X-Ray Fluorescence (XRF)
Exploration Technique: X-Ray Fluorescence (XRF)
|Exploration Technique Information|
|Exploration Group:||Lab Analysis Techniques|
|Exploration Sub Group:||Rock Lab Analysis|
|Parent Exploration Technique:||Rock Lab Analysis|
|Information Provided by Technique|
|Lithology:||Bulk and trace element analysis of rocks, minerals, and sediments.|
XRF analysis depends on the fundamental principles of electron beam and x-ray interactions with solid materials, similar to other analytical techniques. Other techniques that operate on these principles include X-Ray Spectroscopy (through Energy-Dispersive X-ray Spectroscopy (EDX)) and Wavelength Dispersive Spectroscopy (WDS) typically performed using a SEM or EPMA, and X-Ray Diffraction (XRD) analyses.
- Rock Lab Analysis
During analysis, materials are excited using a high-energy incident beam of short wavelength radiation (X-rays) and become ionized. Inner electrons are ejected from lower energy orbitals (usually the K and L orbitals), making atoms in the sample unstable until the electron holes are filled by electrons from a higher energy outer orbital. When an electron moves from a high energy orbital to a lower energy orbital, energy is released in the form of X-rays that are characteristic of the type of atom present. Continuous ionization of the sample and absorption of energy from the incident beam allows for analysis of the complex X-ray spectrum emitted by the excited material. This X-ray spectrum is separated into characteristic wavelengths unique to each element present in the sample using a Wavelength Dispersive X-Ray Spectrometer. The energy intensity of the different wavelengths measured following separation of the emitted spectrum is proportional to the abundance of the elements associated with those wavelengths in the sample. The exact quantitative abundance of each element is determined by comparing these data to mineral and rock standards of known composition.
Bulk elemental analysis by XRF can detect trace elements (Ba, Ce, Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, and Zn) present in abundances >1 ppm in a sample, with typical detection limits on the order of a few ppm. Abundances of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in rock and sediment samples are also detected in the same analysis. 
Rock, mineral, and sediment samples are typically ground into a fine powder prior to analysis. This is done to achieve greater homogeneity in the sample and ensure a representative bulk chemical analysis. While this level of sample preparation is usually sufficient for trace element analysis, accurate measurement of major element abundances requires further homogenization of the material being examined. This is accomplished by mixing the powdered sample with a chemical flux and then melting the mixture using a furnace or gas burner. This preparation method produces a more homogenous (albeit diluted) glass sample whose elemental abundances can be easily calculated following XRF analysis. 
Powdered samples can be analyzed directly for their trace element contents without additional treatment. However, the wide range of grain sizes in a powdered sample and the wide range of abundances of various elements (particularly iron) create considerable difficulties in comparing the proportions of the measured X-ray emission intensities to the standards. Additional treatment of the sample can resolve this issue (see the Best Practices section above).  
Additional limitations of XRF analysis include:
- The elements detectable by XRF analysis are typically limited to the range between Magnesium and Uranium for most commercially available instruments.
- The technique is largely restricted to analysis of materials that can be prepared into a powder and homogenized.
- XRF analysis cannot distinguish between isotopes of the same element, so additional analysis using other instruments is necessary if this type of data is desired.
- XRF analysis cannot distinguish ions of an element in different valence states (e.g. Fe2+ from Fe3+).
- Relatively large sample sizes are needed for analysis, usually in excess of 1 gram.
- How XRF Works [Internet]. Tawanda Scientific. [cited 2014/09/22]. Available from: http://www.tawadascientific.com/main.php?page=how-xrf-works&lang=en
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