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2. Interpretation of XRF Spectra

  • Page ID
    74150
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    XRF Spectra

    Consecutive elements in periodic table

    consecutive elements.png

    • Plotting only a portion of the XRF spectra of several different elements
    • Note periodicity - energy is proportional to Z2 (Moseley’s law)

    Periodic Table of XRF Fluorescence Data

    Including K and L line energies & detection limits

    periodic table.png

    XRF Energies for Various Elements

    Generalizations based on use of field portable analyzers

    • ORGANIC ELEMENTS (i.e., H, C, N, O) DO NOT GIVE XRF PEAKS
      Fluorescence photons from these elements are too low in energy to be transmitted through air and are not efficiently detected using conventional Sibased detectors
    • LOW Z ELEMENTS (i.e., Cl, Ar, K, Ca) GIVE ONLY K PEAKS
      L peaks from these elements are too low in energy (these photons are not transmitted through air and not detected with conventional Si-based detectors)
    • HIGH Z ELEMENTS (i.e., Ba, Hg, Pb, U) GIVE ONLY L LINES
      K peaks from these elements are too high in energy (these electrons have high binding energies and cannot be removed with the limited voltage available in field portable analyzers)
    • MIDDLE Z ELEMENTS (i.e., Rh through I) MAY GIVE BOTH K AND L LINES

    XRF – More Detailed Description

    Note energy level diagrams are not drawn to scale

    metal sample.png

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

    energy level diagrams.png

    • Since XRF affects inner shell and not bonding electrons, the XRF spectrum of an element is independent of its chemical form (i.e., spectra of lead, lead arsenate, and tetraethyl lead will ALL show peaks at 10.61 and 12.55 keV)

    K Line Series

    ~10% As in Chinese supplement

    Chinese supplement.png

    • L lines not observed (1.28 and 1.32 keV - too low in energy to be excited)
    • Kα and Kβ peak energies are often close together (1.2 keV apart for As)
    • K lines observed for low to medium Z elements (i.e., Cl, Fe, As)
    • Kα and Kβ peaks have typical ratio of ~ 5 to 1

    L Line Series

    ~10% Pb in imported Mexican tableware

    Mexican tableware.png

    • K lines not observed (75.0 and 94.9 keV - too high in energy to be excited)
    • Lα and Lβ peak energies are often further apart (2.1 keV apart for Pb)
    • L lines observed for high Z elements (i.e., Hg, Pb, Th)
    • Lα and Lβ peaks have typical ratio of ~ 1 to 1 Pb Lγ line

    More Complex XRF Spectrum

    Chinese supplement containing 4% As and 2% Hg

    Chinese supplement 2.png

    • Line overlaps are possible and users must evaluate spectrum to confirm the presence or absence of an element

    Effect of Detector Resolution

    Spectra of 900 ppm Pb added into Pepto-Bismol

    Older Si(PIN) detector

    Older Si(PIN) detector.png
    • Resolution ~0.2 keV (FWHM)
    • Cannot resolve Pb and Bi peaks

    Newer SDD

    Newer SDD.png
    • Resolution ~0.15 keV (FWHM)
    • Can resolve Pb and Bi peaks

    Adapted from Bruce Kaiser, Bruker AXS

    Artifact Peaks

    Arising from X-ray tube source

    • Electrons with high kinetic energy (typically 10-50 kV) strike atoms in the X-ray tube source target (typically Rh or Ag) and transfer energy
    • The interaction of X-ray source photons with the sample generates several characteristic features in an XRF spectrum which may include the following:
      • Bremsstrahlung
      • Rayleigh peaks
      • Compton peaks

    Bremsstrahlung

    Continuum/backscatter from cellulose sample

    photons.png

    E0 = initial energy of electron in X-ray tube source
    E1, E2 = energy of X-ray

    Bremsstrahlung.png
    • Very broad peak due to backscattering of X-rays from sample to detector that may appear in all XRF spectra
    • Maximum energy of this peak limited by kV applied to X-Ray tube, maximum intensity of this peak is ~ 2/3 of the applied keV
    • More prominent in XRF spectra of less dense samples which scatter more of X-ray source photons back to the detector

