has_units has value type peptide modification details unidentified modification monoisotopic mass delta MS:1003081 Monoisotopic mass delta in Daltons of an amino acid residue modification whose atomic composition or molecular identity has not been determined. This term should not be used for modifications of known molecular identity such as those available in Unimod, RESID or PSI-MOD. This term MUST NOT be used inside the <Modification> element in mzIdentML. MS dalton