Determination of monoisotopic masses and ion populations for large biomolecules from resolved isotopic distributions

TLDR

ESI–FTMS provides sub‑ppm mass accuracy for large biomolecules, yet the rarity of the all‑monoisotopic peak can lead to ±1 Da misassignments despite sub‑0.1 Da precision beyond the decimal. By statistically comparing the observed isotopic pattern to that of a model molecule of identical average mass, the authors assign the monoisotopic mass even when the peak is missing and derive ion‑cell counts from error levels inversely related to molecule number. The approach shows that 128 charges are required to achieve a 3:1 signal‑to‑noise ratio, matching earlier experimental findings.

Abstract

The coupling of electrospray ionization with Fourier-transform mass spectrometry allows the analysis of large biomolecules with mass-measuring errors of less than 1 ppm. The large number of atoms incorporated in these molecules results in a low probability for the all-monoisotopic species. This produces the potential to misassign the number of heavy isotopes in a specific peak and make a mass error of ±1 Da, although the certainty of the measurement beyond the decimal place is greater than 0.1 Da. Statistical tests are used to compare the measured isotopic distribution with the distribution for a model molecule of the same average molecular mass, which allows the assignment of the monoisotopic mass, even in cases where the monoisotopic peak is absent from the spectrum. The statistical test produces error levels that are inversely proportional to the number of molecules in a distribution, which allows 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 previous experimental methods.