A high-purity result on a Certificate of Analysis reflects the material at the moment it was tested. Whether it stays that way is largely a question of storage and handling once it reaches your bench.
Most peptides ship lyophilized — freeze-dried into a dry powder. In this form they are at their most stable, because the absence of water slows the chemical reactions that lead to degradation. The two enemies of a lyophilized powder are moisture and heat. Sealed, dry, and cold is the goal: many labs keep unopened lyophilized material in a freezer, well below room temperature, with desiccant to guard against humidity.
A common mistake is opening a cold vial straight from the freezer. Condensation forms on the cold glass and powder, introducing exactly the moisture you're trying to avoid. Letting a sealed vial equilibrate to room temperature before opening is a simple habit that prevents this.
Once a peptide is reconstituted into solution, the stability clock speeds up considerably. Material in solution is far more prone to degradation than dry powder, so reconstituted stock is generally prepared in smaller working volumes and kept cold. Where repeated use is expected, dividing stock into single-use aliquots avoids the biggest avoidable source of degradation: repeated freeze-thaw cycles. Every thaw-and-refreeze stresses the molecule, and the damage accumulates.
Good labeling closes the loop. Recording the lot number, reconstitution date, and concentration on each aliquot keeps your samples traceable back to the original COA — which is what lets you trust your own results months later.
None of this is exotic. It's the same cold-chain discipline used across analytical chemistry, applied consistently.