Micro x-ray fluorescence (µ-XRF) gets its name because these instruments are designed to analyze very small spot sizes.
They are fundamentally like EDXRF systems, and have seen a similar development path through the years. There are a number of differences between u-XRF and EDXRF to make it worthwhile to classify them separately.
The typical µXRF system has the usual EDX hardware, but has several key differences, most notably, downward facing optics. Special x-ray tubes are used with a smaller spot size on the target, so that there is less beam spread after collimation. Usually direct incident radiation is used, because filters tend to make the x-ray pattern broader. Systems usually have several collimators ranging from 12 microns to a millimeter or more, and can automatically switch between them. Polycapillary optics that focus the x-rays are becoming increasing common in high end systems, and they are capable of producing spot sizes in the 50-200 micron range at much higher x-ray flux rates than with an ordinary through hole collimator. Samples are normally presented on a movable XYZ stage that can be manually operated in some low cost systems, or automatic and programmable in higher end system. The most sophisticated µXRF analyzers can even raster the sample producing a dot map of the surface. Cameras and lasers are typical devices used to aid in sample positioning. The detectors can vary from proportional counters in the low end systems to PIN diodes, Si(Li), or SDD detectors in higher end systems. There is even a µXRF system that uses WDX optics.
The biggest application by far is plating analysis, particularly for analyzing printed circuit boards. Literally thousands of instruments have been sold into this application, primarily for analyzing copper, gold, and tin-lead solder. But there are numerous other applications, and more sophisticated plating analysis systems can measure several layers at once.
High-end µXRF systems are normally sold into forensic, fine art, and archeological applications. These systems can do an excellent job of elemental fingerprinting to either match evidence taken from a crime scene or match materials from a painting or other work of art with known authentic materials.