Isotopident

Isotopident is a tool that helps you to estimate the theoretical isotopic distribution of a peptide or protein, a polynucleotide and a chemical compound from its composition (sequence of amino acids expressed in either 1-letter code, sequence of amino acids expressed in 3-letter code, sequence of nucleotides or its chemical formula). Moreover, Isotopident is capable of computing its monoisotopic mass. Finally, this tool is also able to predict the most likely isotope combination and the exact mass of the given input.

Link: http://education.expasy.org/student_projects/isotopident/htdocs/