Postcolumn reaction detectors

In spite of its attractive features solid-phase analytical derivatization is not widely used at present. It is most useful for trace analysis but requires careful optimization. Reaction yields are rarely quantitative (although reproducible) and background contamination from by-products derived from the extraction sorbent can be a problem. Most applications so far have been set up with gas chromatography as the determinant step rather than liquid chromatography. 

The parabolic-low profile in an open-tubular reactor restricts applications to fast reactions if extensive band broadening is to be avoided [70,71]. The reaction time can be extended by using reactors prepared from optimally deformed capillary tubes. The introduction of secondary flow, even at low flow rates, breaks up the parabolic profile and minimizes band broadening. These reactors are now commercially available in 

Photolysis can be used to convert some compoimds into easier-to-detect products for subsequent on-line derivatization [29,31,68,69,73]. Photolysis causes degradation into smaller molecules (photolysis) and possibly their further reaction with water (photohydrolysis), intramolecular rearrangements, photodimerization, or photoionization as well as possible electron transfer. These photolysis products can be detected at low concentrations directly or after conversion to suitable derivatives by UV—Visible, fluorescence, or electrochemical detection. The primary reagent for photolysis reactions are photons, and their reactivity can be controlled to some extent by varying the excitation wavelength and intensity. However, the main limitation remains the limited number of compoimds that yield useful detection products by photolysis. 

Photoreactors are simple in design, consisting of one or more high-intensity discharge lamps with a PTFE KOT tube reactor woimd aroimd one of the lamps. The PTFE tube creates a light-tube effect, which 

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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). 

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.)... 

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... 

D. T. Gjerde and J. V. Benson, Suspension postcolumn reaction detector for liquid chromatography. Anal. Chem., 62,612,1990. 

In these systems, a high-energy intermediate excites a suitable fluorophore, which then emits its characteristic fluorescence spectrum consequently, they are termed indirect or sensitized chemiluminescence. The most common analytical application has been as a postcolumn reaction detector for liquid chromatography. Various fluorescent analytes (polycyclic aromatic hydrocarbons and polycyclic aromatic amines) and compounds derivatized using dansyl chloride, fluorescamine, or o-phthalaldehyde have been determined with sub-femtomole detection limits. 

The lack of suitable detectors in LC for trace and ultratrace analysis of complex matrices has catalyzed this trend. The major advantage of postcolumn reaction detectors is the absence of artifact information, and the only requirement is good reproducibility. A disadvantage is the great influence of the mobile phase on the reaction medium. 

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