FIGURE 4.

Statistical analysis demonstrates that peptides from isolated neurons are more similar to GdFFD, such that GdFFD is likely the major contributor to m/z 484. 9 in MALDI-TOF MS. A, PCA demonstrates the clustering of GdFFD standard (inverted triangle), GFFD standard (upright triangle), and biological samples (circles) based substantially on two principal components (PCs), with the biological samples clustering more closely with the GdFFD standard based primarily on PC1. Each symbol corresponds to the fragmentation profile of one sample. The average peak area from three acquisitions of each sample, normalized to the sum of the peak areas of all ions in the respective MS/MS spectrum, is used for PCA analysis. B, seven fragment ions are the variables used in the PCA analysis. Their impact regarding to the two principal components are shown in the figure. There are two distinctive groups regarding PC1. Fragment ions such as b3 and the Fi ion are in one group showing similar impact in group forming, and fragment ion y2 is another. The differences in these two groups of variables have contributed to assigning the two clusters. C, table of Eigenvalues shows that only PC1 and PC2 have an Eigenvalue higher than 1, and these two are the components that explain the majority of the variations. Cum, cumulative.