APPROACHES AND LIMITS FOR ACCURATE MASS CHARACTERIZATION OF LARGE BIOMOLECULES

The use of the average mass for mass characterization of large biomole cules is examined in light of the latest achievements in mass spectrom etry, and factors affecting the accuracy of both theoretical calculati on 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 l ead to a systematic underestimation of the average mass during the mea surements. The procedure proposed earlier (Zubarev, R. A. Int. J. Mass . Spectrom. Ion Processes 1991, 107, 17-27) in order to correct for th is effect is now extended to the case of multiply-charged ions and the ir use for mass scale calibration. A formula is derived for the relati onship 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, ot her additive quantities, such as the ratio of intensities of the first isotopic peak to the monoisotopic peak, can be employed.