biforked nematogens

Three laterally substituted liquid crystals were synthesized in order to investigate the effect of a lateral biforked chain on the thermal and analytical properties. The mesogenic molecules have the same core containing four aromatic rings connected by two ester and one diazo linkages, they differ by the length of one chain within the lateral biforked substituent. The phase transition temperatures were obtained by polarized light microscopy and differential scanning calorimetry (DSC). The clearing temperature and the nematic range decrease with increasing length of the lateral biforked chain. The stationary phases derived from these nematogens provide excellent resolution of various classes of compounds, including aromatic hydrocarbons (AH), substituted benzenes, polycyclic aromatic hydrocarbons (PAH), phenols and volatile organic compounds (VOC) present in the essential oils. The selectivities of the stationary phases were found to decrease according to the length of the side chain.     

Analytical performances of two liquid crystals and their mixture as stationary phases in capillary gas chromatography

Comparative gas chromatographic applications of two new liquid crystals called LCa and LCb and their equimolar mixture LC(a+b) were investigated. The thermal properties of LCa, LCb and LC(a+b) were established with differential scanning calorimetry (DSC) and polarizing microscopy. Differential scanning calorimetry of LC(a+b) showed that the melting or clearing temperature was intermediate between the corresponding temperatures of the pure compounds. Polarizing microscopy showed that the liquid crystal phase of A + B was nematic. The chromatographic separation abilities LCa, LCb and LC(a+b) were studied using fused silica capillary columns. Interesting analytical performances were obtained: isomeric separation of aromatics, polyaromatics, phenols.     

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.