Peak thin layer chromatography

Chemical stabiUty studies are monitored by siUca gel thin-layer chromatography (dc) or by high performance Hquid chromatography (hplc) using a reverse-phase C g coated column (24). Hplc peaks or dc spots are visualized by thek uv absorption at 245 nm the tic spots can also be detected by ceric sulfate or phosphomolybdic acid staining. 

The progress of the reaction was monitored hy injecting after each 24-hour period an aliquot into a gas chromatograph and checking the peak corresponding to isophorone. Alternatively, thin-layer chromatography (E. Merck 0.25-mm. silica gel plates developed with ethyl acetate) can be used,... 

Trinitro-N-methylaniline (N-methyl picra-mide), yellow needles from EtOH contg acid, mp /l4.8° (Refs 4 7) the UV spectrum has a peak at 340 and a shoulder at 410—20m (Ref 19) CA Registry No 1022-07-7. It is prepd by the hydrolysis of the N nitroso compd (see below) at 70—80° (Ref 5) by the action of coned sulfuric acid (Ref 8) or UV light (Ref 12) on Tetryl by the action of methyl amine on Ethyl Picrate (Ref 18) or Tetryl (Ref 7) or by the action of nitric acid (d 1.42g/cc) at 0° on N,N-dimethylaniline (Ref 10). It is present in crude Tetryl and is the active der-matitic agent in the latter (Ref 12). It can be isolated from crude Tetryl and identified by thin layer chromatography (Ref 21)... 

Thin-layer chromatography usually involves the adsorption chromatographic separation of substance mixtures into polarity groups. It is well known that clean looking chromatographic peaks can hide several substances. For instance, primary, secondary and tertiary alcohols are to be found at very nearly the same hRf. 

Uniformly labeled 2,4-dichlorophenol- C (purchased from New England Nuclear Corp, Boston, Mass.) was used in the tracer preparation. This provided a label at all carbon positions in the dibenzo-dioxin structure. 2,7-Dichlorodibenzo-p-dioxin- C after initial cleanup by fractional sublimation, contained approximately 5% of an impurity, detected by thin layer chromatography (TLC) which gave mass peaks at 288, 290, 292, and 294 in the mass spectrometer, consistent with a trichloro-hydroxydiphenyl oxide. This is probably the initial condensation product of the Ullman reaction and is most likely 2-(2,4-dichlorophenoxy)-4-chlorophenol. It was removed easily by extractions with aqueous... 

Furthermore CE and thin layer chromatography (TLC) have the advantage to detect retained substances in the original injection zone. In CE, either pressure can be applied or the EOE itself transports uncharged or slow molecules past the detector. In this case, a blank sample solvent injection should be compared with the sample to elucidate whether the peaks are caused by the injection plug itself or are compound-related. 

The two potato glycoalkaloids in the potato extract were identified as follows (a) comparison of thin-layer chromatography of standards a-chaconine and a-solanine and of samples of each peak collected from the HPLC column containing the individual glycoalkaloids and (b) HCl hydrolysis of the HPLC samples into sugars and the aglycon solanidine. These were identified by GLC-MS (Kozukue et al., 1999, 2008 Kozukue and Friedman, 2003). 

Confirmatory analysis of suspected liquid chromatographic peaks is usually accomplished by a photodiode array detector that continuously collects spectral data during the chromatographic separation and further compares the spectrum (200-550 nm) of the eluted suspected compound with that of a standard (37, 38, 66, 161, 163, 166-168, 178, 180, 181). Online absorbance ratio techniques combined witlr off-line thin-layer chromatography have been also reported (171). Although tliese confirmation techniques are relatively simple, their sensitivity is not generally adequate to identify trace levels of residual nitrofurans in edible animal products. 

Tentative confirmation of suspected liquid chromatographic peaks has been achieved in the analysis of carazolol and seven sedatives in swine kidney, by using photodiode-array detection in the wavelengUi range of 220-320 nm. It was reported (526) that further identification could be made possible if the corresponding fractions of the eluate were submitted off-line to two-dimensional thin-layer chromatography. 

Dimethyl bromosuccinate was prepared from bromosuccinic acid by the diazomethane method (26) using the procedure of Eisenbraun, Morris, and Adolphen (27). It was distilled under vacuum (0.08-0.1 Torr) at 45°-49°C to yield a clear colorless oil. Thin layer chromatography with benzene as the solvent on SiC>2 yielded a symmetrical single spot, indicating either a pure compound or no separation with this particular solvent. Its mass spectrum had a very small peak corresponding to the parent compound, but none to a dibromo compound. The mass spectrum for dimethyl bromosuccinate was not found in the literature, but that for dimethyl succinate also has a small peak corresponding to the parent compound (25). 

Identification. Chromatographic methods (interpretation of fluorescing spots in thin layer chromatography or of UV absorption peaks and fluorescence light emission peaks in high performance liquid chromatography) allow fairly rapid and certain identification of FWAs [158-164], The same methods can be used for qualitative and quantitative characterization of the FWA purity in terms of fluorescent or nonfluorescent byproducts. 

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