Chemical compounds, discrete bands

While chromatographic methods separate chemical compounds, they do not identify them as specific compounds. For this purpose one needs a good knowledge of what is most likely to be in the mixture. Known control substances can be analyzed along side the unknown samples. Discrete bands of the various components can then be identified against the known material that travels the same distance on the paper or glass sheet (Fig. 4.34). 

The detection system in gas chromatography (GC), as in other chromatographic techniques, provides the response signal for the chemical compounds separated by the chromatographic column. A flow of finite amounts of chemical entities arrives at the detector in discrete bands in GC, these entities are in the gas phase. 

Both ORD and CD are sensitive to conformational changes and chemical transformation. ORD has the following advantages over CD (1) It is easier to visualize the Cotton effect with ORD because of the three distinct points in the ORD curve the peak, the crossover, and the trough (in that order or in reverse order). (2) An optically active compound that does not show the band in the wavelength range of interest in the absorption spectrum will not show a CD curve but will show a plain ORD curve. CD, on the other hand, possesses an intrinsic discreteness and is a more sensitive tool in examining the environmental effect on the conformation of macromolecules. 

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