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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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2楼2010-01-14 22:15:35
wangwei2008
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3楼2010-01-14 22:51:16
wangwei2008
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4楼2010-01-14 23:07:22
wangwei2008
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5楼2010-01-14 23:17:39
shirongjiu
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yinlingling7839(金币+60,VIP+0): 1-15 14:24
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如果杂质不能有效减少,有必要先对杂质进行鉴定. 一旦杂质的结构确定了, 该杂质形成的机制就会有个总体解释, 才可能设计(实验) 条件以避免形成该杂质. 对于较高浓度杂质来说,1 H和13C的核磁共振图谱对于杂质结构的鉴定是适用的.如果杂质在一连串的反应过程中得到富集, 例如, 在洗脱提取液或者母液中, 则可以通过比较杂质图谱与已知反应物的图谱鉴定出杂质结构. 杂质浓度在更高水平时, 为了便于鉴定,要把已知成分的图谱信号从核磁共振图谱中扣减掉. 上述方法对于有机化学家来说通常是十分方便的. 当杂质结构不是很明确或是想确定其结构归属时, 质谱法可以用于杂质的结构鉴定. 高效液相色谱-质谱联用或气相色谱-质谱联用可使鉴定过程变得容易些. 这种方法可在色谱图上确定杂质的分子量。如果实验室没有配备这些仪器，这些工作可以交由相应的公司完成。通常，上述这些分析最好由专业人士完成。结构分析化学家为实验室规模或中试规模的药厂建立杂质纯化分析方法是有很多帮助的。质量控制化学家对于辨识加工过程中的一种新杂质是一种关键。识谱专家知道如何让他们的仪器得到最有效的运行分析。分析人士与其他化学家的团队协作对于有效确立实验流程是十分关键的。为了给分析员提供信息帮助，详细介绍反应的本质以及预期的分析结果也是很重要的。当在册的分析员在项目帮助下完成相关分析，最后完整的实验结果才会更快产出。 |
6楼2010-01-15 10:37:27
syh9288
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7楼2010-01-15 12:15:01