小中大请教一个美国药典有关物质检查的方法
美国药典对一个有关物质的检测方法如下：
Related compounds—
Diluent, Solution A, Solution B, Peak identification solution, and Chromatographic system— Proceed as directed in the Assay.
Standard solution— Dissolve an accurately weighed quantity of USP Capecitabine RS in Diluent, and sonicate if necessary, to obtain a solution having a known concentration of about 0.6 mg per mL
Test solution— Dissolve an accurately weighed quantity of Capecitabine in Diluent, and sonicate if necessary, to obtain a solution having a known concentration of about 0.6 mg per mL
Procedure— Separately inject equal volumes (about 10 µL) of the Standard solution and the Test solution into the chromatograph, record the chromatograms, and measure the peak responses. Calculate the percentage of each impurity in the portion of Capecitabine taken by the formula:
100(1/F)(CS / CU)(rI / rS)
请高手帮忙分析下，使用单点外标法吗？还是需要做标准曲线，谢谢！
