Mannitol
In short A sugar alcohol sometimes added before lyophilisation to stabilise the powder.
Mannitol is a sugar alcohol widely used as an excipient in freeze-dried products. Added to a peptide solution before lyophilisation, it crystallises during freezing and forms the structure of the dry cake, which helps the cake keep its shape and dissolve evenly later.
Excipients like mannitol are common and not a sign of a lower-quality product. They do, however, change what is in the vial: the total mass of powder is no longer only peptide, and for some analytical work the presence of an excipient needs to be known.
Where a lot contains mannitol, the certificate of analysis says so, and the UKPeps certificate library marks the row "with mannitol". Some substances in the library have both a lot with mannitol and a later lot without it; the older one may also be marked "superseded".
If your protocol requires an excipient-free peptide, check the certificate for the lot before use or write to us with the lot number.