The most common cause of composition modification to a specimen during electron probe microanalysis is the field induced migration of light elements. This is an indirect effect which occurs in response to the long range electric fields that form when dielectric specimens suffer charge imbalance. The result is that the ions are redistributed within the sample according to their respective mobilities and the affect is enhanced rather than eliminated when the sample is coated. The ions typically move radially outward in thin samples because of the excess production of secondary electrons from the specimen surfaces, Cazaux(1986)and downwards in conventional SEM samples when the field is due primarily to the deposition of electrons within the bulk of the specimen

Field induced migration is responsible for most of the elemental signal variations observed during the microanalysis of silicate glasses containing sodium or potassium ions.

Although elements of low atomic number (Z<11) are detectable in TEM specimens using a modem energy-dispersive (EDX) detector, quantification remains a challenge. Problems include the large absorption corrections needed for specimens of typical thickness and the need for calibration specimens which provide k-factors appropriate to a particular detector/window arrangement. We have addressed this second problem by fabricating thin films containing several light elements in combination with silicon, which typically acts as the reference element when specifying k-factors.

Because the EDX peaks of adjacent light elements overlap considerably, it is preferable to have two specimens: one containing C, O and Si, the other B, N, F and Si. We have used both e-beam evaporation and sputtering techniques to deposit thin films on a substrate which can later be removed to leave a freestanding film mounted on a TEM grid.

In conventional quantitative electron microprobe analysis, methods like ZAF and φ(ρz), are used to convert x-ray intensity data to chemical composition utilizing equations that describe electron solid interactions, the x-ray generation process, absorption of the generated x-rays and secondary fluorescence effects. These equations capture the overall response of a sample or standard rather than consider the fate of individual electrons over time. For example, f(χ) is simply the ratio of x-rays emitted to the number generated. While for the most part these methods are built on solid physical models, the models were never exact enough to completely match measured data. Consequently, they have been modified by various adjustable parameters to ensure a high level of accuracy for a wide variety of systems and experimental conditions. However, since most of the adjustments were based on data taken at normal incidence, there has always been a question of how to obtain accurate results from tilted samples.

Wavelength dispersive spectrometry of long wavelength lines by EPMA can give information on the site symmetry and bonding of the x-ray-emitting atom. For fully focussing spectrometers, the energy resolution can be as high as 2eV or better at a spatial resolution of 1 μm, but electron beam currents often must be set to damaging levels. Fluorescent spectroscopy of the same lines in the commercial laboratory XRF spectrometer is far less damaging but spatial resolution is non-existent or, with collimators, relatively poor. With a combination of electron induced and x-ray induced fluorescent spectroscopy, and the insight provided by molecular orbital calculations, speciation or state analysis can be achieved even on damage prone specimens.

Electron and x-ray induced fluorescence has been employed to investigate surface coatings on magnesium metal. Oxygen Kα spectra for crystalline MgO, MgCO3 and powdered 4MgCO3.Mg(OH)2.5H2O were recorded using a TAP(2d=2.5757 nm) analysing crystal on a JEOL 8900R EPMA and a Philips PW 1404 XRF.

The efficiency and reliability of WD analysis can be enhanced substantially by means of computer simulation carried out before embarking on the actual analysis, thereby eliminating the need for a time-wasting ‘trial and error’ process and preventing errors in quantitative analysis due to line overlaps etc. which otherwise might be undetected. By displaying on-screen the combined spectra of elements known or expected to be present in a given sample, overlaps can be studied in detail, which may indicate the preferability of using an alternative analytical peak (e.g. L(Lβ1 instead of Lα1). The selection of optimum background positions with minimal overlap from neighbouring elements is also facilitated. These choices can be made much more rapidly and reliably than by depending on wavelength tables, while also saving the considerable instrument time required to carry out wavelength scans for each new type of sample.

Each field of electron microprobe analysis has its own specific analytical difficulties. Problems relating to the analysis of geologic materials can be primarily attributed to the fact that the samples are nonconductive, some are unstable under the beam, and they have a complex matrix. There are various techniques to solve these analytical problems, but the selection of appropriate standards is one of most important factors.

