Electron Probe MicroAnalysis (EPMA) is a technique capable of producing chemical analyses of microvolumes at the surface of polished solid samples utilising a fine beam of electrons to bombard the surface of the sample. The electron 'probe' generates X-rays whose wavelengths (or energies) are characteristic of the elements present in the sample.

The intensities of the X-rays are a measure of the proportions of the elements present and the analysis may be quantified by reference to the X-ray intensities obtained from standard materials. Since the electron beam may be focussed to a diameter of less than 1 micron and penetrates the surface to a depth of only a few microns, analysis of very small sample volumes may be possible in suitable circumstances.

The Electron Microprobe in the Department of Geology is a JEOL 8600 S automated instrument. The wavelength-dispersive system (WDS) comprises three crystal spectrometers which allow the quantitative analysis of elements from F upwards in the Periodic Table. An Oxford Instruments (née Link AN10000) energy-dispersive system (EDS) is currently used only for qualitative analysis.

The Microprobe is managed by Rob Wilson, with help from both Colin Cunningham and Lin Marvin, under the overall umbrella of the Analytical Services Group of the Department.

Access to the Microprobe Facility is free to all staff and students of the Department. After training, and with initial supervision, these users are expected to operate the instrument themselves.

Instrument time is generally available for external users. Access and costs will depend on whether the user requires professional assistance or wishes to use the instrument on a self-help basis (with training and initial supervision). Free access may be considered in the case of collaborative projects leading to joint publications. For details of competitive commercial costings or to discuss possible joint projects please contact Rob Wilson by telephone, fax or e-mail.

To achieve the best possible quantitative data the specimens must be polished to a 1/4 micron diamond finish. Specimens may be prepared as polished thin sections or as polished blocks, but either must be mounted using 'Araldite' or a similar vacuum compatible epoxy resin.

The specimen holder is designed to accommodate 3 polished thin sections 48 - 50 mm in length and 28 mm in width.

Smaller polished thin sections may be supported on 50 mm cover slip glasses. Larger sizes of polished thin section may be accommodated, up to a maximum of 50 x 78 mm.

A separate specimen holder accommodates four 25 mm diameter polished blocks up to 20 mm deep, although other (smaller) sizes may be accommodated.

Potential microprobe users should make themselves thoroughly familiar with their specimens before embarking on any microprobe work. Although the microprobe is equipped with an optical microscope with both reflected and transmitted modes, the fixed high magnification limits the field of view to c 400 mu (0.4 mm !). In addition, the lowest magnification available for electron imaging (backscattered or secondary) limits that field of view to a maximum of c 2 mm x 1.5 mm. As a polished thin section may extend over a length of typically 40 - 45 mm, location of features previously identified outside the microprobe may not be so easy once the section is less accessible within the microprobe. Experienced optical mineralogists will also find that the transmitted light facilities are extremely limited.

These viewing restrictions mean that photographs, sketches or prints from a microfiche reader or slide scanner should be considered as essential navigational aids. Rough co-ordinates (± 1 mm) of areas of interest may be determined only once the specimens have been mounted in the holder. A preliminary investigation of specimens using the energy dispersive system fitted to our SEM is often useful for identification purposes.

Non-conducting specimens such as rocks must be coated with a thin film of electrically conducting carbon prior to microprobe analysis. Previous carbon coatings obtained elsewhere must be removed prior to re-coating in the Department.