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.     

Modifications of poly(methyl-3-propylthiol)siloxane to give stationary phases for open tubular supercritical fluid chromatography

Summary Poly(methyl-3-propylthiol)siloxane has been evaluated for use as stationary phase in open tubular columns for gas and supercritical fluid chromatography. Immobilization of the stationary phase was achieved by crosslinking with 2,4,6-trichloro-1,3,5-triazine. The poly(methyl-3-propylthiol)siloxane stationary phase film was in situ oxidized to the disulphide and sulphonic acid forms and both have been evaluated for use in supercritical fluid chromatography. Good selectivity towards polar polycyclic aromatic compounds has been shown. All the modified forms of the stationary phase were loaded with silver ions and were evaluated with regard to ligand exchange chromatography using supercritical fluid carbon dioxide as a mobile phase. The utility of the stationary phase, having sulphonic acid groups loaded with silver ions, has been demonstrated by separation of fatty acid methyl esters according to their unsaturation.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Characterization of hexacationic imidazolium ionic liquids as effective and highly stable gas chromatography stationary phases

Polycationic ionic liquids (ILs) are an attractive class of ILs with great potential applicability as gas chromatography stationary phases. A family of hexacationic imidazolium ILs derived from the cycloalkanol family was chemically first prepared in a straightforward manner and then applied for analytical separation purposes. Four tuneable engineering vectors, namely cation ring size structure, anion nature, spatial disposition of cycloalkanol substituents and O‐substitution, were considered as experimental parameters for the design of the desired ionic liquids. A total number of five new phases based on a common benzene core respectively exhibited column efficiencies around to 2500 plates/m, broad operating temperature ranges and also, even more importantly, good thermal stabilities (bleeding temperature between 260 and 365°C), finding variations in the selectivity and analytes elution orders depending on the IL structures. Their solvation characteristics were evaluated using the Abraham solvation parameter model, establishing clear correlations between their cation structure and retention capability with respect to certain analytes. The study of relationships between the ILs structure and solvation parameters gives us an idea of the IL stationary phase to be used for specific separations.     

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.     

Polymeric imidazolium ionic liquids as valuable stationary phases in gas chromatography: Chemical synthesis and full characterization

Seven new functionalized polymerizable ionic liquids were chemically prepared, and later applied for the preparation of polymeric stationary phases in gas chromatography. These coated GC columns, which exhibited good thermal stabilities (240–300 °C) and very high efficiencies (3120–4200 plates/m), have been characterized using the Abraham solvation parameter model. The chromatographic behavior of these polymeric IL columns has been deeply studied observing excellent selectivities in the separation of many organic substances such as alkanes, ketones, alcohols, amines or esters in mixtures of polar and non polar solvents or fragrances. Remarkably, the challenging separation of xylene isomers has been possible using a bis(trifluoromethylsulfonyl)amide based imidazolium IL coated column as a gas chromatography stationary phase.     

Synthesis of siloxane stationary phases for capillary gas chromatography and supercritical fluid chromatography

A comparison is made between dichlorosilanes and cyclic siloxanes as starting materials in the synthesis of stationary phases for capillary gas chromatography (CGC) and supercritical fluid chromatography (SFC). Siloxanes containing one or more of the side groups methyl, vinyl, phenyl, and cyanoethyl in various ratios were synthesized and compared. These phases were characterized by chromatographic (gel permeation, GPC), spectroscopic (IR, ¹H NMR, ²⁹Si NMR), and thermal (DSC) methods. Coated fused silica columns were evaluated with respect to polarity, crosslinkability with several free-radical initiators, and thermal stability. A new liquid phase, 7% cyanoethyl, 7% phenyl, 1% vinyl methyl polysiloxane is shown to be more polar than OV-1701, more temperature stable, easily crosslinked and suitable for use in supercritical fluid chromatography.     

Evaluation of the separation characteristics of application-specific (fatty acid methyl esters) open-tubular columns for gas chromatography

The solvation parameter model is used to characterize the separation properties of the polar stationary phases EC-Wax and PAG with a poly(ethylene oxide) backbone (substituted with propylene oxide in the case of PAG) and the cyanopropyl-substituted polysilphenylene-siloxane stationary phase BPX90 at five equally spaced temperatures between 60 and 140 degrees C. The separation characteristics of these stationary phases are compared to four PEG and two poly(cyanopropylsiloxane) stationary phases (HP-20M, HP-Innowax, SolGel-Wax, DB-WAXetr, HP-88, and SP-2340) characterized in the same way. The database of system constants for these polar stationary phases is used to provide insight into the separation mechanism for fatty acid methyl esters and to determine selectivity differences that can be expected for generically similar stationary phase types. The discussion is not structured to indicate which stationary phase should be used for a particular separation but to provide a general framework to demonstrate the relationship between the retention mechanism and stationary phase chemistry.     

