The specimen is placed in the SEM chamber, and images collected for the backscattered electrons and selected elements. Typical accelerating voltage, probe current, and collection times for the images are 12 kV, 3 nA, and 2.5 h, respectively. An accelerating voltage of 12 kV is used to balance the need for high spatial resolution and low X-ray absorption of the lighter elements, and to provide a satisfactory over-voltage for excitation of heavier elements such as iron.
Brightness and contrast settings are adjusted so that the brightest constituents (ferrite) are at the peak-white gray level, and the darker constituents (periclase) are close to black. Image averaging further reduces noise through collection of multiple images, facilitating discrimination from the BE image. X-ray images are collected for elements Ca, Si, AI, Fe, S, K, Na, and Mg. Because these XR images are collected simultaneously with the backscattered electron image, the image set may be combined to classify the mineral phase present at each location (pixel) in the two-dimensional image. Imaging magnification of 500x provides a resolution of 0.5 µm per pixel (512 pixels by 408 pixels in size) which is approximately the resolution of the X-ray images. Data from replicate samples indicate that about four arbitrarily- selected fields of view provide a reasonably representative sampling for many cements. The figures used to illustrate the procedure in this paper were collected at a greater magnification (1000x) to provide clarity in publication.