Electrochemical detectors in flowing liquid systems: an assessment of their use

Electrochemical detection in flowing liquid systems such as liquid chromatography and autoanalyzers has attracted considerable interest in recent years. Electrochemical detectors are particularly suited to such systems because of their selectivity and sensitivity and their linear response over a wide concentration range.     

Beiträge zur axiomatischen Inhaltstheorie

Ohne Zusammenfassung     

Nichtstationäre Kinetik bei SN1, SN2, E1, E2 und E1cb-Mechanismen

For the S_N1, S_N2, E₁, E₂, AND E₁cb mechanisms exact solutions of the kinetic equations are compared with solutions obtained under the hypothesis of stationarity. Exact integrals are calculated numerically for a set of the essential reaction rate constants. Comparison with the stationary state solution demonstrates that both types of solutions are approximately equal in many cases if the transient particle concentration remains low, whereas the conventional requirement | \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop x\limits^. $\end{document} | = 0 is unimportant. For approximation of exact kinetics by stationary kinetics, however, neither condition is sufficient nor necessary. Practical criterions for recognition of deviations from stationary kinetics are given.     

Freie Objekte und multiplikative Strukturen

Ohne Zusammenfassung     

High-speed ion-pair partition chromatography in pharmaceutical analysis.

Ion-pair chromatography offers attractive possibilities in pharmaceutical analysis. The specificity of the separation systems can be varied over a wide range by appropriate selection of the stationary phase. The choice of a suitable counter-ion can also drastically improve the detection limit, permitting the determination of drug substances in low dosage and possibly of by-products or breakdown products. Ion-pair chromatography of tropane and ergot alkaloids has been investigated using picrate as counter-ion. Alumina, Kieselguhr and various grades of silica gel have been tested as supports. Partition properties studied in a batch procedure have been compared with the actual chromatographic conditions. Columns (10 cm) filled with silical gel (particle size, 5 mum; pore size, 1000 A) show the best performance in the separation of hyoscyamine, scopolamine and ergotamine as picrate ion-pairs. Close control of pH and temperature is essential for reproducible separations. Improvements in detection limits between 100 and 300 times have been observed with these systems. Ion-pair extractions of these alkaloids from dosage forms can be used for sample preparation prior to injection on the the column. This provides an added degree of selectivity and sensitivity.