Electrochemical detection, cationic

Multiple electrodes have been used to obtain selectivity in electrochemical detection. An early example involved the separation of catecholamines from human plasma using a Vydac (The Separation Group Hesperia, CA) SCX cation exchange column eluted with phosphate-EDTA.61 A sensor array using metal oxide-modified surfaces was used with flow injection to analyze multicomponent mixtures of amino acids and sugars.62 An example of the selectivity provided by a multi-electrode system is shown in Figure 2.63... 

Flow injection analysis (F1A). In this technique, introduced by Ruzicka and Hansen, a small amount of sample is injected into a liquid flow (see Fig. 5.16), which apart from being automated is normally continuous, but can include the use of stopped-flow, merging zones extraction techniques in addition to FIA scanning and methods based on intermittent pumping89. The principles of FIA and the versions just mentioned will now be briefly discussed on the basis of the excellent review of Ruzicka and Hansen89 in order to understand the appli-cational possibilities of electrochemical detection in this technique. 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

Assay of ascorbic acid by chromatography with a cationic ion-pairing reagent and electrochemical detection... 

Analysis of adrenaline (adr), noradrenaline (nadr) and dopamine (DA) in urine using (A) An ODS column with ionpairing agent in citrate buffer pH 5.0 with 2% tetrahydrofuran (THF) as the mobile phase (B) a strong cation exchange column with citrate buffer pH 5.0 with 7% THF as the mobile phase. The eluent was monitored by electrochemical detection at a potential of 0.7 V. Methyidopamine (MDA) was used as an internal standard and added to the urine before extraction. Reproduced with permission from J. Chromatogr. Biomed. Apps. (see Reference 10). 

Mobile phases useful for suppressed conductivity detection of anions include sodium hydroxide, potassium hydroxide, and the sodium and potassium salts of weak acids such as boric acid. In nonsuppressed conductivity detection, the ionic components of the mobile phase are chosen so that their conductivities are as different from the conductivity of the analyte as possible. Large ions with poor mobility are often chosen, and borate-gluconate is popular. For cations, dilute solutions of a strong acid are often used for nonsuppressed conductivity detection. For more information on the application of electrochemical detection to inorganic analysis, see Ion Chromatography Principles and Applications by Haddad and Jackson,17 which provides a comprehensive listing of the sample types, analytes, sample pretreatments, columns, and mobile phases that have been used with electrochemical detection. 

High pressure liquid chromatography with electrochemical detection has been used for castanospermine, 1-deoxynqjirimycin, 1-deoxymannojirimycin and swainsonine using a cation exchange column. With this technique it was possible to detect the compounds when added to urine and plant extracts [119],... 

Fig. 11.5 Electrochemical detection of immunocomplexes using on-chip typed cation-exchange chromatography and ferrocene-conjugated antibody. Flow rate 200 pl/min Column volume 30 pi, Buffer change point 350, 650 s...

HPLC strong cation exchange resin electrochemical detection... 

Le Floch, F. Ho, H. A. Harding-Lepage, P. Bedard, M. Neagu-Plesu, R. Leclerc, M. Ferrocene-functionalized cationic polythiophene for the label-free electrochemical detection of DNA. Adv. Mater. 2005, 17, 1251. 

Fig. 5. Two chromatograms obtained by HPLC and electrochemical detection. Left norepinephrine (ME) 85 pg and dopamine (DA) ISO pg, eluted with 0,1 M perchloric acid from a cation-exchange column, detected with a thin-film cell with carbon paste electrode (redrawn). Right perphenazine (PPZ), clopenthixol (CLO), fluphenazine (FPZ) and flupenthixol (FLU), 0.7 ng each, eluted from a methyl-bonded silica column with methanol-aqueous phosphate buffer (pH 7,4), detected with a thin-film cell with glassy carbon electrode. ...

Floch, F. L., Ho, H. A., Leclerc, M. (2006). Label-free electrochemical detection of protein based on a ferrocene-bearing cationic polythiophene and aptamer. Anal Chem 76, 4727-4731. 

Flink. S. Boukamp, B.A. van den Berg. A. van Veggel. F.C.J.M. Reinhoudt. D.N. Electrochemical detection of electrochemically inactive cations by self-assembled monolayers of crown ethers. J. Am. Chem. Soc. 1998, 120. 4652-4657. 

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