An electrochemical detector with a dropping mercury electrode for high-performance liquid chromatography

The electrochemical flow-through cell described has an active volume of less than 1 μl, and incorporates a dropping mercury electrode with drop times of about 0.05 s. Its performance as a detector for high-performance liquid chromatography is assessed for p-nitrophenol and nitrobenzene. The detection limits are 4–5 ng; the relative standard deviation of the peak height is better than 5% for the range 5–150 ng. The dependence of the response on flow rate, mercury pressure and drop time is described.     

A flow detector based on square-wave polarography at the dropping mercury electrode

Square-wave polarography (s.w.p.) at the DME has been adapted for detection in various analytical flow systems. The combination of high sensitivity with rapid potential scan rates results in detector characteristics significantly superior to those previously reported for techniques involving constant applied potential. In an automated flow system for the analysis of discrete samples, s.w.p. allows sensitive and reproducible multi-component sample analysis, at a sampling rate of 22.5 samples per hour (at a 1:1 sample/wash ratio and with relatively low sample volumes). The electrochemical selectivity of the detector may be exploited for monitoring Chromatographic column effluents, in cases where the chromatographic separation is incomplete: species eluted simultaneously but having different reduction potentials can be determined with satisfactory sensitivity. The in-situ monitoring system based on s.w.p. allows rapid simultaneous determinations (~ 300 per hour) of a number of contaminants present at the sub-ppm level; because of its long-term stability and reproducibility, it seems well suited for continuous contaminant control.     

Effect of methylene blue on the capacity of the dropping mercury electrode

Summary The effect of methylene blue under different conditions of p_H, concentration and frequency of the a. c. ripple used has been investigated on the capacity of the dropping mercury electrode. All the peaks caused by methylene blue at different p_H values have been interpreted as due to the reduction of the methylene blue, as indicated by the comprehensive study of the peaks. The halt at about 0.58 Volt at p_H 5.8 is due to desorption which develops into a satellite peak if potassium nitrate is used as indifferent electrolyte. It is found that the peak at 1.35 Volt at p_H 12.3, first increases with increase in concentration and then diminishes at still higher concentrations. The causes of this have been discussed. The effect of frequency reveals that the reduction reactions involved are rather slow.

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Zusammenfassung Der Effekt von Methylenblau unter verschiedenen Verh?ltnissen von Wasserstoff-Ionen-Konzentration, Konzentration und Frequenz des überlagerten Wechselstroms auf die Kapazit?t der Quecksilbertropfelektrode war Gegenstand der Untersuchung. Alle dem Methylenblau zuzuschreibenden Maxima bei verschiedenen p_H- Werten lassen sich interpretieren durch eine Reduktion des Methylenblau, wie die eingehende Untersuchung der Maxima zeigt. Der Konstanzbereich bei etwa 0,58 Volt und p_H 5,8 ist einer Desorption zuzuschreiben, die ein Begleitmaximum entwickelt, wenn Kaliumnitrat als indifferenter Elektrolyt verwendet wird. Das Maximum bei 1,35 Volt und p_H 12,3 w?chst zun?chst bei Steigerung der Konzentration und nimmt dann bei noch h?heren Konzentrationen wieder ab. Die Gründe hierfür werden diskutiert. Der Frequenzeffekt zeigt an, da? die Reduktionsreaktionen ziemlich niedrig sind.

     

Optimization of the operational parameters of the supported mercury drop electrode detector in high-performance liquid chromatography

The pendent (or supported) mercury drop electrode detector for high-performance liquid chromatography is examined with reference to the effects on its performance of solvent flow, the residual current, the flow cell characteristics, the size, shape and stability of the mercury drop, and some other factors. The signal-to-noise ratio varies widely with respect to these factors, whose optimization is discussed. Under the optimized conditions, picogram levels of organic nitro and nitrate compounds can be detected.     

The polarographic behaviour of tungstate at the dropping mercury electrode

Summary The polarographic behaviour of W(VI) is investigated in some supporting electrolytes of varying H⁺ concentration. In HCl solution the reduction yields two waves; the first one is essentially reversible and corresponds to the process W(VI) W(V). The second one is irreversible and composed of two strongly overlapping waves representing the stepwise reduction of W(V) to W(III). In H₂SO₄ solution, three irreversible waves of almost equal heights are observed. The reduction in H₃PO₄ solution indicated that different types of phosphotungstic acid (PTA) are formed which change to either a mixture of 9- and 12-PTA or to 9-PTA only, depending on the H₃PO₄ concentration. In HCl or H₃PO₄ solutions i _l is a linear function of the W(VI) concentration.

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Zusammenfassung Das polarographische Verhalten von Wolframat ist in einigen Trägerlösungen verschiedener H⁺-Konzentration untersucht worden. In HCl-Lösung ergibt die Reduktion zwei Stufen. Die erste Stufe ist reversibel und stellt die Reduktion W(VI) W(V) dar, während die zweite dem Reduktionsverlauf W(V) W(III) entspricht und aus zwei stark über-lappenden Stufen besteht. In H₂SO₄-Lösung sind drei Stufen mit fast gleichen Höhen zu erkennen. In H₃PO₄-Lösung scheint aus dem polarographischen Verhalten von Wolframat hervorzugehen, daß verschiedene Formen der Wolframatophosphorsäure (WPS) gebildet werden, welche sich entweder in eine Mischung der 9- und 12-WPS oder nur in die 9-WPS umwandlen. Die Art der Umwandlung hängt von der H₃PO₄-Konzentration ab. Der Grenzstrom in HCl- und H₃PO₄-Lösungen ist eine lineare Funktion der W(VI)-Konzentration.

