Amperometric systems

The cell medium must favour electrochemical reaction. The mobile phase must be conducting but not necessarily aqueous nonpolar solvents and hence adsorption chromatography are not compatible with electrochemical detection. The mobile phase must not contain any chlorides or hydroxycarboxylic acids. The electrochemical yield is not higher than between 1 and 10% so that most of the sample leaves the cell unchanged. A conversion level of 100% is known as coulometric detection", but the equipment is not more sensitive than amperometric systems. 

The precision of the HPLC column-switching-amperometric system was consistently excellent. For example, 11 injections of a 5.28 x 10 M NADH solution gave a relative standard deviation of 0.3% (x = 9.65 nA 5 = 0.03 nA). 

E. L. Guldberg, A. S. Attiyat, and G. D. Christian, Amperometric Systems for the Determination of Oxidase Enzyme Dependent Reactions by Continuous Flow and Flow Injection Analysis. J. Autom. Chem., 2 (1980) 189. 

Figure 14-5. Schematic setup of a) a two-electrode and b) a three-electrode amperometric system with an enzyme electrode.

Photosystem II (PSII) is part of the photosynthetic apparatus in cyanobacteria, algae and higher plants and catalyzes the light-induced transfer of electrons from water to plastoquinone via a set of delicately arranged cofiictors. It has a well known bindii site for diverse chemical compounds in its so-called D1 subunit and the ability to convert such a binding event into signals which can be easily detected by optical, potentiometric or amperometric systems. Due to these inherent properties PSII can be considered as a natural biosensor and has consequentially been used for the detection of herbicides and other pollutants in pilot studies. ... 

An efficient H gas diffusion amperometric system for the determination of ethanol in beverages was developed by Kunnecke and Schmid [46]. The same authors also used the system to achieve on-line monitoring of ethanol content of cultivation media, with amperometric as well as photometric detection [47]. The system combined the selec-... 

Lundback and Olsson [55] determined lactose in cow milk using a FI amperometric system incorporating an on-line dialyzer and an immobilized galactose oxidase reactor used mainly for the determination of galactose in urine (cf Sec. 9.3.2 ). The response factor of lactose (including dialysis) is 16 times lower than galactose, but the latter is not present in cow milk. This makes it possible to determine lactose using the same system. 

Fig.9.1 Manifold of a FI amperometric system for the detennination of galactose in urine. S, sam> pie DS, on-line dialyzen GOR. immobilized galactose oxidase reactor, Cu(II), copperfll) saturated Bond-Elut column for removal of ascorbic acid, POD, peroxidase reactor, B, phosphate buffer. pH 7 R. 5 mM potassium hexacyanoferrate and 0.1 M sodium phosphate, pH 7 D. amperometric detector and W. waste.

V and therefore the cell reaction occurs spontaneously. These cells are sometimes referred to as Mackereth sensors . Such galvanic sensors are also often termed fuel cell sensors and it is possible to measure either the resulting current or the cell voltage. In the former case, the term amperometric sensor in its widest definition is still correct, although the cell is fundamentally different from the usual Faradaic amperometric systems. Note that for the determination of atmospheric oxygen an alternative has become available in recent years in the form of optical sensors based on fluorescence. These sensors are very robust as they do not contain electrodes or a liquid phase and show very fast response times. 

On the other hand, in terms of the necessary interdisciplinarity required for the realization and exploitation of novel amperometric systems, it is not so easy to convince chemists who are experienced in chemical synthesis to give support to realizing an ad hoc material possessing the physical and chemical characteristics suitable for best efficiency as an electrode modifier. 

Under conditions of complete (i.e. 100%) EC conversion of the analyte, the electrode is said to be operating in the coulometric mode and the total charge which passes depends on the total amount of analyte. Partial electrolysis is found in amperometric systems when the % conversion typically ranges between <1 and 5%. Since in ED systems the analyte is moving past the electrode the reaction is often incomplete and amperometry is more readily achieved than a coulometric response. In such cases the current is proportional to the analyte concentration. 

Mass transport in amperometric systems in which the reagent stream is forced to flow along the surface of the electrode may be described in terms of convective diffusion. Effectively this means that at sufficiently high values of Pg, the Peclet number, the liquid above an electrode may be divided into two distinct zones. In one zone, far away from the electrode surface, convection is important, and the concentration profile is substantially flat. In the other zone, adjacent to the electroactive surface, there is a sharp concentration gradient here diffusion is the predominant mass transport process. The Peclet number is given by v l/D, where is the main stream fluid velocity, and / is the length of the electrode (measured in the direction of fluid flow). Under these conditions, the mass transport limited current z l for a reversible electrode couple (i.e. the concentration of the electroactive form is zero at the electrode surface) is given by... 

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