Pesticide direct fluorescence

The rims of the CD are often derivatized to improve sensitivity by enhancing analyte association. Hydroxypropyl modification in general has proven useful for the detection of analytes by direct fluorescence and some assays have been successfully developed [250,251], For instance, Aaron and Coly have used both native and hydroxypropyl 3-CD to detect aromatic pesticides [252], Likewise, Zhang and Gong [253] have detected the biologically relevant substance, tryptamine, by monitoring its fluorescence within hydroxypropyl 3-CD. Ethyl modification of the bucket rim was sufficient to allow de Rossi et al. [254] to detect tetracycline by direct fluorescence. 

Fig. 1 Photograph taken with a videodensitometer of a Whatman Multi-K SC5 plate showing the 2-D separation of a 16-component mixture of pesticides. The fluorescence-quenched zones are outlined for clarity. Direction 1 ethyl acetate-diisopropyl ether (2.5 97.5) mobile phase on the 20 cm x 3 cm K5F silica gel strip (normal phase TLC) direction 2 acetonitrile-water (85 15) on the adjacent 20 cm x 17 cm C-18F bonded silica gel layer (RPTLC). X Origin. Pesticides 1, propaquizafop 2, quizalofop-P 3, triadimefon 4, triadimenol 5, fenoxycarb 6, quinoxyfen 7, cyromazine 8, oxyfluorfen 9, fluoroglycofen 10, acetochlor 11, metazachlor 12, piperonyl butoxide 13, fur-alaxyl 14, pyriproxyfen 15, buprofezin 16, clofentezine. Source From Separation of a mixture of pesticides by 2D-TLC on two-adsorbentlayer Multi-K SC5 plate, in J. Liq. Chromatogr. Relat. Technol. with permission of Marcel Dekker, Inc.

Direct determination of urea pesticides by high-performance liquid chromatography has been widely reported in the literature (10,32-36,127-130). Ultraviolet detection has often been used (32,33,35,36,60,127) with usually acceptable sensitivity, although this detector is nonspecific and the sensibility is, in general, low. To overcome this problem, several techniques have been assayed, such as precolumn enrichment (60), postcolumn derivatization (34,10), and the use of other detection techniques such as the electrochemical (129), photoconductivity (128,130), and fluorescence detectors (9,10,34). Table 9 summarizes representative papers using these techniques in HPLC analysis. 

Carbamates and substituted ureas are a numerous group of pesticides widely used to control weeds, pests, and diseases in fruit trees, vegetables, and cereals. Carbamate residues in foods are commonly extracted with water-miscible solvents and determined by using a liquid chromatograph equipped with a sensitive detector, frequently a UV detector. In addition, to obtain adequate detection selectivity, the postcolumn fluorimetric labeling technique is used for methyl carbamates. Substituted ureas are normally extracted from foods with organic solvents, and they can be determined directly by HPLC-UV or after postcolumn derivatization by fluorescence determination of their derivatives. 

The use of pH-sensitive fluorescent indicators as spray reagents has been recently examined for the determination of sulphur-containing pesticides and amino acids separated by TLC [158,159]. The procedure is adapted from a ligand-exchange method of Frei and Mallet [160]. The separated pesticides are brominated directly on the TLC plate. This treatment oxidizes the pesticides and liberates hydrobromic acid as a side product. On... 

For purposes of this symposium, I have limited myself to still another of these approaches. That approach includes extraction, separation by HPLC, and direct measurement of the relatively high natural fluorescence inherent in the molecular structure of the pesticides themselves. You will find that solvent polarity and instrumental parameters are important variables when I attempt to contrast the chromatograms obtained in the absorbance mode with those obtained in the fluorescence mode. 

There are various alternatives to obtain fluorescence with a pesticide. For instance, fluorescence may be derived from the pesticide by treatment with heat, acid, base, inorganic salts or a combination of these. Another approach is to prepare a derivative Cfluorogenic labelling) in solution the derivative is then extracted and applied directly on a chromatogram for separation. A fluorogenic reagent that will become fluorescent on contact with the pesticide may also be used. These alternatives are summarized in Figure 5. 

Pesticides Possessing Native Fluorescence. The number of pesticides that possess enough natural fluorescence to enable their quantitative determination directly on thin-layer chromatograms is rather limited. Typical examples are given in Table X which includes their spectral characteristics [9]. With the exception of diphacinone, most fluoresce in the blue region of the spectrum. The limit of detection for each compound is in the low nanogram per spot range. 

Fluorescence of Pesticides by the Direct Approach. The fluorescence obtained by the methods just mentioned comes from the reagents, i.e., fluorogenic sprays and labelling compounds. There is sometimes a lack of selectivity since the fluorescence is nearly always the same for all the compounds on a particular chromatogram whether they are pesticides or impurities. In some cases there is also background fluorescence in addition to interferences from co-extractives which fluoresce under the experimental conditions. But for many pesticides there is no other choice. 

In addition to direct metal analysis, indirect analysis for ligands that pull away the metal ion from the fluorescent metal chelate, is also possible. In this case, a decrease in fluorescence is observed. For example, fluoride causes the fluorescence of the alumi-num-Eriochrome Red B complex to decrease. In a similar way, nonfluorescent metal chelates can be used for the fluorimetric analysis of nonfluorescent analytes via a ligand-exchange reaction, according to the scheme shown in Figure 9. In the first step a nonfluorescent complex is formed between a fluorescent chelator and a metal ion. In the next step the addition of the pesticide induces the displacement of the fluorescent ligand and then combines with... 

In pesticide trace analysis, efforts have been made to observe directly the coupling of antigen-antibody in the quenching of the fluorophore bound to a tracer molecule, by the use of time-resolved. laser-induced fluorescence [31]. On formation of an Ag -AB complex, a significant change in the fluorescence lifetime is observed. However, the fluorescence quantum yield suffers drastically, and this test has never been used. 

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