Thin-layer chromatography Additives

In conclusion, synthetic dyes can be determined in solid foods and in nonalcoholic beverages and from their concentrated formulas by spectrometric methods or by several separation techniques such as TEC, HPLC, HPLC coupled with diode array or UV-Vis spectrometry, MECK, MEECK, voltammetry, and CE. ° Many analytical approaches have been used for simultaneous determinations of synthetic food additives thin layer chromatography, " " derivative spectrophotometry, adsorptive voltammetry, differential pulse polarography, and flow-through sensors for the specific determination of Sunset Yellow and its Sudan 1 subsidiary in food, " but they are generally suitable only for analyzing few-component mixtures. 

Percentage expresses HPLC recovery of 2-bromo-4,6-dinitroaniline (BDNA). Additional thin layer chromatography measurements indicate anaerobic transformation BDNA... 

Several additional thin layer chromatography systems have been reported by Kajfez, et al.11 ... 

Diacetates. The alkenylglycerols are isolated by thin-layer chromatography on silica gel G in a solvent system of hexane-diethyl ether (80 20, v/v). These compounds can be located by use of the TNS spray or by running an additional thin-layer chromatography plate for charring with sulfuric acid. 

In addition, thin-layer chromatography was employed for the separation of technetium and molybdenum. Neutron-irradiated molybdate was separated from produced 99niTc04 on cellulose MN 300 using butanol saturated with I M HCl [180]. Molybdate was identified in pertechnetate solutions by means of thin layers of silica gel or AI2O3 with mixtures of 1 M HCl/methanol or 1 M HCI/ethanol as solvents. ITic TCO4 spot revealed a higher mobility than the M0O4 spot [181]. 

Thin-Layer Chromatography. Chiral stationary phases have been used less extensively in tic as in high performance Hquid chromatography (hplc). This may, in large part, be due to lack of avakabiHty. The cost of many chiral selectors, as well as the accessibiHty and success of chiral additives, may have inhibited widespread commerciali2ation. Usually, nondestmctive visuali2ation of the sample spots in tic is accompHshed using iodine vapor, uv or fluorescence. However, the presence of the chiral selector in the stationary phase can mask the analyte and interfere with detection (43). 

Analysis. Dilute aqueous solutions of hydroxyhydroquiaone turn blue-green temporarily when mixed with ferric chloride. The solutions darken upon addition of small amounts, and turn red with additions of larger amounts of sodium carbonate. Derivatives used for identification are the picrate, which forms orange-red needles (mp of 96°C), and the triacetate (mp 96—97°C). Thin-layer chromatography and Hquid chromatography are well suited for the quahtative and quantitative estimation of hydroxyhydroquiaone (93,94). 

Immobilization. The abiUty of cyclodextrins to form inclusion complexes selectively with a wide variety of guest molecules or ions is well known (1,2) (see INCLUSION COMPOUNDS). Cyclodextrins immobilized on appropriate supports are used in high performance Hquid chromatography (hplc) to separate optical isomers. Immobilization of cyclodextrin on a soHd support offers several advantages over use as a mobile-phase modifier. For example, as a mobile-phase additive, P-cyclodextrin has a relatively low solubiUty. The cost of y- or a-cyclodextrin is high. Furthermore, when employed in thin-layer chromatography (tic) and hplc, cyclodextrin mobile phases usually produce relatively poor efficiencies. 

As a general procedure, a mixture of the steroidal ketone (50 mg) and lithium aluminum deuteride (20 mg) in dry ether (5 ml, freshly distilled from lithium aluminum hydride) is heated under reflux until the reduction is complete according to thin layer chromatography test. The excess deuteride is then decomposed by the careful addition of a few drops of water and the reaction mixture is worked up by the usual procedure. For hindered ketones or esters the use of other solvents, such as tetrahydrofuran or dioxane, may be preferable to allow higher reaction temperatures. 

In addition to the reagent sequences with clearly detectable reaction mechanism, whict have already been described, many sequences of reagents not covered by any of th< reaction types described have also found appUcation in thin-layer chromatography. Th< reaction sequences that remain to be described were all designed to provide as speciflt a detection of the separated substances as possible. 

High performance liquid chromatography (HPLC) has been by far the most important method for separating chlorophylls. Open column chromatography and thin layer chromatography are still used for clean-up procedures to isolate and separate carotenoids and other lipids from chlorophylls and for preparative applications, but both are losing importance for analytical purposes due to their low resolution and have been replaced by more effective techniques like solid phase, supercritical fluid extraction and counter current chromatography. The whole analysis should be as brief as possible, since each additional step is a potential source of epimers and allomers. 

Not only in HPLC, but also in modem thin-layer chromatography, the application of reversed-phase stationary phases becomes increasingly important. The advantage of the hydrophobic layers in comparison with the polar, surface-active stationary phases is the additional selectivity and a reduced hkehhood of decomposition of sensitive substances. 

Thin-layer chromatography is employed in many areas of QC and routine monitoring of product quality [458]. Fluorescence scanning, densitometry or videodensitometry are used for quantification. Not all polymer additives are amenable to TLC analysis. Some fatty acid amides are virtually insoluble in organic solvents and cannot be isolated by thin-layer or column chromatography. 

Many organic additives can be analysed using the technique of high performance liquid chromatography (HPLC), particularly using MS for positive identification. HPLC methods have now largely superseded thin layer chromatography (TLC)-based procedures. 

Van de Vaart et al. [45] used a thin-layer chromatographic method for the analysis of miconazole and other compounds in pharmaceutical creams. The drugs in creams were analyzed by thin-layer chromatography on silica gel plates with ether in pentane-saturated chamber or with butanol-water-acetic acid (20 5 2). Both active ingredients and vehicle components were detected and Rf values of 67 active ingredients are tabulated. Additional eluents may be needed to separate certain combinations of ingredients. 

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