In study of geologic materials a convention prevails that the summation approximates 100%. For most analyses summation is an indication of the quality of analysis, although some elements could be over or under corrected. Significant progress has been made in developing correction procedures, yet some fundamental parameters still are not well known, and the results will vary depending on which procedures or fundamental parameters are used. One of the most practical solutions to eliminate significant matrix effects is to select reference materials with a close composition to the unknown.

Well-characterized calibration standards for microanalytical applications are difficult to obtain, often poorly characterized, and often not homogeneous from piece to piece. In addition, many microanalytical standards are available only in very small quantities making inter-laboratory comparisons difficult. To further complicate the situation, destructive microbeam techniques such as secondary ion mass spectrometry (SIMS) and laser source mass spectrometries (LSMS) require larger quantities of material than nondestructive techniques.

The U.S. Geological Survey, Geologic Division is in the process of evaluating ways to produce relatively large quantities of well-characterized standards. We are interested in producing standards of geological materials appropriate for multiple microbeam techniques including electron probe microanalysis (EPMA), SIMS and LSMS. The microbeam standards are produced by melting powders of standards of naturally occurring materials that the USGS has previously provided as bulk analytical standards.

The certification process for a microanalysis standard can be quite lengthy. In addition to certifying the reference material for composition, the extent of heterogeneity between specimens and within specimens must be determined. NIST (or formerly, NBS) 260 Special Publications have been used in the past to describe procedures used for individual SRM certifications. Some of these publications describe in detail how the extent of microheterogeneity (or, microhomogeneity) as well as the specimen to specimen heterogeneity can be determined. The intent here is to describe a general statistical approach used at NIST to determine and report the extent of heterogeneity. This approach can be readily used by other laboratories either for certification or for evaluation of standards.

When evaluating a material for use as a standard, there are several physical characteristics which must be satisfied. The material must be robust under the electron beam at the voltages and currents to which it will be subjected in its proposed use. When mounted and polished, it should be stable on exposure to the atmosphere. However, if the material will not be mounted and polished in use, it should be in the same physical form as the final certified standard material.

We have used several techniques in the field of scanning electron microscopy (SEM) to examine ceramic pot sherds from a number of archaeological sites in New Mexico in an effort to characterize the ceramic material and determine the origin of some of the vessels. Typical archaeological sites belonging to Pueblo Indian ancestors in New Mexico are littered with numerous fragments of vessels which served different purposes such as cooking, food/water storage, eating, and ceremonial. often different styles of painting and slipping found on these fragments relate to different temporal and cultural influences. An important question that arises during examination of the many types of sherds involves the origin of each of the vessels. Analysis of mineral content and mineral chemistry can help answer these questions.

Thin sections of pot sherds from 4 prehistoric Anasazi sites (A.D. 1150-1250) from south and central New Mexico, a prehistoric Anasazi site in northwestern New Mexico (Chaco Canyon, Pueblo Alto A.D. 1000 -1150), and an historic Pueblo site from the Spanish period (A.D. 1540-1700) were examined.

Bulk chemical characterizaton with techniques such as neutron activation analysis (NAA) has been used for many years to identify sources of archaeological artifacts by comparing bulk chemical profiles of archaeological artifacts to profiles determined for samples of raw material for source identification. The chemical approach to determining the provenance of archaeological pottery is complicated by the paste preparation activities of potters and by post-depositional processes. In a ceramic provenance investigation, the investigator typically identifies a series of relatively homogeneous subgroups, which most often are assumed to represent geographically restricted raw material sources. Although the source-specific interpretation is often the most parsimonious explanation of compositional patterning, it is really only the first of several competing hypotheses which, singly or in combination, might account for patterns of similarity and difference in a compositional data set. In this case, two white firing pottery types (SAWH and Ivory) made during the Late and Terminal Formative periods (300 B.C. - 200 A.D.) in southeastern Mesoamerica (Guatemala) were found to be chemically distinct.

