Trace electrochemical detection

Figure 1 Electrochemical detection of catechol, acetaminophen, and 4-methyl catechol, demonstrating the selectivity of differential pulse detection vs. constant potential detection. (A) Catechol, (B) acetaminophen, and (C) 4-methylcatechol were separated by reversed phase liquid chromatography and detected by amperometry on a carbon fiber electrode. In the upper trace, a constant potential of +0.6 V was used. In the lower trace, a base potential of +425 mV and a pulse amplitude of +50 mV were used. An Ag/AgCl reference electrode was employed. Note that acetaminophen responds much more strongly than catechol or 4-methylcatechol under the differential pulse conditions, allowing highly selective detection. (Reproduced with permission from St. Claire, III, R. L. and Jorgenson, J. W., J. Chromatogr. Sci. 23, 186, 1985. Preston Publications, A Division of Preston Industries, Inc.)...

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

Perhaps the most important of all electrochemical detection schemes currently in use is the electrical conductivity detector. This detector is specifically useful for ion exchange, or ion, chromatography in which the analyte is in ionic form. Such ions elute from the column and need to be detected as peaks on the recorder trace. 

Pocurull E, Marce RM, Borrull F. 1996. Determination of phenolic compounds in natural waters by liquid chromatography with ultraviolet and electrochemical detection after on-line trace enrichment. J Chromatogr 738 1-9. 

Biogenic amines are of great interest to researchers because of their potential roles in several psychiatric and neurological disorders. They include dopamine (DA), noradrenaline (NA), 5-hydroxytryptamine (5-HT, serotonin), histamine, and trace amines such as 2-phenylethylamine (PEA), tyramine, octopamine, phenylethanolamine, and tryptamine (Coutts and Baker, 1982). Although GC assays for DA, NA, and 5-HT are available, HPLC analysis with electrochemical detection has for many years now been the method of choice for analysis of these neurotransmitter amines. 

An interesting feature of the electrochemical detection is its relatively small variation in sensitivity for various substances for which it responds. This relatively constant molar response is due to the small number of electrons, usually two or three, involved in electrochemical reactions. This feature is very convenient in trace analysis, because the analyst can predict the sample size, dilutions, and other manipulations drat must be used to produce the desired analytical sensitivity. 

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. 

An alternative approach, which additionally favors the separation of sugars from interfering compounds, is the precolumn formation of the derivatives in this case trace sugar detection is sometimes possible, as nmol or pmol orders are approached. Examples include the use of dan-sylhydrazine (with either fluorimetric or colorimetric detection) (39) and l-phenyl-3-methyl-5-pyrazolone (UV absorption or electrochemical detection) (40). 

Because the vitamins occur in food in trace quantities, detection sensitivity is often an issue. Ultraviolet absorbance is the most common detection method. Fluorescence and electrochemical detection are used in specific cases where physicochemical properties permit and where increased sensitivity and selectivity are desired. Refractive index is seldom used, due to its lack of specificity and sensitivity. 

The analysis of trace substances in environmental science, pharmaceutical and food industries is a challenge since many of these applications demand a continuous monitoring mode. The use of immunosensors based on AuNPs in these applications should also be appropriate. Although there are many recent developments in the immunosensor field, which have potential impacts [36], nevertheless there are few papers concerning environmental analysis with electrochemical detection based on AuNPs. The application of some developed clinical immunosensors can also be extended to the environmental field. 

C. M. selvaka and I. S. Krull, Trace determination of barbiturates with LC-photolysis electrochemical detection. In Analytical Meth-... 

C. M. Selavka and I. S. Krull, Trace determination of barbiturates with LC-photolysis-electrochemical detection (LC-hv-EC). In Analytical Methods in Forensic Chemistry (M. H. Ho, ed.), Ellis Horwood, New York, 1990, pp. 195-209. 

This method is free from interferences, which when coupled with the specificity and the high sensitivity of the electrochemical detection mode, renders it suitable for the determination of trace levels of nitrate and nitrite in surface, ground and main water. 

Investigators from the Department of Chemistry of the University of Liverpool tried using ECL with enzyme IM for trace explosive detection (TNT and PETN). The detection step uses ECL and triggers light emission by an electrochemical reaction. The ECL step... 

J. B. F. Lloyd, Clean-up Procedures for the Examination of Swabs for Explosives Traces by High-Performance Liquid Chromatography with Electrochemical Detection at a Pendant Mercury Drop Electrode, Journal of Chromatography 261 (1983) 391. 

Lloyd JBF. 1983a. Clean-up procedures for the examination of swabs for explosive traces by high-performance liquid chromatography with electrochemical detection at a pendent mercury drop electrode. J Chromatogr 261 391-406. 

Liu, Y.-C., Yu, C.-C. and Hsu, T.-C. (2007) Trace molecules detectable by surface-enhanced Raman scattering based on newly developed Ag and Au nanoparticles-containing substrates. Electrochem. Commun. 9 639-644. 

Despite the potential for direct aqueous injection of water samples into reverse phase systems, there are very few cases where this is possible due to the low detection levels normally required for environmental analysis. Using direct aqueous injection and coulometric electrochemical detection, the analysis of phenol and chlorophenols and 2-mercaptobenzothiazole have been achieved at trace levels (methods with limits of detection for phenol 0.034 ngp and 0.8 pgl for mercaptobenzothiazole have been achieved). There is a potential for the use of direct aqueous injection for the analysis of phenol in effluents using fluorescence detection which would be expected to detect down to low mg T. Direct aqueous injection has been used in an automated system similar to that shown in Figure 11.1. The trace enrichment cartridge was replaced by a large sample loop (50 pi) and a coulometric electrochemical detector used instead of the UV detector. 

Armentrout, D.N. McLean, J.D. Long, M.W. Trace determination of phenolic compounds in water by reversed phase liquid chromatography with electrochemical detection using a carbon-polyethylene tubular anode. Anal. Chem. 1979, 51, 1039-1045. 

Andrea GD, Terrazzino S, Fortin D, Farruggio A, Rinaldi L, Leon A. HPLC electrochemical detection of trace amines in human plasma and platelets and expression of mRNA transcripts of trace amine receptors in circulating leukocytes. Neurosci Lett 2003 346 89-92. 

D.L. Rabenstein (University of California, Riverside) is investigating the biological chemistry of selenium compounds in intact erythrocytes to characterize transport across the erythrocyte membrane and their intracellular metabolism. Reactions will be characterized in erythrocytes, plasma, and aqueous solution by nuclear magnetic resonance (NMR) spectroscopy. Methods will be developed for the determination of selenols, diselenides, and selenosulfides in biological fluids and erythrocytes by high performance liquid chromatography (HPLC) and electrochemical detection. The HPLC methods will be used to determine metabolites formed at trace levels. 

MS/MS) is the standard detector for bioanalytical assays and drug discovery screening, its use for routine assays of drug substances and products is still limited due to its high cost and lower precision. Nevertheless, LC/MS/MS methods are increasingly used for ultra trace analysis or screening of complex samples. Other detection options include conductivity detection for ionic species and electrochemical detection for neuroactive species in biochemical research. 

The advantage of LC-MS detection over UV or electrochemical detection is its tremendously improved selectivity. Flavonoids, phenolic acids but also certain oxidation and degradation products of procyanidins can be suppressed by monitoring specific ion traces. Sample clean up procedures can therefore be minimized. Fig. (10) illustrates the benefit of LC-MS detection in the analysis of complex matrices. A major drawback of this technology in the analysis of procyanidins is its low sensitivity, even compared to UV detection [282],... 

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