https://www.rochester.edu/newscenter/nasa-bennu-asteroid-spa...

Key to the curation process was the use of so-called “witness plates”—flat plates made of aluminum and sapphire—that were exposed to all the same conditions as the sample from Bennu, creating a detailed record of potential contaminants.

If a compound found in the Bennu sample wasn’t on the witness plate, scientists could confidently identify it as originating from Bennu. This is critical when dealing with organic compounds, where contamination can make it hard to distinguish what is truly extraterrestrial.