Total Reflection x-ray fluorescence (TXRF) and the fundamentally related Grazing emission x-ray fluorescence (GEXRF) rely on scatter properties near and below the Bragg angle to reduce background intensities and improve detections limits an order of magnitude or more over more traditional XRF instruments.
If light is directed at a smooth surface at a very small angle (typically less than 0.5 degree for x-rays) virtually 100% of the light will be reflected at an equally small angle. This is the same principle relied on for polycapillary optics. A few x-rays will excite atoms immediately at the surface, and those atoms emit their characteristic radiation in all directions. Because there is virtually no backscatter into the detector, extraordinary detections limits can be achieve.
GEXRF turns the theory around and takes advantage of the fact that when x-rays are directed at a surface they will not be scattered at an angle below the Bragg angle. A detector that only detects x-rays coming off a surface at an angle less than the Bragg angle, will only detect fluorescence x-rays and not background scatter.
TXRF instruments are usually very sophisticated and expensive pieces of equipment with finely tuned optics. The x-ray tubes are usually very high in power, several kilowatts, and must have a small spot size on the anode. A long collimator or wave-guide is needed to restrict the angle to less than the Bragg angle. Using multilayers in the wave-guide can improve the efficiency. The sample needs to be finely and reproducibly polished and positioned precisely with respect to angle and height. A detector is positioned above the surface. Given the sophistication of these systems, Si(Li) or other high resolution detectors are used in most systems.
Some people prefer the GEXRF variation. The x-ray tube can be directed at the sample with little regard to spot size or angle. This saves on a lot of hardware expense. A detector and collimator assembly is positioned so that only x-rays coming from less than the Bragg angle are counted.
Advantages and Disadvantages
While these techniques can achieve amazing performance, they are seldom used. The principle problems are that only a few products are suitable for TXRF analysis without a substantial amount of sample preparation. The other problem is that the optical alignment is so critical that minor vibrations and temperature changes make it necessary to re-align the optics, and/or calibrate the instrument. These problems, in addition to the high cost of most existing systems, have limited the use of these techniques to date.