Reaction detector

Reaction detectors are a convenient means of performing online postcolumn derivatization in HPLC. The derivative reaction is performed after the separation of the sample by the column and prior to detection in a continuous reactor. The mobile phase flow is not interrupted during the analysis and reaction, although it may be augmented by the addition of a secondary solvent to aid the reaction or to conform to the requirements of the detector. Reaction detectors are finding increasing application for the analysis of trace components in complex matrices where both high detection sensitivity and selectivity are needed. Many suitable reaction techniques have been published for this purpose [641-650]. 

Figure 8.38 Schexatic diagraiM of soaa typical reaction detectors used in HPLC. A, non-segxented tubular reactor B, segnented tubular reactor C, extraction segmented reaction detector. P pump, PS - phase sepeurator, B - device for introducing bubbles and 0 = detector.

This sets an upper limit of about 20 minutes for the slowest reaction that can be easily exploited. The time required for r reaction most profoundly influences the design of the reaction detector. 

The photochemically-induced change of an analyte in a flowing stream can be used as the basis of a reaction detector. Photolysis may be u to convert a substance into more readily... 

Scholten, A. H. M. T. and Frei, R. W., Identification of ergot alkaloids with a photochemical reaction detector in liquid chromatography, /. Chromatogr., 176, 349, 1979. 

Determination of organolead metabolites of tetraalkyllead in urine can be carried out after solid-phase enrichment and end analysis using reversed-phase HPLC with chemical reaction detector and by LC-MS (thermospray127). The chemical derivation consists of conversion to the dialky Head form, as shown in reaction 1, followed by complex formation with 4-(2-pyridylazo)resorcinol (11) and spectrophotometic measurement at 515 nm128. 

Igawa, M., J. W. Munger, and M. R. Hoffmann, Analysis of Aldehydes in Cloud- and Fogwater Samples by HPLC with a Postcolumn Reaction Detector, Environ. Sci. Technol, 23, 556-561 (1989). 

Fig.4.72. Diagram of the luminol reaction detector and chromatograph for analysis of trace amounts of metals (see text for details). (From ref. 200 with permission of Marcel Dekker, New York.)...

Aqueous samples analyzed by HPLC using a postcolumn reaction detector, formaldehyde separated on a reversed phase C-l 8 column derivatized with 3-methyl-2-benzothiazolinone hydrazone and detected at 640 nm (Igawa et al., 1989). (The method was developed for cloud and fogwater analysis.)... 

Applicability of the conductivity detector can be extended by chemical derivatization or by the use of postcolumn photochemical reactions [78]. The use of a photochemical reaction detector, also known as a photoconductivity detector, can also be very selective. Only certain organic compounds such as trinitroglycerin, chloramphenicol, and hydrochlorothiazide will undergo photolytic decomposition to produce ionic species. 

Miles and Moye [171] have shown that several classes of nitrogen containing pesticides responded to a high performance liquid chromatography post-column reaction detector that employed ultraviolet photolysis with optional reaction with o-phthalicdicarboxaldehyde-2-mercaptoethanol followed by fluorescence detection. It was applied to the determination of jV-methylcarbamates, carbamoyl oximes, carbamethoic acids, dithiocarbamates and phenyl ureas, phenyl amides and phenyl carbamates in groundwater. See also Table 4.3. 

Column chromatography [50] has been used to determine sulphate in waste water. A post-column solid phase reaction detector was employed in conjunction with an anion exchange separation column to determine 5 to 40mg L 1 sulphate. 

Engelhardt H, Meister J, Kolia P. 1993. Optimisation of post-column reaction detector for HPLC of explosives. Chromatographia 35 5-12... 

The so-called micro-total analytical systems (/tTAS) can integrate sample handling, separation, and detection on a single chip [9]. Postcapillary reaction detectors can be incorporated as well [10]. Fluorescence detection is the most common method employed for these chip-based systems. A commercial instrument (Agilent 2100 Bioanalyzer) is available for DNA and RNA separations on disposable chips using a diode laser for LIF detection. In research laboratories, polymerase chain reaction (PCR) has been integrated into a chip that provides size separation and LIF detection [11]. 

By postcolumn derivatization, the analyte is deriva-tized after the separation and before the detection by using a reaction detector. The simplest way is to add a reagent solution to the column effluent with an extra pump. After the mixing T-piece, a reactor with a suitable holdup volume is inserted to allow reaction to take place. The benefits of this approach are that chromatographic separation is not affected and reaction is not required to be complete. 

Problems are often found in many analytical methods due to the complex nature of the mixture and the lack of adequate detection means, thus leading to poor quantitation techniques. For the routine separation of a broad range of surfactants, high-performance liquid chromatography (HPLC) appears to be the most cost-effective [7-18]. Ultraviolet (UV) and fluorescence detectors are commonly used in HPLC analysis of surfactants because of their compatibility with separation techniques requiring gradient elution. However, these detectors have two inherent limitations (a) the detector response is dependent on molecular structure (i.e., degree of aromaticity and type of substitution) and (b) only species with a chromophore can be detected. To overcome those limitations, postcolumn reaction detectors, based on extraction of fluorescent ion pairs, were introduced for on-line detection of alkylsul-... 

Electrolytic - Postcolumn reaction detector for Halogen-, S-, 0.1 to l.OpgCl on FID... 

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