购买的标样还需要分析校正吗?

购买的标样，证书提供的含量应该可以直接用。但如ASTM D5599，“before preparing the standards,determine the purity of the oxygenate stocks and make corrections for the impurities found. Whenever possible,use stocks of at least 99.9% purity. Correct for the purity of the components for water content determined by Test method D1744."

为什么？如何矫正？