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.     

Separation characteristics of phenyl-containing stationary phases for gas chromatography based on silarylene-siloxane copolymer chemistries

The solvation parameter model is used to characterize the retention properties of five open-tubular column stationary phases (ZB-5 ms, DB-5 ms, DB-XLB, DB-17 ms, and DB-35 ms) based on silarylene-siloxane copolymer chemistries at five equally spaced temperatures over the range 60-140 degrees C. System constant differences and regression models for varied compounds are used to establish the selectivity equivalence of the silarylene-siloxane copolymer stationary phases and to compare their separation characteristics with poly(dimethyldiphenylsiloxane) stationary phases containing a nominally similar concentration of phenyl groups. These studies demonstrate that ZB-5 ms and DB-5 ms are selectivity equivalent. DB-XLB is significantly more dipolar and polarizable than DB-5 ms. In general terms, the silarylenesiloxane copolymer stationary phases are slightly less cohesive and more dipolar and polarizable with similar hydrogen-bond basicity to the poly(dimethyldiphenylsiloxane) stationary phases they were designed to replace. None of the silarylenesiloxane copolymer or poly(dimethyldiphenylsiloxane) stationary phases are hydrogen-bond acidic. Selectivity differences between the two types of stationary phase are temperature dependent and tend to be smaller at higher temperatures within the temperature range studied. Consequently, selectivity differences cannot be globalized without reference to the temperature for the comparison.     

System constants of synthesized poly(methyl-3,3,3-trifluoropropyl) siloxanes

The method of solvation model has been applied to five poly (methyl-trifluoropropyl) siloxanes (TFPSXX) prepared in our laboratories, at five trifluoropropyl (TFP) group contents, XX = 0, 11.5, 26.3, 35.5 and 50.0%, at 80, 100, 120 and 140 degrees C. Previously, specific retention volumes of 60-odd solutes of varied polarities were measured upon each of these stationary phases within the above temperature range. Constant s prevails over all other constants, TFPSXX stationary phases showing strong dipole/induced dipole forces with the solutes, moderate acidity and no basicity at all. Constant e is zero in the stationary phase without TFP groups, but has negative low-medium values for the other fluorine contents, XX from 11.5 to 50.0%, hinting at repulsive forces, as expected. Normal values for constant l, decreasing from the less cohesive TFPS00 to the more cohesive TFPS50, were found. For each TFP content constants s, a and l show a negative temperature dependence, while constant e increases as temperature increases. Constant c also decreases with increasing temperature. At each temperature, constants s and a increase with increasing %TFP (or increasing stationary phase polarity), whereas constants e and l show the opposite trend, diminishing with increasing polarity of the stationary phase. Principal component analysis shows that the five stationary phases presented in this work conform a group with other earlier synthesized trifluoropropyl siloxanes and other fluorinated stationary phases taken from literature: VB-210, QF-1, DB-200, DB-210 and PFS6, showing the same selectivity which only the fluorine atom confers. A dendrogram of 38 stationary phases supports these results.     

The effect of the trifluoropropyl group in polysiloxane stationary phases used for capillary gas chromatography

Four poly(methyl 3,3,3-trifluoropropyl siloxanes) with trifluoro-propyl group content (group substitution) between 8 and 35 percent have been synthesized and characterized as stationary phases for gas chromatography in borosilicate glass capillary columns. Results are compared with those from two commercial stationary phases–a polydimethylsiloxane and a poly(methyl 3,3,3-trifluoropropyl siloxane) with a fifty percent trifluoropropyl group content (group substitution). Retention index values, McReynolds constants, polarity (as defined by McReynolds) and retention polarity (as defined by Takács) increase regularly with the trifluoropropyl group content of the stationary phase. The temperature coefficient of the retention indices of the McReynolds probes, and that of the polarities, have been determined at temperatures between 60 and 180 °C. Specific retention volumes do not follow the linear dependence on trifluoropropyl group content observed for retention indices or polarities. Substances with electron-donor groups show maximum retention for a trifluoropropyl group content of ca 30%, whereas the retention of hydrocarbons, halogenated compounds, and alcohols decreases as the degree of trifluoropropyl group substitution increases from 0 to 50%. It is felt that a polysiloxane with a trifluoropropyl group content of ca 30 to 35% would be the best choice for the separation of ketones, nitro compounds or amines.     

