Peptide Mass Shift

True covalent modifications (like unremoved protecting groups or oxidations) are distinct chemical entities. They will typically have a different HPLC retention time than your target peptide. Conversely, LC-MS artifacts—such as metal adducts (Sodium +22 Da, Potassium +38 Da) or in-source fragmentation—occur during the ionization process inside the mass spectrometer. If the shifted mass perfectly co-elutes with your main product peak, it is highly likely to be an ionization artifact rather than a synthesis failure.

If a mass shift does not match a single entry, it is frequently a combination of multiple side reactions or adducts. For instance, a +38 Da shift might not be a single event; it could be an oxidation (+16 Da) combined with a Sodium adduct (+22 Da). When troubleshooting an unknown mass, always subtract common ionization adducts first, then look for combinations of expected protecting groups based on your sequence.

Chemical artifacts are highly residue-specific. You can drastically reduce your troubleshooting time by cross-referencing your observed mass shift with the Affected Residue(s) column. For example, if you see a +16 Da shift but your sequence lacks Methionine, Cysteine, Tryptophan, or Histidine, you can safely rule out standard oxidation and start investigating alternative causes, such as a synthesis deletion or a solvent contaminant.

Chemical artifacts are highly residue-specific. You can drastically reduce your troubleshooting time by cross-referencing your observed mass shift with the Affected Residue(s) column. For example, if you see a +16 Da shift but your sequence lacks Methionine, Cysteine, Tryptophan, or Histidine, you can safely rule out standard oxidation and start investigating alternative causes, such as a synthesis deletion or a solvent contaminant.

Yes. While this mass shift table is designed for post-synthesis LC-MS troubleshooting, you can take a proactive approach using the Chemical Stability Audit in Peptalyzer™. By inputting your target sequence before beginning your synthesis, Peptalyzer automatically scans for high-risk sequence motifs (such as Asp-Gly for aspartimide formation, or N-terminal Gln for pyroglutamate). This allows you to anticipate potential mass shifts and adjust your synthetic strategy—such as utilizing Dmb-protected dipeptides—before wasting reagents.

The mass shift table is an exhaustive diagnostic reference that includes unpredictable, environmentally dependent artifacts—such as metal adducts (+22 Da Na⁺), solvent contaminants (formylation), or incomplete scavenger trapping. In contrast, the Peptalyzer™ Chemical Stability Audit focuses purely on sequence-dependent chemical risks (like oxidation-prone Met/Trp/Cys residues or specific degradation motifs). Peptalyzer predicts what is structurally likely to happen based on your sequence, while this table helps you diagnose what actually happened in the flask.