**4. On-line liquid chromatography in bioactivity determination of compounds**

The methods of determination of antioxidant activity are popular, which does not mean that they are only used in scientific research. A number of modifications of those methods along with methods which are not presented here are still used in analytical procedures applied in examination of bioactive substances. Owing to constantly broadening knowledge on the mechanism of oxidation and action of antioxidants, the choice and development of analytical methods is also changing. Increasing awareness of biological activity and the availability of analytical methods has changed the way substances are analysed. Regarding the different transport mechanisms of substances in organisms, observations have been conducted with different test cells. An analysis of different substances in mixtures has revealed differences in their biological activity. Due to such differences within a sample, it may contain both strong antioxidants and biologically inactive compounds as well as pro-oxidants. Their separation may obtain individual substances, or their mixtures, with beneficial biological properties. In search of rich sources of bioactive substances, screening studies are conducted in which isolated components of mixtures are analysed for their activity, e.g. as antioxidants. Time and money which must be spent on such analyses, as well as new testing capabilities, combined with chromatographic methods have made looking for such sources much cheaper and easier. Examples of using different chromatographic methods to inhibit or promote oxidation reactions are presented below.