Retention behaviors of novel ionic liquid stationary phases and their selectivity for capillary gas chromatography

<正>One chloride-terminated ionic liquid(CTIL) and two hydroxyl-terminated ionic liquids(HTILs) were synthesized and used as stationary phases for capillary gas chromatography(CGC).Molecular interactions of these stationary phases were evaluated by Abraham solvation parameter model,indicating that the CTIL exhibits remarkably strong H-bond basicity and the HTILs possess both H-bond basicity and acidity.The molecular interactions were further confirmed by separation of a complex mixture consisting of ketones,aldehydes,esters,alcohols and aromatic compounds.It was found that the obtained solvation parameters correlate well with the chromatographic performances of the analytes in terms of elution order and resolution.The well correlated relationship between the solvation parameters and the selectivity of the CTIL and HTILs stationary phases is quite helpful in predicting and understanding the retention behaviors of different types of analytes on these stationary phases.     

Solvation enthalpies and heat capacities of n-alkanes in four polymer phases by capillary gas chromatography

Molar solvation enthalpy (deltasol H(o)298) and molar heat capacity changes (deltasol C(o)p) were determined by gas chromatography for the C6-C12 n-alkanes on four preferred stationary phases (100% polydimethyl siloxane, 50% diphenyl-50% dimethyl polysiloxane, 50% trifluoropropyl methylsiloxane, and polyethylene glycol) in commercial FSOT. Statistical evaluation indicated the temperature independence of deltasol C(o)p in the range 303-393 K. Deltasol H(o)298 depends linearly on the number of carbon atoms in the n-alkanes, but no linearity could be established for deltasol C(o)p of higher homologues on polar columns, which may be due to a more ordered state on the liquid phase. The homologues for which a linear temperature dependence exists demonstrated that deltasol C(o)p is related linearly to the van der Waals volume and the temperature derivative of the density of the stationary phase. The results are consistent with a simple physical explanation at the molecular level.     

Preparation of polysiloxane stationary phases for capillary column chromatography: A New methoxyphenyl phase

Contemporary methods for the synthesis of alkyl- and arylsubstituted polysiloxane stationary phases are reviewed. A new, moderately polar phase containing the 3-(4-methoxyphenyl)-propyl group is reported.     

Evaluation of smectic biphenylcarboxylate ester liquid-crystalline polysiloxane stationary phases for capillary column gas chromatography

Mesomorphic biphenylcarboxylate esters were coupled via flexible aliphatic hydrocarbon spacers to a polysiloxane backbone. The influence of spacer length, percent mesomorphic substitution, and crosslinking of the stationary phase on liquid-crystalline transition temperatures and on chromatographic performance was investigated. Unique selectivity and good efficiency over a wide temperature range for gum and cross-linked liquid-crystalline phases were demonstrated by the separation of various isomeric polycyclic aromatic compounds.     

Column packing for gas chromatography containing immobilized poly(ethylene glycol) stationary phase

Summary Carbowax 20M poly(ethylene glycol) stationary phase was immobilized on Chromosorb W by cross-linking with pluriisocyanate. The properties of the prepared packing material were investigated. Column efficiencies of 10,960 and 7,510 theoretical plates/meter were obtained for n-pentadecane and 1-heptanol, respectively.     

Gas chromatography in qualitative analysis Part 16. An investigation of the oxidation of reference stationary phases used in column characterization by the Rohrschneider and McReynolds methods

Summary The on-column oxidation of hydrocarbons commonly used as stationary phases in column characterization is investigated. In each case significant shifts of retention are observed, which may be negative or positive depending upon the polarity of the test solutes and the adsorptivity of the support material. In order to reduce the consequent uncertainties in the Rohrschneider [1] and McReynolds [2] polarity values, a new method of column characterization is proposed. The applicability of the new method is illustrated by a consideration of a range of silicone liquid phases.     

Chromatographic evaluation of some stationary phases based on poly(methyloctylsiloxane) immobilized onto silica

Several stationary phases were prepared by thermal immobilization of poly(methyloctylsiloxane) onto a silica surface using different amounts of poly(methyloctylsiloaxane), and different times and temperatures of immobilization to provide different carbon contents for the phases. The chromatographic properties were evaluated using the Tanaka test. Comparison of the results obtained with literature data using hierarchical cluster analysis showed dissimilarity with most of the commercial phases. Some basic pharmaceuticals, including six benzodiazepines were separated on one of the better PMOS phases.