Electrochemical interferences uric acid

The problem of selectivity is the most serious drawback to in vivo electrochemical analysis. Many compounds of neurochemical interest oxidize at very similar potentials. While this problem can be overcome somewhat by use of differential waveforms (see Sect. 3.2), many important compounds cannot be resolvai voltammetrically. It is generally not possible to distinguish between dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) or l tween 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA). Of even more serious concern, ascorbic acid oxidizes at the same potential as dopamine and uric acid oxidizes at the same potential as 5-HT, both of these interferences are present in millimolar concentrations... 

Biosensors fabricated on the Nafion and polyion-modified palladium strips are reported by C.-J. Yuan [193], They found that Nafion membrane is capable of eliminating the electrochemical interferences of oxidative species (ascorbic acid and uric acid) on the enzyme electrode. Furthermore, it can restricting the oxidized anionic interferent to adhere on its surface, thereby the fouling of the electrode was avoided. Notably, the stability of the proposed PVA-SbQ/GOD planar electrode is superior to the most commercially available membrane-covered electrodes which have a use life of about ten days only. Compared to the conventional three-dimensional electrodes the proposed planar electrode exhibits a similar... 

Table 13.2 summarises the different approaches used to construct enzyme electrochemical biosensors for application to food analysis based on the different types of enzymes available. Generally, the main problems of many of the proposed amperometric devices have been poor selectivity due to high potential values required to monitor the enzyme reaction, and poor sensitivity. Typical interferences in food samples are reducing compounds, such as ascorbic acid, uric acid, bilirubin and acetaminophen. Electrocatalysts, redox mediators or a second enzyme coupled reaction have been used to overcome these problems (see Table 13.2), in order to achieve the required specifications in terms of selectivity and sensitivity. 

Another application of Zinc oxide nanostructure is immobilization of uricace onto ZnO nanorod and fabrication a sensitive biosensor for uric acid detection [167], The biosensor successfully used for micromolar detection of uric acid in the presence serious interferences, glucose, ascorbic acid, and 1-cysteine. The apparent KM value for the uric acid biosensor is 0.238 mM, showing high affinity of the biosensor. Direct electron transfer of SOD at a physical vapor deposited zinc oxide nanoparticles surface was investigated [168], In comparison to SOD immobilized onto ZnO nanodisks [169], the electron transfer rate constant is small and a quasi- reversible electrochemical behavior observed. A novel... 

Laccase also catalyzes the 02-dependent oxidation of ascorbic acid, ferrocyanide, iodide, and uric acid. These reactions have been utilized to eliminate electrochemical interferences in amperometric hydrogen peroxide detection at membrane-covered enzyme electrodes (Wollen-berger et al., 1986). The capacity of the laccase membrane to oxidize ferrocyanide has been characterized by anodic oxidation of ferrocyanide at +0.4 V (Fig. 62). When a fresh enzyme membrane is used, a current signal appears only at substrate concentrations above 5 mmolA the current increases linearly with increasing concentration. This threshold concentration decreases with increasing membrane age until the remaining enzyme activity is too low for complete substrate oxidation. 

In view of the low physiological uric acid concentrations, in the assay of uric acid electrochemical interferences by other anodically oxidizable substances become particularly troublesome. Kulys et al. (1983) coimmobilized HRP with uricase in order to eliminate these interferences. This approach permits uric acid measurement to be performed at a potential of 0 V vs SCE. However, the autoxidation of ferrocyanide used as HRP substrate is a serious drawback of this method. 

Many other similar applications have been reported such as the electrochemical determination of electroinactive cationic medicines,313 determination of urea,314 uric acid,315 and application to glucose biosensors to decrease interference of ascorbate, urate, and acetaminophen.316 Enzyme immobilized membranes are also sensing membranes, e.g. urea responsive membranes, poly(carboxylic acid) membranes in which urease is immobilized,317 fructose responsive membranes, and polyion complex membranes in which fructose dehydrogenase is immobilized.318 Such applications will expand further in the future and contribute to human life. 

Uric acid, a serum component having a maximal normal level of 6 mg/100 ml (Orten and Neuhaus, 1982), is a major electroactive interference. A hydrodynamic voltammogram for uric acid displayed an i/, of 430 mV versus Ag/AgCl. Consequently potential selection could not be used to resolve NADH from the more easily oxidizable uric acid. Immunoassay reagents were also sources of electrochemical interference. 

The precolumn was first characterized by evaluating the retention times of NADH and the known electrochemical interferences. Spectroscopic detection was used for this, since the electrode passivators (IgG, HSA), uric acid, and NADH absorb at 280 nm, and IgG and HSA are electroinactive at 750 mV versus Ag/AgCI. Figure 16 shows a chromatogram of a blank human serum—NADH mixture which was injected into the Lichrosorb-DIOL precolumn. The first peak (retention time of 113 sec) was primarily composed of IgG and HSA. The second peak (173 sec) was NADH the shoulder that appears at approximately 190 sec was uric acid. Therefore a heart-cut should be from 150 sec (to remove the macro-molecular fraction) to 185 sec (to remove the late-eluting uric acid among other possible interferences). 

Interference studies. In the conventional electrochemical method for H2O2 determination based on the oxidation at either platinum or carbon electrodes, electroactive compounds such as ascorbic acid, acetaminophen, and uric acid interfere. Various approaches, such as the incorporation of peroxidases or... 

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