Absorption detectors derivatizing reagents

Chemical derivatization has become a very popular technique for increasing the sensitivity of a specific type of detector to compounds for which it normally exhibits little or no response. An examples of this procedure would be the reaction of an aliphatic alcohol, which contains no UV chromophore, with benzoyl chloride to form the benzyl ester which could then be detected by the UV detector. Derivatization can also be used to permit the use of an alternative type of detector to increase sensitivity. For example an amino acid may exhibit only weak absorption in the UV, but when reacted with a suitable fluorescing reagent, could be detected by means of the fluorescence detector at concentration levels one or two orders of magnitude lower than with the UV detector. It is clear that derivatization procedures can increase significantly the versatility of many detectors. 

Quite often, neither of the three aforementioned detection systems will provide adequate sensitivity and selectivity to complete the analysis with specified lower limits of detection. In these cases, an increasingly popular trend has been to either pre- or postcolumn derivatize the analyte or analytes to convert them to species that can be adequately detected with reliable and commercially available detectors. Tables VII and VIII list a variety of reagents for the derivatization of specific organic moieties that provide increased UV absorption and fluorescence, respectively (39). 

Many compounds are not sensitive to uv detection. Currently, two main methods are used to improve this sensitivity, both requiring either a pre- or a postcolumn reaction. The first method is to chemically add a highly absorptive group to the compound to be detected, and the second is to add a compound that fluoresces to the compound being detected. Once the compounds are through the column and have been separated, they then are reacted with other reagents in small-volume reaction chambers before they get to the detector. One postcolumn apparatus to derivatize compounds is the Pickering apparatus discussed later. 

The electrochemical detector has been used for the analysis of roquefortine and the zearalenones and refractive index detection has bean used for trichothe-cenes such as T-2 toxin with no native fluorescence or useful UV absorption. For molecules such as T-2 toxin, with a free hydroxyl group, it is possible to derivatize with a reagent such as an aromatic acid, which confers UV absorption on the derivative. 

Derivatization procedures have been used to enhance the sensitivity of different solutes to a variety of other detecting systems. Reagents have been suggested for rendering solutes amenable to electrochemical detection, radioactivity detection and even atomic absorption spectrometers when used as detectors. However, further discussion on precolumn derivatization techniques is outside the scope of this book and those interested are recommended to read the book by Frei and Lawrence (41) that deals exclusively with derivatization procedures. 

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