1. Michael Barber Centre for Mass Spectrometry, Manchester Interdisciplinary Biocentre, The University of Manchester, Manchester, U.K., Shimadzu Research Laboratory (Europe) Ltd., Wharfside, Trafford Wharf Road, Manchester M17 1GP, U.K., and Molecular Biophysics Group, School of Biological Sciences, The University of Liverpool, Liverpool L69 7ZB, U.K.