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  • 1
    ISSN: 1612-1112
    Keywords: RP-HPLC ; Optimization ; Complex mobile phases ; Iso-eluotropic mixtures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The stepwise procedure described previously [1] for the selection of optimal mobile phase compositions in reversed phase liquid chromatography (RPLC) is improved in two ways. The optimization criterion is modified to account for variations in analysis time. The iterative approach to the optimal solvent composition makes a more efficient use of previous data. An example illustrates the resulting gain in speed and accuracy.
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  • 2
    ISSN: 1612-1112
    Keywords: Liquid chromatography ; Alumina ; Basic drugs ; lon-exchange ; Water-organic mixtures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary It is shown that the selectivity of ion-exchange chromatography on alumina can be substantially improved by adding organic solvents to the aqueous mobile phase. With increasing modifier content the retention of organic cations generally passes through a maximum. This is attributed to the competing effects of enhanced solvation of the buffer cation and reduced solute ionization. The mutual interaction of pH, buffer composition, solute pK and eluent composition produces a very flexible system that can be optimized for the separation of a particular sample. As an example of practical significance, the separation of six basic drugs at pH 6 in a mixture of acetonitrile and water is presented.
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  • 3
    ISSN: 1612-1112
    Keywords: Solubility parameter ; Theory of liquid chromatography ; Retention ; Selectivity ; Specific effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Das Konzept des Löslichkeitsparameters wird kurz diskutiert. Danach wird es benutzt, um einige Erscheinungen der Flüssig/Flüssig-Verteilungs-(LLC) und Adsorptions-Chromatographie (LSC) zu erklären. Es werden unterschiedliche Phasensysteme bezüglich dreier charakteristischer Erscheinungen besprochen:Retention, Selektivität (erhöhte Trennkraft eines Systems) undSpezifität (erhöhte Trennkraft für bestimmte Substanzpaare). Die Erscheinung zweier wesentlich unterschiedlicher Techniken, der Normalphasen- und der Umkehrphasen-Technik, ergibt sich als logische Konsequenz der vereinfachten Theorie. Es wird auch klar, warum die Reversed Phase. Anwendungen sehr viel zahlreicher als die übrigen sind. Es werden einige Anregungen zur Entwicklung neuer stationärer Phasen und der Verbesserung der bereits bestehenden gegeben. Es wird dargestellt, wie nützlich das Löslichkeitsparameter-Konzept zur Vorhersage der Lösemittelstärke von Lösemittelgemischen in der Umkehrphasen-Chromatographie ist. Zum Schluß werden einige praktische Regeln zur Auswahl und dem Betrieb von stationären und mobilen Phasensystemen formuliert.
    Notes: Summary The solubility parameter concept is briefly discussed. It is then used to explain some of the current features of liquid partition and adsorption chromatography. Various phase systems are discussed on the basis of three characteristics:retention, selectivity (the general separation power of a system) andspecificity (increased separation power towards certain pairs of solutes). The emergence of two essentially different techniques, the ‘normal phase’ and ‘reversed phase’ modes, will appear as a logical consequence of simplified theory. It also becomes obvious why ‘reversed phase’ applications are so much more numerous. Some suggestions are given for the development of new stationary phases and the improvement of existing ones. The usefulness of the solubility parameter concept to predict the solvent strength of mixed eluents in reversed phase liquid chromatography (RPLC) is demonstrated. Some practical rules for the selection and operation of stationary and mobile phase systems are formulated.
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  • 4
    ISSN: 1612-1112
    Keywords: Liquid Chromatography ; Modification of alumina ; Extreme pH ; Ion exchange
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The applicability of alumina as a stationary phase in liquid chromatography is considered. Despite the presence of hydroxyl groups on wetted alumina, chemical modification with silanes by analogy to silica is not successful. Although depolarisation can be achieved with hexamethyldisilazane, the product is not stable to hydrolysis in aqueous media. The use of alumina as an ion-exchanger is more promising, especially because alumina appears to be stable over a very broad pH range. As a result of its amphoteric character alumina can be used either as an anion-exchanger or as a cation-exchanger. The latter possibility is more profitable in alkaline solvents and allows the separation of strongly basic compounds at pH values as high as 12. Retention can be controlled by judicious choice of ionic strength, counter ion and pH.
