Electrochemical detectors – tailormade techniques for liquid chromatography and capillary electrophoresis?

Electrochemical detectors for liquid chromatography and capillary electrophoresis are reviewed with special emphasis on electrode materials that allow the amperometric detection of otherwise non-electroactive compounds such as aliphatic alcohols, carbohydrates or amino acids. Noble metal electrodes can catalyze the oxidation of aliphatic compounds in alkaline media if multistep potential-time waveforms are employed. Various metal and metal oxide electrodes such as Ni, Cu or Co allow the detection of carbohydrates and similar compounds under constant potential conditions. Metallic copper electrodes operating in an amperometric mode or in a potentiometric mode can also serve as selective detectors for complexing species. A range of applications in combination with chromatography and electrophoresis is summarized. The current state of electrochemical detectors indicates that both amperometric and potentiometric detectors are on the verge of becoming tailormade detectors for micro-separation techniques.     

A method for the determination of the hepatic enzyme activity catalyzing bile acid acyl glucuronide formation by high-performance liquid chromatography with pulsed amperometric detection

A method for the determination of the activity of hepatic glucuronyltransferase catalyzing formation of bile acid 24-glucuronides using high-performance liquid chromatography (HPLC) with pulsed amperometric detection (PAD) has been developed. Bile acid 24-glucuronides were simultaneously separated on a semimicrobore column, Capcell Pak C18UG120, using 20 mM ammonium phosphate (pH 6.0)-acetonitrile (27:10 and 16:10) as the mobile phase in the stepwise gradient elution mode. A 1 M potassium hydroxide solution for the hydrolysis of the 24-glucuronides, which liberates the corresponding bile acids and glucuronic acid, was mixed with the mobile phase in a post-column mode, and the resulting eluant was heated at 90 degrees C, the 24-glucuronides being monitored using a pulsed amperometric detector; the limit of detection was 10 ng. The proposed method was applied to the determination of the hepatic enzyme activity catalyzing bile acid 24-glucuronide formation and the result exhibited the efficient 24-glucuronide formation of the monohydroxylated bile acid, lithocholic acid.     

Comparison of amperometric and UV-spectrophotometric monitoring in the HPLC analysis of pesticides

Summary Studies have been carried out to compare different detection techniques including amperometric detection in the reductive and oxidative mode and UV detection following the HPLC separation of pesticides. By the example of 4,6-dinitro-o-cresol (DNOC), 2-sec-butyl-4,6-dinitrophenol (Dinoseb) and 2-tert-butyl-4,6-dinitrophenol (Dinoterb), it is shown that the electrochemical detection exhibits higher sensitivities than the UV-technique. The detection limits are in case of the oxidative amperometric technique 0.1 ng for all pesticides, in the reductive mode 0.3 ng for DNOC and 1 ng for Dinoseb and Dinoterb. By UV-monitoring the detection limits were found to be 2 ng for DNOC and 24 ng for Dinoseb and Dinoterb.     

Evaluation of the potentiometric detection of acetylcholine and other neurotransmitters in capillary electrophoresis

It is demonstrated that acetylcholine may be determined potentiometrically in capillary electrophoresis down to 6 x 10(-7) M with a miniature coated-wire ion selective electrode. The more established amperometric method is not possible for this compound. Similarly, 4-aminobutyric acid (GABA) is not accessible amperometrically; although potentiometric detection was possible, the sensitivity is not good. Other neurotransmitters may be determined by either method, but in these cases amperometric determination is generally favorable. For serotonin the potentiometric detection limit was 4 x 10(-5) M, while for the amperometric method a detection limit of 10(-7) M was found.     

The potentiometric and amperometric determination of silver with mercaptophenylthiothiadiazolone and mercaptobenzthiazole

Summary The previously described gravimetric determinations of silver were used for the potentiometric and amperometric determinations of silver in the presence of interfering cations. The potentiometric titration of silver with MFTD (MBT) is a simple matter, if enough complexone is present, in both ammoniacal and neutral solutions. The amperometric determination of silver with MFTD (MBT) is carried out in neutral solution, containing complexone, with rotated platinum electrode and mercury-mercuric iodide half-cell, short-circuited through a microammeter. The only interfering elements are mercury, gold, and some of the platinum metals.     

