Transmittance electron microscopy

TEM Transmittance electron microscopy (used in the determination of asbestos, silica, and other particulates, and as a tool in toxicological studies)... 

More recently techniques have been developed to enhance the resolution of electron microscopy the techniqne is called High resolntion transmittance electron microscopy (HRTEM). Figure 21.3 shows KI (potassium iodide) incorporated in a single-walled nanotube (see Chap. 11 for nanotubes). Individual potassium (K+) and iodine (L) atoms are clearly seen embedded in a tube of about 1.6 mn (1,600 pm) diameter. You can see clearly individual atoms (actually ions) in the crystalline solid of SrTiOj as shown in Fig. 21.4. What you are seeing in these pictures is the image of the electron cloud around a nucleus, and that is essentially an atom. 

Two separate lines of research led to the proposal that transmitter released in response to neuronal excitation is derived from a vesicle-bound pool rather than from the neuronal cytoplasm. One piece of evidence came from electron microscopy which... 

When an action potential traveling down the axon of a motoneuron reaches the myoneural endplate, a process occurs that releases acetylcholine into the synaptic cleft and consequently depolarizes the postsynaptic membrane. A similar process probably occurs at cholinergic synapses in the central nervous system. In 1950 Fatt and Katz discovered a spontaneous subthreshold activity (MEPP) of motor nerve endings and were thereby led to the concept that acetylcholine is released in definite units (quanta) of 10 to 10 molecules. Electron microscopy subsequently revealed characteristic vesicles about 40 nm in diameter, clustered near presynaptic membranes. Subcellular fractionation procedures were devised by Whittaker and de Robertis for the isolation of these vesicles from brain homogenates in sucrose density gradients, and it was soon demonstrated that they were indeed concentrated reservoirs of acetylcholine. The hypothesis that the vesicles discharge the quanta of transmitter became irresistible. 

The samples have been prepared by e-beam evaporation of a dielectric layer followed by thermal evaporation of the silver fraction, which builds the island film, while the sandwich is completed by a further dielectric film. In every sample, intentionally the same amount of silver (corresponding to an average thickness of 4 nm, as recorded by quartz monitoring) has been embedded in a 6 nm thick dielectric film, formed from either Mgp2, LaFs, Si02, or AI2O3. The optical transmittance T and reflectance R of all films have been measured by a Perkin Elmer Lambda 19 spectrophotometer. To correlate the optical properties with the sample morphology, transmission electron microscopy (TEM) has been applied. 

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