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.     

气相色谱固定相研究新进展

发展高选择性固定相是实现气相色谱(GC)高效分离样品组分及其分析测定的关键。近年,材料科学的快速发展促进了新型色谱固定相的研究和应用。该文综述了近5年有关多孔材料、石墨烯及类似物、三聚茚类材料和蝶烯类材料等作为GC固定相的研究进展,并对GC固定相研究进行了总结和展望。     

The orthogonal character of stationary phases for gas chromatography

A database of system constants for 32 open-tubular columns at 100 degrees C is used to identify stationary phases for obtaining a wide selectivity space in comprehensive GC. Three parameters based on the Euclidean distance (D-parameter) or vectors (d-parameter and costheta) in hyperspace are used to establish the chemical similarity and retention correlation as an inverse scale of selectivity differences. It is shown that the poly(methyloctylsiloxane) stationary phase is the best candidate for a low-selectivity stationary phase and affords a wider selectivity space when combined with a selective polar stationary phase than poly(dimethylsiloxanes). The most suitable polar stationary phases are poly(ethylene glycols) or bis(cyanopropylsiloxane-co-silarylenes and to a lesser extent poly(methyltrifluoropropylsiloxanes). No systems are truly orthogonal but angles between individual stationary phase vectors of about 75 degrees are possible by choosing the correct combination of stationary phases.     

Ionic liquid stationary phases for gas chromatography

This article provides a summary of the development of ionic liquids as stationary phases for gas chromatography beginning with early work on packed columns that established details of the retention mechanism and established working methods to characterize selectivity differences compared with molecular stationary phases through the modern development of multi-centered cation and cross-linked ionic liquids for high-temperature applications in capillary gas chromatography. Since there are many reviews on ionic liquids dealing with all aspects of their chemical and physical properties, the emphasis in this article is placed on the role of gas chromatography played in the design of ionic liquids of low melting point, high thermal stability, high viscosity, and variable selectivity for separations. Ionic liquids provide unprecedented opportunities for extending the selectivity range and temperature-operating range of columns for gas chromatography, an area of separation science that has otherwise been almost stagnant for over a decade.     

Fullerene-impregnated ionic liquid stationary phases for gas chromatography

A new method has been developed to facilitate the use of fullerenes as stationary phases (SPs) in gas chromatography (GC). In this method, ionic liquids (ILs) are used as solvents to coat fullerenes (C(60), amino-C(60) and hydroxy-C(60)) onto GC columns. However, the ILs serve not just as coating solvents but also act synergistically with fullerenes to provide unique properties as stationary phases, namely dual modal characteristics. They act as non-polar SPs when separating non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures), and as polar SPs for polar analytes (e.g., alcohol mixtures). The polarity of the stationary phase can be adjusted by changing either the type of the IL and/or by adding either C(60) (or its amino or hydroxy derivatives) to the IL. It was found that C(60) and its derivatives produce not only a change in the polarity of the SP but also substantial enhancement in separation efficiencies for both non-polar and polar analytes. More importantly, when added to the IL SP, C(60) improves separation efficiencies not just for non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures) but also for polar analytes (mixtures of ortho-, meta- and para-xylene and alcohol mixtures) as well. Moreover, C(60) SP provides higher efficiencies than amino-C(60) and hydroxy-C(60) for separation of polar analytes. This is rather surprising considering that not only are amino-C(60) and hydroxy-C(60) more polar than C(60), but that the IL used to coat the amino- and hydroxy-C(60) (i.e., N-ethylpyridinium trifluoroacetate, [EtPy(+) CF(3)COO(-)]) is more polar than the IL used to coat the C(60) (i.e., octylmethylimidazolium bis(trifluoromethyl)sulfonyl)amide], [OMIm(+) (CF(3)SO(2))(2)N(-)]). Moreover, compared to its amino and hydroxy derivatives, the concentration of C(60) in the column was 10 times lower.     

Polymer-coated fibrous stationary phases in packed-capillary gas chromatography.

Fibrous synthetic polymers have been introduced as the support material for packed capillaries in gas chromatography. The filaments of the polymers were packed longitudinally into a fused-silica capillary, followed by the conventional coating process for open-tubular capillaries. With various polysiloxane-based polymeric materials coated onto these filaments, it was demonstrated that the retentivity was significantly improved over conventional wall-coated capillaries of the same length and that the selectivity can be tuned by selecting different coating materials chosen for the various purposes. The results clearly showed the contribution of the fibrous support and the polymer-coating to the retention of analytes. They also showed the bright possibility for a novel usage of fine fibrous polymers as the support material, which can be combined with a newly-synthesized coating materials designed for particular separations.     