Determination of Organic Peroxides by High Performance Liquid Chroma-Tography with Electrochemical Detection

Abstract

The application of electrochemical detection to the high performance liquid chromatographic determination of organic peroxides has been studied. The use of a buffered mobile phase was found to be critical to the successful analysis of samples containing hydroperoxides. Using amperometric detection, mixtures of peroxide containing compounds were readily determined. The sensitivity of the amperometric detector was in the one nanogram range for both benzoyl peroxide and cumene hydroperoxide. Polar-ographic detection was found to be a highly reproducible method for the analysis of samples containing peroxides as components of mixtures in the range of 5-2000 ng. The peroxide containing compounds determined in this manner were t-butyl hydroperoxide, cumene hydroperoxide, and 13-hydroperoxy-9(Z)-11(E)-octadecadien-oic acid. The polarographic detection system was used to obtain observed half-wave potentials for the peroxides under different chromatographic conditions. These observations correlated closely with literature results on the polarography of these compounds.     

Determination of traces of osmium by the catalysed hydrogen peroxide-cyanocuprate(I) reaction

The reaction between hydrogen peroxide and a mixture of cyanocuprate(I) species at pH 11.2 is selectively catalysed by traces of osmium. With potentiometric or amperometric monitoring, osmium concentrations up to 1 ng ml can be determined, with a lowest determinable concentration of 0.03 ng ml . In the presence of luminol a Landolt-type reaction proceeds and visual or instrumental monitoring of the chemiluminescence can be used. A large number of other cations can be tolerated. The kinetics, mechanism, rate constants and Arrhenius parameters have been investigated.     

Comparison between amperometric and true potentiometric end-point detection in the determination of water by the Karl Fischer method

A rapid and sensitive method using true potentiometric end-point detection has been developed and compared with the conventional amperometric method for Karl Fischer determination of water. The effect of the sulphur dioxide concentration on the shape of the titration curve is shown. By using kinetic data it was possible to calculate the course of titrations and make comparisons with those found experimentally. The results prove that the main reaction is the slow step, both in the amperometric and the potentiometric method. Results obtained in the standardization of the Karl Fischer reagent showed that the potentiometric method, including titration to a preselected potential, gave a standard deviation of 0.001(1) mg of water per ml, the amperometric method using extrapolation 0.002(4) mg of water per ml and the amperometric titration to a pre-selected diffusion current 0.004(7) mg of water per ml. Theories and results dealing with dilution effects are presented. The time of analysis was 1-1.5 min for the potentiometric and 4-5 min for the amperometric method using extrapolation.     

Electrolytically generated silver(II) as a coulometric titrant

The use of electrolytically generated silver(II) as a coulometric titrant has been studied. Difficulties arising from the current efficiency for generation of silver (II) at a platinum or gold electrode and from the reduction of silver(II) by water, have been overcome. The precautions necessary for accurate titrations of oxalic acid, cerium(III), arsenic(III) and vanadium(IV) by amperometric or potentiometric methods are detailed. Manganese and chromium could not be determined directly. Substances which reduce nitric acid could be determined if care was taken.     

Determination of Aztreonam by liquid Chromatography with UV and Amperometric Detection

Abstract

A comparative study of UV and amperometric detection of aztreonam after HPLC separation is presented. At pH 2.0 and a detection potential of +1.15 V (vs. Ag/AgCl), the detection limits with amperometric detection are about two times higher (3–5 ng) than those obtained with UV detection (1–3 ng) for aztreonam and its main decomposition products, the E-isomer and open-ring aztreonam. With the advantage of specificity for the aminothiazole group of the aztreonam molecule, amperometry can be used as an alternative or complementary mode to UV detection for the determination of aztreonam in injectable formulations and in human serum.     

