Comet parameter: add_Nterm_peptide
         
         - Specify a static modification to the N-terminus of all peptides.
         
- The specified mass is added to the unmodified N-terminal mass (mass of H or 1.007825).
         
- The default value is "0.0" if this parameter is missing.
         
Example:
         
add_Nterm_peptide = 14.01