A significant loss in electron probe current can occur before the electron beam enters the specimen chamber of an environmental scanning electron microscope (ESEM). This loss results from electron scattering in a gaseous jet formed inside and downstream (above) the pressure-limiting aperture (PLA), which separates the high-pressure and high-vacuum regions of the microscope. The electron beam loss above the PLA has been calculated for three different ESEMs, each with a different PLA geometry: an ElectroScan E3, a Philips XL30 ESEM, and a prototype instrument. The mass thickness of gas above the PLA in each case has been determined by Monte Carlo simulation of the gas density variation in the gas jet. It has been found that the PLA configurations used in the commercial instruments produce considerable loss in the electron probe current that dramatically degrades their performance at high chamber pressure and low accelerating voltage. These detrimental effects are minimized in the prototype instrument, which has an optimized thin-foil PLA design.

Many of the techniques that have been developed in X-ray crystallography are being applied in electron crystallographic studies of proteins. Electron crystallography has the advantage of measuring structure factor phases directly from high resolution images with an accuracy substantially higher than is common in X-ray crystallography. However, electron diffraction amplitudes are often not as precise as those obtained in X-ray work. We discuss here some approaches to maximizing the reliability of the diffraction amplitudes through choice of exposure and data processing schemes. With accurate measurement of diffraction data, Fourier difference methods can be used in electron crystallographic studies of small, localized changes of proteins that exist in two-dimensional crystals. The mathematical basis for the power of these methods in detecting small changes is reviewed. We then discuss several issues related to optimizing the quality of the diffraction data and derive an expression for the best exposure for recording diffraction patterns. An application of Fourier difference maps in localizing drug binding sites on the protein tubulin is discussed.

Micrometer sized particles have been studied to show that a high-quality transmission electron microscope (TEM) specimen can be produced, without the use of embedding media, from a site-specific region of chosen particles using the focused ion beam (FIB) lift-out (LO) technique. The uniqueness of this technique is that site-specific TEM LO specimens may be obtained from particles and from regions which are smaller than the conventional ∼10–20 μm × 5 μm × ∼0.1 μm dimensions of the LO specimen. The innovative FIB LO procedures are described in detail and TEM images of electron transparent specimens obtained from specific micrometer-sized particles are presented.

A Mo-rich precipitate in an Fe-rich matrix and the heterophase interface bounding the precipitate have been examined on an atomic scale by three-dimensional atom-probe (3DAP) microscopy and transmission electron microscopy (TEM). The Mo-rich precipitate was generated by isothermal solid-state decomposition at 500°C of an Fe-15 at.% Mo-5 at.% V alloy. After this heat treatment, TEM reveals a characteristic modulated strain contrast structure. The precipitate has a composition of Mo-13.0 at.% Fe-4.9 at.% V as measured by 3DAP microscopy. The alloy decomposes at a nearly constant 5 at.% V, constituting a metastable tie-line between Fe-rich and Mo-rich bcc solid solutions. The formation of the equilibrium intermetallic phases is kinetically impeded. A local vanadium enrichment of 21 at.% is detected at the interface of the precipitate in a matrix region about 0.27 nm or 1–2 atomic layers in thickness. The corresponding value of the Gibbsian interfacial excess of vanadium is 3.3 ± 1.3 nm−2.