Specifies a mass adjustment to be applied to all indicated amino acids prior to spectral analysis. Amino acids are identified by their single letter designation. N-terminal and C-terminal fixed modifications are designated by n and c, respectively. The relative mass difference, positive or negative, is listed after the amino acid, separated by a space. If multiple fixed modification masses are desired, use multiple fixed_modification declarations.
[Amino Acid] [Mass]
1.0 - present
To indicate alkylation of cysteine using iodoacetamide:
fixed_modification = C 57.02146