Natural rock-forming minerals and many synthetic materials have traditionally been characterized by electron microprobe analysis (EMPA). However, recent advances in technology have created attractive alternatives to EMPA that focus instead on bulk chemistry and high-resolution surface studies. The purpose of this abstract is to demonstrate the continued usefulness of careful electron microprobe analysis as part of a well-coordinated analytical study. To this end, a brief summary is presented of the role the electron microprobe played in the recent discovery of yoshiokaite, a new Ca, Al-silicate mineral and one of only three new minerals discovered on the Moon.[l]

Regolith breccias returned by several Apollo missions were subjected to extensive bulk chemical studies at the University of California, Los A n geles.[2] These bulk chemical studies focused particularly on key trace elements that might identify exotic regolith breccias.

Is average atomic mass or average atomic number more appropriate to describe backscattered electron (BSE) effects in electron-solid interactions? In electron microprobe literature, either one or both of the terms A and Z may be found utilized in various theoretical models to describe electron backscattering. Physical theory would seem to favor atomic number averaging, because backscattering ultimately represents an electromagnetic interaction between the electron and the nucleus. At this scale, mass and its associated property of gravity, is 10 times less powerful than the electromagnetic force. Nonetheless, some empirical results show a strong relation between atomic mass and electron scattering, thus endorsing mass averaging.

The periodic table presents an opportunity for distinguishing between these two influences: those pairs of elements for which an increase in Z yields a decrease in average A (e.g., Ar-39.95 and K-39.10; Co-58.93 and Ni-58.71; Te-127.60 and 1-126.90) due to natural isotope abundances.

The use of backscattered electron (BSE) imaging in low voltage scanning electron microscopy (SEM) has increased over the past few years. This appears to be due to several factors including improved performance of SEMs at low voltages, reduced beam penetration, more reliable metrology, improved atomic number (Z) contrast information (for low Z) and reduced charging artefacts over secondary electron (SE) imaging. Understanding the factors involved in low voltage BSE detection may assist in improving the information attainable.

It has been shown that the signal Sdet from a BSE detector, for EB ≫ Ew is given by

where η is the BSE yield, Ω is the solid angle subtended by the detector to the specimen, D is the internal conversion efficiency of the detector, EB is beam accelerating voltage, Ew is the energy barrier of the dead layer on the detector's surface, IB is the beam current, F(Z) and F(Ω) are functions which take into account the variation of BSE energy with atomic number Z and collection angle respectively.

Low voltage scanning electron microscopy with a field emission source allows characterization of materials with high spatial resolution. This high resolution comes from the low incident energy which gives a small interaction volume (about 10 nm in Fe at 1 keV ), from the field emission source which gives a small probe size (about 2.5 nm in the most recent FE-SEM) and from virtual, or through the lens, secondary electron detectors with gives high collection efficiency and eliminates some of the SEII and all the SEIII- For example, it has been shown that 10 nm NbC inclusions in steels can be imaged in such FE-SEM at 2 keV (this work was performed with a HITACHI S-4500). However, quantitative x-ray analysis of such precipitates are difficult because the critical ionization energy of the Nb Lα lines is equal to 2.37 keV, an incident electron energy of at least 5 keV must be used to get significant x-ray counts rates.

The Secondary Electron Detector (SED) is the first link in the chain of image formation after the initial illumination of a specimen in a Scanning Electron Microscope (SEM), therefore it is most crucial that the efficiency of the SED be as great as possible in order to maximize Signal-to- Noise Ratio (S/N) as any imperfections, inadequacies, and noise are then amplified along with the signal by the subsequent electronic amplifiers in the signal chain.

In the Everhart-Thornley SED, matching the scintillator's output spectrum to the photomultiplier tube's photocathode spectral sensitivity is one factor along with other characteristics of the PM tube, like quantum efficiency and noise characteristics. Light-pipe material along with as optical coupling techniques are another factor which determines the overall efficiency of converting SE electron strikes on the scintillator to a video output signal.

