Clusters similar to those of the IR spectra are formed. Figure shows a so-called biplot of the first two principal components, which together explain about 60 % of the variance of the data. Within the biplot, Important variables are I0 and Ds. An increase in surface area is described by an increase in I0, while an increase in Ds describes an increase in surface roughness. I0 and Ds describe the progression along PC 2.