Figure 2 of González, Mol Vis 2012; 18:1572-1582.

Figure 2. Proteome/peptidome profile gel view and principal component analysis (PCA) of tear samples purified with four different solid-phase extraction (SPE) methods. A: Virtual gel, representing the mass/charge (m/z) ratio (x-axis) of the detected peaks for each spectrum acquired (y-axis). The intensity of each peak is represented as a gray intensity scale corresponding virtually to the z-axis. In this gel representation, the same tear pretreated with four different SPE methods can be observed: Magnetic Beads (MB), Empore Disc (EM) C8, Empore Disc C18, and Zip Tip (ZT). In all the cases, each tear was spotted in quadruplicate (top profile) to verify the technique reproducibility and enhance statistical validity. B: PCA analysis showing that those samples treated with column-based systems (Empore C8 and C18) group together with higher dispersion, whereas the samples treated with magnetic beads and Zip Tip grouped separately and homogeneously.