Determination of enkephalin peptides by nonaqueous capillary electrophoresis with electrochemical detection

Nonaqueous capillary electrophoresis with electrochemical detection (NACE-ED) was applied to the analysis of enkephalin peptides. The effect of different buffer compositions on the electrophoretic behavior of methionine enkephalin, leucine enkephalin, and [D-Ala2]-leucine enkephalin was studied. Separation of the protonated and the deprotonated peptides was obtained using ACN/methanol-based electrolyte systems. The electrochemical behavior of the enkephalins was studied by the capillary batch injection analysis technique. NACE-ED yielded well-defined signals in the oxidation mode only for the negatively charged analytes. The optimized BGE for the counterelectroosmotic separation consisted of 10 mM ammonium acetate in ACN/methanol (3:1 v/v). Using a platinum microdisk electrode set to an actual potential of +0.65 V detection limits in the submicromolar range were observed which are about one order of magnitude lower compared to UV detection. Problems concerning EOF instability and electrode fouling caused by water and other neutral sample impurities transported by the EOF can be avoided in the EOF-inverted mode using poly(ethylene glycol)-coated capillaries and an actual working electrode potential of +1.0 V. For the quantification of the enkephalins [D-Ala2]leucine enkephalin was used as internal standard. The practical utility for the determination of enkephalins in spiked plasma samples after SPE was demonstrated.     

High-Performance Thin-Layer Chromatography Combined with Densitometry for Quantitative Analysis of Phenolic Acids in Complex Mixtures

JPC – Journal of Planar Chromatography – Modern TLC - Protocatechuic acid and caffeic acid in plant extracts, both free and bound as glycosides and ester derivatives, have been...     

Electrochemically assisted injection – a new approach for hyphenation of electrochemistry with capillary-based separation systems

A new concept based on the electrochemical conversion of analyte species during the injection into capillary flow systems is presented. This approach is termed electrochemically assisted injection (EAI). In a specially designed injection cell containing the analyte solution a conversion efficiency of about 83% can be achieved. Potassium octacyanotungstate(IV) served as a model compound for the analytical characterisation of EAI applying capillary flow injection analysis with double-pulse amperometric detection. Capillary electrophoresis experiments were performed using EAI to study the electrochemical oxidation of various ferrocene derivatives in acetonitrile solution. The electropherograms recorded with UV detection show separated signals for the ferrocene compounds and their oxidation products. The migration behaviour and the stability of ferrocenium cations and other reaction products were investigated.     

Adsorption of water on zeolites of different types

We have investigated the interaction of water with Na⁺-ion exchanged zeolites of different structures (LTA, FAU, ERI, MOR and MFI) by means of temperature-programmed desorption (TPD). The non-isothermal desorption of water shows, depending on the zeolite type, differently structured desorption profiles. In every case the profiles have, however, two main ranges. Using a regularization method, desorption energy distribution functions have been calculated. The desorption energy distributions between 42–60 kJ mol^?1, which can be attributed to a non-specific interaction of water, show two clearly distinguished energy ranges. The water desorption behaviour of this range correlates with the electronegativity of the zeolites and the average charge of the lattice oxygen atoms calculated by means of the electronegativity equalization method (EEM). The part of the desorption energy distributions in the range of 60–90 kJ mol^?1, reflecting interactions of water with Na⁺ cations, shows two more or less pronounced maxima. In agreement with vibrational spectroscopic studies in the far infrared region, it may be concluded that all samples under study possess at least two different cation sites.     

Gradient thin-layer chromatography of extracts fromDigitalis lanata andDigitalis purpurea

Summary Glycosides isolated fromDigitalis lanata andDigitalis purpurea (1985–1991 crops) were chromatographed on High-Performance precoated silica layers using stepwise gradient elution, the eluents containing increasing concentrations of methanol in ethyl acetate. The extracts were separated into about 20 components, presumably glycosides, some of which were identified using a mixture of standards. Densitograms at 365 nm were also obtained. The adsorbent-eluent system was very efficient and the separation was rapid owing to the use of a non-aqueous mobile phase. The method is suitable for the comparative analysis of lanatosides in plant extracts.     

Analytical possibilities of microelectrode use for stripping voltammetry

The analytical utility of microelectrodes for stripping voltammetry is discussed from several points of view. The application of microelectrodes for microanalysis is demonstrated using a novel capillary flow injection system. Heavy metals at g l^–1 concentrations have been determined in l-samples. The influence of electrode size and convection during the deposition period of anodic stripping voltammetry on the reproducibility of trace metal determination was studied for various types of electrodes. In the case of mercury film microelectrodes, the precision can be improved if the accumulation of the analyte is performed under quiescent conditions. Practical examples of stripping voltammetry with microelectrodes such as copper determination in whisky and trace metal measurements in drinking water are given.     

Stepwise gradient thin-layer chromatography and densitometric determination of taxol in extracts from various species ofTaxus

Summary Extracts fromTaxus species needles (T. media, T. baccata, T. cuspidata) were separated on thin layers of silica Si 60 by two-stage development (first stage: 60 % heptane solution of a mixture of 5% methanol and 95% chloroform; second stage: 70% heptane solution of the same solution of methanol in chloroform). The peak of taxol was satisfactorily separated from the neighbouring peaks to permit its densitometric determination. Content of taxol for various samples of Taxus species is compared.     

Stepwise gradient thin-layer chromatography and densitometric determination of taxol in extracts from various species ofTaxus

Summary Extracts fromTaxus species needles (T. media, T. baccata, T. cuspidata) were separated on thin layers of silica Si 60 by two-stage development (first stage: 60 % heptane solution of a mixture of 5 % methanol and 95 % chloroform; second stage: 70 % heptane solution of the same solution of methanol in chloroform). The peak of taxol was satisfactorily separated from the neighbouring peaks to permit its densitometric determination. Content of taxol for various samples of Taxus species is compared.     

Application of Multiple-Gradient Thin-Layer Chromatography for the Quantitative Determination of Convallatoxine in Herba Convallariae

JPC – Journal of Planar Chromatography – Modern TLC - A high-performance thin-layer chromatographic method with densitometric detection has been used to determine the convallatoxine...     

Densitometric Determination of Persilben in some Species of Polygonum L.

JPC – Journal of Planar Chromatography – Modern TLC - Persilben (3,5-methoxy-2-hydroxy-E-stilbene) was satisfactorily separated on thin layers of silica gel, by use of two-step gradient...