Ascorbic Acid interference

Ascorbate oxidase is added to the sensor for ascorbic acid interference elimination. 

Lowry JP, O Neill RD. Homogeneous mechanism of ascorbic acid interference in hydrogen peroxide detection at enzyme-modified electrodes. Analytical Chemistry 1992, 64, 453 156. 

Amperometric detection was achieved on two patches of C films (formed by CVD of 3,4,9,10-perylenetetracarboxylie dianhydride) on a glass chip. The microchannels were formed using a 23- im-thick photoresist as a spacer. Glucose oxidase and lactate oxidase were immobilized with HRP on the C films via a coated film of osmium PVPD polymer. Simultaneous measurements of glucose and lactate in rat brain cerebrospinal fluid (first perfused with 50 mM veratridine) were achieved. These two films were spatially separated in order to avoid interdiffusion of H202 formed from the two separate enzymatic reactions. Moreover, the two films were preceded by a third C film immobilized with ascorbate oxidase in order to remove ascorbic acid interference [759]. 

Ascorbic acid interferes with the autoanalyser determination of serum transaminases and lactic dehydrogenase (7,9). Serum bilirubin concentrations may be reduced by ascorbic acid, so that the presence of liver disease may be masked (9). 

More recently, Olivia et al. reported glucose biosensors using Pt nanoparticles deposited boron-doped diamond microfiber (BDDMF) electrode on which GOx was immobilized by overoxidized polypyrrole [140]. Authors reported that amperometric measurement of glucose with the GOx/over-oxidized polypyrrole/Pt-modified BDDMF electrode showed a hnear response in the range of 1-70 mM and was free from ascorbic acid interference. 

PEDOT/Au NP nanocomposite DPV 0.12pA/nM 20-130 pM 2 nM Electrochemical polymerization, ascorbic acid interference [43]... 

Kojima, J., and Hess, J. W. Ascorbic acid interference with estimation of beta-glucosamlnidase activity. Annals Biochem., 23 474-483,1968. Krasner, N., and Dymock, I. W. Ascorbic acid deficiency in malignant diseases a clinical and biochemical study. Br. J. Cancer, 30 142-145, 1974. 

The successful exploitation of the multienzyme composition of plant tissues in eliminating interferences and, hence, improving the selectivity and stability of tissue-based biosensors have also been reported. The presence of AAO in zucchini has been successfully used to eliminate ascorbic acid interference from the determination of dopamine or norepinephrine. Also, the incorporation of papaya tissue into a carbon paste matrix has been effectively used to destroy surface active proteins. 

The former crmsists of a significant enhancement of sensor selectivity when Nafion is added as the outermost layer of an electrode coating the electrocatalytic properties of many electrode coatings that are the object of different chapters of this monograph are combined with the selectivity conferred by this ion-exchange polymer. In this respect, a most frequently reported applicatiOTi is addressed to lower the ascorbic acid interference in amperometric sensors for glucose or for dopamine determination. 

A chitosan-modified carbon fiber microelectrode for in vivo detection of serotonin was described. It was demonstrated that chitosan has the ability to reject physiological levels of ascorbic acid interferences and facilitate selective and sensitive detection of in vivo levels of serotonin. In vivo results demonstrated that the chitosan-modified electrode could measure serotonin produced in the zebrafish intestine with high spatial and temporal resolution. A serotonin concentration of 30.8 ( 3.4) nM could be recorded in vivo with the implanted chitosan-modified microelectrode in normal physiological conditions. Due to its inherent biocompatibility and remarkable adherence, chitosan was proved to be an excellent coating for use in implantable sensors, able to selectively detect and monitor levels of m VIVO neurotransmitters. 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Ibrahim and co-workers developed a new method for the quantitative analysis of hypoxanthine, a natural compound of some nucleic acids. " As part of their study they evaluated the method s selectivity for hypoxanthine in the presence of several possible interferents, including ascorbic acid. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Biochemical Functions. Ascorbic acid has various biochemical functions, involving, for example, coUagen synthesis, immune function, dmg metabohsm, folate metaboHsm, cholesterol cataboHsm, iron metaboHsm, and carnitine biosynthesis. Clear-cut evidence for its biochemical role is available only with respect to coUagen biosynthesis (hydroxylation of prolin and lysine). In addition, ascorbic acid can act as a reducing agent and as an effective antioxidant. Ascorbic acid also interferes with nitrosamine formation by reacting direcdy with nitrites, and consequently may potentially reduce cancer risk. 

