Pulse amperometry

For detectors without membranes, the application of a potential pulse from a region of electroinactivity to a region of electroactivity has some advantages. The duty cycle of the pulse can be arranged such that the electrode spends the majority of the time at the electroinactive potential. Under these circumstances, electrode reactions that give rise to products that poison the electrode are of diminished significance, simply because the deleterious products are produced at a much reduced rate. Secondly, the natural processes of convection can redistribute the electroactive species whilst the electrode is at the rest potential, and thus limit the spread of the diffusion layer into the bulk 

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The use of non-inert and chemically modified electrodes and other strategies for the detection of species that are difficult to analyze with the normal electrode materials have been reviewed.55 Photosensitization prior to amperometric detection is another tactic that has proved useful for the analysis of substances that are normally considered to be electrochemically inert.56 The use of pulsed amperometry has recently been reviewed.57... 

Specifications for modem detectors in HPLC are given by Hanai [538] and comprise spectroscopic detectors (UV, F, FUR, Raman, RID, ICP, AAS, AES), electrochemical detectors (polarography, coulometry, (pulsed) amperometry, conductivity), mass spectromet-ric and other devices (FID, ECD, ELSD, ESR, NMR). None of these detectors meets all the requirement criteria of Table 4.40. The four most commonly used HPLC detectors are UV (80%), electrochemical, fluorescence and refractive index detectors. As these detectors are several orders of magnitude less sensitive than their GC counterparts, sensor contamination is not so severe, and... 

F. Gonon, M.F. Suaud-Chagny, and M. Buda, Fast in vivo monitoring of electrically evoked dopamine release by differential pulse amperometry with untreated carbon fibre electrodes, in Proceedings of Satellite Symposium on Neuroscience and Technology (A. Dittmar and J.C. Froment, eds), p. 215. Lyon... 

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

NaOH H20 Anion-exchange resin Ion exchange Pulsed amperometry High selectivity for mono-, di-, and trisaccharides high sensitivity Need for specific instrumentation instability of some products at basic pHs... 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

Moreover, a thick-film sensor array suitable for automation combined to readout based on intermittent pulse amperometry (IPA) has been commercialised by Alderon Biosciences [27,28]. These genosensors and the readout instruments provide a simple, accurate and inexpensive platform for patient diagnosis. 

Martens, D. A., and Loeffelmann, K. L. (2003). Soil amino acid composition quantified by acid hydrolysis and anion chromotography-pulsed amperometry. /. Agric. Food Chem. 51, 6521-6529. 

L. G. McLauglin and J. D. Henion, Determination of aminoglycoside antibiotics by reversed-phase ion-pair high-performance liquid chromatography coupled with pulsed amperometry and ion spray mass spectrometry, J. Chromatogr., 597 195-206 (1992). 

Table 1 Separation of Carbohydrates, Alditols, Alcohols, and Glycols using a CarboPac MAI Column and Pulsed Amperometry...

Fig. 3-105. Separation of various sugar alcohols and saccharides. - Separator column CarboPac PA-1 eluent 0.15 mol/L NaOH flow rate 1 mL/min detection pulsed amperometry at a Au working electrode injection volume 50 pL solute concentrations 10 ppm xylitol, 5 ppm sorbitol, 20 ppm each of rhamnose, arabinose, glucose, fructose, and lactose, 100 ppm sucrose and raffmose, 50 ppm maltose.

Fig. 3-110. Gradient elution of various mono- and disaccharides. - Separator column IonPac AS6A eluent (A) water, (B) 0.05 mol/L NaOH gradient linear, from 7% B to 100% B in 15 min flow rate 0.8 mL/min detection pulsed amperometry at a Au working electrode with post-column addition of NaOH injection volume 50 pL solute concentrations 15 ppm inositol (1), 40 ppm sorbitol (2), 25 ppm fucose (3) and deoxyribose (4), 20 ppm deoxyglucose (5), 25 ppm arabinose (6), rhamnose (7), galactose (8), glucose (9), xylose (10), mannose (11), fructose (12), melibiose (13), isomaltose (14), gentiobiose (15), and cellubiose (16), 50 ppm turanose (17), and maltose (18).

Pulsed amperometry in a three-electrode detector cell is carried out in a non-flowing solution, in which solute ions are dissolved in the supporting electrolyte. A pulsed potential is applied and is increased stepwise after each pulse. The resulting current... 

While the working potential required for the desired electrochemical reaction may be determined with voltammetric experiments, amperometry is used as the detection method in ion chromatography. A distinction is made between amperometry with constant working potential and pulsed amperometry. 

With the aid of pulsed amperometry, this compound can be directly oxidized without prior conversion into the dihydrolipoic acid. The sensitivity of this method is in the lower pmol range for such compounds. 

Fig. 6-10. Schematic representation of the detector cell for pulsed amperometry.

Carbohydrate analysis is much simpler in such samples. Pulsed amperometry is a very sensitive detection method and beverages to be analyzed can often be strongly diluted, so interferences caused by the matrix are usually not observed. To determine sorbitol in apple juice, for example, apple juice is simply diluted 1 1,500 with de-ionized water and injected directly (Fig. 8-65). Comparison measurements with enzymatic techniques... 

Like all carbohydrates, palatinitol may be separated in alkaline environment on a strongly basic anion exchanger in the hydroxide form and may be detected via pulsed amperometry. Fig. 8-81 shows a corresponding chromatogram with the separation of both isomers. Sorbitol, mannitol, and isomaltose can be detected as impurities in the same run. 

Lipoic acid acts as one of the coenzymes in the oxidative decarboxylation of pyruvate and other a-keto acids. It can be separated in an alkaline environment on a strongly basic anion exchanger in the hydroxide form, and can be detected like carbohydrates via pulsed amperometry at a Au working electrode. The corresponding chromatogram of a lipoic acid standard is shown in Fig. 8-88. This method allows to accurately detect 0.1 nmol lipoic acid. 

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