Txrf Analysis Essay

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.

Hardware

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?

The analysis of major and trace elements in geological materials by XRF is made possible by the behavior of atoms when they interact with X-radiation. An XRF spectrometer works because if a sample is illuminated by an intense X-ray beam, known as the incident beam, some of the energy is scattered, but some is also absorbed within the sample in a manner that depends on its chemistry. The incident X-ray beam is typically produced from a Rh target, although W, Mo, Cr and others can also be used, depending on the application.

An XRF spectrometer, with the sample port on top, and a set of samples in silver metallic holders in the sample changer in front.

When this primary X-ray beam illuminates the sample, it is said to be excited. The excited sample in turn emits X-rays along a spectrum of wavelengths characteristic of the types of atoms present in the sample. How does this happen? The atoms in the sample absorb X-ray energy by ionizing, ejecting electrons from the lower (usually K and L) energy levels. The ejected electrons are replaced by electrons from an outer, higher energy orbital. When this happens, energy is released due to the decreased binding energy of the inner electron orbital compared with an outer one. This energy release is in the form of emission of characteristic X-rays indicating the type of atom present. If a sample has many elements present, as is typical for most minerals and rocks, the use of a Wavelength Dispersive Spectrometer much like that in an EPMA allows the separation of a complex emitted X-ray spectrum into characteristic wavelengths for each element present. Various types of detectors (gas flow proportional and scintillation) are used to measure the intensity of the emitted beam. The flow counter is commonly utilized for measuring long wavelength (>0.15 nm) X-rays that are typical of K spectra from elements lighter than Zn. The scintillation detector is commonly used to analyze shorter wavelengths in the X-ray spectrum (K spectra of element from Nb to I; L spectra of Th and U). X-rays of intermediate wavelength (K spectra produced from Zn to Zr and L spectra from Ba and the rare earth elements) are generally measured by using both detectors in tandem. The intensity of the energy measured by these detectors is proportional to the abundance of the element in the sample. The exact value of this proportionality for each element is derived by comparison to mineral or rock standards whose composition is known from prior analyses by other techniques.

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