Indirect laser-induced analysis

NACE with indirect detection has been applied to the determination of fatty acids (FAs) and ascorbic acid (AA), respectively. C2-C18 FAs have been separated in less than 12 min using 8-hydroxy-7-iodoquinoline sulfonic acid as chromophores in NACE with indirect absorbance. The dissociation constant (p/linear plot -log[(mu 0/mu) - 1] versus pH, using 20% isopropanol and 40% ACN as the organic modifier in NACE, are all above about two units than those obtained in aqueous solution. NACE with indirect laser-induced fluorescence, using merocyanine 540 MC540) asfluorophores, has been performed for the analysis of AA and its stereoisomer, isoascorbic acid (lAA), and the limits of detection of AA and lAA are 0.30 and 0.17 pM, respectively. This method has been applied to the determination of AA in a lemon juice spiked with lAAasthe internal standard in less than three minutes, and its concentration is 76.7 + 0.4 mM. 

Indirect UV detection is a common technique which has been applied to the analysis of cations and anions. A UV-absorbing anion plus an electroos-motic flow modifier is added to the electrolyte. The displacement chromophore permits indirect photometric detection. Optimum separations can be achieved by choosing an electrolyte anion which has a mobility similar to the ions of interest. Some investigators have used conductivity detection, while others have used indirect laser-induced fluorescence for anion detection. Detection of the nonfluorescence analyte is obtained by charge displacement of the fluorophore (339,340). 

Capillary electrophoresis has been applied to the analysis of enzymically glucosylated flavonoids and of monosaccharides from glycosaminoglycans, using UV detection and indirect laser-induced fluorescence detection, respectively, and the separation of the cyanogenic glycosides amygdalin and prunasin from their isomers has been achieved by a micellar capillary electrophoresis method. ... 

Electrochemical (amperometric) detection is also the method of choice for the analysis of nitroaromatic explosives on microchip devices since it offers up to three orders of magnitude higher sensitivity than indirect laser-induced fluorescence. ° Microfluidic device for detection of five TNT-related explosive compounds with exchangeable carbon thick-film screen-printed amperometric detector can be mentioned in this context. ... 

Many applications for ion analysis use a UV detector with indirect detection, though other electrochemical, laser-induced fluorescence (LIE), or mass spectrometry detectors have been described. The main advantage of UV detection is its availability on commercial instruments and that both UV-absorbing and non-UV-absorbing analytes may be detected. Nowadays, electrochemical detectors are also available specific background electrolytes (BGEs) must be used and the detector has to be adapted to existing CE instruments. 

Experimental Analysis Despite the large number of works devoted to the investigation of the radical R" reactivity, the determination of the experimental values of the radical addition rate constants to a monomer double bond remains difficult. Only a small part of these values is based on a direct detection of the produced radical (flash photolysis, ESR). In fact, radicals are very often difficult to observe because of their absorption band below or close to 300 nm. Moreover, the classical overlap of the absorption spectra in the UV (due to the presence of different transient and ground-state species inherent to the production route when using laser flash photolysis) often prevents the direct observation. Indirect methods were tentatively used but they involve a complex set of reactions and are rather difficult to carry out. The same holds true for laser-induced photocalorimetry [265]. 

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