Ladislav Feltl

Study of the properties of cyclodextrins as stationary phases in gas chromatograpy

Abstract The formation and properties of inclusion compounds of α- and β-cyclodextrin were studied in a gas-solid chromatographic system. Substances of various structural types and geometries (hydrocarbons, hydrocarbon halogeno derivatives, alcohols, aromatic compounds) were used as sorbates. The results of chromatographic measurements are correlated with polarizability and polarity data for the sorbate molecules, and interpreted in terms of the possible interactions involved.

Gas chromatographic study of the inclusion properties of calixarenes. II. Selective properties of cyclic tetra- to octamers derived from phenol, and some problems associated with the use of calixarenes in capillary gas chromatography

Fused-silica capillary columns have been prepared containing calixarene oligomers derived from p-tert.-butylphenol as the selectors. Using the quantitative structure-retention relationships (QSRR) method, general selectivity of the studied calixarenes toward aromatic analytes and highly pronounced selectivity of p-tert.-butylcalix[4]arene and p-tert.-butylcalix[8]arene toward positional isomers of xylenes, ethyltoluenes and diethylbenzenes, and toward butylbenzene isomers, have been demonstrated. p-tert.-Butylcalix[8]arene exhibits selectivity toward chloromethanes. The efficiency of these columns was rather low because of the very poor solubility of the basic, unsubstituted calixarenes in common GC stationary phases.

Gas chromatographic study of the inclusion properties of calixarenes I. p-tert.-Butylcalix[4]arene in a micropacked column

Abstract Quantitative structure-retention relationships (QSRR) in gas-solid chromatography (GSC) were employed as an auxiliary method to study the inclusion properties of p-tert.-Butylcalix[4]arene. Selected experimental retention data were studied thermodynamically. The sorbates involved homologous series of aliphatic and alicyclic alkanes and alkenes, aromatics, halo derivatives, alcohols and ethers. It is presumed that p-tert.-Butylcalix[4]arene forms inclusion complexes with benzene, its lower n-alkyl derivatives (toluene to n-butylbenzene), p-dialkylbenzenes (p-xylene, p-ethyltoluene), m-xylene, di- and trichloromethane, methanol and ethanol under GSC conditions.

Cyclodextrins and methylated cyclodextrins as stationary phases in gas-solid chromatography

Abstract Cyclodextrins and their methylated derivatives were used as stationary phases in gas-solid chromatography and the effect of inclusion on the retention of compounds with various structures and geometries (alkanes, aromatic hydrocarbons, alcohols) was followed. It has been found that inclusion predominates in the gas-solid interaction of compounds with suitable geometric dimensions, while the retention of polar substances is primarily determined by the interactions with the cyclodextrin hydroxyl groups. On decreasing the cyclodextrin polarity by means of methylation, stereoselectivity was evident even in the separation of alcohols. On the other hand, the presence of methyl groups causes steric hindrance to penetration of the guest molecule into the cyclodextrin cavity. The selectivity of the inclusion process is especially important for separations of positional isomers.

Comparison of methods employing gas chromatography retention data to determine vapour pressures at 298 K

Validity of five models suggested for expressing the relationship between vapour pressures and GC retention times measured on a non-polar capillary column were tested on a common set of compounds [five homologous series of the type H-(CH2)n-Y, where Y denotes Cl, Br, CHO, OCOCH3 and COOCH3, and n varies from 6 to 14]. Standard methods of statistical analysis, as well as vapour pressure values obtained independently from direct vapour pressure measurements were used as validity criteria. For the 40-compound data set examined, the methods provided vapour pressures agreeing within 9.2-24.7% (average absolute percent error) with direct experimental data.

Chromatographic study of the inclusion properties of cyclodextrins: Study of inclusion from the gaseous phase

The ability of cyclodextrins to form inclusion compounds with substances in the gaseous phase was studied chromatographically. Chromatographic measurements enabled the mechanism of the inclusion process to be clarified and have shown that other forces, mainly hydrogen bonding, play a role in the process in addition to the geometric properties of the interacting substances. These facts are expressed in terms of the changes in the chromatographic quantities and of the changes in the thermodynamic characteristics that are derived from the former quantities. The selective properties of cyclodextrins have been utilized in separations of substances that are otherwise separated with difficulty (xylenes, diethylbenzenes, trimethylbenzenes, etc.).

Comparative studies of the chromatographic properties of α- and β-cyclodextrin in gas-solid and gas-liquid chromatography

Abstract A systematic study was carried out of the properties of α- and β-cyclodextrins as selective agents for the separation of various compounds, mainly isomers, in gas-liquid and gas-solid chromatographic systems. The test compounds included aliphatic and aromatic hydrocarbons (xylenes, trimethylbenzenes, diethylbenzenes) and alcohols (propanols, butanols). The possible interaction mechanism is discussed in terms of molecular inclusion, the effect of the nature of the matrix solvent on the chromatographic properties of the cyclodextrin solutions and the selectivity due to the inclusion process. Examples of chromatographic separations are given.

Study of the stereospecific properties of cyclodextrins as gas-solid chromatographic stationary phases

Abstract A comparative study was carried out of the stereospecific properties of α-, β-and γ-cyclodextrins, using a set of monoalkyl-, dialkyl- and trialkylbenzenes. The relationship between the relative retention and the boiling points of the test substances were studied and, on this basis, the retention mechanism was discussed. The elution order of variously substituted aromatic hydrocarbons, affected by the steric arrangement of the substances and by the size of the cyclodextrin cavity, permits separations of complex mixtures, as demonstrated in the chromatograms given.

Study of sorption properties and structure of porasils over a wide temperature range

Abstract Various types of Porasils used in gas chromatography have been studied over a wide temperature range and their sorption properties and changes in structure have been followed. Adsorption isotherms of benzene were measured on Porasils that were thermally pre-treated from 100° to 1000° and their specific surface areas were calculated. The adsorption isotherms had concave shapes in all instances. At pre-treatment temperatures above 700°, a pronounced decrease in the amount adsorbed per gram of the adsorbent occurred. Only Porasil F yielded identical results over the whole range of pre-treatment temperatures. Porasil A and D exhibited irreversible changes at temperatures above 700°. In this region, emanometric and X-ray analyses, porosimetric analysis and grain size measurements were carried out, and microscopic photographs were taken. The results show that Porasil pre-treated at temperatures of 900–1000° meets the requirements for an ideal support better than thermally untreated silica gels of low surface area.

Gas chromatographic determination of vapour pressures of pheromone-like compounds IV. Acetates, a reinvestigation

Abstract The equilibrium vapour pressures P and enthalpies of vaporization ΔH for an acetate series with carbon numbers varying from 10 to 18 were determined using a GC technique. The P and ΔH quantities at 298.15 K were calculated for more than 100 (un)saturated linear-chain compounds based on the experimental retention time measurements of the test acetates and five reference alkane standards with known vapour pressures. The advantage of utilizing several standards in the GC based vapour pressure determinations was demonstrated by comparison with previously published data. Additionally, empirical equations were developed for correlating 1n P and ΔH with the number of carbon atoms ( N ) and double bond positions (Δ) in the chain. It was shown that with these two-parameter equations the vapour pressures of acetates can be predicted with a considerable degree of confidence. In the case of E 2-, E 3- and Z 4-alkenyl acetate isomers, however, anomalies in retention time/vapour pressure-structure relationships were observed, indicating specific types of polar interactions.