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专业: 药物学其他科学问题

[求助] 腺嘌呤美国质量标准翻译求助，请各位大侠帮帮忙

Organic impurities— （usp）
1、pH 7.0 Phosphate buffer— Dissolve 4.54 g of monobasic potassium phosphate in water to make 500 mL of solution. Dissolve 4.73 g of anhydrous dibasic sodium phosphate in water to make 500 mL of solution. Mix 38.9 mL of the monobasic potassium phosphate solution with 61.1 mL of the dibasic sodium phosphate solution. Adjust, if necessary, by the dropwise addition of the dibasic sodium phosphate solution to a pH of 7.0.
2、Standard preparations— Dissolve a suitable quantity of USP Adenine RS, accurately weighed, in hot water, cool, and dilute quantitatively with water to obtain a solution having a known concentration of about 0.19 mg per mL. Pipet 5-mL portions into three 100-mL volumetric flasks, dilute with 0.10 N hydrochloric acid, 0.010 N sodium hydroxide, and pH 7.0 Phosphate buffer, respectively, to volume, and mix.
3、Test preparations— Proceed as directed for Standard preparations, using the specimen in place of the Reference Standard.
4、Procedure— Obtain the UV absorption spectra of the Test preparations and the Standard preparations concomitantly, in 1-cm cells, covering the range between 220 nm and 320 nm, using water as the blank. The respective absorptivities, calculated on the dried basis, at the wavelengths of maximum absorbance, for each pair of corresponding solutions do not differ by more than 2.0%.
这是有机杂质的检测方法，第1、2、3段是试剂和对照液及供试品液的配制，比较容易看懂，但是第4段就看迷茫了，不知道其测定的原理是什么，以下为自己翻译的内容，不知道对否，请各位战友指导指导，解释一下其测定的原理：
有机杂质（usp）：
1、Ph7.0缓冲液：取4.54g磷酸二氢钾于500ml水中溶解，配制磷酸二氢钾溶液，取4.73g无水磷酸氢二钠于500ml水中溶解，配制磷酸氢二钠溶液。取上述磷酸二氢钾溶液38.9ml和磷酸氢二钠溶液61.1ml混合，如果有必要，用磷酸氢二钠溶液将混合液ph调整为7.0.
2、对照溶液：取适量腺嘌呤对照品，精密称重，与热水中溶解，冷却，用水定容配制成含腺嘌呤0.19mg/ml的溶液，各量取5ml于三个100ml量瓶中，分别用0.10mol/l盐酸，0.010mol/l氢氧化钠，Ph7.0缓冲液稀释，定容。
3、供试品溶液：按照对照品溶液配制的方法配制供试品溶液。
4、检查：采用1cm比色皿，使用水作为空白溶剂，用UV法对以上对照品和供试品溶液共6份分别进行波长扫描，波长范围为220nm~320nm。以无水物计算，每对对应的溶液在最大吸收波长处的吸收率差异不得大于2.0%。

第4段真的不懂其测定的原理，帮帮忙解释一下啊（腺嘌呤美国usp标准可在药物在线查询）

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