Electrochemical, detection stability

Xie Q, Perez-Cordero E and Echegoyen L 1992 Electrochemical detection of and enhanced stability of fullerides in solution J. Am. Chem. Soc. 114 3978-80... 

Electrochemical detection places a number of restrictions on the mobile phase used in the chromatographic separation but is well suited to reversed-phase separations. Even so, the mobile phase must be oxygen-free, which requires bubbling an inert gas such as helium through the mobile phase reservoir as well as the sample. Additionally, both the mobile phase and the sample must be free of metals to ensure baseline stability during the readings. Despite these constraints, electrochemical... 

Xie Q, Perez-Cordero E, Echegoyen L (1992) Electrochemical detection of Cgo and C-jo. enhanced stability of fullerides in solution. J Am Chem Soc 114 3978-3980... 

The ubiquitous electrochemical behavior of ferrocene and its relative chemical stability have made this organometallic complex a useful group for the preparation of redox-active devices. The incorporation of ferrocene-modified amino acids into larger polypeptide structures can therefore lead to electrochemically active de novo designed proteins. In addition, the attachment of ferrocene derivatives to peptides make them electroactive and eligible to electrochemical detection. Hence, it is not surprising that the first synthesis of a ferrocene-modified amino acid dates back to the 1950s. 

Liquid chromatography with electrochemical detection (LCEC) is in widespread use for the trace determination of easily oxidizable and reducible organic compounds. Detection limits at the 0.1-pmol level have been achieved for a number of oxidizable compounds. Due to problems with dissolved oxygen and electrode stability, the practical limit of detection for easily reducible substances is currently about 10-fold less favorable. As with all detectors, such statements of the minimum detectable quantity must be considered only with the proverbial grain of salt. Detector performance varies widely with the analyte and the chromatographic conditions. For example, the use of 100- m-diameter flow systems can bring attomole detection limits within reach, but today this is not a practical reality. 

MD Lucock, M Green, R Hartley, MI Levene. Physicochemical and biological factors influencing methylfolate stability use of dithiothreitol for HPLC analysis with electrochemical detection. Food Chem 47 79-86, 1993. 

Anodically oxidized BDD thin-film electrodes have been shown to be best suited for the electrochemical detection of CPs. Anodic pretreatment of diamond is crucial for achieving high stability and reproducibility of the... 

Derivatization is another form of sample preparation. It is utilized for the analysis of labile analytes or to enhance retention or detection with a preferred type of detector. Derivatization can be performed to enhance detection by UV/Vis, fluorescence, or electrochemical detection. Consideration must be given to the stability of the derivatize to solvolysis and thermal degradation. In our labs alendronate, a bisphosphonate with a primary amine functionality, was derivatized with FMOC to enhance detection by UV/Vis as well as to increase retention in RPLC mode [19]. An acylchloride was derivatized with... 

Mechanistic aspects of the action of tyrosinase and the usual transduction schemes have been summarized on several occasions [166,170-173]. In short, this copper enzyme possesses two activities, mono- and di-phenolase. Due to the predominant presence of the mono phenolase inactive form (met-form), the enzyme is inherently inefficient for the catalysis of these monophenol derivatives. However, in the presence of a diphenol, the catalytic cycle is activated to produce quinones and the scheme results in an efficient biorecognition cascade. This activation is achieved more efficiently when combined with electrochemical detection through the reduction of the produced quinones [166], as illustrated in Fig. 10.5. Consequently, a change in the rate-hmiting step can be observed through kinetic to diffusion controlled sensors with a concomitant increase in stability and sensitivity, as depicted in Fig. 10.6. 

When using BrMAC, postcolumn hydrolysis of separated PG esters is neccessary for on-line detection of the fluorophore, resulting in detection limits of about 10 fmol. Hydrazone or amide derivatives are also suitable for fluorescence detection as well as 3-bro-momethyl-6,7-dimethoxy-l-methyl-2(l//) quinoxali-none (YMC Pack C8) or 9-anthroyldiazomethane (ADAM) characterized by its low stability (Nucleosil ODS silica). Electrochemical detection of active com-... 

The determination of catecholamines requires a highly sensitive and selective assay procedure capable of measuring very low levels of catecholamines that may be present. In past years, a number of methods have been reported for measurement of catecholamines in both plasma and body tissues. A few of these papers have reported simultaneous measurement of more than two catecholamine analytes. One of them utilized Used UV for endpoint detection and the samples were chromatographed on a reversed-phase phenyl analytical column. The procedure was slow and cumbersome because ofdue to the use of a complicated liquid-liquid extraction and each chromatographic run lasted more than 25 min with a detection Umit of 5-10 ng on-column. Other sensitive HPLC methods reported in the literature use electrochemical detection with detection limits 12, 6, 12, 18, and 12 pg for noradrenaline, dopamine, serotonin, 5-hydroxyindoleace-tic acid, and homovanillic acid, respectively. The method used very a complicated mobile phase in terms of its composition while whilst the low pH of 3.1 used might jeopardize the chemical stability of the column. Analysis time was approximately 30 min. Recently reported HPLC methods utilize amperometric end-point detection. 

