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yinlingling7839木虫 (小有名气)
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If impurities cannot be minimized readily, it may be necessary to first identify the impurities . Once the structure of the impurities is known, the mechanism of formation can generally be elucidated, and conditions to avoid forming these impurities may be designed. For impurities formed at relatively high levels, 1H NMR and 13C NMR spectroscopy may be suitable for identification. If the impurities are enriched in a stream of the reaction, for instance in a wash extract or in the mother liquor, one can identify the impurities simply by comparing the spectra to those of known reaction components. At relatively high levels of impurities, signals from known compounds may be mentally subtracted from the spectra to ease impurity identification. This approach is usually very convenient for the organic chemist. Mass spectroscopy (MS) may be used to identify impurities whose structures are not obvious or to corroborate the structural assignment. Identification is facilitated by HPLC-MS or GC-MS, which can be used to assign molecular weights to impurities seen in the chromatograms. If access to such equipment is limited,these services can be provided on a contractual basis by other firms. Often the analysis of a compound is best left to specialists. Analytical chemists may be very helpful in developing an in-process assay for the laboratory or pilot plant and in purifying an impurity. Quality control (QC) chemists may be key in recognizing a new impurity in a manufacturing batch. Spectroscopists know how to get the best performance from their instruments. Good teamwork among the analysts and other chemists is essential to the efficient development of a process. To best support the efforts of analysts, thoroughly explain the nature of the reaction and the expected outcome of the analyses. Definitive results are generated more quickly when the analyst is enrolled in helping with a project. |
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