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P Jandera

Publications and source records attributed to P Jandera.

24 records · Page 2Linked to original sources
PubMed1996

Chromatographic behaviour in reversed-phase high-performance liquid chromatography with micellar and submicellar mobile phases: effects of the organic modifier.

Continuing our earlier study of the retention behaviour in reversed-phase systems with aqueous mobile phases containing surfactants in concentrations lower (submicellar systems) and higher (micellar systems) than the critical micellar concentration (CMC), we investigated the chromatographic behaviour of various non-ionic solutes in mixed aqueous-organic micellar and submicellar mobile phases and their dependence on the methanol concentration. CMC values were measured for two cationic surfactant and one anionic surfactant in mixed aqueous-methanolic solvents, and were found to increase slightly with increasing methanol concentration. Depending on the character of the surfactant, a limiting concentration of methanol was found, above which micelles do not occur anymore. Sorption isotherms of the surfactants on an octylsilica gel column were measured as a function of the concentration of methanol in aqueous-methanolic solvents. A modified Langmuir equation was used to describe the distribution of the surfactants between the stationary and the mobile phases in the concentration range below CMC. The retention of several polar solutes was measured on an octylsilica gel column both in micellar and submicellar mobile phases containing methanol. The dependencies of the capacity factors of the solutes studied on the concentration of methanol in the mobile phase can be suitably described by the same form of equation as that conventionally used for aqueous-organic mobile phases that do not contain surfactants, but the slopes of the dependencies for a given solute are different in the two ranges of surfactant concentrations. The ratio of the two slopes is controlled by the interaction with micelles and is approximately equal to, below or above 1, depending on whether the solutes do or do not associate with the micelles, or are repulsed from them. Simultaneous control of the concentrations of the organic solvent and of the surfactant in the mobile phase can be used for fine tuning the selectivity of separation as a complement to commonly used adjusting concentrations of two organic solvents in ternary aqueous-organic mobile phases. These effects are illustrated by practical examples of submicellar HPLC with mobile phases containing methanol.

Adsorption↗
PubMed1993

Determination of ivermectin in bovine plasma by column-switching LC using on-line solid-phase extraction and trace enrichment.

Ivermectin is a potent avermectin anthelmintic, derived from the soil organism Streptomyces avermitilis. Because of its potency, it is administered in very low doses (200 micrograms kg-1), giving rise to plasma levels in the ppb region, and hence sensitive methods are required for its determination in biological samples. A method is described for the determination of ivermectin in bovine plasma, based on a commercially available C18 column employing a mobile phase composed of acetonitrile-water-isopropanol (40:40:35, v/v/v). Extraction from plasma is carried out on-line using coupled-column technology; switching between the extraction and analytical columns is achieved using a six-port column-switching valve. The method is linear over the range 2-100 ng ml-1 (r = 0.9994) and has a limit of detection of 0.8 ng ml-1 (signal-to-noise ratio = 3). Reproducibility of the method, expressed as the overall mean relative standard deviation, is 6.7% (intra-assay) and 3.8% (inter-assay). The mean recovery of the drug from plasma is 76.4%. The method is simple and economical in its execution and is sufficiently reproducible to obviate the need for an internal standard.

1-Propanol↗
PubMed1987

Liquid chromatography of phenylurea herbicides and related compounds on chemically bonded ion-exchange materials.

The possibilities of using new chemically bonded anion-exchange (Silasorb DEA) and cation-exchange (Silasorb S) materials for high-performance liquid chromatography of phenylurea herbicide compounds were investigated. The columns packed with Silasorb DEA or with Silasorb S ion-exchange materials can be used for efficient separations in normal-phase systems using n-propanol-n-hexane as mobile phases. Here, specific selectivity effects for phenylurea-type herbicides can be achieved in comparison to chromatography on unmodified silica or on chemically bonded aminosilica or cyanosilica, especially with Silasorb S columns. The influence of the mobile phase composition and of the structure of phenylurea herbicides on the chromatographic behaviour on Silasorb S and Silasorb DEA columns was investigated in detail and the possibilities of separation are illustrated by several examples.

Chromatography, High Pressure Liquid↗
PubMed1986

Method for characterization of selectivity in reversed-phase liquid chromatography. III. Retention behaviour in gradient-elution chromatography: application to the chromatography of pesticide compounds.

The method introduced recently for the characterization and prediction of absolute and relative retention in reversed-phase liquid chromatography under isocratic conditions has been applied to gradient-elution chromatography. The method is based on two indices (one lipophilic, nce, and the other polar, qi, to characterize the behaviour of each solute in methanol-water mobile phases of various compositions. The indices nce and qi of phenylurea and triazine herbicides were found to be composed of additive contributions of structural elements (functional groups) in the molecules of the solutes. Using the present approach, it is possible to predict retention volumes in gradient-elution chromatography with a precision comparable to those under isocratic conditions (i.e., less than 10% relative for most compounds tested). Simultaneous adjustment of the initial mobile phase composition and the slope of the linear gradient makes it possible to control to a certain extent the selectivity in gradient-elution chromatography using binary solvent gradients and offers an alternative approach to the use of multi-solvent gradients for selectivity control. This method may be especially useful in optimizing gradient-elution separations of pairs of compounds with positive differences in both indices nce and qi, delta nce greater than 0 and delta q greater than 0. Selection of the optimal gradient profile is illustrated on a practical example of gradient-elution chromatography of phenylurea herbicides.

Chromatography, Liquid↗