When working with mass spectrometry, particularly ms/ms spectra, a number of tables are essential in order to understand these.
A number of tables can be downloaded through this page:
General mass table (elemental mass values, amino acid residues and immonium ions). Word, pdf
Amino acid residue and fragment ion mass values. Word, pdf.
Monoisotopic peptide residue values up to 300 Da. Word, pdf.
a2-b2 ion pairs (integer values, Cys as carbamido). Word, pdf. With decimals, free Cys. Word, pdf.
Spreadsheet containing most of the above tables. Excel.