Amperometric detectors, liquid chromatography

For selective estimation of phenols pollution of environment such chromatographic methods as gas chromatography with flame-ionization detector (ISO method 8165) and high performance liquid chromatography with UV-detector (EPA method 625) is recommended. For determination of phenol, cresols, chlorophenols in environmental samples application of HPLC with amperometric detector is perspective. Phenols and chlorophenols can be easy oxidized and determined with high sensitivity on carbon-glass electrode. 

Enzyme linked electrochemical techniques can be carried out in two basic manners. In the first approach the enzyme is immobilized at the electrode. A second approach is to use a hydrodynamic technique, such as flow injection analysis (FIAEC) or liquid chromatography (LCEC), with the enzyme reaction being either off-line or on-line in a reactor prior to the amperometric detector. Hydrodynamic techniques provide a convenient and efficient method for transporting and mixing the substrate and enzyme, subsequent transport of product to the electrode, and rapid sample turnaround. The kinetics of the enzyme system can also be readily studied using hydrodynamic techniques. Immobilizing the enzyme at the electrode provides a simple system which is amenable to in vivo analysis. 

For amperometric techniques at a fixed potential (e.g., liquid chromatography detectors), the classical charging current is zero since dE/dt = 0. There is still a background current, however, due to surface redox processes or slow... 

K. Bratin, P. T. Kissinger, and C. S. Brunlett, Reductive mode thin-layer amperometric detector for liquid chromatography, J. Liquid Chromatogr., 4-.Y111 (1981). 

We believe that this symposium on Recent Advances in Pesticide Analytical Methodology fulfills this plea. High-performance liquid chromatography (HPLC) has made the greatest advances. Chapters on HPLC cover subjects on metabolism studies automation of HPLC evaluation of LC columns the effect of the mobile phase on reversed-phase chromatography the electrochemical or amperometric detector and fluorogenic detection. 

In a potentiostatic step experiment on an ultramicrodisc electrode, the current attains a limiting value in a time of the order r ID. In addition to this time scale becoming shorter with decreasing r, the mass transport rate of species to the electrode increases. As a consequence, one motivation for the development of devices using UMEs has been the resulting insensitivity of the limiting current to fluctuations of solution flow rate [62]. Hence UMEs have found widespread use as electrochemical detectors in fluid flow experiments, such as amperometric liquid chromatography (e.g., Ref. [63]) and flow injection analysis (e.g., Ref. [64]). Incidentally, this implies that the current should also be insensitive to vibration. 

To many analysts the major limitation of electrochemical detection for liquid chromatography (LCEC) is its limited applicability to gradient elution techniques. Amperometric electrochemical detectors exhibit both the best and the worst characteristics of solute property and bulk property detectors. While the Faradaic current arises only from the solute, the non-Faradaic current arises from... 

Kissinger, P.T. 1977. Amperometric and coulometric detectors for high-performance liquid chromatography. Anal. Chem. 49 447A-456A. 

Lund, W. Hannisdal, M. Greibrokk, T. Evaluation of amperometric detectors for high-performance liquid chromatography analysis of benzodiazepines. J. Chromatogr. 1979, 173, 249-261. 

With respect to chromatography, electrochemical detection means amperometric detection. Amper-ometry is the measurement of electrolysis current versus time at a controlled electrode potential. It has a relationship to voltammetry similar to the relationship of an ultraviolet (UV) detector to spectroscopy. Whereas conductometric detection is used in ion chromatography, potentiometric detection is never used in routine practice. Electrochemical detection has even been used in gas chromatography in a few unusual circumstances. It has even been attempted with thin-layer chromatography (TLC). Its practical success has only been with liquid chromatography (LC) and that will be the focus here. 

Reversed-phase liquid chromatography with thin-layer amperometric detector operated 271 at +0.720 V vs. Ag/AgCI... 

An important application of flow cells is their use as detectors in liquid chromatography (LC), capillary zone electrophoresis (CZE), and flow injection (FI) methods (46-50). Such cells may be coulometric ones, where all of the material flowing into the cell is electrolyzed, but more frequently they are amperometric or voltammetric cells, sometimes utilizing UMEs as described in Section 9.7. 

Oosterhuis, B. van Boxtel, C.J. Determination of salbutamol in human plasma with bimodal high-performance liquid chromatography and a rotated disc amperometric detector. J.Chromatogr., 1982, 232, 327-334 [SPE electrochemical detection plasma LOD 0.5 ng/mL pharmacokinetics]... 

Until recently the main disadvantage of this technique was the poor sensitivity of the detectors. Development of more sensitive detectors such as a reductive amperometric electrochemical detection, ultraviolet detection, ICP-AES, ICPMS, AFS, and AAS has resulted in wider applications in environmental studies. The main advantage of liquid chromatography is the possibility to separate a great variability of organomercury compounds. Applications of HPLC for Hg speciation studies have been reviewed by Harrington. ... 

Photo-acoustic spectroscopy has been used for ultratrace levels of Hg in air and snow (de Mora etal. 1993). X-ray fluorescence is nondestructive, rapid, requires minimal sample preparation, and was, for example, used successfully to determine the maximal level of mercury in maternal hair to assess fetal exposure (Toribora et al. 1982). However, the procedure is less sensitive compared to AAS and INAA if no pre-concentration is used. Electrochemical methods have been replaced as detectors in chromatography by other instrumental techniques because of poorer detection limits. High-performance liquid chromatography (HPLC) with reductive amperometric electrochemical reduction, however, was shown to be capable of speciating Hg(II), methyl- ethyl- and phenylmercury, with detection limits <2pgL (Evans and McKee 1987). 

Amperometric detectors are easily miniaturized with preservation of performance, since their operation is based on reactions at the electrode surface. Using a single carbon fiber or microelectrode as a working electrode allows detector cells of very small volume and in-column detectors to be constructed for use in open tubular and packed capillary column liquid chromatography [189-192]. These microcolumn separation techniques combined with amperometric detection are exploited for the quantitative analysis of volume-limited samples such as the contents of single cells [193,194]. 

Many simple carbohydrates and other polyhydroxy compounds can be oxidized at a silver oxide surface. The oxidation is via an electrocatalytic mechanism involving a Ag(I) oxide. This forms the basis of a flow stream detector operated in an amperometric mode which may be used for either flow injection or high performance liquid chromatography (HPLC) applications. The title electrode has been applied to the detection of simple carbohydrates, triglycerides and nucleic acid components. 

The Kissinger type twin electrode thin layer cell is a widely used tool in the everyday analytical practice especially in the field of flow injection techniques and as an amperometric detector in liquid chromatography. Less attention is paid to the possibilities offered by this cell 2LS a microanalytical tool when it is filled with a quiescent solution sample. In this way the determination of electroactive components in a volume of about 50-100 pi can be carried out by applying a proper excitation potential program. 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

---