     

The study of mutual coagulation of colloids by the dropping mercury electrode capacity measurements

Summary It has been shown that the dropping mercury electrode capacity measurements can be employed for studying the mutual coagulation of congo red or congo rubin with methylene blue. It has also been shown that the congo red or congo rubin is never completely removed by coagulation by methylene blue. On the other hand the end point only indicates the region of maximum mutual coagulation. It is of interest to study this technique for determining the dye number of sugar house-products.

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Zusammenfassung Messungen der Kapazit?t der Quecksilberelektrode gestatten die Verfolgung der wechselseitigen Koagulation von Kongorot und Kongorubin mit Methylenblau. Es ergibt sich, da? Kongorot und Kongorubin durch Methylenblau niemals vollst?ndig ausgeflockt werden. Andererseits zeigt der Endpunkt nur den Bereich der maximalen gegenseitigen Flockung. Es bestünde Interesse, diese Technik zur Bestimmung der Farbnummer für Zucker-Haus-Erzeugnisse anzuwenden.

     

Polarography of thiodiglycolic acid and thiodipropionic acid at the dropping mercury electrode

The polarographic behaviour of thiodiglycolic acid (R?S?R) and thiodipropionic acid (R?CH₂?S?CH₂R) at the DME, has been investigated in presence of 0.1 M KCl and 0.01% thymol with respect to the effects of changes in pH, concentration, drop time, and temperature on the wave characteristics. Well-defined irreversible cathodic waves were obtained in the pH range 2.40 to 4.76 for R?S?R and 2.3 to 4.98 for R?CH₂?S?CH₂R. At lower (<2.0) and higher (>5) pH values, ill-defined waves were obtained. The mechanism of the electrode process in aqueous solutions has also been elucidated.     

The effect of size and location of adsorbed intermediates in the double layer on the tafel slope the reduction of hydroxylamine on the dropping mercury electrode

The reduction of hydroxylamine on the dropping mercury electrode was studied. Kinetic parameters were determined from measurements at the foot of the polarographic wave. Tafel plots were extended to higher current densities with the use of the Koutecký correction.This molecule was chosen as an example of a relatively small species for which the effect of competition with water is small and the importance of the location of the adsorbed intermediate in the double layer can be demonstrated. The experimentally observed value of b = 96 ± 3 mV (combined with reaction orders of unity both with respect to hydroxylamine and hydrogen ions) was found to be consistent with the notion of an adsorbed intermediate whose reduction is brought about by a potential difference φ_M - φ_x, where φ_x has an intermediate value between φ_M and φ_s. These results were compared with earlier results found in a study of the reduction of nitroalkanes under similar conditions.The advantages of making measurements on the DME at the foot of the polarographic wave are emphasized.     

Computer calculation of activation energies of irreversible electrode processes at a dropping mercury electrode

A computer program has been written for calculation of the activation energies of irreversible electrode processes at a dropping mercury electrode, based on the variation of current and half-wave potential with temperature. The program calculates the half-wave potential, the transfer coefficient, and the activation energies of the diffusion process and the irreversible electrode process. It has been written in both ALGOL and FORTRAN.     

A comparison of different HPLC detector designs based on the dropping mercury electrode principle

Summary Different designs of dropping-mercury electrode (DME) detectors for HPLC were tested and compared to solid-state and UV detectors. The time constants and the corresponding response volumes were measured. A qualitative explanation for the flow pattern of these cells was attempted. With nitrobenzene as a test substance the detection limits and reproducibilities of the best DME detectors were comparable to those of a good UV detector. The linear dynamic ranges and response times for optimally designed electroanalytical cells are better than for UV detectors.

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Zusammenfassung Vergleiche zwischen diesen Detektoren sowie einem Festkörperelektroden und einem UV-Detektorentyp wurden angestellt. Zeitkonstanten und Detektionsvolumen wurden gemessen und eine Erklärung für die Flußprofile gegeben. Die mit Nitrobenzol gemessenen Detektionsgrenzen und Reproduzierbarkeiten der besten Typen sind vergleichbar mit denen von guten UV-Detektoren. Der dynamisch lineare Bereich und die Ansprechzeit sind für optimal gebaute elektro-analytische Detektoren deutlich besser als für UV-Detektoren.

     

Detection of Nepsilon-monomethyllysine using high-performance liquid chromatography and high-performance liquid chromatography-mass spectrometry

Nepsilon-Monomethyllysine was identified in the serum, urine, brain, and liver samples of rats treated per os with L-deprenyl. The identification procedure included reaction with Fmoc chloride, clean-up, and analysis using HPLC-UV-MS. Oral administration of (-)-N-14C-methyl-N-propynyl(2-phenyl-1-methyl)ethylammonium hydrochloride L-deprenyl) to rats resulted in transfer of the radiolabelled methyl group to the Nepsilon-amino group of the endogenous lysine. The radiolabelled Nepsilon-monomethyllysine was urinary eliminated together with the other radiolabelled deprenyl metabolites, such as deprenyl-N-oxide and methamphetamine. The presence of Nepsilon-monomethyllysine has also been traced, and its concentrations were compared in the serum, liver and brain of rats subjected to L-deprenyl treatment. Methyl group transfer from the L-deprenyl to endogenous compounds; and the urinary elimination of their products may offer a vital way to eliminate or to decrease the degree of drug transmethylation to the lysine constituents of blood vessels' proteins.