The feasibility of using a high-resolution scanning transmission electron microscope to study the diffusion of heavy atoms on thin film substrates of low atomic number has been investigated. We have shown that it is possible to visualize the diffusion of individual uranium atoms adsorbed to thin carbon film substrates and that the observed motion of the atoms does not appear to be induced by the incident electron beam. Advances in high-resolution transmission electron microscopy during the past several years have made possible the observation of individual heavy atoms on thin substrates of low atomic number (e.g., refs. 1-5). Studies at atomic resolution of atoms adsorbed to light element substrates could be of value in acquiring an improved understanding of chemisorption, catalysis, and the earliest stages of thin film nucleation. It is the purpose of this paper to demonstrate that a scanning transmission electron microscope (STEM) capable of atomic resolution may be a useful instrument for surface science studies and, in particular, that it is capable of reliably observing the diffusion of individual atoms on thin film substrates.In the STEM, an atomic resolution image can be formed by scanning an electron beam less than 5 A in diameter across a specimen in a raster fashion while collecting the transmitted elastically scattered electrons with an annular detector located beneath the specimen (6) (see Fig.

K E Y w ORDS. X-ray microanalysis, microchemical microscopy, biological cryosections, electron microscopy, X-ray mapping, cell physiology, computer imaging. S U M M A R YRecent advances in widely available microcomputers have made the acquisition and processing of digital quantitative X-ray maps of one to several cells readily feasible. Here we describe a system which uses a graphics-based microcomputer to acquire spectrally filtered X-ray elemental image maps that are fitted to standards, to display the image in real time, and to correct the post-acquisition image map with regard to specimen drift. Both high-resolution quantitative energy-dispersive X-ray images of freeze-dried cyrosections and low-dose quantitative bright-field images of frozenhydrated sections can be acquired to obtain element and water content from the same intracellular regions. T h e software programs developed, together with the associated hardware, also allow static probe acquisition of data from selected cell regions with spectral processing and quantification performed on-line in real time. In addition, the unified design of the software program provides for off-line processing and analysing by several investigators at microcomputers remote from the microscope. The overall experimental strategy employs computer-aided imaging, combined with static probes, as an essential interactive tool of investigation for biological analysis. This type of microchemical microscopy facilitates studies in cell physiology and pathophysiology which focus on mechanisms of ionic (elemental) compartmentation, i.e. structurefunction correlation at cellular and subcellular levels; it allows investigation of intracellular concentration gradients, of the heterogeneity of cell responses to stimuli, of certain fast physiological events in uiuo at ultrastructural resolution, and of events occurring with low incidence or involving cell-to-cell interactions.

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