SummaryWe present a new model for the gas amplification effect used in many environmental scanning electron microscopes, wherein molecular complexity is shown to be the critical factor. Monte Carlo simulations, based on experimental electron scattering cross-sections, are used to deduce a predictive model for the amplification process that is superior to the Townsend gas capacitor model. These predictions are compared with experimentally obtained amplification curves. Significantly, it is shown that the ionization efficiency of the electrons changes dramatically over the gap distance, and a constant value cannot be assumed. Atomic and molecular excitations affect the amplification process in two ways: first, they serve to lower the average kinetic energy of the imaging electrons, thereby keeping a greater fraction near the ionization threshold energy. Second, molecular normal modes determine the effectiveness of positive gas ions in producing additional secondaries upon surface impact. Practical implications such as signal gain and fraction of useful signal as a function of operating conditions are discussed in the light of the new model. Finally, we speculate on potential new contrast mechanisms brought about by the presence of an imaging gas.

A commonly adopted model for the microstructure of Nephila clavipes major ampullate silk (MAS) is similar to that used for Bombyx mori (silkworm) silk: a simple composite wherein discrete, essentially perfect crystals are dispersed throughout an amorphous protein matrix. However, inconsistencies arise when researchers using complementary microstructural characterisation techniques attempt to explain their results within that framework. We present here the findings of our parallel studies in x‐ray diffraction, electron microscopy, and molecular modeling. These results, combined with other data gleaned from the literature, lead us to propose a revised description of the spider silk microstructure. The new model recognizes that the 70–500 nm sized ordered regions in MAS cannot be constructed from a simple motif of repeating monomers, and develops the concept of non‐periodic lattice (NPL) crystals to characterize these structures. The local composition, symmetry, and perfection of order vary over distances that are small compared to the size of an NPL crystal. © 1997 John Wiley & Sons, Inc. Biopoly 41: 703–719, 1997

SYNOPSISWe report the first direct observations of the physical and chemical microstructure of spider dragline, revealed by analytical transmission electron microscopy. Individual crystallites were imaged within the amorphous matrix. They are irregularly shaped, approximately 70-100 nm in diameter, and uniformly distributed throughout the matrix. Electron diffraction determined their space group to be P2,. The corresponding orthogonal cell has lattice parameters of a = 13.31 A (0-sheet repeat), b = 9.44 8, (interchain repeat within j3-sheets) , and c = 20.88 8, (repeat along polypeptide chain). Electron energy loss spectroscopy indicated compositional variations within the matrix, and between the crystallites and matrix. Most notably, calcium was found exclusively in the crystallites. Attempts to produce synthetic analogues of dragline, which exhibits an unparalleled combination of strength, stiffness, and toughness, cannot depend solely on duplicating the constituent proteins. The complex hierarchical microstructure of the natural material must be taken into account.

SummarySpecimen damage from the electron beam poses a considerable problem with electron microscopy. This damage is particularly acute in environmental scanning electron microscopy (ESEM) for two reasons. Firstly, owing to its ability to stabilise insulating and hydrated specimens, ESEM lends itself to polymeric and biological materials that are typically highly beam-sensitive. Secondly, water acts as a source of small, highly mobile free radicals, which accelerate specimen degradation. By taking the results of single-particle simulations of electron±water interactions, we determine the concentration of reactive species in a water specimen under ESEM conditions. We consider 12 species, which are produced in a Gaussian distribution, and annihilate according to a second-order reaction scheme. Self-diffusion along the concentration gradient is also modelled. We find that the dominant reactive species is the hydroxyl (.OH) radical. Annihilation of this species is suppressed due to the lower concentration of reactants. The relatively stable hydrogen peroxide is also found at large concentrations. By comparing two beam energies, 5 and 25 keV, we find a drastic increase in the quantities of reactive species produced with beam energy. The longer range of 25 keV primary electrons spreads reactive species over a wider region, which then decay far more slowly.