X Ray Flourescence (XRF)

Transcription

1 X Ray Flourescence (XRF) Aspiring Geologist XRF Technique XRF is a rapid, relatively non destructive process that produces chemical analysis of rocks, minerals, sediments, fluids, and soils It s purpose is to identify the elemental abundances of the Indentifies both major and trace elements It can quantify a broad range of amounts. From 100% X Ex) Gold Impurities To a few parts per million X Ex) Basalt Flows Phillips PW1606 X ray Fluorescence Spectrometer How It Works Similar process to X Ray Spectroscopy and XRD The is zapped by short wavelength X Rays or Gamma Rays This radiation excites the and dislodges inner orbital electrons causing ionization of the X Ray Schematic Representation of XRF With space in the lower orbitals open, electrons in higher orbitals fall into the lower ones This releases a secondary radiation, the fluorescense, from the Fluorescent Radiation Modified from How It Works (cont) Fluorescent radiation is energy released as photons from the as electrons move down to lower orbitals to stabilize the atoms. Primary Radiation Characteristic Radiation Secondary Radiation Energy is released during this process because the binding energy of a low orbital is less than that of a higher orbital. The energy released is roughly equal to the difference in the binding energies of the two orbitals involved Both the energy and the wavelengths of the secondary radiation is much less than the original X Ray 1

2 How It Works (cont) Characteristic of the secondary radiation such as energy and wavelength are specific to element whose atom they were released from These can be detected and converted into computer generated data The XRF machines output, coupled with a little quantitative analysis, will report what percent of each element is within the Typical XRF Graph With Spikes ray_fluorescence Specifics of XRF Because the secondary radiation produces such weak photons, the tubes that involve radiation transport must be kept in a vacuum After secondary radiation occurs, the fluorescence is channeled into a solid state detector, like a crystal, that is able to produce a steady beam of photons for further detection Specific wavelengths will come off the crystal at specific angles, which allows for isolated detection This technique is called Wavelength Dispersive Spectroscopy (WSD) Wavelength Dispersive Spectrometer ray_fluorescence Isolating Individual Elements WSD machines have many detectors, and the capability of isolating individual wavelengths of fluorescent radiation Detection Principles Complicated process that varies with each of the many detection methods Detectors contain their own, specific atoms The secondary radiation will ionize these atoms contained in the dt detectors t producing a charge The charge is proportional to the incoming photons energy Next the charge is collected and the process starts over for the next photon 2

3 Detectors Two very common types of detectors used to measure the intensity of the secondary radiation are Gas Flow Proportional and Scintillation detectors Gas Flow Proportional detectors are used to measure smaller wavelengths (K spectra of elements less than Zn(30)). They are filled with gas(90% Argon), and when the photons pass through, the argon is ionized and the electric field multiplies the charge into a measurable pulse Scintillation detectors analyze shorter wavelengths (K spectra from NB(41) I(53) and L spectra from Th(90) U(92)) An embedded crystal produces scintillations that are proportional to the energy of the photon absorbed. This pulse is translated into a voltage proportional to the original photon Where The K And L Spectra Come From on.svg Last Step Ideally, experimenters can compare their results to a standard whose composition is very similar Typical XRF Detection Arrangement The standards elemental abundance has been calculated by other techniques and is considered accurate Sample Preparation Because preparation is relatively cheap and easy, XRF is a widely used technique You at least 1 gram of uncontaminated One type of preparation technique involves converting the powdered intodisks Powdered Disk Preparation The must be grinded into a completely homogeneous powder The Powder is then fused and pressed into a pellet that will adequately absorb the primary X Rays This technique is best suited for trace element analysis The other common preparation technique consists of converting the into a glass bead Some labs do both, others use only one technique 3

4 Glass Bead Preparation The is melted in a 1000 furnace It is added to a flux. For example Washington State uses a 2:1 ratio of di lithium tetraborate flux to Lithium and Boron are commonly used in fluxes because they are light and absorb few X Rays After cooling, the glass is shaped and polished into a bead suitable for XRF analysis Preparation (cont) The flux in the glass bead makes it very durable The glass bead s can survive many decades of analysis Any standards used in comparison with XRF results must be prepared the same way the was There is usually a correction made after the initial XRF analysis and using a glass can make this process easier By using a watered down, the correction is much smaller Calibration Two approaches: empirical and fundamental parameters Empirical calibration is based on the analysis of known standards. Standards should have similar elemental abundances to the Fundamental Parameters Calibrationmay be done for standardless s It relies on built in algorithms that refer to the physics of the detector s response to a of one pure element It may be checked by running a standard after calibration is completed Limitations Cannot analyze elements lighter than Na(11) Cannot distinguish between various isotopes of elements Cannot distinguish between different ions of the same element Most machines have some limitations on elemental analysis and cannot identify every element with atomic mass > 11 Potential Error Calibration mistakes can throw of the results Improper preparation Mechanical failure of the machine Improper orientation of critical parts Crystals detectors X ray beams Practical Uses XRF is used in the commercial production of metals, glasses, ceramics, and building materials Utilized in research fields such as geochemistry!, archaeology, construction, forensic science, environmental studies, the petroleum industry, and mining 4

5 Ray Gun? No it s a handheld XRF! Sources Papachristodoulou, Christina. Elemental analysis using the XRF Technique. XRF Laboratory. 27 Sept 2009 < > Rollinson, Hugh. Using Geochemical Data. Essex: Pearson Limited, Wirth, Karl, and Andy Barth. X Ray Fluorescence. Geochemical Instrumentation and Analysis. Carleton College. 24 Sept < chnique s/xrf.html> < X ray Fluorescence. Wikipedia. 2 Oct 2009 ray_fluorescence> X Ray Fluorescence Spectroscopy (XRF). Amptek. 4 Oct 2009 < ndt.com/xrf.html Gneiss XRF Machine! Panalytical Epsilon 5 m 5