Subhash C. Dhanesar

Survey of organic molten salt phases for gas chromatography

Abstract The organic molten salts tetra- n- hexylammonium benzoate, tetra- n- heptylammonium chloride, 1-methyl-3-ethylimidazolium chloride, ethylpyridinium bromide, tri- n- butylbenzylphosphonium chloride, tetra- n- butylammonium tetrafluoroborate, and sodium isovalerate are evaluated as stationary phases for gas chromatography. They all behave as stable isotropic liquids over a temperature range of at least 50°C, and in most cases, greater than 100°C. As a series these salts provide a range of liquid phases spanning temperatures from room temperature to ca . 290°C. The principal solute-retaining forces are orientation and proton donor interactions, the magnitude of which depends on the identity of the salt. The physical and chemical properties of the organic molten salts are compatible with their use in gas chromatography to separate a wide range of sample types including alcohols, halocarbon compounds, substituted benzene and naphthalene derivatives, and essential oils. Calcium nitrate tetrahydrate is also evaluated as an example of a hydrated melt. Excessive retention of polar solutes and poor peak shape make its general use impractical.

Evaluation of tetraalkylammonium tetrafluoroborate salts as high-temperature stationary phases for packed and open-tubular column gas chromatography

Abstract Four tetraalkylammonium tetrafluoroborate salts were evaluated as high-temperature polar stationary phases for packed and open-tubular column gas chromatography. Tetrapropylammonium and tetraoctylammonium tetrafluoroborates were found to have too limited a temperature operating range to be generally useful. Tetrahexylammonium and tetrabutylammonium tetrafluoroborates had liquid ranges from 90 to 230 and 162 to 285°C, respectively, and were studied in more detail. Both phases showed strong, selective dipole interactions these being slightly stronger for the tetrabutylammonium salt than for the tetrahexylammonium salt. The tetrabutylammonium tetrafluoroborate salt also provided higher column efficiencies, by about a factor of two, when used in packed-column gas chromatography. Retention of a wide range of organic solutes was found to occur by gas—liquid partition on the tetrafluoroborate salts. Five surface roughening techniques were evaluated for the preparation of efficient, long-lived, open-tubular columns. Whisker-modified surfaces, alone or coated with a layer of microcrystalline sodium chloride, provided columns of acceptable efficiency (2000–2500 plates per meter) and greater durability than other column types. Barium carbonate-modified surfaces provided columns of the highest efficiency but of poor durability. Columns modified with a layer of Chromosorb R6470-1 were the least efficient and of poor durability. Examples of the separation of complex mixtures on packed and open-tubular columns, coated with tetraalkylammonium tetrafluoroborates, are given to illustrate their practical utility.

Mobile phase optimization of a urea-linked chiral stationary phase for the high-performance liquid chromatographic separation of optical isomers

Abstract A silica-based urea-linked chiral stationary phase, ( R )-N-(1-naphthylethyl-N′-propylsilyl urea), was studied with binary, ternary, and single-component mobile phases, to determine the factors which most affect retention and selectivity. Binary mobile phases were prepared with hexane as the non-selective solvent, and 2-propanol, dichloromethane, or chloroform as the selective solvent. (±)-1-Phenylethyl-3,5-dinitrobenzamide was used as the enantiomer test mixture. Increasing the mobile phase strength reduced the retention of test solutes, while selectivity remained largely unchanged. Also, modifiers improved resolution and did not affect selectivity. Single-component mobile phases reduced the retention in the order of increasing solvent strength, selectivity being solvent-dependent.

Synthesis and stationary phase properties of bromophenyl ethers

Abstract Electrophilic bromination is shown to be a useful reaction for the site-specific bromination of phenyl ethers containing from two to seven benzene rings. Compared to the phenyl ether stationary phases used in gas chromatography, bromination primarily raises the lower and upper operating temperatures, generally leads to a reduction in the efficiency of column packings and changes the selectivity and polarity of the phases only to a small extent. The three-ring bromophenyl ether, 1,3-dibromo-4,6-bis(4-bromophenoxy)benzene is shown to have the most useful stationary phase properties for the separation of organic compounds with a useable temperature range of 80–200°C.

Synthesis and chromatographic properties of cyano- phenyl ethers

Abstract A general route for the preparation of cyanophenyl ethers to be used as polar thermally stable liquid phases in gas chromatography is described. Electrophilic bromination with thallium acetate catalysis occurs exclusively at positions para to the ether bond. Under vigorous reaction conditions bromination at ortho positions in the terminal phenyl rings may also occur. On subsequent reaction with cuprous cyanide all para substituted bromine groups may be exchanged but those in ortho positions are resistant to reaction. To monitor the synthetic reactions effectively and to establish the isomeric purity of the final products high-performance liquid chromatography and proton nuclear magnetic resonance spectroscopy were used.