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  • 5
    ISSN: 1612-1112
    Keywords: Solubility parameter ; Theory of liquid chromatography ; Retention ; Selectivity ; Specific effects
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The solubility parameter concept is briefly discussed. It is then used to explain some of the current features of liquid partition and adsorption chromatography. Various phase systems are discussed on the basis of three characteristics.retention, selectivity (the general separation power of a system) andspecificity (increased separation power towards certain pairs of solutes). The emergence of two essentially different techniques, the ‘normal phase’ and ‘reversed phase’ modes, will appear as a logical consequence of simplified theory. It also becomes obvious why ‘reversed phase’ applications are so much more numerous. Some suggestions are given for the development of new stationary phases and the improvement of existing ones. The usefulness of the solubility parameter concept to predict the solvent strength of mixed eluents in reversed phase liquid chromatography (RPLC) is demonstrated. Some practical rules for the selection and operation of stationary and mobile phase systems are formulated.
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  • 6
    ISSN: 1612-1112
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary An analytical technique has been developed for the separation and quantitative determination of glycine peptides, from glycine to heptaglycine, with a possible extension to octaglycine and nonaglycine. Using an amino acid analyzer, a cation exchange resin and a citrate buffer of pH 3.35 (gradient elution is not required), the complete separation takes 3 hours 30 minutes. The quantitative precision is about 3%. The elution order can be explained on the basis of two mechanisms: - the pKa-values determine the positions of glycine up to tetraglycine - tetraglycine up to heptaglycine, with equal pKa-values, follow Andrews' equation, Ve=a−b log M, derived for gel chromatography.
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  • 7
    ISSN: 1612-1112
    Keywords: Reversed-phase LC ; Ternary mobile-phases ; Optimization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary A simple rapid procedure is described for estimating optimum compositions of ternary mobile phase mixtures for the separation of samples by reversed-phase liquid chromatography (RPLC). Retention data in two iso-eluotropic binary mobile phase mixtures (mixtures with equal retention times) are required to initiate the procedure. The logarithm of the capacity factor is assumed to vary linearly with the composition of isoeluotropic ternary mixtures formed by mixing the two limiting binaries. Using the product of resolution factors of adjacent peaks as the criterion, an optimum ternary composition is then calculated. After a chromatogram has been obtained with the predicted optimum ternary mobile phase, the procedure is repeated until no further improvement can be achieved. Examples of the application of the present procedure are described to illustrate its effectiveness.
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  • 8
    ISSN: 1612-1112
    Keywords: Column liquid chromatography ; Ion-pair chromatography ; Sulfonate pairing ion ; Adsorption isotherms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The retention of positively and negatively charged and neutral solutes was studied in an aqueous phosphate buffer eluent, with constant inorganic-counterion concentration, on ODS-Hypersil stationary phase in the presence of various sulfonic acid ion-pairing reagents. The adsorption isotherms of the ion-pairing reagents d-camphor-10-sulfonic acid, sodium cyclohexylsulfamic acid and sodium alkylsulfonates (butyl-, hexyl- and octyl-) were determined by the breakthrough method.
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  • 9
    ISSN: 1612-1112
    Keywords: Capillary electrophoresis ; Chiral separation ; γ-Cyclodextrin ; Mandelic acid enantiomers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The separation of the optical isomers of mandelic acid by capillary electrophoresis using γ-cyclodextrin as a chiral selector is presented. The mobility of the enantiomers can be regulated by adjustment of the pH, the concentration of the chiral selector and by the molarity and the temperature of the buffer. Different organic modifiers can either increase or decrease resolution. The importance of the nature of the electrolyte is evidenced by the fact that acceptable enantioseparation has only been achieved using phosphate buffer. The factors affecting the mobility are discussed. A simple equation describing the apparent mobility of the analytes in buffers containing cyclodextrins is presented. The equilibrium constant for the inclusion complex formation of mandelic acid with γ-cyclodextrin was found to be 2.8 M−1 for D-mandelic acid and 2.4 M−1 for the L-enantiomer.
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  • 10
    ISSN: 1520-6882
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
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