Amino acids amperometric

Pulsed amperometric detection (PAD), introduced by Johnson and LaCourse (64, 65) has greatly enhanced the scope of liquid chromatography/electrochemistry (66). This detection mode overcomes the problem of loss of activity of noble metal electrodes associated with the fixed-potential detection of compounds such as carbohydrates, alcohols, amino acids, or aldehydes. Pulsed amperometric detection couples tlie process of anodic detection with anodic cleaning and cathodic reactivation of a noble metal electrode, thus assuring a continuously cleaned and active... 

Alcohol dehydrogenase, 178 Alkaline error, 149 Alkaline phosphatase, 185 Alkanethiols, 46, 123 Alkoxide precursor, 120 Amino acids, 92, 187 Ammonium sensor, 181, 182 Amperometric sensors, 172 Aniline, 35, 39... 

G.G. Guilbault and G.J. Lubrano, Amperometric enzyme electrodes. Amino acid oxidase. Anal. Chim. Acta 69, 183-185 (1974). 

Fortier [6] found that AQ polymer from Eastman was not deleterious for the activity of a variety of enzymes such as L-amino acid oxidase, choline oxidase, galactose oxidase, and GOD. Following mixing of the enzyme with the AQ polymer, the mixture was cast and dried onto the surface of a platinum electrode. The film was then coated with a thin layer of Nafion to avoid dissolution of the AQ polymer film in the aqueous solution when the electrode was used as a biosensor. These easy-to-make amperometric biosensors, which were based on the amperometric detection of H202, showed high catalytic activity. 

A study was made of RP-HPLC with constant-potential (1.2 V vs SCE) and pulsed-potential amperometric detection using platinum or gold electrodes, of the derivatives of the common amino acids, obtained from phenyl and methyl isothiocyanates. All the thiohydantoins (98) were oxidized at both electrodes LOD was less than 0.2 pM for lysine and glycine, for 50 pL injection268. 

Amperometric biosensors incorporating certain enzymes on the electrode for the determination of D- and L-amino acids were investigated. The parameters included enzyme immobilization procedure, composition of the immobilizing matrix, amount of enzyme,... 

Copper electrodes have been used to determine amino acids and carbohydrates [10]. Metal oxide electrodes (including thin-film semiconductors) show some promise, but nothing of substance has yet been published with regard to LCEC. Pulsed amperometric detection (PAD) takes advantage of metal oxides formed in situ. This approach is discussed later. 

Another approach to dealing with the nonelectrochemically active nature of most amino acids is to generate, in situ, chemical reactions at the electrode surfaces to produce electrochem-ically active products for detection. Related to this concept, is the online use of immobilized enzymes (142) to react with amino acids. A by-product of this reaction is hydrogen peroxide, which is then quantified by amperometric detection. 

R Mogele, B. Pabel, R Galensa. Determination of organic acids, amino acids and saccharides by high-performance liquid chromatography and a postcolumn enzyme reactor with amperometric detection. J Chromatogr 591 165-173, 1992. 

M. Varadi, N. Adanyi, E.E. Szabo and N. Trummer, Determination of the ratio of d- and L-amino acids in brewing by an immobilised amino acid oxidase enzyme reactor coupled to amperometric detection, Biosens. Bioelectron., 14(3) (1999) 15335-15340. 

P. Sarkar, I.E. Tothill, S.J. Setford and A.P. Turner, Screen-printed amperometric biosensors for the rapid measurement of l- and D-amino acids, Analyst, 124(6) (1999) 865-870. 

Amperometric detection can be then applied for the determination of analytes such as glucose [131-137], ascorbic acid [138-141], uric acid [142,143], amino acids 1144—1491, peptides or DNA [150,151]. Con-ductimetric detection has been also employed in combination with CE microchip for amino acids [152,153], peptides/proteins [152,154,155], carbohydrates [153] or DNA [152],... 

Amino acids (isoleucine, phenylalanine, arginine and alanine) have been analysed on a microchip with a post-channel reaction with amino acid oxidase reaction [144], Pre-channel derivatisation of amino acids with naphthalene-2,3-dicarboxyaldehyde (NDA) has been described for facilitating its amperometric detection [145]. Separation and direct detection of amino acids without derivatisation have also been achieved in microchips [89,109,122,132,146-148]. 

C.D. Garcia and C.S. Henry, Direct determination of carbohydrates, amino acids, and antibiotics by microchip electrophoresis with pulsed amperometric detection, Anal. Chem., 75 (2003) 4778-4783. 

J. Wang, S. Mannino, C. Camera, M.P. Chatrathi, M. Scampicchio and J. Zima, Microchip capillary electrophoresis with amperometric detection for rapid separation and detection of seleno amino acids, J. Chromatogr. A, 1091 (2005) 118-177. 

Pre-column OPA derivatization was also employed to analyze biogenic amines prior to MEKC separation on a PDMS chip [654]. Pre-column OPA derivatization and MEKC were also performed on a glass chip to analyze amino acids. Usually, OPA was used for fluorescent detection. However, in this report, amperometric detection was used as the OPA derivatives were also electroactive. Voltage (needed for separation) programming was used to decrease the migration time of late migrating species [655]. 

Pulse amperometric detection (PAD) has been used for the detection on a PDMS chip. This method is useful for analysis of underivatized compounds, such as carbohydrates, amino acids, and sulfur-containing antibiotics, which easily caused electrode fouling. In PAD, a high positive potential (1.4-1.8 V) is first applied in order to clean the electrode (e.g., Au) surface. This is followed by a negative potential step (-0.5 V) to reactivate the electrode surface. A third moderate potential (+0.5 to +0.7 V) is applied for detection of the target analyte [752]. 

Dual LIF and amperometric detection are used to detect a five-component mixture (see Figure 7.24). The labeled amino acids (i.e., NBD-Arg, NBD-Phe, and NBD-Glu) are fluorescently detected. The EOF marker (DA) and internal standard (CAT) are amperometric ally detected. The fluorescent peaks are normalized to the internal standard, CAT. As a result, the RSD of migration time was improved from 2.7% to 0.8% [670]. 

Electrochemical transducers work based on either an amperometric, potentio-metric, or conductometric principle. Further, chemically sensitive semiconductors are under development. Commercially available today are sensors for carbohydrates, such as glucose, sucrose, lactose, maltose, galactose, the artificial sweetener NutraSweet, for urea, creatinine, uric acid, lactate, ascorbate, aspirin, alcohol, amino acids and aspartate. The determinations are mainly based on the detection of simple co-substrates and products such as 02, H202, NH3, or C02 [142]. 

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