Preparation of yeast cells for surface analysis by XPS

Analysis of the chemical composition of the surface of microbial cells by X-ray photoelectron spectroscopy (XPS) requires drying. The influence of the sample preparation procedure (washing, freezing, freeze drying, storage) has been examined with brewing yeasts. The recommended procedure involves washing at least once and pouring a concentrated suspension into a vial, precooled at liquid nitrogen temperature. The freeze-drying conditions are not critical provided that no melting occurs; this would cause cell lysis and release of intracellular compounds which alter the XPS results. Prolonged storage of freeze-dried samples prior to surface analysis leads to oxidation of surface groups. The cooling rate is high enough to prevent migration of intracellular constituents to the surface; however removal of surface polymers or appendages during washing or rearrangements of the polymer network at the surface, resulting from freezing, recrystallization or water removal, cannot be excluded.