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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| X-ray fluorescence | 6/6 | https://en.wikipedia.org/wiki/X-ray_fluorescence | reference | science, encyclopedia | 2026-05-05T10:06:44.876284+00:00 | kb-cron |
X-ray absorption X-ray enhancement sample macroscopic effects All elements absorb X-rays to some extent. Each element has a characteristic absorption spectrum which consists of a "saw-tooth" succession of fringes, each step-change of which has wavelength close to an emission line of the element. Absorption attenuates the secondary X-rays leaving the sample. For example, the mass absorption coefficient of silicon at the wavelength of the aluminium Kα line is 50 m2/kg, whereas that of iron is 377 m2/kg. This means that fluorescent X-rays generated by a given concentration of aluminium in a matrix of iron are absorbed about seven times more (that is 377/50) compared with the fluorescent X-rays generated by the same concentration of aluminium, but in a silicon matrix. That would lead to about one seventh of the count rate, once the X-rays are detected. Mass absorption coefficients are well known and can be calculated. However, to calculate the absorption for a multi-element sample, the composition must be known. For analysis of an unknown sample, an iterative procedure is therefore used. To derive the mass absorption accurately, data for the concentration of elements not measured by XRF may be needed, and various strategies are employed to estimate these. As an example, in cement analysis, the concentration of oxygen (which is not measured) is calculated by assuming that all other elements are present as standard oxides. Enhancement occurs where the secondary X-rays emitted by a heavier element are sufficiently energetic to stimulate additional secondary emission from a lighter element. This phenomenon can also be modelled, and corrections can be made provided that the full matrix composition can be deduced. Sample macroscopic effects consist of effects of inhomogeneities of the sample, and unrepresentative conditions at its surface. Samples are ideally homogeneous and isotropic, but they often deviate from this ideal. Mixtures of multiple crystalline components in mineral powders can result in absorption effects that deviate from those calculable from theory. When a powder is pressed into a tablet, the finer minerals concentrate at the surface. Spherical grains tend to migrate to the surface more than do angular grains. In machined metals, the softer components of an alloy tend to smear across the surface. Considerable care and ingenuity are required to minimize these effects. Because they are artifacts of the method of sample preparation, these effects can not be compensated by theoretical corrections, and must be "calibrated in". This means that the calibration materials and the unknowns must be compositionally and mechanically similar, and a given calibration is applicable only to a limited range of materials. Glasses most closely approach the ideal of homogeneity and isotropy, and for accurate work, minerals are usually prepared by dissolving them in a borate glass, and casting them into a flat disc or "bead". Prepared in this form, a virtually universal calibration is applicable. Further corrections that are often employed include background correction and line overlap correction. The background signal in an XRF spectrum derives primarily from scattering of primary beam photons by the sample surface. Scattering varies with the sample mass absorption, being greatest when mean atomic number is low. When measuring trace amounts of an element, or when measuring on a variable light matrix, background correction becomes necessary. This is really only feasible on a sequential spectrometer. Line overlap is a common problem, bearing in mind that the spectrum of a complex mineral can contain several hundred measurable lines. Sometimes it can be overcome by measuring a less-intense, but overlap-free line, but in certain instances a correction is inevitable. For instance, the Kα is the only usable line for measuring sodium, and it overlaps the zinc Lβ (L2-M4) line. Thus zinc, if present, must be analyzed in order to properly correct the sodium value.
== Other spectroscopic methods using the same principle == It is also possible to create a characteristic secondary X-ray emission using other incident radiation to excite the sample:
electron beam: electron microprobe; ion beam: particle induced X-ray emission (PIXE). When radiated by an X-ray beam, the sample also emits other radiations that can be used for analysis:
electrons ejected by the photoelectric effect: X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA) The de-excitation also ejects Auger electrons, but Auger electron spectroscopy (AES) normally uses an electron beam as the probe. Confocal microscopy X-ray fluorescence imaging is a newer technique that allows control over depth, in addition to horizontal and vertical aiming, for example, when analysing buried layers in a painting.
== Instrument qualification ==
A 2022 review addresses the application of portable instrumentation from quality assurance/quality control perspectives. It provides a guide to the development of a set of Standard Operating Procedures if regulatory compliance guidelines are not available.
== See also == Emission spectroscopy – Frequencies of light emitted by atoms or chemical compoundsPages displaying short descriptions of redirect targets List of materials analysis methods Micro-X-ray fluorescence Mössbauer effect – Resonant and recoil-free emission and absorption of gamma radiation by atomic nuclei, resonant fluorescence of gamma rays X-ray fluorescence holography
== Notes ==
== References == Beckhoff, B., Kanngießer, B., Langhoff, N., Wedell, R., Wolff, H., Handbook of Practical X-Ray Fluorescence Analysis, Springer, 2006, ISBN 3-540-28603-9 Bertin, E. P., Principles and Practice of X-ray Spectrometric Analysis, Kluwer Academic / Plenum Publishers, ISBN 0-306-30809-6 Buhrke, V. E., Jenkins, R., Smith, D. K., A Practical Guide for the Preparation of Specimens for XRF and XRD Analysis, Wiley, 1998, ISBN 0-471-19458-1 Jenkins, R., X-ray Fluorescence Spectrometry, Wiley, ISBN 0-471-29942-1 Jenkins, R., De Vries, J. L., Practical X-ray Spectrometry, Springer-Verlag, 1973, ISBN 0-387-91029-8 Jenkins, R., R.W. Gould, R. W., Gedcke, D., Quantitative X-ray Spectrometry, Marcel Dekker, ISBN 0-8247-9554-7 Penner-Hahn, James E. (2013). "Chapter 2. Technologies for Detecting Metals in Single Cells. Section 4, Intrinsic X-Ray Fluorescence". In Banci, Lucia (ed.). Metallomics and the Cell. Metal Ions in Life Sciences. Vol. 12. Springer. pp. 15–40. doi:10.1007/978-94-007-5561-1_2. ISBN 978-94-007-5560-4. PMID 23595669.electronic-book ISBN 978-94-007-5561-1 ISSN 1559-0836electronic-ISSN 1868-0402 Van Grieken, R. E., Markowicz, A. A., Handbook of X-Ray Spectrometry 2nd ed.; Marcel Dekker Inc.: New York, 2002; Vol. 29; ISBN 0-8247-0600-5
== External links ==