Chain of custody (revision 13)

A chain of custody is the documented sequence of individuals who held a sample between the moment it was drawn and the moment it was analysed, together with the conditions under which it was held and the transfers between them. Its purpose is to exclude substitution, tampering and undocumented storage excursions as explanations for a result.^[1]

In the context of peptide testing, an unbroken chain of custody is uncommon. Most independent reports arise from material bought, shipped, stored for an unknown period and then submitted, with no documentation of any of it. This does not make the results worthless; it bounds what they can establish.^[2]

Chain of custody is complementary to blind sampling. Blind sampling addresses selection — whether the sample was chosen to be favourable. Chain of custody addresses handling — whether the sample analysed is the sample drawn, unchanged.

A chain-of-custody record identifies the sample uniquely, names each person who held it with the dates of transfer, and states the storage conditions maintained at each stage. Where temperature matters, a record without temperature is incomplete.^[1]

For peptides, temperature and time are the conditions that matter most, because the degradation routes — deamidation, oxidation, aggregation — are all time- and temperature-dependent. A sample held at ambient temperature for three weeks before analysis may return a lower purity than the same material analysed on receipt, and without a record it is impossible to attribute the difference.^[3][4]

This is the single most common confound in community testing. A result reported as characterising a supplier's material may be characterising the purchaser's storage, and the two are indistinguishable without a custody record. See Temperature excursion.

The ordinary sequence for a community-submitted report is: material is bought; it is shipped by a carrier under unknown conditions; it is received and stored by the purchaser for some period; a vial is selected; it is posted to a laboratory. No step in that sequence is documented in a way that would satisfy a custody requirement.^[2]

The consequences run in both directions and are not symmetric in the way people usually assume. Degradation in transit or storage produces a result worse than the material as shipped, so an unfavourable result from an uncontrolled chain is weak evidence against a supplier. A favourable result is less affected, since degradation does not improve purity — but it says nothing about whether that vial was representative.