Sometimes the metal may be transformed into a different oxidation state thus copper(II) may be reduced in acid solution by hydroxylamine or ascorbic acid. After rendering ammoniacal, nickel or cobalt can be titrated using, for example, murexide as indicator without interference from the copper, which is now present as Cu(I). Iron(III) can often be similarly masked by reduction with ascorbic acid. 

Describe the rationale of using electrodes coated with Nation films for selective detection of the cationic neurotransmitter dopamine in the presence of the common interference from anionic ascorbic acid. 

Drugs can also Interfere with laboratory results by negating certain nonspecific oxidation and reduction reactions essential for the chemical assay. Penicillin, streptomycin and ascorbic acid are known to react with cupric Ion thus, false positive results for glucose may occur If a copper reduction method Is used. If the specific enzymatic glucose-oxidase method Is employed, ascorbic acid can cause a false negative result by preventing the oxidation of a specific chromogen In the reaction. 

Colorimetric procedures used In steroid assays are often subject to drug Interference. In the determination of 17-Ketosterolds by the Zimmerman reaction, drugs with the 17-Keto basic structure such as ascorbic acid, morphine and reserplne will cause Increased values. In the determination of 17,21 -dlhydroxysterolds by the Porter-Sllber reaction the dlhydroxy-acetone chain Is the reactive unit. Drugs like meprobamate, chloral hydrate, chloropromazlne and potassium Iodide will Interfere with this reaction and cause elevated values. In the colorimetric determination of vanlllylmandellc acid (VMA) by a dlazo reaction, drugs like methocarbamol and methyl dopa cause... 

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... 

Another approach to improve selectivity is to use an enzyme electrode. The enzyme ascorbate oxidase has been used successfully to remove ascorbate as an interference of in vivo voltammetric electrodes 219,320) Ascorbate oxidase converts the ascorbic acid to dehydroascorbate which is not electroactive in the potential region used for in vivo analysis. 

Tetrazolium salts can be reduced nonselectively by many endogenous reductants such as thiol-containing proteins, as well as exogenous ones such as ascorbic acid.521 -523 This can lead to serious interferences and several measures have been described to reduce or eliminate their effect.524,525,650... 

A poly(aniline boronic acid)-based conductimetric sensor for dopamine consisting of an interdigitated microarray electrode coated with poly(aniline boronic acid) has also been developed by the Fabre team. The sensor was found to show a reversible chemoresistive response to dopamine without interference by ascorbic acid from their mixtures.42... 

A similar study has also been conducted to determine the suitability of ascorbic acid 2-phosphate (AAP) as an alternative substrate to 4-AP for AP under identical conditions [48], Although 4-APP and AAP were suitable substrates for amperometric immunosensors, 4-APP was superior owing to its sixfold faster enzymatic reaction and lower detection potential (approximately 200-400mV). Notably, the lower detection potential for the hydrolysis product of 4-APP minimizes interferences from other species and hence improves the sensitivity of the immunosensor. 

Further improvement of the Prussian blue-based transducer presents two principal problems. First, Prussian blue layers are not mechanically stable, especially on smooth electrode surfaces because of their poly crystalline nature. Second, despite the low electrode potential used, the most powerful reductants like ascorbic acid still interfere with sensor response if present in excessive concentrations. 

The Folin-Ciocalteu assay is the most widely used method to determine the total content of food phenolics (Fleck and others 2008). Folin-Ciocalteu reagent is not specific and detects all phenolic groups found in extracts, including those found in extractable proteins. A disadvantage of this assay is the interference of reducing substances, such as ascorbic acid (Singleton and others 1999). The content of phenolics is expressed as gallic acid or catechin equivalents. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

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