Here is a neat program, created by Robert Winkler, which allows you to calculate the Mw of proteins using data from your electrospray ionization (ESI) mass spec source.
ESIprot 1.0 [http://www.bioprocess.org/esiprot/]
Reference this journal article: Winkler R. "A universal tool for charge state determination and molecular weight calculation of proteins from electrospray ionization mass spectrometry data." Rapid Commun Mass Spectrom, 24(3), 285-294, 2010.
