Surface spectroscopy of adsorbed proteins: input of data treatment by principal component analysis

X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), two surface-sensitive spectroscopic methods, are commonly used to characterize adsorbed protein layers. Principal component analysis (PCA) is a statistical method which aims at reducing the number of variables in complex sets of data while retaining most of the original information. The aim of this paper is to review work carried out in our group regarding the use of PCA with a view to facilitate and deepen the interpretation of ToF-SIMS or XPS spectra acquired on adsorbed protein layers. ToF-SIMS data acquired on polycarbonate membranes after albumin and, or insulin adsorption were treated with PCA. The results reveal the preferential exposure of particular amino acids at the outermost surface depending on the adsorption conditions (nature of the substrate and of the proteins involved, concentration in solution), giving insight into the adsorption mechanisms. PCA was applied on XPS data collected on three different substrates after albumin or fibrinogen adsorption, followed in some cases by a cleaning procedure with oxidizing agents. The results allow samples to be classified according to the nature of the substrate and to the adsorbed amount and, or the level of surface coverage by the protein. Chemical shifts of particular interest are also identified, which may facilitate further peak decomposition. It is useful to recall that the outcome of PCA strongly depends on data selection and normalisation.