The automatic amperometric and potentiometric microtitration of pharmaceutically important sulfanilamide derivatives by diazotization with nitrite

The automatic amperometric and potentiometric microtitration of pharmaceutically important sulfanilamide derivatives by diazotization with nitrite is described. Optimal conditions for diazotization and amperometric indication are discussed. A new cathodic amperometric indication of nitrite in M hydrochloric acid-25% ($\text{w}\text{v}$) of potassium bromide medium is proposed; the electrode processes involved are interpreted on the basis of current-potential curves. The effects of the hydrochloric acid and bromide concentrations, and of temperature on the limiting current are described. Sulfamethazine, sulfadimethoxine, sulfamethoxypyridazine and sulfamethoxypyrazine were titrated amperometrically in the range 10^?3–10^?6 M. Potentiometric titrations of these sulfanilamides were possible in the range 10^?3–10^?4 M.     

Design and use of electrochemical sensors in enantioselective high throughput screening of drugs. A minireview

The importance of reliable detection systems for enantiomeric assays increases with the necessity of high throughput screening analysis of raw materials for the pharmaceutical industry. The utilization of electrochemical sensors in enantioselective analysis is an accurate and precise alternative to chromatographic techniques. The reliability of the response characteristics as well as of the analytical information obtained by using electrochemical sensors is strictly correlated with the design of the sensors. The designs evaluated for sensors have been based on PVC, imprinting polymers and carbon paste matrices. Among these, carbon paste sensors have been the most reliable and have been utilized for the construction of potentiometric, enantioselective membrane electrodes as well as for amperometric biosensors, and immunosensors. There are two ways to use the electrochemical sensors in enantioselective screening analysis: selective binding and catalyst selectivity. A molecule with a special chemical architecture is required for selective binding: a lock for a key. The high reliability of analytical information obtained using these sensors has made possible the automation of potentiometric and amperometric techniques by integration of enantioselective sensors as detectors in flow injection analysis and sequential injection analysis techniques.     

Amperometric and constant-current potentiometric determinations of the Landolt effect

An amperometric method with one platinum indicator electrode as well as the constant-current potentiometric method with one and two indicator electrodes were successfully applied to the Landolt effect determination by means of the hydrogen peroxide-iodideascorbic acid and bromate-bromide-ascorbic acid indicator reactions. The possibilities of application of these methods are illustrated by the determination of small amounts of iron(III), vanadium(V), and molybdenum(VI) with satisfactory accuracy and precision.     

Simultaneous Determination of Catecholamines and Related Metabolites by Capillary Electrophoresis with Amperometric Detection

An electrophoretic method was developed for the determination of several important catecholamine markers, namely norepinephrine, epinephrine, dopamine, metanephrine, vanillylmandelic acid and homovanillic acid in urine samples. Under the optimum conditions, the six marker compounds could be well separated with the major coexisting interference compound uric acid within 24 min at a separation voltage of 16 kV in a borate running buffer (80 mmol/L, pH=9.48). Highly linear response can be obtained over three orders of magnitude for the above markers with the low limits of detection ranging from 1.0 ng/mL to 5.0 ng/mL(S/N=3). The proposed capillary electrophoresis with amperometric detection(CE-AD) method has been used to simultaneously determine the six catecholamine markers in urine samples of healthy volunteers and patients suffering from different diseases avoiding redundant measurements and high assay cost; and the electrochemical profiles can suggest more diagnostic information of multiple diseases, which provides a promising and convenient entry into primary diagnoses of several clinical diseases.     

