Methionine is the weakest point: its sulphur easily oxidises to methionine sulfoxide, which adds 16 Da to the mass. Cysteine can form unwanted disulfide bridges or dimers; tryptophan and histidine are also sensitive, especially to light.
Oxidation is promoted by dissolved oxygen, exposure to light, trace metals and certain contaminants from materials. That is why some peptides are designed by replacing methionine with norleucine, an almost identical amino acid without sulphur, as in melanocortin analogues.
The practical measures are simple: keep vials closed and protected from light, avoid shaking solutions (which brings in air) and reduce the time the peptide spends in solution at room temperature.
A +16 Da peak in mass spectrometry is the typical fingerprint of oxidation. If it appears in a batch or grows over time, it points to a storage or manufacturing problem.
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Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).