Working standards shelf life

Why the shelf life of API working standard is only one year ? Even it is taken from approved batch as it’s shelf life is allotted as 5 years.

Working standard is preared from approved batch of API by comparing analytically with Reference standard and also performing its characterization to establish its identification and structure etc.
The comparison with Reference standard is made by utilizing current lot of Reference standard obtained from authentic sourcers like Pharmacopoeia. The current lot keeps on changing routinely. Hence in order to keep working standard always comparable with current lot of Reference standard generally working standard is retested annually. Also, since working standard once selected is used multiple times during analysis there are chances of moisture pick up and degradation during its storage. Hence its shelf life or retesting frequancy is once in a year.

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