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Basic XRF Workshop
Tim ElamUniversity of Washington - Applied Physics Lab
and Ametek, Inc. Materials Analysis Division(EDAX Research Group)
andGeorge Havrilla
Los Alamos National Laboratory
Denver X-ray Conference August 2, 2010
Workshop Outline
1. What is an XRF instrument?2. How does an XRF spectrum look?
And why does it look like that3. How do we do quantitative analysis?
Specimen effects Instrument effects
4. Applications (George)
Material -> sample -> specimen
Material to measure - solid, powder,liquid, suspended particulates
Sample - chip off rock, scoop,precipitate, pre-concentrate, etc.
Specimen - an individual preparationfor measurement, unit placed ininstrument
Continuum Ag Kα
Ag KβAg L
30 kilovolts tube voltage
What happens when X-rayshit your specimen?
Coherent or Rayleigh scatterWave phenomenon, no energy lossSame process that causes rainbowsAtoms are the right size to scatter X-rays
Incoherent or Compton scatterX-ray hits a single electronLoses energy and changes directionDepends on angle (fixed for most instruments)
Photoelectric absorption (dominant process)Leaves atoms excited (electron shell vacancy)Independent of angle
XRF Mechanism
Atomic vacancy created by:X-rays - XRFProtons - Proton Induced X-ray Emission (PIXE)Electrons - Electron microprobe Picture from AmpTek, Inc.
Used with permission.
A vacancy in theelement must becreated to get XRF
XRF Spectrum
Quantitative overview
Source: James WillisUsed with permission
XRF Explanation - Detailed
Specimen
DetectorSource
(usually an X-ray tube)
Incident Beam Optics(Minimum - aperture) Emergent Beam Optics
(Mono for WDXRF)
Inst
rum
ent
Spe
cim
enIn
tera
ctio
ns
Start with Specimen
X-ray processes in specimen -matrix and scatter
Incident beam is absorbed by all atoms, including analyteEmitted beam is absorbed by all atoms, including analyteX-rays absorbed by Ni atoms emit Ni Kα x-rays,
which can excite additional Fe atomsThis produces both absorption and enhancement matrix effects
Cr Fe Ni
The incident X-rays can also scatter from the atoms. This produces background fromthe continuum primary radiation and peaks from the primary characteristic lines. Raleigh(coherent) scatter is at the same energy while Compton (incoherent ) is shifted.
X-ray processes in specimen -thickness, homogeneity, etc.
Highenergy
Lowenergy
Inhomogeneousspecimen
XRF Explanation - Detailed
Specimen
Detector
Source (usually an X-ray tube)
Incident Beam Optics(Minimum - aperture)
Emergent Beam Optics(Mono for WDXRF)
Instrument - Sources• Isotope and monochromatic SR - less background• Optics - filter, secondary fluorescer, polarizer, or
focusing optic• All optics modify spectrum - this has implications for
analysis, especially sensitivity and background
Data from R. Gorgl, P. Wobrauschek, Ch. Streli, H. Aiginger andM. Benedikt, "Enery-Dispersive Measurement and Comparison of DifferentSpectra from X-ray Tubes", X-ray Spectrometry, 24, 157-162 (1995).
Tungsten X-ray Tube 45 kVKey consideration:primary spectrum energyvselement absorption edges
Instrument - geometry andatmosphere
Detector
2!
Geometry factor (independentof elements or lines; includesarea of specimen, acceptanceangle of detector, etc)
4π steradians
Source solid angleDetector acceptance solid angle
1!
Attenuation of bothbeams in chamberatmosphere
Incident angle Emergence angle
Both angles affect path, especially for thin specimens.
Instrument - detectors
Monochromator for WDXRF, slitsFocusing optics for confocalFilters (used occasionally)
Specimen
Amplifier andPulse shaper
Spectrum
Active layer - Si diode or prop ctrElectrode and/or dead layerBe or plastic window
time
volta
ge
time
volta
ge
Pulse height analyzer
energy
resp
onse
QuantificationSpectrum processing (ED)
Energy
Inte
nsity
1) Subtract background to getnet intensity
2) Sum all counts under peak
Energy
Inte
nsity
Resolve overlaps and interferences•Fixed regions•Fixed energy
Linear least squaresBayesian
•Full least squares
Identify peaks and associate intensitywith analyte elements
Scan setup (WD)
Energy
Inte
nsity
Energy
Inte
nsity
3-point scan setup for peak and background
Fewer interferencesbut harder to handle
Conversion of net intensity toconcentration
Mostly, concentration is proportional to intensity
!
Ci" I
i
!
Ci=Ii
I(i)
Where I(i) is the net intensity from a pure elementspecimen.But there are absorption and enhancement matrix effects,and we may not know the pure element intensity,so
!
Ci= K
i" I
i"M
i" S
i
•Ci concentration of analyte i•Ki proportionality factor•Ii net intensity of analyte i•Mi interelement effect of specimen matrix •Si specimen preparation and heterogeneity
Matrix correction
Ignore S, since we don't know and often can't control homogeneity.
Source: James WillisUsed with permission
This ratio is MFe
In general, M is a function of allconcentrations (or intensities)
Ci = Ki x Ii x Mi x Si
Empirical Matrix Correction
• Empirical methods require many standards• They only cover the composition space of the
standard set• They can be in terms of intensity or
concentration• M is closer to linear function of concentration• Completely dependent on quality of standards
Empirical methods can still provide the best possibleaccuracy (provided the standards are adequate).
Ci = r0 + Ii [ ri + ∑nrin In ]
Theoretical Matrix Correction
• Fundamental parameters method allows calculation of I from C• Can get M from intensity, this allows matrix correction• In principle no standards required - "standardless"• Actually involves some standards, typically measured at factory• If all elements are observed in spectrum, can use sum=100%,
"true standardless"
Ii = G • ∫
S-1S • Ci • pi • ωi • τi(E)
µ(E)Sin ψ1
µ(i)Sin ψ2
+• I(E) • dE
E
Quantification Results
Source: James WillisUsed with permission
Uncertainty and accuracy• Unlike optical spectra, always count limited• X-rays arrive at random, unpredictable times• Sources of uncertainty
– Statistics always lower limit– Calibration and matrix effects– Specimen inhomoqeneity effects
• Accuracy– “Standardless”: limited by atomic data– Calibration– Specimen preparation
• Minimum Detection Limits– Usually calculated from calibration, assuming statistical
fluctuations of background– XRF- background relatively small (vs electron microprobe,
for example)
Summary of XRF capabilities
• Qualitative and quantitative analysis• Solids and liquids• Atomic number ≥ 10 (Na) [special cases ≥ 5]• Energy 0.5 to 25 keV (Wavelength 25 to 0.5 Å)• Sample preparation relatively simple• Concentration range ppm to 100%• Typically about 1% relative accuracy• Time required - a few seconds to several
minutesFrom John GilfrichDon’t tell him.
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