光电化学传感器的研究进展

光电化学传感器以光作为激发源,以光电流或光电压作为检测信号,具有响应快速、灵敏度高、设备简单等特点,目前已在环境、食品、医学等多个领域的分析测试中得到广泛应用。该文阐述了光电转换材料与光电化学传感器的制备方法,介绍了光电化学传感器的原理和分类。光电化学传感器包含光寻址电位型传感器和电流型光电化学传感器,其中,电流型光电化学传感器由于优良的光电性能、检出限低、所需材料低廉且易加工等优势而被广泛应用。文中着重介绍了电流型光电化学传感器在金属离子、有机污染物、核酸、蛋白质、细胞等方面的应用,并对光电化学传感器的发展前景进行了展望。     

Operational parameters of voltammetric high-performance liquid chromatographic detectors with copper electrodes and application to a determination of some fodder biofactors

Two versions of an amperometric detector with a copper working electrode have been constructed and tested for high-performance liquid chromatography (HPLC). The performance of the detectors was studied using selected amino acids. The dependence of the detector response on the mobile phase flow-rate was studied in the range common in both macro- and microcolumn HPLC (5 microliter/min to 1.0 ml/min). It has been found that the detection sensitivity generally increases with decreasing flow-rate, i.e., the detector response is governed by the rate of the complexation reaction between the cupric ions and the solutes. This fact makes amperometric detection with a copper electrode especially useful for microcolumn separations. For all 20 amino acids studied, calibration curve parameters and detection limits have been determined; the latter vary from 0.4 to 18 ng in the injected volume. The amino acids can also be sensitively detected in a medium of 0.1-1.0 M ammonia, which is promising for the use of strong anion exchangers in amino acid separations. Choline can also be detected at a copper electrode, with a detection limit of 40 ng. An HPLC method with amperometric detection at a copper electrode has been developed for the determination of lysine, methionine and choline in fodder biofactors.     

Capillary batch injection analysis and capillary flow injection analysis with electrochemical detection: a comparative study of both methods

Capillary batch injection analysis (CBIA) and capillary flow injection analysis (CFIA) in combination with electrochemical detection as well as optical detection methods were studied and compared with respect to their performance. Despite the differences in technical equipment both techniques share the same idea of reproducible transport and washout of nanolitre samples over sensing surfaces. Thus the same electrochemical flow cell can be used for both CBIA and CFIA. The amperometric and potentiometric CBIA responses were studied under various experimental conditions in order to optimise the CBIA set-up. In particular, the density of the sample solution relative to that of the cell electrolyte had a remarkable effect on the hydrodynamic characteristics of CBIA. Dispersion in CFIA was investigated using on column UV-detection for electroosmotic flow (EOF) conditions as well as for gravity flow conditions. It is demonstrated for a 75 μm capillary that the relative band broadening of the sample plug under gravity flow is only about twice as large as under EOF. Furthermore, dispersion in a system that involves a chemical reaction between the sample and the carrier solution, namely CrO₇ ^2– and Fe²⁺ has been investigated by amperometric detection and exploited for the determination of dichromate microsamples.     

Determination of L-ascorbic acid in fruit and vegetable juices by flow injection with immobilised ascorbate oxidase

Ascorbate oxidase was immobilised on cyanogen bromide activated-Sepharose 4B and incorporated in a flow-injection system with amperometric detection at a glassy carbon electrode at +0.6 V. On passage through the immobilised ascorbate oxidase a fraction of the L-ascorbic acid was converted into dehydroascorbic acid and the decrease in signal was measured. This could be directly related to the amount of L-ascorbic acid present. The calibration graph was linear over the range 0-400 ng ml(-1) with a correlation coefficient of 0.9994. The detection limit (2 sigma) in phosphate buffer (0.08 M, pH 5.5) was 4.0 ng ml(-1). The relative standard deviation for a 200 ng ml(-1) standard was 1.0% (n = 10) and the sampling throughput was 30 samples h(-1). The method was used for the simple and rapid determination of L-ascorbic acid in fruit and vegetable juice.     

Non-aqueous redox determination of ascorbic acid with copper (II)

Hydrated copper (II) perchlorate (in acetonitrile) has been used for the direct visual and potentiometric determination of ascorbic acid in acetic acid-acetonitrile media. Diphenylamine and diphenylbenzidine are suitable indicators. A bright platinum wire is used as indicator electrode and a modified calomel or an antimony electrode as reference electrode for the potentiometric titration. Ascorbic acid is oxidized to dehydroascorbic acid. The proposed method is simple, accurate and reliable. The reverse titration also works well.