Fig. 4

Score scatter plot of the PCA model for RPE cells cultured during 4 (gray, n = 3 biological replicates, each measured 3 times), 12 (red, n = 3 biological replicates, each measured 3 times), 17 (beige, n = 3 biological replicates, each measured 3 times), and 25 (blue, n = 3 biological replicates, each measured 3 times) weeks after LC–MS analysis in positive (A) and negative (B) ionization modes (see Additional file 4 for individual data values). X-axis and Y-axis reveal the scores of the first and the second component, respectively. The principal components 1, 2, and 3 (PC1, PC2, and PC3) could explain 57% of total variance. Points of different colors represent run samples from each experimental group (3 analytical replicates and 3 biological replicates) and ellipses, the 95% confidence region