Chromatography is a science of separations.
High Performance Liquid Chromatography (HPLC) like other forms of chromatography, is used to separate complex mixtures into their components. There are many flavors of HPLC, but what they have in common is that the separation takes place in solution. Having separated a mixture, you need to see the components. The most popular detectors use either UV/VIS light absorption, or fluorescence. Unfortunately, many substances are difficult to detect. Moreover, you want to see the components of interest without distraction from the background.
Post-column derivatization, also known as post-column reaction, renders visible certain compounds that are normally invisible. This trick is accomplished after the separation by performing a chemical reaction on the substances that gives them an easily-detectable physical property. Typically you use a reaction that produces a strong color or makes a fluorescent product. You can increase the sensitivity of detection by several orders of magnitude in favorable cases. Most reagents are selective for a particular class of substances, so analytes of that class are more easily seen against a complex background. So, post-column derivatization is used to increase sensitivity and selectivity in HPLC analysis.
The post-column reaction system mixes the stream of eluant from the HPLC column with a stream of reagent solution. The mixture usually flows through a reactor to allow enough time for the chemical reactions to complete. If the reaction is slow, the reactor may be heated to speed things up. Some reactions need two or more reagents added in sequence. Finally the mixed streams pass into the detector, typically UV/VIS absorbance or fluorescence. Of course a practical system requires metering pumps, pulse-dampeners, thermostats, and safety systems to give reliable results.
Examples of the chemistry and hardware are given in the catalog and user’s manuals.