DETERMINATION OF MONOISOTOPIC MASSES AND ION POPULATIONS FOR LARGE BIOMOLECULES FROM RESOLVED ISOTOPIC DISTRIBUTIONS

The coupling of electrospray ionization with Fourier-transform mass sp ectrometry allows the analysis of large biomolecules with mass-measuri ng errors of less than 1 ppm. The large number of atoms incorporated i n these molecules results in a low probability for the all-monoisotopi c species. This produces the potential to misassign the number of heav y isotopes in a specific peak and make a mass error of +/-1 Da, althou gh the certainty of the measurement beyond the decimal place is greate r than 0.1 Da. Statistical tests are used to compare the measured isot opic distribution with the distribution for a model molecule of the sa me average molecular mass, which allows the assignment of the monoisot opic mass, even in cases where the monoisotopic peak is absent from th e spectrum. The statistical test produces error levels that are invers ely proportional to the number of molecules in a distribution, which a llows an estimation of the number of ions in the trapped ion cell. It has been determined, via this method that 128 charges are required to produce a signal-to-noise ratio of 3:1, which correlates well with pre vious experimental methods.