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1. Introduction

  • Page ID
    73747
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    The Electromagnetic Spectrum

    How does light affect molecules and atoms?

    Electromagnetic spectrum.PNG

    D.C. Harris, Quantitative Chemical Analysis, 7th Ed., Freeman, NY, 2007.

    X-Ray Interactions With Matter

    When X-rays encounter matter, they can be:

    • Absorbed or transmitted through the sample (Medical X-Rays – used to see inside materials)

      hand.jpg

      www.seawayort.com/hand.htm

    • Diffracted or scattered from an ordered crystal (X-Ray Diffraction – used to study crystal structure)

      600px-X-ray_diffraction_pattern_3clpro.jpg

      http://commons.wikimedia.org/wiki/Fi...ern_3clpro.jpg

    • Cause the generation of X-rays of differentcolors” (X-Ray Fluorescence – used to determine elemental composition)

    Atomic Structure

    atomic structure.png

    • An atom consists of a nucleus (protons and neutrons) and electrons
    • Z is used to represent the atomic number of an element (the number of protons and electrons)
    • Electrons spin in shells at specific distances from the nucleus
    • Electrons take on discrete (quantized) energy levels (cannot occupy levels between shells
    • Inner shell electrons are bound more tightly and are harder to remove from the atom

    Adapted from Thermo Scientific Quant’X EDXRF training manual

    Electron Shells

    Shells have specific names (i.e., K, L, M) and only hold a certain number of electrons

    electron shells.png

    X-rays typically affect only inner shell (K, L) electrons

    Adapted from Thermo Scientific Quant’X EDXRF training manual

    Moving Electrons to/from Shells

    Binding Energy versus Potential Energy

    moving electrons.png

    • The K shell has the highest binding energy and hence it takes more energy to remove an electron from a K shell (i.e., high energy X-ray) compared to an L shell (i.e., lower energy X-ray)
    • The N shell has the highest potential energy and hence an electron falling from the N shell to the K shell would release more energy (i.e., higher energy X-ray) compared to an L shell (i.e., lower energy X-ray)

    Adapted from Thermo Scientific Quant’X EDXRF training manual

    XRF – A Physical Description

    Step 1: When an X-ray photon of sufficient energy strikes an atom, it dislodges an electron from one of its inner shells (K in this case)

    Step 2a: The atom fills the vacant K shell with an electron from the L shell; as the electron drops to the lower energy state, excess energy is released as a Kα X-ray

    Step 2b: The atom fills the vacant K shell with an electron from the M shell; as the electron drops to the lower energy state, excess energy is released as a Kβ X-ray

    XRF-Excitation-Model.png

    www.niton.com/images/XRF-Excitation-Model.gif

    XRF – Sample Analysis

    fluorescence-metal-sample.png

    www.niton.com/images/fluoresc...tal-sample.gif

    • Since the electronic energy levels for each element are different, the energy of X-ray fluorescence peak can be correlated to a specific element

    Simple XRF Spectrum

    ~10% As in Chinese supplement

    Chinese supplement.png

    • The presence of As in this sample is confirmed through observation of two peaks centered at energies very close (within ±0.05 keV) to their tabulated (reference) line energies
    • These same two peaks will appear in XRF spectra of different arsenic-based materials (i.e., arsenic trioxide, arsenobetaine, etc.)

    ~10% Pb in imported Mexican tableware

    Mexican tableware.png

    • The presence of Pb in this sample is confirmed through observation of two peaks centered at energies very close (within ±0.05 keV) to their tabulated (reference) line energies
    • These same two peaks will appear in XRF spectra of different lead-based materials (i.e., lead arsenate, tetraethyl lead, etc.)

    Box Diagram of XRF Instrument

    Box Diagram of XRF Instrument.PNG

    X-ray Source Detector Sample Digital Pulse Processor XRF Spectrum (cps vs keV) Results (elements and conc’s) software

    • High energy electrons fired at anode (usually made from Ag or Rh)

      Can vary excitation energy from 15-50 kV and current from 10-200 µA

      Can use filters to tailor source profile for lower detection limits

    • Energy-dispersive multi-channel analyzer – no monochromator needed, Peltiercooled solid state detector monitors both the energy and number of photons over a preset measurement time

      The energy of photon in keV is related to the type of element

      The emission rate (cps) is related to the concentration of that element

    • Concentration of an element determined from factory calibration data, sample thickness as estimated from source backscatter, and other parameters

    Different Types of XRF Instruments

    XRF Instruments - Easy to Use.png
    • EASY TO USE (“point and shoot”)
    • Used for SCREENING
    • Can give ACCURATE RESULTS when used by a knowledgeable operator
    • Primary focus of these materials
    XRF Instruments - Complex Software.png
    • COMPLEX SOFTWARE
    • Used in LAB ANALYSIS
    • Designed to give ACCURATE RESULTS
      (autosampler, optimized excitation, report generation)

    This page titled 1. Introduction is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Contributor.

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