K. Markides

Peroxide-initiated in situ curing of silicone gums for capillary column gas chromatography

Abstract The combination of leaching, silanization with cyclic siloxanes and peroxide-initiated in situ vulcanization of silicone stationary phases for gas chromatography gives capillary columns that show very high efficiencies, very low adsorptivity as reflected by stringent tests and very high efficiencies, very low adsorptivity as reflected by stringent tests and very high stability, e.g., low bleeding rates. Such columns have been prepared using AR-glass and fused silica as the supports and SE-30, SE-52, SE-54 and polar OV-215 as stationary phases. The versatility of these columns is demonstrated by the analysis of underivatized drugs, barbiturates and tricyclic antidepressants. The separation of aqueous solutions is also described.

Cyanosilicones as stationary phases in gas chromatography : II. Gum and rubber phases on fused silica

Abstract The presence of cyano groups in the stationary phase may provide useful selective interactions during separation. In capillary gas chromatography, the full utility of such interactions will be evident when the columns show high efficiency, good deactivation, high thermal stability and reproducibility in retention characteristics. Columns that fulfil these requirements were prepared. Fused silica capillaries were treated with hydrochloric acid, silanized at a high temperature with cyclic cyanopropylsiloxane and statically coated with cyanopropylsilicone gum, which was finally immobilized by in situ cross-linking. Vulcanization is normally inhibited by cyanopropyl groups, but a certain percentage of tolyl groups in the gum made cross-linking possible. The usefulness of the cyanosilicone columns is demonstrated by the separation of several samples such as fatty acid methyl esters and pheromones.

Glass capillary columns for gas chromatography coated with non-extractable films of cyanosilicone rubbers

Abstract A procedure for the preparation of glass capillary columns using a bonded cyanosilicone rubber as stationary phase is presented. Two types of cyanosilicone pre-polymer were synthesized, with 25% and 100% cyano-substitution, respectively. Glass capillaries that had been leached with HCl or treated with a sodium chloride sol were coated with the pre-polymer, and a rubber was formed situ. Such columns show very good thermal stability up to 300°C; further, activity is low and the efficiency good. The utility of these thermally stable polar columns is demonstrated by the analysis of aldononitrile derivatives prepared from sugars and also polynuclear aromatics (PNAs), especially aza-PNAs.

Cyanosilicones as stationary phases in gas chromatography. I.

Abstract Access to medium-polarity and polar capillary columns would greatly increase the separation power of gas chromatography. A prerequisite for this, however, is that the columns must possess high efficiency, a high degree of deactivation and high thermal stability. A procedure is presented for the preparation of fused silica and soda-glass capillary columns that fulfil these requirements. Efficiency and deactivation are obtained by leaching the capillary followed by silanization at high temperatures using bis(cyanopropyl)cyclotetrasiloxane. Column stability is achieved by in situ peroxide-initiated cross-linking in a silicone gum to form a rubber stationary phase. Three types of phases were synthesized, having 33, 50 and ca . 95s% cyano substitution. Some tolyl and vinyl substitution was included in order to facilitate cross-linking.

Immobilization of polyethylene glycol in capillary columns for gas chromatography

Abstract Two methods are described for the immobilization of polyethylene glycol. Immobilization was achieved by reaction with methyl(vinyl)cyclopentasiloxane (V 5 ) and dicumyl peroxide or ethylene oxide. Fused-silica capillaries coated with V 5 -immobilized polyethylene glycol 40M could be used over a wide temperature interval, 30–300°C. Further, immobilization makes feasible the use of thick films. The utility of this column type is demonstrated by several examples. Especially notable is the excellent performance achieved with water solutions and also free acids.

Cyanosilicones as stationary phases in gas chromatography : III. Synthesis, characterization and evaluation

Abstract Methods for the reproducible synthesis of curable cyanosilicone stationary phases have been developed. Different paths of synthesis has been compared, and the influence of these on the phase properties examined. A series of phases having 33, 50, 60, 75% cyano substitution, respectively, was prepared, in a manner so as to avoid the formation of amides. The effects of the use of methyltolyl, bis(tolyl) and methylvinyl blocks and bis(dimethyl)silylarylene units on cross-linking of cyano-containing organosiloxanes were examined. These phases were characterized by various chromatographic, spectroscopic and thermal analytical methods. Columns coated with the phases were tested with respect to efficiency, activity and thermal stability. The usefulness of the phases for selective separations of fatty acid methyl esters is demonstrated both with standards and actual samples of different origins.

Evaluation of bonded methylsilicone rubber as a stationary phase for glass capillary columns

Abstract A detailed description of the preparation of glass capillary columns for gas chromatography, coated with non-extractable methylsilicone rubber, is given. The advantages and disadvantages of such columns are examined and compared with the properties of columns coated with a methylsilicone gum. The excellent qualities of the new columns for the separation of polynuclear aromatics are shown and discussed.

Ozone-initiated in situ cross-linking of non-polar and medium-polar silicone stationary phases for capillary gas chromatography

Abstract Non-polar and medium-polar stationary phases have been immobilized by an ozone-curing method in fused-silica capillaries. Phases such as SE-33, SE-54, OV-215 and methyl(tolyl)siloxanes (33% and 50% tolyl, respectively) could be cross-linked at room temperature. However, SE-30, SE-52, OV-1701, bis(cyanopropyl)methyltolylsiloxane (33% CN and 33% tolyl) and the methyl(phenyl)tolylsiloxanes (25% phenyl and 25% tolyl) required a higher temperature to obtain suitable immobilization. Tests made with columns coated with these ozone-immobilized silicones showed that non-polar and moderately polar phases were of high quality, and that the thermal stability was very good. Infrared spectra of the silicone phases showed no or a very weak absorption band in the region of 1700–1770 cm -1 .

Gas chromatographic—mass spectrometric analysis of compounds generated upon thermal degradation of some stationary phases in capillary gas chromatography

Abstract The thermal degradation products of some immobilized stationary phases for gas chromatography were studied. Column bleed products were analyzed by gas chromatography—mass spectrometry (GCMS); pyrolysis GCMS was also utilized in this study. The phases studied were: SE-30; methyltolyl silicone (33% tolyl); cyanopropyl (methyl)tolylsilicone (50% cyanopropyl, 25% methyl, 25% tolyl); Carbowax 40M. Cyanopropyl- and tolyl-substituted silicones were degraded mainly to three- to five-membered cyclics, but the cleavage of substituent groups seems to be relatively limited. the degradation products of Carbowax 40M consisted of homologous series of polyethylene glycol derivatives.

Some aspects of current techniques for the preparation of capillary columns for gas chromatography

Abstract Desirable column properties and their relation to different steps in column preparation are discussed. The properties of different capillary materials are compared, the different methods for high-temperature silylation, which are being used to achieve deactivation and wettability, are considered and the advantages of immobilized silicone stationary phases are described. Immobilization leads primarily to improved stationary phase film stability and this is of particular utility in connection with polar columns; some polar silicone gums have a high temperature coefficient of viscosity and may therefore be redistributed on a capillary surface at elevated temperatures, leading to deterioration in column performance. Immobilization very efficiently counteracts such an effect. Silicones containing cyanopropyl and phenyl substitution cannot readily be cross-linked. It is demonstrated that a convenient way to solve this problem is to include some tolyl substitution in the gum that is to be cross-linked.