APPROACHES AND LIMITS FOR ACCURATE MASS CHARACTERIZATION OF LARGE BIOMOLECULES

The use of the average mass for mass characterization of large biomolecules is examined in light of the latest achievements in mass spectrometry, and factors affecting the accuracy of both theoretical calculation and experimental determination are analyzed. It is concluded that, in practice, the accuracy of average mass measurements is limited to +/-0.1 Da for molecular masses below 10 000 Da and to 10 ppm for masses above that value. Inherent properties of the isotopic distributions lead to a systematic underestimation of the average mass during the measurements. The procedure proposed earlier (Zubarev, R. A. Int. J. Mass. Spectrom. Ion Processes 1991, 107, 17-27) in order to correct for this effect is now extended to the case of multiply-charged ions and their use for mass scale calibration. A formula is derived for the relationship between mass accuracy and both the instrumental resolving power and molecular ion peak statistics. Monoisotopic mass measurements are recommended to be used whenever possible. As a complement to that, other additive quantities, such as the ratio of intensities of the first isotopic peak to the monoisotopic peak, can be employed.