This small workflow takes as input a peptide sequence and calculates the elemental composition of the peptide (Calculate_Elemental_Composition). This is then transferred as a numerical vector to the Rshell Calculate_Isotopic_Distribution, that calculates the (integer mass) isotopic distribution using the FFT method. To shift to approximate real masses (or m/z values), transpose the distribution with the difference between the integer and real monoisotopic masses (both easily calculated using ...