Samples (2.5 ml) were concentrated and desalted onto a micro C18 precolumn (500 mm  2 mm; Michrom Bioresources, Auburn, CA, USA) with H2O:CH3CN (98:2, 0.05% heptafluorobutyric acid) at 20 ml minÀ1. After a 4 min wash the precolumn was switched (Valco 10 port valve; ...