Pulsed Electrochemical Detection PED

The most common electrode materials in PED electrochemical detection are Au, Pt, and carbon. Electronic resonance in aromatic molecules stabilizes freeradical intermediate products of anodic oxidations, and, as a consequence, the activation barrier for the electrochemical reaction is lowered significantly. Even for reversible redox couples that are considered to be weU-behaved, dc amperometry is often accompanied by the practice of disassembling the electrochemical ceU and mechanically polishing the working electrode daily . In this maimer, fouling from nonspecific adsorption processes and/or mechanistic consequences is physically removed from the electrode surface. 

In contrast to aromatic moieties, absence of w-resonance for aliphatic compounds results in very low oxidation rates even though the reactions may be favored thermodynamically. StabiUzation of free-radical products from aliphatic compounds can be achieved alternatively via their adsorption to the surface of noble metal electrodes. Unfortunately, adsorption of organic molecules and free radicals also has the consequence of fouling of the electrode and loss of its activity [57]. The historical perspective of nonreactivity for ahphatic compounds at noble metal electrodes can be attributed to surface fouling as a result of high, but transient, catalytic activity. An alternative approach is to combine electrochemical detection with on-hne cleaning. Hence, in order to maintain uniform and reproducible electrode activity at noble metal electrodes for polar aliphatic compounds. 

Although increased sensitivity and reproducibility has also been reported for pulsed potential cleaning at carbon electrodes by several researchers [60, 61], these electrodes have not generally been successful for the detection of polar aliphatic compounds. This effect is attributable to the absence of appropriate electro-catalytic properties of carbon surfaces to support the anodic oxygen-transfer reaction mechanisms of polar aliphatic compounds. 

Most frequently, PED is appUed at Au electrodes under alkaline conditions 

Voltammetric resolution of complex mixtures is futile, since electrocatalytic-based detection of various members within a class of compounds is controlled primarily by the dependence of the catalytic surface state on the electrode potential rather than by the redox potentials (E°) of the reactants. Therefore, general selectivity is achieved via chromatographic separation prior to electrocatalytic detection. This does not preclude some limited selectivity from control of detection parameters. 

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HPAEC analyses were carried out to determine the oligomeric products released from various pectic substrates after depolymerization by the PL isoenzymes. Action pattern analyses for the concerted action of PL isoenzymes utilized 68% esterified pectin as substrate. One-ml reaction mixtures in a buffer system as detailed in section 2.2. comprising 0.5% (w/v) substrate and 5 U of enzyme were incubated for 30 s to 18 h, and then thermoinactivated. Samples of 750 pi were applied to a Carbopac PA-1 (Dionex) column before the carbohydrates were eluted over a period of 70 min using a gradient of 0.2 M KOH, 0.05 M K-acetate to 0.2 M KOH, 0.7 M K-acetate. Detection employed a Pulsed Electrochemical Detector (PED, Dionex) in the integrated amperometry mode according to the manufacturer s recommendations. 

The disadvantages listed above are either alleviated or greatly diminished by the use of Integrated Pulsed Electrochemical Detection (IPAD), which is also a form of PED. Figure 4.4 shows the IPAD waveform and Table 4.7 describes the times and potentials that are generally used for amino acid detection. Here, the electrode current is integrated electronically throughout a rapid cycUc or square wave... 

GC, for their direct detection of carbohydrates, with high sensitivity and specificity and without the need for sample derivatization. Specifically, the technique of high-performance anion-exchange chromatography with pulsed electrochemical detection (HPAEC-PED) is fast becoming the technique of choice for this. HPLC techniques with R1 detection are greatly limited in their ability to analyze complex carbohydrates in foods. 

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