Electron Microprobe Analysis (EMPA) Laboratory
(run by the Stanford School of Earth Sciences Mineral Analysis Facility)

Uses for EMPA

EMPA (also known as EPMA, Electron Probe Micro-Analysis) is useful for determining the elemental composition of a solid sample, down to a spatial resolution of about 1 micron. Because the instrument can resolve and display small features that appear on the surface of a sample, the method is particularly adept at identifying the composition of small inclusions embedded in a matrix, characterizing multi-phase materials, and measuring micron-scale compositional variations. The user can view the surface at high magnification (up to 40,000X), identify the features of interest, and then analyze the composition of those features. The technique will also identify trace elements or bulk contaminants. However, unlike XPS, EMPA samples a volume about 1-3 microns in radius extending into the sample, so it is not especially sensitive to the composition of the very near surface. EMPA data are usually calibrated using well-established standards for each element, and can be highly accurate as well as precise. Compositional images (maps) can readily be obtained with backscattered electrons (sensitive primarily to mean atomic number), with calibrated wavelength dispersive (WDS) x-ray data, or with more qualitative energy dispersive (EDS) data.

Cathodoluminescence (CL)

Cathodoluminescence is the emission of visible light on stimulation by an electron beam. In the new EMPA instrument, scanning the electron beam can product a full-color CL image that can be very useful for rapid phase identification and observation of compositional variation such as zoning in either major or trace elements. In addition, a full optical spectrum can be collected on a point-by-point basis, providing detailed information about optical emissions and fluorescence. CL data can be combined with x-ray and electron signals at each point to provide great flexibility in mapping composition and phase distribution.

The instrument

The new JEOL JXA-8230 SuperProbe, commissioned in January of 2013, has 5 dual-crystal wavelength dispersive spectrometers and an energy dispersive detector. The instrument is fully automated by Advanced Microbeam hardware and PROBE-PRBSE microanalysis and digital imaging software. The lab has hundreds of standard reference materials for quantitative bulk or thin film analysis. Quantitative analysis of bulk specimens (> 1000 nm thick) is performed using the CITZAF matrix correction algorithms incorporated into the PROBE software. Thin film (10-1000 nm) analysis is performed using the program STRATA. The JEOL EDS system has a 10 mm2 silicon drift x-ray detector with 133 eV resolution. Cathodoluminescence data is produced by an “xCLENT III Advanced hyperspectral” system.

Basic theory of operation

The electron microprobe is used for chemical analysis of very small solid samples, and for mapping and imaging compositional variations. A finely focused (approx. 1 micron) beam of electrons is directed onto a flat polished specimen. The high energy electrons ionize the inner shells of the target elements in the sample. The decay of the ionized state results in the emission of characteristic x-rays.

Using Bragg's and Moseley's laws to determine the energy spectrum of x-rays permits the identification of the wavelengths unique to every element in the periodic table. Identifying which elements comprise a specific sample constitutes qualitative analysis. The method is simple and very fast.

Quantitative analysis is accomplished by measuring the intensity of the characteristic wavelengths for each element in the sample. The unknown sample intensities are then compared to intensities measured on standard reference materials of known compositions. Necessary corrections are applied by computer and the results can be displayed as weight % or atomic proportions.

The spatial relations of different materials or particles can be displayed either by moving the beam over the sample or moving the sample under the beam.

Detection limits and spatial resolution

The instrument is capable of analyzing the light elements C-N-O-F with a detection limit and sensitivity of about 500-1000 ppm and the heavier elements Na up to U with a detection limit and sensitivity of about 300 ppm (depending on the mean atomic number of the matrix and x-ray counting times).

The spatial resolution for quantitative analysis is of the order of 1-3 microns. The spatial resolution for secondary and backscattered electron imaging depends on the accelerating voltage and beam current and is approximately 100-200 nm.

Key features

Detectable element range

4Be to 92U

Detectable wavelength range

0.087 to 9.3 nm

Number of X-ray spectrometers

1 to 5 (plus EDS)

Speciment size

150 mm x 150 mm x 50 mm

X – Y range

90 mm x 90 mm

Speciment stage drive speed

15 mm/s max

Accelerating voltage

0.2 to 30 kV (100 V steps)

Probe current range

e(-12) to e(-5) A

Probe current stability

e(-3) / h

BEI (backscattered electron image)

TOPO and COMPO

Scanning image magnification

x40 to 300,000 (WD: 11 mm)

 

Restrictions on samples

Samples must be solid and vacuum compatible. Small grains of solid materials can readily be mounted and polished, and standard polished petrographic thin sections (e.g. 25x38 mm glass slides) easily accommodated. Other unmounted samples can be bonded to a mounting block with epoxy. A sample can range in size from about 0.5 mm x 0.5 mm x 0.5 mm to about 1 cm x 5 cm x 5 cm. The sample must be conducting. If it is not conducting, we can sputter coat it with about 30 nm of carbon, but you must inform us in advance if the sample is insulating or if you are uncertain.

Before analyzing thin films (< 1000 nm thick), we generally simulate the expected results based on the estimated composition using the STRATA software to determine if EMPA will have the desired sensitivity. If the same elements appear in both the film and the substrate, then determining the amounts of those elements in the film will generally become increasingly difficult as the film becomes thinner. EMPA can determine the film thickness if the density is accurately known, or the density if the film thickness is accurately known.

Other sources of information on EMPA

For further information about EMPA at Stanford, please contact Bob Jones at
robjones@stanford.edu or (650) 725-1677.