Reversed-phase liquid amperometric detection

Before considering the special requirements for automated on-line determination of metals from industrial effluents, it is worthwhile examining the features of standard laboratory procedures associated with the off-line determination of copper as a dithiocarbamate complex by liquid chromatography with electrochemical detection. The off-line determination of copper as its diethyldithiocarbamate complex in aqueous samples, zinc plant electrol3d e, and urine have been described [3, 7, 10] using reverse phase liquid chromatography with amperometric detection. A standard instrumental configuration for the conventional laboratory off-line method as used in these studies is depicted in Fig. 7.2. 

F Dionisi, J Prodolliet, E Tagliaferri. Assessment of olive oil adulteration by reversed-phase high-performance liquid chromatography/amperometric detection of tocopherols and tocotrienols. J Am Oil Chem Soc 72 1505-1511, 1995. 

Alawi [319] has discussed an indirect method for the determination of nitrite and nitrate in surface, ground and rain water by reaction with excess phenol (nitrite ions first being oxidised to nitrate) and extraction of the o-nitrophenol produced, followed by separation on a reversed phase high performance liquid chromatography column with amperometric detection in the reduction mode. Recoveries were 82% for nitrate and 77% for nitrite in the concentration range 10-lOOOpg L 1. The method is claimed to be free of interferences from other ions. 

We believe that this symposium on Recent Advances in Pesticide Analytical Methodology fulfills this plea. High-performance liquid chromatography (HPLC) has made the greatest advances. Chapters on HPLC cover subjects on metabolism studies automation of HPLC evaluation of LC columns the effect of the mobile phase on reversed-phase chromatography the electrochemical or amperometric detector and fluorogenic detection. 

The determination of catecholamines requires a highly sensitive and selective assay procedure capable of measuring very low levels of catecholamines that may be present. In past years, a number of methods have been reported for measurement of catecholamines in both plasma and body tissues. A few of these papers have reported simultaneous measurement of more than two catecholamine analytes. One of them utilized Used UV for endpoint detection and the samples were chromatographed on a reversed-phase phenyl analytical column. The procedure was slow and cumbersome because ofdue to the use of a complicated liquid-liquid extraction and each chromatographic run lasted more than 25 min with a detection Umit of 5-10 ng on-column. Other sensitive HPLC methods reported in the literature use electrochemical detection with detection limits 12, 6, 12, 18, and 12 pg for noradrenaline, dopamine, serotonin, 5-hydroxyindoleace-tic acid, and homovanillic acid, respectively. The method used very a complicated mobile phase in terms of its composition while whilst the low pH of 3.1 used might jeopardize the chemical stability of the column. Analysis time was approximately 30 min. Recently reported HPLC methods utilize amperometric end-point detection. 

Tan, Y.K. Soldin, S.J. Determination of salbutamol in human serum by reversed-phase high-performance liquid chromatography with amperometric detection. J.Chromatogr., 1984, 311, 311-317... 

I. Svagrova, K. Stulik, V. Pacakova, P. Caliceti and F.M. Veronese, Determination of narciclasine in serum by reversed-phase high-performance liquid chromatography comparison of amperometric, ultraviolet photometric and fluorescence detection, J. Chromatogr., 1991, 563, 95-102. 

Z Liu, T Li, J Li, E Wang. Detection of menadione sodium bisulfite (vitamin Kj) by reversed-phase high performance liquid chromatography with series dual-electrode amperometric detector. Anal Chim Acta 338 57-62, 1997. 

This applies to another possible way of non-electrolytic accumuladcm, based on spontaneous transfer and distribution of a species between two liquid phases, typically from an aqueous solution into a non-polar medium, when the role of exchanged species can be attributed to neutral molecules or ion associates with compensated charge. This is accompUshed with the liquid binder in carlxMi paste mixtures or by liquid membranes coating the tips of potentiometric or amperometric sensors. In both cases, extraction (Eq. 5.12a) and the reverse process, re-extraction, take place in the electrode bulk ( ELB ) before the release from the interior to the aqueous phase ( AQ ) and the final disintegration during detection (Eq. 5.12b) ... 

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