Introduction

The role of radicals in (bio-)chemical reactions is currently becoming increasingly significant. Free radicals may be generated by any kind of irradiation and contribute essentially to many aging processes in many materials, particularly in the presence of oxygen. They are even able to cause manifold organic damages as in lipid peroxidation or in inflammatory diseases. Organic reperfusion injuries after ischaemia are currently the subject of intensive research activities. On the other hand, stable free radicals are used practically in a number of applications in many fields, e.g. as additives in industrial processes such as polymerization or as analytical tools in research on membrane, emulsion and surface properties of materials or formulations. The utilization of such substances as protective additives, e.g. for process control or as research tools, has stimulated interest in the synthesis of new compounds of this class. The increasing search for radicals is paralleled with a rising demand for methods to detect, identify and quantify them. In the context of separation techniques, this means having a technique at hand to trace them in eluted fractions.

High chemical reactivity combined with low specificity is typical of the majority of radicals. As a consequence, solutions containing such substances are likely to alter their composition within a short time owing to the decreasing content of reactive components and to the accumulation of reaction products. This kind of change can easily be monitored by chromatographic methods. Having separated all the constituents at a given time, it may become necessary

Krull IS, Szulc ME and Dai J (1997) Derivatizations in HPCE. A Primer. Thermo Bioanalysis Corporation, San Jose, CA.

Lawrence JF (1981) Organic Trace Analysis by Liquid Chromatography. New York: Academic Press.

Lingeman H and Underberg WJM (eds) (1990) Detection-Oriented Derivatization Techniques in Liquid Chrom-atography. New York: Marcel Dekker.

to identify original or intermediate radicals to evaluate the particular stage of an ongoing reactive process. However, the detectors routinely used in HPLC cannot indicate directly any radical present in the separated fractions. The most advanced method suitable for this purpose is electron spin resonance spec-troscopy (ESR), but most ESR spectrometers are currently installed as large and heavy instruments, not at all suitable as detectors for chromatographic methods. It will be shown here that this kind of spectroscopic method can be realized with devices of table-top size that can easily be integrated into any chromatographic separation line.

Electron spin resonance (or electron paramagnetic resonance, EPR) spectroscopy is the only direct method to measure radicals since it is based on the existence of unpaired electrons. Likewise, paramagnetic metal complexes are also sensitive to this spec-troscopic method. A substantial advantage for the study of radical reactions would be the rapid analysis of a fraction directly upon separation to avoid changes caused by putative consecutive reactions. This can only be achievable by direct coupling of the separator (HPLC) with the specific detector (ESR) in the shortest possible way without any unnecessary dead volume (long tubing lines or valves). An absolute prerequisite for such an instrumental set-up is a spectrometer of a size that allows installation at the site of the sample separation, and not necessarily vice versa. The other problem is synchronization of sample separation and recording of a spectrum which requires a deRnite period of time. Both problems have been solved in the on-line coupling of HPLC and ESR spectroscopy described here.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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