Structure and mass analysis by scanning transmission electron microscopy

In the scanning transmission electron microscope (STEM) an electron beam of a few angstroms diameter is raster scanned over a thin sample and the scat tered electrons are sequentially measured for each sample element irradiate d. The mass, the elemental composition and the structure of a protein can b e simultaneously assessed if all detector systems of the STEM are used. Asp ects affecting the accuracy of the mass measurement technique and the deman ds placed on the instrument's dark-field detector system are outlined. In a ddition, the influences of some sample preparation techniques are noted and the mass-loss induced at ambient temperatures by the incidence of 80 kV el ectrons on various biological samples is reported. Finally, the importance of the STEM for the structural analysis of proteins is documented by exampl es. (C) 2000 Elsevier Science Ltd. All rights reserved.