    Rayleigh Peaks

    Elastic scattering from metal alloy sample

    scattering.png

    E0 = initial energy of X-ray from target element in x-ray tube source
    E1 = energy of X-ray elastically scattered from (typically dense) sample

    metal alloy sample.png
    • Peaks arising from target anode in X-ray tube source (Rh in this case) that may appear in all XRF spectra acquired on that instrument
    • No energy is lost in this process so peaks show up at characteristic X-ray energies (Rh Lα and Lβ at 20.22 and 22.72 keV in this case)
    • Typically observed in spectra of dense samples as weak peaks (due to increased absorption of X-ray source photons by sample)

    Compton Peaks

    Inelastic scattering from cellulose sample

    inelastic scattering.png

    E0 = initial energy of X-ray from target element in x-ray tube source
    E1 = energy of X-ray inelastically scattered from (typically non-dense) sample

    Compton peaks.png
    • Peaks arising from target element in X ray tube (again, Rh in this case) that may appear in all XRF spectra acquired on that instrument
    • Some energy is lost in this process so peaks show up at energies slightly less than characteristic X-ray tube target energies
    • Typically observed in spectra of low density samples as fairly intense peaks (note these peaks are wider than Rayleigh peaks)

    Artifact Peaks

    Arising from detection process

    • The interaction of X-ray fluorescence photons from the sample with the detector can generate several different types of artifact peaks in an XRF spectrum which may include the following:
      • Sum peaks
      • Escape peaks

    Sum Peaks

    Example from analysis of Fe sample

    two photons at detector.pngsum peak.png

    • Artifact peak due to the arrival of 2 photons at the detector at exactly the same time (i.e., Kα + Kα, Kα + Kβ)
    • More prominent in XRF spectra that have high concentrations of an element
    • Can be reduced by keeping count rates low

    Escape Peaks

    Example from analysis of Pb sample

    energy absorption by Si atoms in detector.pngescape peaks.png

    • Artifact peak due to the absorption of some of the energy of a photon by Si atoms in the detector (Eobserved = Eincident – ESi where ESi = 1.74 keV)
    • More prominent in XRF spectra that have high concentrations of an element and for lower Z elements
    • Can be reduced by keeping count rates low

    Artifact Peaks Due to Blank Media

    blank media artifact peaks.png

    • May observe peaks due to contaminants in XRF cups, Mylar film, and matrix
    • In this case, the cellulose matrix is highly pure and the peaks are due to trace elements in the XRF analyzer window and detector materials
    • This can complicate interpretation (false positives)

    Summary of Factors That Complicate Interpretation of XRF Spectra

    Elements in the sample may produce 2 or more lines

    • Kα, Kβ, Lα, Lβ, (we use simplified nomenclature and discussed only α and β lines)
    • Lγ, Lα1, Lβ1, Lβ2 (can also have α1 and α2 lines, β1 and β2 lines, γ lines, etc.)

    Peak overlaps arising from the presence of multiple elements in the sample and limited detector resolution

    Peaks from X-ray source

    • Bremsstrahlung (more prominent in less dense samples)
    • Rayleigh peaks from X-ray source target (typically Ag Lα, Lβ)
    • Compton peaks from X-ray source target (typically at energies < Ag Lα, Lβ)

    Sum peaks (two X-ray photons arriving at the detector at the same time)

    • E = Kα + Kα
    • E = Kα + Kβ

    Escape peaks (Si in the detector absorbing some of the energy from a X-ray)

    • E = Kα – Kα for Si (where Si line energy = 1.74 keV)
    • E = Lα – Kα for Si

    Other artifact peaks

    • Product packaging, XRF cup, Mylar film, (measure what you want to measure)
    • Contaminants on XRF window or trace levels of elements in XRF window or detector materials (analyze blanks to confirm source of these artifacts)

    This page titled 2. Interpretation of XRF Spectra is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Contributor.

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