The high mass accuracy achievable by FT-ICR MS at 21T allows for the unambiguous assignment of molecular formulae to molecular ions. The presence of Cu was confirmed in all eleven CuLs, with an experimental mass difference between 63 Cu and 65 Cu of 1.99821 ± 0.00001 Da (Dmtheoretical, 1.99819 Da) and a 65 Cu to 63 Cu ratio of 2.3 ± 0.4 (SI Fig. ). Molecular formulae were assigned to all CuLs with absolute errors between 6 and 100 ppb (Table All molecular formula assignments were confirmed by their isotope fine structure -the presence of heavy isotope peaks (e.g. N, C, O, 65 Cu) at a RA predicted from the number of each element in the formula obtained using LC-FT-ICR MS at 21T. The theoretical isotope fine structure for each formula assignment was simulated using IsoPro 3.1 at the achieved resolving power for that m/z and compared to the measured isotope fine structure (Fig. ).