干燥失重

　　R927.1

　　cnki:ISSN:1007-306X.0.2004-03-029

　　几乎是化学原料药的必测项目。其准确的测定与否影响着化学结构的判断和含量测定的结果。含结晶水药物的水分(或含结晶溶剂时溶剂)测定,是结构确定的一个很重要因素;同一化合物含不同的结晶水往往会视为不同的药物。那种数据不够水分凑,用随意的加××%吸附水,以改变计算值,使之与理论值相符是不科学的、也是不可取的。严格的测定含结晶水的方法,首选晶体X 射线衍射(SXRD)。晶体X衍射是很繁琐的工作,单晶的培养、测试、计算工作量都很大,一般并不常做。用常规的四大波谱(IR、UV、NMR、MS)进行结构确证。

　　<731> LOSS ON DRYING
　　The procedure set forth in this chapter determines the amount of volatile matter of any kind that is driven off under the conditions specified. For substances appearing to contain water as the only volatile constituent, the procedure given in the chapter, Water Determination 921 , is appropriate, and is specified in the individual monograph.
　　Mix and accurately weigh the substance to be tested, and, unless otherwise directed in the individual monograph, conduct the determination on 1 to 2 g. If the test specimen is in the form of large crystals, reduce the particle size to about 2 mm by quickly crushing. Tare a glass-stoppered, shallow weighing bottle that has been dried for 30 minutes under the same conditions to be employed in the determination. Put the test specimen in the bottle, replace the cover, and accurately weigh the bottle and the contents. By gentle, sidewise shaking, distribute the test specimen as evenly as practicable to a depth of about 5 mm generally, and not more than 10 mm in the case of bulky materials. Place the loaded bottle in the drying chamber, removing the stopper and leaving it also in the chamber. Dry the test specimen at the temperature and for the time specified in the monograph. [NOTE—The temperature specified in the monograph is to be regarded as being within the range of ±2 of the stated figure.] Upon opening the chamber, close the bottle promptly, and allow it to come to room temperature in a desiccator before weighing.
　　If the substance melts at a lower temperature than that specified for the determination of Loss on drying, maintain the bottle with its contents for 1 to 2 hours at a temperature 5 to 10 below the melting temperature, then dry at the specified temperature.
　　Where the specimen under test is Capsules, use a portion of the mixed contents of not fewer than 4 capsules.
　　Where the specimen under test is Tablets, use powder from not fewer than 4 tablets ground to a fine powder.
　　Where the individual monograph directs that loss on drying be determined by thermogravimetric analysis, a sensitive electrobalance is to be used.
　　Where drying in vacuum over a desiccant is directed in the individual monograph, a vacuum desiccator or a vacuum drying pistol, or other suitable vacuum drying apparatus, is to be used.
　　Where drying in a desiccator is specified, exercise particular care to ensure that the desiccant is kept fully effective by frequent replacement.
　　Where drying in a capillary-stoppered bottle* in vacuum is directed in the individual monograph, use a bottle or tube fitted with a stopper having a 225 ± 25 µm diameter capillary, and maintain the heating chamber at a pressure of 5 mm or less of mercury. At the end of the heating period, admit dry air to the heating chamber, remove the bottle, and with the capillary stopper still in place allow it to cool in a desiccator before weighing.

　　本章中给出的方法阐述了在特定的条件下物质中的挥发性成分的测定。对于水分为唯一挥发性成分的物质，按各论中规定的<921>水分测定法测定。
　　除各论中另有规定外，取混匀后（如果样品是比较大的结晶，应先迅速捣碎使成2mm左右的颗粒）的待测样品，精密称定1－2克置于与样品相同条件下干燥30分钟的具塞扁形称量瓶中，称量瓶先称取皮重，待样品加入后，盖上瓶盖，称取总重。轻轻地平移振摇，尽可能使样品均匀平铺成约5mm厚，对于比较松散的、体积大的也不应超过10mm厚。将装有样品的称量瓶放在干燥室内，瓶盖打开，也放入干燥室，按各论中规定的温度和时间干燥。[注意：各论中规定的干燥温度应该被视为规定温度±2℃。]打开干燥室，立即盖上瓶盖，移入干燥器中冷却到室温，然后称量。
　　当待测样品未达到规定的干燥温度时就融化时，应先在低于上述融化温度5-10℃下干燥1－2小时，然后在规定温度下干燥。
　　假如样品是胶囊，应至少取4粒胶囊的内容物，混匀后取样。
　　假如样品是片剂，应至少取4片，研细混匀后取样。
　　假如各论中要求采用热重分析法测定干燥失重，需要用到高精度电子天平。
　　假如各论中要求真空干燥，需要用到真空干燥器、真空干燥枪或者其他真空干燥设备。
　　当用到干燥器时，应该特别注意及时更换干燥剂以保持其有效状态。
　　假如各论中要求采用毛细管塞称量瓶测定干燥失重，应该使用直径为225±25um的毛细管塞称量瓶，毛细管塞放在干燥器中，装有样品的称量瓶按规定干燥，维持干燥室内的压力在5毫米汞柱或者更低。在加热结束时，可以把干燥的空气放进干燥室内，取出称量瓶，移入干燥器，盖上毛细管塞，冷却，称重。

　　干燥失重=（W1-W2)/W1*100％。
　　W1为干燥前的样品重量，
　　W2为干燥后的样品重量。
　　W1-W2就是干燥中减失的重量。