Liquid crystal stationary phases for gas chromatography and supercritical fluid chromatography

Novel monomeric and polymeric liquid crystalline compounds were synthesized as stationary phases for gas chromatography (GC) and supercritical fluid chromatography (SFC). Monomeric liquid crystalline compounds were used in packed column gas chromatography for the separation of isomeric aromatic compounds and insect sex pheromones. Liquid crystalline polymers possess long nematic ranges and a uniform coating was easily achieved in glass and fused silica capillaries, which could stand temperatures up to 250°C in GC and pressures of 200 MPa at 160°C in SFC. The columns provide excellent selectivity and resolution for fused ring aromatic compounds such as the isomers anthracene and phenanthrene or triphenylene and chrysene.     

Crosslinked methylphenylsilicones as stationary phases for capillary gas chromatography

Summary Methyl(phenyl)silicones offer useful selectivities when used as stationary phases in gas chromatography (GC). Such phases have, however, hitherto been of restricted importance in capillary GC due to the lack of phases having a viscosity high enough to ensure stationary phase film stability. Further, to utilize fully the possibilities of a methyl(phenyl)silicone capillary column, it must also possess high efficiency and a high degree of deactivation.In this work, the preparation of soda-glass capillary columns coated within situ cured methyl(phenyl) and methyl(tolyl)-silicones is presented. Vulcanization was made possible by the introduction of some vinyl substitution in the gum to be cured: tolyl-containing gums could be cured without the presence of vinyl groups. In addition, fused silica capillary columns coated with OV-1701 were prepared.The columns show a coating efficiency of above 80%, a thermal stability up to 320 °C and a high degree of deactivation. Their utility is demonstrated by the separation of samples containing polynuclear aromatics, antidepressants and some potent mutagens.Presented at the 14th International Symposium on Chromatography London, September, 1982     

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.     

Dicyanobiphenyl polysioloxane stationary phases for capillary column gas chromatography

Summary The chromatographic performance of newly developed dicyanobiphenyl polysiloxane stationary phases were evaluated and compared with the performance of other polar stationary phases, including the previously reported monocyanobiphenyl polysiloxanes. Due to the unique combination of polarizable biphenyl and polar cyano functionalities in the side chains of the flexible polysiloxane backbone, and by virtue of their mild liquid crystalline properties, the new stationary phases provide excellent resolution of a wide variety of analytes, both polar and nonpolar, in both GC and SFC. They can be easily coated and cross-linked in open tubular columns, and the resultant columns demonstrate excellent efficiency and performance at temperatures up to 280–300°C. The new stationary phases exhibit enhanced selectivities for various types of isomeric compounds.     

Macroporous polymeric monoliths as stationary phases in gas adsorption chromatography

The influence of the conditions of preparation of divinylbenzene-based polymer monoliths on the properties of monolithic capillary columns for use in gas adsorption chromatography was examined. It was found that the polymerization time and the temperature and composition of the polymerization mixture have an effect on the dynamic and chromatographic properties of the columns. The monolithic capillary columns prepared under the optimal synthesis conditions had the height equivalent of a theoretical plate at the level of 20–30 μm, which is an order of magnitude below that of conventional open tubular columns.     

The preparation of non-extractable methylphenylpolysiloxane stationary phases for capillary column gas chromatography

Summary The synthesis of methylphenylpolysiloxane polymers and their use in the preparation of crosslinked, non-extractable stationary phases for fused-silica capillary columns are described. By preparing more viscous phenyl-containing polymers than are commercially available, stationary phase films of these polymers could be efficiently coated on fused-silica capillary columns and stabilized by a free radical crosslinking mechanism using peroxides. Four methylphenylpolysiloxane polymers containing different phenyl concentrations were prepared. These included three polymers containing 50% phenyl and one polymer containing 70% phenyl. Two of the 50% phenyl polymers had one phenyl and one methyl group attached to each silicon atom. One of these also had 1% vinyl incorporated. The third 50% phenyl polymer was synthesized in such a way that one half of the silicon atoms had two phenyl groups attached while the rest contained dimethyl groups. The 70% phenyl polymer also had 4% vinyl incorporated. Due to the intrinsic thermal stability of these phenyl phases and the enhanced film stability achieved by crosslinking, the 70% phenyl phase could be utilized up to 400 °C. Using the methods described in this paper, highly efficient and thermally stable fused silica capillary columns coated with crosslinked methylphenylpolysiloxane stationary phases can be successfully prepared.     

