X-RAY PHOTOELECTRON-SPECTROSCOPY ANALYSIS OF BIOSURFACES - EXAMINATION OF PERFORMANCES WITH YEAST-CELLS AND RELATED MODEL COMPOUNDS

Two brewer's yeast strains were analyzed by X-ray photoelectron spectroscopy (XPS). The C-1s peak was split into components representing carbon related to different chemical functions. Sample manipulation, spectrometer use and data treatment are responsible for variation coefficients of less than 3% for the atom fractions of oxygen, total carbon and major types of carbon. The possible influence of surface degradation and contamination on the XPS results was investigated using model compounds (quartz, porous silica, sorbitol, starch). The degradation of yeast during XPS analysis is marked by a decrease in the amount of oxygen and of carbon singly bound to oxygen or nitrogen, and by an increase in the amount of carbon only bound to carbon and hydrogen. This is due to X-rays or photoelectrons and not to flood gun electrons. The degradation increases proportionally with X-ray intensity and varies according to the strain. For both of the strains studied, the surface concentration of carbon only bound to carbon and hydrogen is much higher than expected from the composition of whole cell walls. Correcting the XPS data for contamination and degradation, and accounting for proteins still leaves a high concentration of hydrocarbon-like compounds at the surface; this may either reflect the real surface composition or be due to the migration of lipids through the cell wall during freeze-drying.