A lot number is the only thing on a vial that connects the material inside it to a measurement someone actually made. Everything else on the label — the name, the weight, the purity claim — is a statement. The lot number is a key.

The weak point is not the certificate

The certificate is rarely the thing that is wrong. The failure happens one tier above it. Material moves through brokers and repackers, and at each handover there is an opportunity for a batch number to be reassigned, a retest date to be reset, or a certificate to be inherited from a different batch of the same compound.

The document that arrives can be perfectly genuine and describe material you did not receive. Nothing about the paperwork looks wrong, because nothing about the paperwork is wrong.

What to check

• The number on the vial, the number on the packing slip and the number on the certificate are the same number.
• The date of manufacture is earlier than the date of analysis, and both are earlier than today. A manufacture date that has been reset to the repacking date is a common tell.
• The retest date comes from the manufacturer rather than from the date the reseller received it.
• The lot appears in a published index you can search without asking anyone for it.

Why superseded lots stay published

A library that shows only the lots currently shipping is a brochure. The question a quality team actually has is whether the vial already in their freezer is still good, and that vial is, by definition, from an older lot.