Application of dithiocarbamate resin-metal complexes as stationary phases in gas chromatography

A chelating resin with dithiocarbamate functional groups to which silica gel was used as a matrix and silanes were used with diamino functional groups as a spacer was synthesized. The structure and the conversion of functional groups of the resin were confirmed by IR spectra and elemental analysis. The influence of pH on the adsorption of the resin for metal ions was also examined. The resin under optimum pH conditions formed a 1:1 metal complex with copper ion. The affinity of metal ions toward the synthesized resin decreased in the order Hg(II) >; Cu(ll) >; Cd(II) >; Zn(II). The resin exhibited efficient complexation of transition metal cations. The cadmium, copper and zinc complexes were investigated for application as stationary phase for the gas chromatographic analysis of dialkyl sulphides. The material was packed in a 2.1 m × 3.2 mm I.D. spiral glass column. Factors affecting the retention and sample selectivity were also studied. A shorter retention time and sharp peaks were obtained when ammonia was introduced into the mobile phase. At an oven temperature of 100°C, a flow-rate of 60 ml min⁻¹ and use of a flame ionization detector, the analysis of dialkyl sulphides showed that the copper resin complex as the stationary phase gave the best results. The stationary phase was also used for the separation of dialkyl sulphide from a hydrocarbon mixture.     

Chemometric study of retention on binary stationary phases in gas chromatography

Using gas chromatography, data analysis is performed on a dataset consisting of 486 retention indices, 27 standards (ramified alkanes, aliphatic alcohols, and aromatic compounds), 6 pure and binary stationary phases, and three temperatures. The behavior of the pure stationary phases (OV-3, OV-225, OV-61-OH, and OV-1701-OH) and the binary stationary phases (OV-3/OV-225 and OV-61-OH/OV-1701-OH) at different temperatures (60°C-100°C) is investigated with factor and topological analysis. The influence of temperature and the nature of the mixed stationary phases on the retention indices is studied by correspondence factor analysis (CFA). The non-additivity of the retention properties of the pure phases used as mixed phases is clearly established by CFA. The topological analysis of the substituent's effect is investigated with a DARC/PELCO procedure and shows the particular influence of the stationary phase composition on the retention. The substituent effect is measured for the pure and binary stationary phases at various temperatures. The evolution of the substituent effect from the pure stationary phases to the binary phases is discussed.     

Long chain alkyl-polysiloxanes as non-polar stationary phases in capillary gas chromatography

Summary A new methyl-octadecyl polysiloxane phase with a distinct selectivity in PCB separation and a significantly improved temperature range has been developed. The basic structural element of the separation of PCBs on methyl-octadecyl-polysiloxane is the elution according to the number of the substituents in the ortho-positions 2,2 and 6,6. The 2,2,6,6-substituted congeners are eluted first, while the non-2,2,6,6-substituted coplanar PCB are eluted last. As compared to methyl-phenyl (5%) polysiloxane (SE 54) the standard stationary phase used for the separation of the PCBs, the methyl-octadecyl-polysiloxane has several unique features in an improved PCB separation. The Sil 5–50% C18 column resolves the pair PCB 31/28 and also the sets PCB 5/8, PCB 16/15/32, PCB 25/53, PCB 48/47, PCB 56/60, PCB 118/149, PCB 105, 132, 153, and PCB 170/190. All these separations are not achieved with a SE 54 column. No separation is observed for the pair PCB 138/163 on Sil 5–50% C18. The order of elution of the Cl_xB congeners reveals a rather stringent structure correlation. If the structure/retention correlation is extended to all 209 Cl_xB congeners, 20 groups with one ring fixed / one ring variable substitution can be distinguished.Dedicated to Prof. Dr. V. Krivan on the occasion of his 60th birthday.Presented at 14th International Symposium on Capillary Chromatography, May 25–29, Baltimore, USA.