An electron microprobe (EMP) or electron probe microanalyzer (EPMA) is a high powered microscope that uses electrons, instead of light, to examine a sample. The ARL-SEMQ electron microprobe at Concord University was installed in 2010. It is currently configured with four wavelength-dispersive spectrometers (WDS), a recently installed Bruker large-area silicon drift detector (SDD) energy-dispersive spectrometer (EDS), an EDAX Si(Li) EDS, as well as both secondary and backscatter electron imaging. A digital video camera captures images from the visible light optics, either reflected or transmitted light, and can feed a live image to remote users over the internet. Most analytical, X-ray mapping, and electron image capture functions are automated using modern computer hardware.

ARL SEMQ Electron Microprobe

Most elements from atomic number 6 (C, carbon) through 92 (U, uranium) may be quantitatively analyzed, and elements from atomic number 4 (Be) may be detected. Maximum spatial resolution is about 1 micron (1/1000 of millimeter). For comparison, human hair averages about 50 to 70 microns in diameter. Thus it is possible to both determine what a sample is composed of but also where in the sample specific elements or components (e.g. different mineral phases) are located at a truly microscopic level of detail. Analysis is usually non-destructive, and most natural and synthetic solid materials can be analyzed if properly prepared. For best results, samples should be stable a high vacuum environment, be suitable for polishing to 1 micron smoothness, and contain individual particles at least 5 microns in diameter. Typically samples are prepared in circular 25.4 mm (1-inch) diameter mounts or as petrographic thin sections, although other sizes may be accomodated. Non-conductive samples are coated with a thin film of carbon prior to analysis.

The two EDS systems are used for rapid qualitative and quantitative analyses as well as X-ray mapping - including spectral imaging and phase discrimination. Elements from B and heavier may be quantified at major and minor element concentrations. Combined WDS+EDS quantitative analyses may also be performed to provide additional analytical flexibility and improved throughput. In addition, software is installed for automated particle work.

The four WDS spectrometers are used primarily for quantitative analysis. Each is configured with two diffracting crystals (LIF and PET in spectrometer 1, PET and RAP in spectrometer 2, ADP and LIF in spectrometer 3, TAP and OV60 in spectrometer 4), and each crystal type allows for the analysis of a different range of elements. Quantitative analysis functions are fully automated using the robust and feature-rich Probe for EPMA software. Computer automation allows large numbers of points, lines, or a combination of both to be analyzed unattended, often overnight. The WDS spectrometers are also used to determine the spatial distribution of elements within a sample. This is referred to as x-ray mapping. As the electron beam is scanned across a sample, the resulting x-rays are recorded, and an image is generated.

The minimum detection limits and analytical accuracy of a WDS analysis depend primarily on the electron beam conditions (current and accelerating voltage) and the counting times chosen for the analysis. Detection limits of ~100 - 300 ppm are readily attained in many materials. During a typical multi-element analysis of a few minutes duration, 1σ precision of 0.3 - 1.5% relative is normally attained for major elements (i.e. those present at concentrations > 1% by weight). If desired, detection limits and precision may both be improved by increasing counting times, by increasing beam current, and/or by assigning multiple spectrometers to a single element.

Because the electron microprobe is essentially a specialized scanning electron microscope, it can also be used to collect electron images. Secondary electron images (SEI) are used primarily to reveal surface features and morphology. Backscatter electron images (BSE) can also reveal the size and shape of particles, but are primarily used to provide some spatial information about the sample composition. This is because the intensity of the BSE signal depends on the average atomic number of the sample. Areas richer in heavier elements are brighter in BSE images, and areas consisting primarily of light elements are darker.

The ARL SEMQ design has several advantages. The higher take-off angle at which the spectrometers are positioned (52.5^o in the ARL compared to 40^o in Cameca and JEOL microprobes) increases light-element sensitivity through reduced absorption of X-rays exiting the sample, reduces matrix corrections for quantitative analysis, and reduces the sensitivity to minor surface topography. The instrument and spectrometers are ruggedly built and therefore durable. Although the Concord instrument currently has only 4 WDS spectrometers, up to 6 fully-tunable wavelength dispersive (WDS) spectrometers may be installed when the energy-dispersive spectrometer (EDS) is placed in a rear port. The more common JEOL and Cameca microprobes have at most 5 WDS spectrometers. Some ARL SEMQ instruments were configured with as many as 12 WDS spectrometers. In this arrangement, 2 fixed-position spectrometers fit in the same space as 1 tunable spectrometer. A derivative of the original ARL design is still manufactured and available in the Asian market.

One major application of the microprobe is tephrochronology, the use of volcanic ash and pumice (tephra) as a tool for dating and correlation. Tephrochronology is employed globally with numerous interdisciplinary applications including: environmental and climate change, archaeology, Earth surface processes, ecology, animal and plant evolution, earthquake hazards & neotectonics, volcanic hazards, and even medicine.

Backscatter electron (BSE) image
 



Wavelength-dispersive (WDS) x-ray maps
 



Energy-dispersive X-ray Spectrum



Quantitative WDS analyses