The chain of custody must be maintained for all evidence that may
be used in court. This chain in part insures the integrity of the
evidence. For some items, like a handgun with a serial number, the
chain is maintained a little differently than chemical evidence. Chemical
evidence cannot by identified by any unique marking, and in general,
all white powders look like, well, white powders.
For this reason, chemical evidence is generally kept sealed, and any
person legitimately breaking a seal should do so in a manner that
preserves the previous seals. The person opening, sampling and resealing
chemical evidence in the laboratory should clearly identify ``who''
(initials) and ``when'' (date) in a manner that will indicate
any later tampering.
Please note that the chain does not have to include every person who ever touched the evidence. In practice, the person with custody can testify that the evidence was not altered (or if it was, how it was altered).