1. Quality of Life Division, National Physical Laboratory, Teddington, Middlesex TW11 0LW, U.K.; Laboratory of Biophysics and Surface Analysis, School of Pharmacy, University of Nottingham, Nottingham NG7 2RD, U.K.; and Kratos Analytical, Wharfside, Trafford Wharf Road, Manchester M17 1GP, U.K.