Myra E. Coddens

Organic salts, liquid at room temperature, as mobile phases in liquid chromatography

Abstract The synthesis of six alkylammonium salts of the general formula Rn—a NH+a X−, where R = ethyl, propyl, butyl or sec.−butyl, X− = nitrate or thiocyanate, n It was concluded that the alkylammonium nitrate the thiocyanate salts could be used as mobile phases in liquid chromatography when mixed with a second solvent of low viscosity. Suitable equations are given to predict the viscosity of mixtures of alkylammonium nitrate and thiocyanate salts in all volume proportions with water, methanol, acetonitrile, tetrahydrofuran and dichloromethane. The liquid organic salts form immiscible solvent pairs with non-polar organic solvents and should also be suitable for use in liquid—liquid chromatography and liquid—liquid extraction.

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.

Some quantitative aspects of scanning densitometry in high-performance liquid chromatography

Abstract The optimization of instrumental parameters that influence resolution and signal-to-noise ratios in the recording of thinlayer chromatograms by scanning densitometry are discussed. A standardized method of determining sensitivity and detectability is proposed and used to evaluate the performance of a Shimadzu CS-910 scanning densitometer. Fluorescence enhancement reagents are discussed as a method of improving sample detectability in the fluorescence mode. The influence of the sorbent medium on both the position and intensity of the fluorescence emission signal is discussed.

Qualitative identification of polycyclic aromatic hydrocarbons by high-performance thin-layer chromatography and fluorescence scanning densitometry

Abstract A procedure is described for the accurate measurement of emission response ratios for polycyclic aromtic hydrocarbons using fluorescence scanning densitometry and reversed-phase high-performance thin-layer chromatography. By optimizing instrumental conditions and using perylene as a reference standard, the emission response ratios can be reproduced to better than 3% relative standard deviation on a day-to-day basis. The emission response ratios enable the optimum detection conditions for sensitivity or selectivity to be easily identified. The normalized emission response ratios can be used to improve the confidence in the identification of a particular compound made by coincidence of retention in the chromatographic system. The method has been applied to the identification of polycyclic aromatic hydrocarbons in samples of laboratory dust and a diesel engine exhaust particulate sample.

Comparison of methods for separating polycyclic aromatic hydrocarbons by high-performance thin-layer chromatography

A comparison is made of different stationary phases (silica gel, cellulose, acetylated cellulose, and 3-aminopropyl-, ethyl-, octyl-, octadecyl-, and diphenylsilanized silica gel), mobile phases, and development techniques (conventional, continuous, and multiple) for the separation of environmentally important polycyclic aromatic hydrocarbons by high-performance thin-layer chromatography. Reversed-phase separations using octadecylsilanized silica gel and methanol-water or methanol-water-based ternary solvent systems were found to give the best separations. A new development technique, multiple development with application of the mobile phase further up the plate at each successive development, provided superior resolution of the polycyclic aromatic hydrocarbons compared with conventional multiple development.

Mode selection and optimization of parameters for recording high-performance thin-layer chromatograms by transmission measurements

Abstract Optimum conditions to maximize the observed resolution and signal-to-noise ratio of a scanning densitometer for high-performance thin-layer chromatography are described. Transmission measurements are compared in three operating modes; two being pseudo dual-beam methods and the third a dual-wavelength single-beam mode. In all modes resolution is shown to be independent of those parameters which control the dimensions of the measuring beam within their accessible ranges. Signal-to-noise ratios depend on both the operating mode and the dimensions of the measuring beam. In particular, signal shows a marked dependence on the ratio of slit height to spot diameter. Similar values for resolution and better signal-to-noise characteristics may be obtained in the transmission mode compared to reflectance for substances absorbing in the visible range.

Instrumental Evaluation of Thin-Layer Chromatograms

Recent changes in the practice of thin-layer chromatography (TLC) have created a renaissance of interest in this technique and led to its wider acceptance as a powerful tool for qualitative and quantitative analysis of mixtures. The performance breakthrough in TLC, referred to as high performance thin-layer chromatography (HPTLC), was not a result of any specific advances in instrumentation or materials, but was rather a culmination of improvements in practically all of the operations comprising TLC, [1–6]. Improvements in the quality of the sorbent layer, methods of sample application, new development techniques, and the availability of scanning densitometers for in situ quantitative analysis were all important developments in the evolution of HPTLC. The new HPTLC plates were prepared from sorbents of smaller average particle diameter and of narrower particle size distribution than those used to prepare conventional TLC plates. The performance of these plates improved an order of magnitude over those used for conventional TLC. Because of the lower sample capacity of the HPTLC sorbent layer, the amount of sample applied to it had to be scaled down from those commonly used in conventional TLC. Typical sample volumes applied to the plates are in the range 100–200 nl, sample amounts less than ca. 0.1 μg, and starting spot sizes of 1.0–2.0 mm.