After addressing formula errors and typos, we now turn to inconsistencies resulting from miscalculations. A common, albeit subtle, numerical discrepancy (low ppm range) is observed when the exact mass is calculated for a neutral molecule, while the measured mass corresponds to a charged species -typically a cation (Figure , section e). Significantly larger mass errors in adducts ([M+H], [M+Na]) arise if the nominal mass 8 of the added atom (1.0000 for H or 23.0000 for Na) is used instead of its precise isotopic mass (1.0078 for H or 22.9898 for Na) (Figure , section f). Unlike elements such as sodium (Na) and fluorine (F), which are monoisotopic, most elements, like carbon (C) and hydrogen (H), are polyisotopic. In compounds containing these polyisotopic elements, confusion between molecular weight (MW) and monoisotopic mass usually results in significant errors (Figure , section g). This example also illustrates a situation where an apparent miscalculation is misleadingly validated by matching measurements. In rare instances, certain isotopic compositions result in the molecular weight and exact mass being very similar or identical. For example, the exact mass of C₁₁H₂₂BN₂⁺ is 265.1871, while the molecular weight of C₁₁H₂₂BN₂ is 265.1870.