Advanced metal coating methods have greatly improved the localization of secondary electrons (SEI) from biological specimens staged in an in-lens field emission SEM. Using these metal coatings, molecular level structural information from cell membranes and isolated bio-molecules have been more accurately recorded from frozen-hydrated samples due to the presence of bound water rather than from chemically fixed and dried samples. Although in-lens cryo-SEM systems have provided images with equivalent resolution to TEM-replicas, several conditions are encountered which require optimizing techniques and hardware.

A drop of hydrated Bakers yeast on a gold Balzers carrier was plunge-frozen in ethane at its melting point. Thermal properties of ethane, such as the large difference between melting and boiling points, and its low viscosity increase the heat transfer coefficient during plunging thereby leading to faster cooling of the sample than with other cryogens. Carriers were transferred to the cryo-workstation of the Oxford CT 3500 system.

High-resolution secondary electron (SE) imaging was first demonstrated at 100 kV in the STEM a decade ago. High-resolution SE imaging is now routinely obtainable in field-emission SEMs. Although nanometer-scale surface features can be examined at low incident beam voltages we still do not fully understand the factors that affect the contrast of low voltage SE images. At high incident beam voltages, SE1 (SEs generated by the incident probe) and SE2 (SEs generated by backscattered electrons at the sample surface) can be spatially separated. SE1 carries high-resolution detail while SE2 contributes to background. At low incident beam voltages, however, the interaction volume of the incident electrons shrinks rapidly with decreasing incident beam voltage. Thus, both the SE1 and SE2 signals carry high-resolution information. At low incident beam voltages, SE3 (SEs generated by backscattered electrons impinging on the sample chamber, pole pieces and etc.) also carries high-resolution detail and contributes significantly to the collected signal, especially for high atomic number materials and at short working distances.

The elemental composition of polyphosphate bodies (PPB’s) and other components of the cell have previously been determined by using energy dispersive X-ray spectrometers(EDX). In this present study we perform a quantitative analysis or normally grown cells and cells that were grown in the overplus phenomenon.

Cells of Plectonema boryanum were grown in modified Fitzgerald's media and harvested after a logarithmic growth phase of 14 days. The overplus cells were grown as has been previously described2. The cells were exposed to l00ppm of lead, as well as other metals separately, for one hour. The cells were then fixed and embedded in EPON according to Luft's procedure.

For X-ray analysis cells were first located using the TEM mode, and then the microscope was switched to the STEM mode. Analysis of cell components were carried out using the spot mode (75Kv).

Despite the large production and consumption of textile fibers on a global basis, the ability to identify specific types of textile fibers with similar physical-chemical properties is a challenging obstacle for the textile industry. One problem in particular is the identification of specialty animal fibers. Cashmere is a specialty fiber that has a value of 7 to 8 times higher than its similar counterpart wool. It is important, therefore, for textile manufactures to be capable of distinguishing between bails of high value specialty fibers and those containing contamination from lower value products.

Today the universally accepted method of identifying fibers is microscopic analysis. However, the optical microscope is limited because of the subjective nature of the analysis and its limited ability to easily measure different parameters of the fiber such as the fiber diameter, the fiber scale length and the fiber scale thickness.

One of the limitations of microscopy is that only a relatively small area is viewed and that both microscopic and macroscopic information is needed to better understand a process or relationship. Microscopic structures that can only be seen at high magnification may appear insignificant at magnifications required to see macroscopic structures. Montages from the scanning electron microscope (SEM) and optical microscope allow large areas to be displayed at relatively high magnifications revealing both macroscopic and microscopic features.

The use of automated digital microscopy and image software have reduced the barriers for creation of montages and provided new display modes, thereby stimulating their use as an enhanced data acquisition, review, and interpretation technique.

The ability to create digital montages is a valuable tool in the analysis of cement and concrete. It has been used to evaluate the relationship between small scale deleterious phases and their larger scale effects.