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.
X-Ray Fluorescence (XRF)
Karl Wirth, Macalester College and Andy Barth, Indiana University~Purdue University, Indianapolis
What is X-Ray Fluorescence (XRF)
An X-ray fluorescence (XRF) spectrometer is an x-ray instrument used for routine, relatively non-destructive chemical analyses of rocks, minerals, sediments and fluids. It works on wavelength-dispersive spectroscopic principles that are similar to an electron microprobe (EPMA). However, an XRF cannot generally make analyses at the small spot sizes typical of EPMA work (2-5 microns), so it is typically used for bulk analyses of larger fractions of geological materials. The relative ease and low cost of sample preparation, and the stability and ease of use of x-ray spectrometers make this one of the most widely used methods for analysis of major and trace elements in rocks, minerals, and sediment.
Fundamental Principles of X-Ray Fluorescence (XRF)
The XRF method depends on fundamental principles that are common to several other instrumental methods involving interactions between electron beams and x-rays with samples, including: X-ray spectroscopy (e.g., SEM - EDS), X-ray diffraction (XRD), and wavelength dispersive spectroscopy (microprobe WDS).
The analysis of major and trace elements in geological materials by x-ray fluorescence is made possible by the behavior of atoms when they interact with radiation. When materials are excited with high-energy, short wavelength radiation (e.g., X-rays), they can become ionized. If the energy of the radiation is sufficient to dislodge a tightly-held inner electron, the atom becomes unstable and an outer electron replaces the missing inner electron. When this happens, energy is released due to the decreased binding energy of the inner electron orbital compared with an outer one. The emitted radiation is of lower energy than the primary incident X-rays and is termed fluorescent radiation. Because the energy of the emitted photon is characteristic of a transition between specific electron orbitals in a particular element, the resulting fluorescent X-rays can be used to detect the abundances of elements that are present in the sample.
X-Ray Fluorescence (XRF) Instrumentation - How Does It Work?
An XRF spectrometer, with the sample port on top, and a set of samples in silver metallic holders in the sample changer in front.