To construct the probabilistic isotopologue classification model, elemental mass defects (EMDs) were used. The assumption here is that the monoisotopic and isotopologue mass have the same EMD because they have the same molecular structure with the isotopologue having one or more of its atoms being replaced with heavier versions (i.e., isotopes) of the same elements. To calculate the EMD for both the monoisotopic and isotopologue mass, the elemental mass (EM ) needs to be calculated according to equation 1. Here, the ion mass can either be the monoisotopic or the isotopologue mass and the er mass (i.e., elemental ratio mass) depends on the elemental ratio used. For the classification model the elemental ratios CO, CCl, CN, CS, CF, and CH were used, which have an er mass of 27.995, 46.969, 26.003, 43.972, 30.998, and 13.008, respectively. These values are the sum of the elemental masses of each element for a single elemental ratio. For example, the er mass of CO equals the monoisotopic mass of a carbon atom plus that of an oxygen atom (i.e., 12.000 + 15.995 = 27.995). The selected elemental ratios were chosen based on both the frequency they were encountered in the DDS-Tox database (Table ) and the fact that only 0.007% of the database entries contain none of the selected elements.