We have started using the Ten-peptide standard, 71 µg, lyophilized, in a 2 mL vial (https://www.agilent.com/en/products/liquid-chromatography/lc-columns/biomolecule-separations/peptide-standard ) as a quality control sample in our LC-MS/MS analysis runs. We are using MSGF in OpenMS to identify the peptides and are able to find all except peptide number 7:
[Ace-F-3,-2 H-1] Angiotensinogen (1-14) Ace-FFHDRVYIHPFHLLVYS.
According to the Certificate of Analysis (https://www.agilent.com/cs/library/certificateofanalysis/Part%20Number%205190-0583%20CofA%20Lot%20Number%20AGI-13002.pdf ) the molecular weight of this peptide is 2231.61 Da. However, a check in the NIST Mass and Fragment Calculator v1.3 the monoistopic mass of the acetylated peptide is 2231.13 Da
I would be very grateful if someone could explain to me what other modifications are made to this peptide and how I can specify it in a search engine such as MSGF.
Thank you in advance!