Liquid LCEC

Because of this lack of resolving power, much electroanalytical research is aimed at providing increased selectivity. This can be accomplished in two ways. First, electrochemistry can be combined with another technique which provides the selectivity. Examples of this approach are liquid chromatography with electrochemical detection (LCEC) and electrochemical enzyme immunoassay (EEIA). The other approach is to modify the electrochemical reaction at the electrode to enhance selectivity. This... 

Liquid chromatography/electrochemistry (LCEC) has become recognized as a powerful tool for the trace determination of easily oxidizable and reducible compounds. This is because detection of as little as 0.1 pmol of material is readily accomplished with relatively simple and inexpensive equipment. Initial interest in LCEC was generated by the determination of several aromatic matabolites of tyrosine in the central nervous system. However, the application of LCEC into other areas of biochemistry has begun at a growing pace. A bibliography of LCEC applications is available... 

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. 

LCEC Liquid chromatography with L2ToFMS Laser-desorption laser-... 

Whatever the future of electroanalysis really will be, it is worth considering what Faulkner recently said190 at a meeting on "Electrochemistry Faces Reality , where he mentioned more explicitly the significant impact of LCEC, liquid chromatography electrochemistry (see p. 346) and cybernetic instrumentation. 

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. 

The modern interest in electrochemical detectors for liquid chromatography was stimulated by the recognition that this technique was ideal for the study of aromatic metabolism in the mammalian central nervous system. Most of the papers published during the past 20 years have focused on applications of the LCEC technique to neurochemical problems. Since the First commercial detectors became available in 1974, a number of other areas of application have been explored as well. The trade publication Current Separations provides a useful... 

LCEC systems are used for a wide variety of applications, many of which have been published [1,32]. Space does not permit a thorough review however, it is possible to generalize by considering the classes of compounds that have most frequently been studied phenols, aromatic amines, thiols, quinones, and nitro compounds. By analogy with liquid chromatography with UV detection (LCUV), it is frequently desirable to obtain an electrochemical spectrum of a compound to assess Its suitability for LCEC. Cyclic voltammetry (CV) is the electrochemical equivalent of spectroscopy (Chap. 3). It is useful to carry out CV experiments in several possible mobile phases, since electrochemical reactions can be very dependent on the medium. 

A dual-electrode liquid chromatography-electrochemistry (LCEC) system used in the detection and identification of flavanols and procyanidins in wines and grape seeds is a valuable tool (30). Voltammetric behavior of phenolic compounds by LCEC could provide information that cannot be obtained using HPLC with UV detection, for which the identification is usually based on a comparison of the retention time with that of standard compounds, especially for the identification of catechins and procyanidins with a small amount of sample available (30). Figure 10 shows the procyanidins commonly found in wines. 

In conclusion, we have tried to present the principles of LCEC, describe the present applications that have been made, and survey the areas of potential utility by reviewing pertinent chemistry and related methods. It is our opinion that the success of LCEC in neurochemistry can carry over to pesticides owing not only to the advantages of electrochemical detection but also to the tremendous potential of its adjunct, liquid chromatography. 

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... 

Ding, X.D. Krull, I.S. Dual electrode liquid chromatography-electrochemical detection (LCEC) for platinum-derived cancer chemotherapy agents. J. Liq. Chromatogr. 1983, 6, 2173-2194. 

Kissinger P. LCEC The combination of liquid chromatography and electrochemistry. J Chem Ed 1983 60 308-11. 

Furthermore, the voltammetric data established a background for the quantitative determination of catecholamines using a selective am-perometric detector in combination with liquid chromatography (LCEC technique) 254). The tendency of this class of compounds to absorb at electrode surfaces (such as platinum) allowed the determination of... 

LCEC liquid chromatography with electrochemical detection 11.6.4... 

Phenol can be detected electrochemically by oxidation at a carbon paste electrode (Wehmeyer et al., 1983). A convenient means of determining a low concentration of phenol in a small volume of sample is by liquid chromatography with electrochemical detection (LCEC). A diagram of the LCEC system is shown in Fig. 2. The sample is injected by means of 20-pl sample loop into a 5-cm column slurry-packed with lO-pm Cjg stationary phase. The column serves to separate the peak for phenol from other assay constituents in order to achieve a better detection limit. The phenol is detected by oxidation in a thin-layer electrochemical cell with a carbon paste working electrode. 

Fig. 8. Correlation between digoxin concentration in patients by RIA and competitive heterogeneous enzyme immunoassay liquid chromatography/ electrochemistry (EIA-LCEC). A perfect correlation is represented by the solid line. (Reprinted with permission from Wehmeyer et al., 1986. Copyright 1986, American Chemical Society.)...

Liquid Chromalogniphy/Eiectroclieinistry II. BASIC PRINaPLES OF LCEC... 

Fig. 1 One example of a sandwich type thin-layer LCEC detector with adjustable dead volume, flow pattern, and up to four channels. Source From Four channel liquid chromatography/electrochem-istry, in Curr. Sep. ...

SucKOW, R. F., and Cooper, T. B., 1980, The electrochemical detection of imipramine, desipramine, and their 2-hydroxylated metabolites in plasma using reversed-phase paired-ion liquid chromatography. First International Symposium on Neurochemical and Clinical Applications of LCEC, Indianapolis, Abstract No. 8, Bioanalytical Systems, West Lafayette, Indiana. 

Five types of amperometric detection have been applied for enzyme-based ECIA. They are flow-injection analysis with electrochemical detection (FIAEC), liquid chromatography with electrochemical detection (LCEC), amperometric detection with interdigitated array electrodes (IDA), rotating disk electrode (RDE) amper-ometry, and scanning electrochemical microscopy (SECM). Of these, the two conventional types, FIAEC and LCEC, shall be discussed in this section, leaving the discussion of the other types to Section V on miniaturized immunoassays. 

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