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How long should a laboratory keep my sample?
The short answer
Long enough to re-run a disputed result. A retention period of several weeks after reporting is common practice, and it is what makes a challenge meaningful: without the original sample, a disagreement about a result cannot be settled by measurement. Ask about retention before you send, not after you dispute.
In detail
Retention matters most in exactly the situation where you need it most — a failing result the vendor contests. If the sample is gone, the argument becomes rhetorical.
Retained raw data is the companion policy. Chromatograms, integration parameters and the standard curve used should survive the report, so the calculation itself can be re-examined rather than just repeated.
Storage conditions during retention matter too: a sample kept badly is not a sample worth re-running.
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