I. Topalova

Structure-retention correlations of hydrocarbons in GLC and GSC. Alkenylbenzenes

SummaryFor the study of relationships between retention behaviour and molecular structure in GLC and GSC the retention indices of alkenylbenzenes with 2–6 carbon atoms in a straight or branched side chain and of isotopological alkylbenzenes were determined on capillary columns coated with OV-1 and Ucon LB and on a micropacked column filled with graphitized thermal carbon black. From the values obtained index differences corresponding to the increments of the double bond in the side chain were derived. These differences depend significantly on the structure of the side chain and of the position of the double bond.In the case of 1-phenyl-1-alkenes the double bond close to the aromatic ring gives rise to an increase of retention values, especially for the trans-isomers, causing remarkable retention differences between the cis and transisomers. The alkenylbenzenes which exhibit a terminal double bond are less strongly retained than the isotopological alkylbenzenes.

Structure—retention correlations of alkylphenols in gas—liquid and gas—liquid chromatography

Abstract The retention behaviour of alkyl- and dialkylphenols has been investigated by gas—liquid chromatography (GLC) on glass capillary columns, coated with PS-255 and OV-1701-Vi, and by gas—solid chromatography (GSC) on a microcolumn, packed with uncoated graphitized thermal carbon black (GTCB). The dependence of the chromatographic properties of the alkyl phenols on their molecular structure is discussed. Due to electronic and steric effects, alkyl groups neighbouring the phenolic hydroxyl group decrease the retention values in GLC. In contrast, the interaction between vicinal alkyl groups leads to increased retention. GSC on GTCB is advantageous for the distinction of meta - and para -isomers, which are difficult to separate by GLC.

Ring opening of cyclohexane over ZSM-zeolites followed by reversed-flow gas chromatography

SummaryKinetic parameters for the ring opening of cyclohexane over modified ZSM-5 zeolites can be measured simultaneously under non-steady state conditions using reversed-flow gas chromatography. At relatively low temperatures, the main product detected and measured was propene.The mathematical relations used for the calculation of adsorption-desorption rate constants, surface reaction rate constants and adsorption equilibrium constants for the reactant cyclohexane are different from previously studied catalytic reactions, owing to the different experimental arrangement for feeding the catalytic bed. The diffusion bands obtained experimentally for reactant or product are described by the sum of two or three exponential functions of time, respectively. From the exponential coefficients of time and the pre-exponential factors, all determined by simple PC programs, the above kinetic parameters are calculated.

Plasma-modified thermal carbon black as a packing material for gas chromatography

Abstract The chromatographic properties of thermal carbon black (TCB) TG-10 could be improved by treating it with a high-frequency plasma, in the presence of benzene vapour. In comparison with the untreated TCB, chromatographic peaks are more symmetrical and retention volumes are lower. The data obtained show that the surface of TCB after modification becomes more uniform and non-specific, and the properties of modified TCB approach those of graphitized carbon black. TCB modified in this manner is good sorbent for selective separation of structural and geometrical isomers.

Gas chromatographic analysis of organic compounds on non-polar adsorbents

Abstract The applicability of new non-polar adsorbents in gas chromatography has been investigated. Molybdenum and tungsten sulphides are characterized by a lamellar structure, which resembles the structure of graphite, and hence are expected to show chromatographic properties similar to those of graphitized thermal carbon black (GTCB). This assumption was verified chromatographically by comparing the properties of argon plasma-treated thermal carbon black (TCB) with the properties of MoS 2 and WS 2 and MoSe 2 . The adsorption characteristics (relative retentions, heats of differential adsorption) were determined at low levels of adsorbent surface coating with reference compounds. The results indicate that the treatment of TCB in a hydrogen-containing plasma (oxygen-free flow) results in lower retentions. The retention of n -alkanes by MoS 2 and WS 2 is higher than that of monoalkylbenzenes with the same number of carbon atoms. Di- n -butyl ether is adsorbed by MoS 2 and WS 2 more weakly than n -nonane, which indicates a homogenous sorbent surface. The adsorption of di- n -butyl ether on MoSe 2 is stronger than that of n -nonane, which indicates an non-homogeneous adsorbent surface. The results made it possible to derive several practically applicable conclusions. MoS 2 and WS 2 , for instance, can be used for analytical purposes at lower temperatures than thermal carbon black. The adsorbents under consideration are suitable for the separation of structural and geometrical isomers.

