It is expected by regulatory agencies that sound scientifically rationale should be applied to assign lab expiry periods. You can adopt data from recognized references (USP, ASTM, AOAC, Standard Methods, ACS or compendial methods). Otherwise, you have to develop stability data during the method validation. For solid chemicals you can get the expiry dates from the supplier’s certificate of analysis, or from recognized scientifically sources. Also, once you open the chemical’s container, need to establish guideliness, according to chemical substance properties, supplier recommendatios and solution’s stability. What is important is to have scientifically proved back up data to justify any assigned expiration period.
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