Reference chromatographic method impurities appearing

As a third option, impurities appearing in the reference chromatographic method may be isolated using analytical (under overloaded conditions) or semi-prep chromatography and then identified by MS and/or NMR. Mobile-phase components should be removed prior to analysis. Removal of mobile-phase components may entail evaporation, liquid-liquid extraction, freeze-drying, or solid-phase extraction. Identification by MS should be attempted first because its high sensitivity requires only small sample amounts. The less sensitive NMR requires the isolation of greater amounts of the impurity. An alternative to this approach is the use of LC-MS or LC-NMR. 

If the laboratory worker does not know of a reference to the preparation of a commercially available substance, he may be able to make a reasonable guess at the synthetic method used from published laboratory syntheses. This information, in turn, can simplify the necessary purification steps by suggesting probable contaminants. However, for other than macromolecules it is important that at least the NMR and IR spectra of the substance be measured. These measurements require no more than two to three milligrams (which are recoverable) of material and provides a considerable amount of information about the substance. Three volumes on the NMR spectra [C.J.Pouchert and J.Behnke, The Aldrich Library of C and FT-NMR Spectra, Vols 1—3, Aldrich Chemical Co., Inc, Milwaukee, Wl, 1993], and one on the infrared spectra [C.J.Pouchert, The Aldrich Library of FT-IR Spectra, 3nd ed, Aldrich Chemical Co., Milwaukee, Wl, 7959], as well as computer software [FT-IR Peak-search Data Base and Software, for Apple HE, IIC and II Plus computers and for IBM PC computers, Nicholet Instruments, Madison, Wl, 1984] contain data for all the compounds in the Aldrich catalogue and are extremely useful for identifying compounds and impurities. If the material appears to have several impurities these spectra should be followed by examination of their chromatographic properties and spot tests. Purification methods can then be devised to remove these impurities, and a monitoring method will have already been established. 

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