Electrochemical Detection of Ceo and C70 " Enhanced Stability of Fullerides in Solution... 

As mentioned in the previous section, the response, the stability and the enzyme activity found greatly enhanced at the MWCNT platform. Other than CNTs, AuNPs also possess some unique properties and recent years it has been widely employed in the biosensors to immobilize biomolecules. Thus in this section we discuss about the application of AuNP matrix for the immobilization of AChE for pesticide sensor development. With the use of AuNPs, the efficiency and the stability of the pesticide sensor gets greatly amplified. Moreover, the nanoparticles matrix offers much friendly environment for the immobilized enzyme and thus the catalytic activity of the enzyme got greatly amplified. Interestingly, Shulga et al. applied AChE immobilized colloidal AuNPs sensor for the nM determination of carbofuran, a CA pesticide [16], The enzyme-modified electrode sensor was also utilized for the sensitive electrochemical detection of thiocholine from the enzyme catalyzed hydrolysis of acetylthiocholine chloride (ATCl). The fabrication and the enzyme catalyzed reaction at the AuNPs coated electrode surface is shown in Fig. 6. 

E.C.Y. Chan, P.Y. Wee and PC. Ho, Evaluation of degradation of urinary catecholamines and metanephrines and deconjugation of their sulfoconjugates using stability-indicating reversed-phase ion-pair HPLC with electrochemical detection, J. Pharm. Biomed. Anal., 22, 515-526 (2000). 

Electrochemical detection has been regarded as particularly appropriate strategy for microfluidic chip systems. Electrochemical biosensors in microfluidic chips enable high sensitivity, low detection limits, reusability, and long-term stability. And the detection mechanism and instmmentation for realization are simple and cost-effective. These valuable features have made electrochemical devices receive considerable attention [20,94,95]. The electrochemical detectors are commercially available for a variety of analyses [96]. The review written by Wang summarized the principles of electrochemical biosensors, important issues, and the state-of-the-art [97]. Lad et al. described recent developments in detecting creatinine by using electrochemical techniques [98]. 

With regard to biosensors and analytical chip systems, challenging problems of electrochemical detection strategy are deterioration in selectivity and stability of biological functional objects like enzymes and chronic passivation of the underlying electrodes. Insufficient selectivity or specificity is an intrinsic limitation of electrochemical detection, which has been addressed by combining chemically or biologically specific receptors. 

Sensitivities in electrochemical detection are much improved compared to UV detection [26,31,274], although some authors claim it to be of the same order [272], This may again reflect the dependence of detector responses on the condition and type of electrodes on one hand and the type of detector cell on the other. For (-)-epicatechin a detection limit in the pico-molar range has been reported in the dual-mode detection [272]. Other researchers found detection limits of 0.1 mg/1 for (+)-catechin, (-)-epicatechin and procyanidin B3 [74], Me Murrough and Baert [31], who analyzed beer by direct injection HPLC, noticed a 40 % decrease of responses using dual-mode electrochemical detection compared to singlemode detection. Because of improved stability, reliability and accuracy they advocate the use of dual-mode detection after all. 

Chan, D.S. Sato, A.K. Claybaugh, J.R. Degradation of captopril in solutions compounded from tablets and standard powder. Am. J.Hosp.Pharm., 1994,51,1205-1207 [stability-indicating tablets powder] Wakabayashi, H. Yamato, S. Nakajima, M. Shimada, K. Application of an electrochemical detector with a graphite electrode to liquid chromatographic determination of penicillamine and captopril in biological samples. J.Pharm.Biomed.Anal., 1994, 12, 1147-1152 [rat serum liver kidney SPE electrochemical detection LOD 20-300 pg, extracted penicillamine homocysteine (IS)]... 

The most widely used detection methods in HPLC analysis of anilines and phenols are UV (especially diode array) and electrochemical detection (ED). UV detectors provide very good signal stability and in the case of diode-array detectors they can be used for analyte tentative confirmation purposes using UV spectra libraries. Electrochemical detectors are more sensitive than the UV detectors however, their performance is highly dependent on the type of samples analyzed. Components from dirty samples are deposited on the electrochemical cell and the detector sensitivity is rapidly decreased. ° ° The heterocyclic amines can be measured with UV, electrochemical, and fluorescence detectors. 

Webee G (1993) Investigation of the stability of metal species with respect to liquid chromatography with electrochemical detection. Fresenius J Anal Chem 346 639-642. 

The aromatic characters of the linearly and angularly fused tricyclic [l,2,4]triazines were considered <93ACS296>. The exclusive formation of the 6-substituted pseudo base (66a) and the 2-substituted pseudo base (67a) from the linearly and angularly fused derivatives, respectively, could be interpreted by considering the aromatic stabilities of these compounds (Figure 2). Liquid chromatography with electrochemical detection was applied for the detection of the isoquinolino[l,2-/][l,2,4]triazinium derivative <87JC25>. 

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