Eluent amperometric detectors

FIGURE 3-23 Schematic of a carbon-fiber amperometric detector for capillary electrophoresis A, fused silica capillary B, eluent drop C, stainless steel plate RE, reference electrode WE, working electrode, AE, auxiliary electrode. (Reproduced with permission from reference 58.)... 

In amperometric detectors, the eluent flows by the surface of the glassy carbon electrode in which only 5-15% of the electroactive species is present and this undergoes electrolytic conversion (oxidation or reduction) as the surface area of the electrode is relatively small. 

This analysis is performed using a Dionex Model 12 ion chromatograph with 200pL sample loop, an eluent flow rate of 2.3ml min 1 (30% of full capacity) and a sample pump flow rate of 3.8ml min 1 (50% of full capacity). Pulse damper is installed just before the injection valve to reduce flow pulsation. A Dionex amperometric detector... 

A pump delivers the mobile phase through the chromatographic system. In general, either single-piston or dual-piston pumps are employed. A pulse-free flow of the eluent is necessary for the sensitive UV/Vis and amperometric detectors. Therefore, pulse dampeners are used with single-piston pumps and electronic circuitry with dual-piston pumps. 

The classification of eluents into the two categories mentioned above is only meaningful and necessary in the framework of conductivity detection with its different application requirements. Eluent selection is much easier for applications using spectrophoto-metric or amperometric detectors, respectively. In photometric detection, both the photometric properties of the eluent ions and their chemical properties have to be taken into account nevertheless, a large number of eluents are available. The alkali salts of phosphoric acid, sulfuric acid, and perchloric acid have proved successful, because they all feature a good UV transmittance. In the field of amperometric detection, the choice... 

Cells are classified according to how the working electrode is positioned relative to the flow stream. There are three major configurations tubular, thin layer, and wall jet. The tubular cell (open or packed) with its greater working electrode surface area is used for coulometric detection. The thin layer and wall jet designs are used for amperometric detector cells. In thin layer cells, the eluent flow is in the same plane as... 

Conductivity, direct absorbance or a differential refractometer are the most common forms of detection for lEC, PAD and ELSD. A pulsed amperometric detector (PAD) or, more recently, an evaporative light-scattering detector (ELSD) is appropriate for detection of carbohydrates. Both non-suppressed and suppressed conductivity have been used extensively. The need to incorporate a low concentration of a strong acid into the eluent has been an impediment to direct conductivity detection. 

In amperometric detector cells, the analyte is transported to the working electrode surface by diffusion as well as convection migration is suppressed by the supporting electrolyte. Electrolyte concentrations in the eluent of 0.01 - 0.1 mol/L are sufficient. Several cell geometries employing solid electrodes have been designed and tested. The two types shown in Figure 18 are the most commonly used. 

When using amperometric detectors, the com-]x> ition of the eluent should remain as constant as possible (isocratic elution). If gradient elution is used to reduce the analysis time, then base line drift is observed, resulting in a loss of sensitivity. This is particularly so with glassy carbon working electrodes and is due to the slow adjustment of the electrochemical equilibrium at the electrode surface to the changing eluent composition. 

An ECD measures the current generated by electroactive analytes in the HPLC eluent between electrodes in the flow cell. It offers sensitive detection (pg levels) of catecholamines, neurotransmitters, sugars, glycoproteins, and compounds containing phenolic, hydroxyl, amino, diazo, or nitro functional groups. The detector can be the amperometric, pulsed-amperometric, or coulometric type, with the electrodes made from vitreous or glassy carbon, silver, gold, or platinum, operated in the oxidative or reductive mode. Manufacturers include BSA, ESA, and Shimadzu. 

Adamic and Bartak [6] used high pressure aqueous size exclusion chromatography with reverse pulse amperometric detection to separate copper(II) complexes of poly(amino carboxylic acids), catechol and fulvic acids. The commercially available size exclusion chromatography columns were tested. Columns were eluted with copper(II) complexes of poly(aminocarboxylic acids), citric acids, catechol and water derived fulvic acid. The eluent contained copper(II) to prevent dissociation of the labile metal complexes. Reverse pulse electrochemical measurements were made to minimise oxygen interferences at the detector. Resolution of a mixture of DTP A, EDTA and NTA copper complexes was approximately the same on one size exclusion chromatography column as on Sephadex... 

Conductivity detection is the most popular for ion chromatography. Although UV detection is often overlooked, it can be quite powerful. Amperometric detection, for example, offers selectivity and sensitivity, in many cases unsurpassed. The optimum eluent separation pH may not be the optimum pH for detection. An anion may be separated but not detected. This is especially true for some weak acid anions and suppressed conductivity detection. Chapter 4 discusses the use of different detectors for IC. 

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