Collagen (type I from calf skin) adsorption on polystyrene (PS) and plasma-oxidized polystyrene (PSox) was studied, using a quartz crystal microbalance with energy dissipation measurements (QCM-D) and atomic force microscopy (AFM) in tapping mode. Radio-labeled collagen was used to measure the adsorbed amount and the ability of adsorbed collagen to exchange with molecules in the solution. The results show that the collagen adlayer consists of two parts: a dense and thin sheet in which fibrils are formed (directly observed by AFM) and an overlying thick layer (up to 200 nm) containing protruding molecules or bundles which are in very low concentration but modify noticeably the local viscosity. The thickness and viscosity of the semi-liquid adlayer both increase with adsorption time and collagen concentration. Fibril formation near the surface also increases with time and collagen concentration and occurs more readily on PS compared to PSox. Radiochemical measurements show that this may be related to the larger mobility of molecules adsorbed on PS, presumably owing to a smaller number of binding points.

A VAMAS (Versailles Project on Advanced Materials and Standards) interlaboratory study for desorption electrospray ionization mass spectrometry (DESI MS) measurements has been conducted with the involvement of 20 laboratories from 10 countries. Participants were provided with an analytical protocol and two reference samples: a thin layer of Rhodamine B and double-sided adhesive tape, each on separate glass slides. The studies comprised acquisition of positive ion mass spectra in predetermined m/z ranges. No sample preparation was required. Results for Rhodamine B show that very consistent craters may be generated. However, inadequacies of the spray and sample stage designs often lead to variable crater shapes. The average repeatability for Rhodamine B is 50%. Yet, repeatabilities better than 20% can be achieved. Rhodamine B proved to be an excellent reference sample to check the sample erosion crater, the sample stage movement and memory effects. Adhesive tape samples show that their average absolute intensity repeatability is 30% and the relative repeatability is 9%. The constancy of these spectra from relative intensities gives day-to-day average relative repeatabilities of 31%, three times worse than the short-term repeatability. Significant differences in the spectra from different laboratories arise from the different adventitious adducts observed or from contaminants that may cause the higher day-to-day variations. It is thought that this may be overcome by allowing some 20 ppb of sodium to be always present in the solvent, to be the dominating adduct. Repeatabilities better than 5% may be achieved with adequate control.

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