Applications of LCEC

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... 

LCEC has become a widely used analytical technique for biomedical analysis. Several reports appear each month deairibing new LCEC analyses or reporting results obtain-with LCEC. While space does not permit a complete review of all bitwhemical applications, this section will consider applications of LCEC to general classes of compounds to provide an overview of the uses of this technique. For a thorough survey of the literature, a bibliography of LCEC applications is available... 

Blank, C. L., Wong, P., Pike, R., and Wassil, D., 1980, Application of LCEC to the determination of enzymatic activities. First International Symposium on the Neurochemical and Clinical Applications of LCEC, Indianapolis, Abstract No. 11, Bioanalytical Systems, West Lafayette, Indiana. 

The first and still most common LCEC application is determination of the catecholamines in biological samples. The number of papers describing methods for catecholamine determination for certain circumstances are far too numerous to cite, however, several good reviews are available The second major use of LCEC is in the... 

The majority of LCEC applications have used oxidative detection. This is likely because of the perceived difficulties encountered with reductive detection. In particular, dissolved oxygen and trace metal ions must be removed to prevent high background currents. These problems are not difficult to overcome and more applications of reductive detection should appear as this is more generally realized. 

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... 

Most electrode materials that are employed in LCEC can also be used for CEEC. The most commonly employed working electrode is a carbon fiber. Carbon fibers come in many different sizes and can also be etched to smaller diameters. Common applications of CEEC with carbon fiber electrodes are the detection of catecholamines in single neuronal cells and amino acids in brain microdialysis samples following derivatization with NDA/CN. 

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. 

Herein are described the principles, practice, and promise of LCEC in the determination of alkaloids in plant material. First the basic principles of LCEC relative to the determination of alkaloids are explained. Then the most recent methods applicable to LCEC detection of alkaloids in plant material are described, including our own work with Catharanthus alkaloids in cell cultures. Finally, we suggest applications for other alkaloids, which are based on published accounts of LCEC detection of alkaloids in body fluids and in other nonplant matrixes. 

One of the most active areas of electrochemical research and development in the last decade has been the use of electrochemical detectors for flowing stream analysis. Of particular interest is the analysis of complex solutions containing several chemical species. The use of high performance chromatography coupled to electrochemical detection (LCEC) is an important development in this regard. In most applications of conventional LCEC, such as in clinical, biomedical or organic analysis, discrete samples are analysed. 

Sensitivity by itself is not sufficient to completely evaluate an LCEC system for analytical purposes. The minimum detectable quantity (detection limit) is of more practical importance. The detection limit takes into consideration the amount of baseline noise as well as the response to the analyte. The detection limit is then defined as the quantity of analyte which gives a signal-to-noise ratio of three (a S/N of 3 is the generally accepted criterion although other values have been used). For a complete description of an LCEC application, both the sensitivity and detection limit, along with the S/N criteria used, should be provided. 

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. 

Another dynamic measurement is the LCEC technique which can he thought of, simplistically, as FIA using a chromatographic column positioned between the sample injection port and the detector. Bioanalytical systems (BAS) of West Lafayette, Indiana, specializes in instrumentation for LCEC. Their catalogs come with extensive bibliographies covering a variety of applications. 

S. M. Waraszkiewicz, E. A. Milano, and R. L. DiRubio, A stability indicating LCEC method for epinephrine and its application to the analysis of lidocaine hydrochloride injectable solutions, Curr. Sep., 4(2) 23 (1982). 

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... 

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