Chain of custody

Chain of custody is the record connecting a sample to every party who has handled it and every condition it has experienced. In analytical work it exists to answer a specific challenge: how do you know the material tested is the material in question?

An unbroken chain records each transfer with a date, an identified handler and the storage conditions in the interval. A gap does not prove that anything went wrong. It means the question can no longer be answered from the record, which for practical purposes has the same effect on how much weight a result can bear.

Sampling is part of the chain

How a sample was taken determines what the result describes. A sample drawn from one container tells you about that container. Extending the conclusion to a whole batch is an inference that depends on the batch actually being homogeneous and on the sampling plan being designed to test that assumption rather than to assume it.