Derivatisation reactor

Post-column on-line derivatisation is carried out in a special reactor situated between the column and detector. A feature of this technique is that the derivatisation reaction need not go to completion provided it can be made reproducible. The reaction, however, needs to be fairly rapid at moderate temperatures and there should be no detector response to any excess reagent present. Clearly an advantage of post-column derivatisation is that ideally the separation and detection processes can be optimised separately. A problem which may arise, however, is that the most suitable eluant for the chromatographic separation rarely provides an ideal reaction medium for derivatisation this is particularly true for electrochemical detectors which operate correctly only within a limited range of pH, ionic strength and aqueous solvent composition. 

Post-column on-line derivatisation is carried out in a reactor located between the column and the detector. With this technique, the derivatisation reaction does not need to go to completion, provided it can be done reproducibly, and the reaction does not produce any chromatographic interferences. The reaction needs to take place in a fairly short time at moderate temperatures, and the reagent should not be detectable under the same conditions at which the derivative is detected. The mobile phase may not be the best medium in which to carry out the reaction, and the presence of the reactor after the column will increase the extra-column dispersion. 

Fig. 2.4p shows three types of post-column reactor. In the open tubular reactor, after the solutes have been separated on the column, reagent is pumped into the column effluent via a suitable mixing tee. The reactor, which may be a coil of stainless steel or ptfe tube, provides the desired holdup time for the reaction. Finally, the combined streams are passed through the detector. This type of reactor is commonly used in cases where the derivatisation reaction is fairly fast. For slower reactions, segmented stream tubular reactors can be used. With this type, gas bubbles are introduced into the stream at fixed time intervals. The object of this is to reduce axial diffusion of solute zones, and thus to reduce extra-column dispersion. For intermediate reactions, packed bed reactors have been used, in which the reactor may be a column packed with small glass beads. 

Reducing sugars Wines Microwave-assisted sugar oxidation by hexacyanoferrate(III) in alkaline medium UV—Vis 9 pmol I, 1 Flow injection system derivatisation using the oxidising reagent de-bubbler before measurement coiled reactor around the oven antenna [128]... 

Unlike pre-column derivatisation, the post-column derivatisation reaction need not reach completion, although the extent of reaction must be reproducible (Gfeller et al., 1977). It is this requirement, coupled with the desire to limit band spreading, which has led to reactor design being a crucial factor (Schwedt, 1979). Three reactor... 

---