What is a dimer on a peptide certificate?

A dimer is two copies of a molecule joined together. In a peptide vial, it means two peptide chains that have bonded to each other, forming one larger molecule roughly twice the mass of the original.

It isn't a contaminant from outside. It's the target peptide in a different form.

How dimers form

Some form during manufacture, as a by-product of synthesis and purification. Others can form later, particularly when a peptide is exposed to heat, moisture or repeated temperature changes. Peptides containing cysteine can link through the sulphur atoms in that amino acid; others can join or clump in different ways.

How a laboratory finds them

Because a dimer is about twice the mass of the single peptide, mass spectrometry picks it up as a distinct signal at roughly double the expected weight. On HPLC it usually shows as a separate peak.

Some laboratories report dimer content as its own figure. Others include it in the overall impurity total. Neither approach is wrong. They're just different ways of presenting the same information.

Keeping it low

For research stock, the rules are the same ones that protect any lyophilised peptide: keep it sealed, cool, dry and dark, and avoid repeated warming and cooling. Heat, moisture and temperature cycling do far more damage than age. We store and ship our stock the same way.