Gas chromatographic retention on carbon adsorbents coated with monolayers of polynuclear aromatic hydrocarbons

Abstract Monolayers of polynuclear aromatic hydrocarbons (PAHs) were used to modify carbon adsorbents (non-graphitized and graphitized thermal carbon blacks). The thermodynamic adsorption characteristics, retention volumes and initial differential heats of adsorption, of molecules belonging to different classes of organic compounds were measured by gas chromatography on these sorbents. The nature of the retention of molecules with different chemical properties on PAH-modified surfaces is determined predominantly by dispersion interaction by competing electrostatic interaction. The effect of PAH modification is the greatest on the retention of oxygen- and nitrogen-containing compounds. Exposure of thermal carbon blacks, modified with monomolecular PAH layers, to an high-frequency low-temperature plasma helps to improve the symmetry of chromatographic peaks.

Plasma modification of adsorbents of different chemical natures with electron-donor molecules

Abstract Adsorbent supports of different chemical natures, i.e., thermal carbon black and silochrome (a macroporous silica adsorbent), have been modified with planar pyrene molecules which exhibit donor properties. The effect of exposure of the modified adsorbents to a high-frequency low-temperature plasma (HLP) on the retention parameters has been investigated by gas chromatography. It is shown that the HLP treatment modifies the surface properties of the adsorbents, which is manifested in an increased contribution of the electrostatic interaction to adsorption. The modified adsorbents have been applied to the separation of certain multicomponent mixtures.

Gas chromatographic investigation of silica supports modified in a high-frequency low-temperature plasma

Abstract The modification of gas chromatography (GC) supports and adsorbents by glow discharge is of practical interest as it is a method of producing changes in particular surface properties. The adsorption properties of the surface of Silochrom CX-1 modified in a high-frequency low-temperature plasma with argon and benzene vapours were investigated using GC. The thermodynamic characteristics of series of aliphatic and aromatic hydrocarbons and oxygen-containing compounds were measured. The modified silica adsorbents were used for the separation of structural isomers and other mixtures of organic compounds. The modifications of silica described here improve the uniformity of the adsorbent surface and hence the separation properties, increasing the selectivity of the silica supports. This will expand the field of application of silica adsorbents.

Gas chromatographic properties of modified boron nitride

Abstract Boron nitride (α-crystal modification) with significant surface non-uniformity was treated in a high-frequency, low-temperature plasma. The properties of the starting and modified samples were analysed by gas chromatography. Retention values and heats of adsorption for close-to-zero coverage of the surface for several compounds were determined. The chromatographic properties of α-boron nitride samples treated in plasma are improved. The retention time decreases and the symmetry of chromatographic peaks of oxygen-containing compounds is improved. This allows the modified boron nitride to be used for the separation of different mixtures.

Carbon black adsorbents modified with n-hydrocarbons and treated with an high-frequency low-temperature plasma

Abstract A commercial thermal carbon black with a surface area of 10 m 2 /g was modified by adsorption of a monolayer of n -dotriacontane. The capacity of the monolayer was 0.6 mg/m 2 as determined by three methods: calculations using the Van der Waals sizes of molecules; the adsorption isotherm from solutions and gas chromatographic data. The modified adsorbent was then treated in an high-frequency low-temperature plasma. The gas chromatographic properties of the resulting adsorbent were improved and the separation of different mixtures of hydrocarbons and polar substances was achieved.