Electron probe microanalysis EPMAWhat’s the point?Which Elements?Issues for Low eV X-raysLong Wavelength SpectrometryFluorescence YieldsSlide 7Fluorescence Yields…aren’t everythingAbsorptionM.A.C.sPeak Shifts and ShapesDeconvolution of F peaksMeasuring Light Elements: peaks or integrals?Area Peak FactorsAPF compilationsCrystal types and peak shapesSlide 17But not always...Crystallographic effectsInterferencesInterferences (as shown by Virtual WDS)-1Interferences (as shown by Virtual WDS)-2PHA can helpSome (random) particular issuesBoronOxygenCarbon contamination(Traditional) Anti-contaminationBest “Anti-contamination Device” is Oil-free Vacuum Pumps!Other coatings“Soft X-ray Emission Spectrometer”Electron probe microanalysisEPMA Light Element AnalysisModified 4/7/14What’s the point?What are the problems analyzing light elements? How precise/accurate are such analyses?Which Elements?Element Z Wavelength Å Energy (eV) Edge (eV)Be 4 114 109 112B 5 67.6 183 192C 6 44.7 277 284N 7 31.6 292 400O 8 23.6 525 532F 9 18.3 677 687We are concerned here primarily about the K lines of elements Be-F, although many of the concerns affecting their analysis is also true for L and M lines of heavier elements that fall in this low wavelength (low energy) realm.(Now, >2012, Li is doable…see last slide)K X-ray DataIssues for Low eV X-rays• Decreased X-ray generation (low fluorescence yields vs high Auger production) • Absorption of these weak X-rays by mass in “path length” in material• Interferences by L and M lines of higher Z elements, as well as higher order lines • Wavelength shifts and changes in shape of peaks, due to chemical effects (e.g., valence electrons involved in X-ray generation), and sometimes crystallographic orientation (polarization)• Possible errors in matrix correction due to poorly known values especially of m.a.c.s• Low E0 operation beneficial (decreased range, thus decreased path length), but then coating (and contamination) and thin film effects magnifiedLong Wavelength SpectrometryTAP and stearate crystals, or the newer layered synthetic (“pseudo crystals”) have been the two options up to ~2012. Each category has positive and negative features:• TAP and stearate: better spectral resolution (avoid interferences), but peak shape differences accentuated and there are lower count rates; stearates somewhat less stable.• LSMs: poorer spectral resolution (interferences unavoidable), higher orders suppressed, and there are much higher count rates.• Diffraction Gratings: JEOL (and Cameca?) have looked at diffraction gratings, though only JEOL is commercially selling for a very special spectrometer. Possibly combining LSM with groves laid in it.The yields of K lines of B-F are <0.05, as well as the L and M lines of the higher Z elements that fall below 1 keV (longer than 12 Å).2006 comment: We are discussing this in one context--light elements -- but need to view the wider issue too.While the above may be strictly correct, it is also somewhat disingenuous. Recall the F.Y. means only the fraction of x-rays vs fraction of Auger electrons being produced by inner shell ionizations…it says nothing about the actual number of x-rays you can count on your spectrometers! At 7 kv and 20 nA I can easily generate 50,000 counts a second of B Ka -- in pure B. Fluorescence YieldsFluorescence YieldsThe yields of K lines of B-F are <0.05, as well as the L and M lines of the higher Z elements that fall below 1 keV (longer than 12 Å).…it says nothing about the actual number of x-rays you can count on your spectrometers! This will be a function of the “efficiency” of the crystal, the sin theta position, and the ionization efficiency of the P10 gas for the x-ray.Example: The chart would imply that Hf* La counts>>Hf Ma counts.But at 18 keV, 20 nA, Hf Ma (TAP) yields 8362 cpsHf La (LIF low pressure) yields 844 cpsHf La (LIF high pressure) yields 2509 cps why?* Hf: Z=72Fluorescence Yields…aren’t everythingLine counts xtal Sin Gas P Over-voltageMAC Hf by ArHf Ma 8362 TAP .293 Low P 10.8 870Hf La 844 LIF .390 Low P 1.9 124Hf La 2509 LIF .390 High P 1.9 124AbsorptionX-rays with low energies (e.g. < 1 keV) have increased problems with absorption:• within the sample and standard (compare emitted from generated C K in B4C matrix, Fig. 1)• by any intentional (e.g. C-coat) or unintentional (contamination, oxidized) thin film or coating on sample and standard• by windows or diffracting crystal (though these are the same for both standard and sample and thus ‘cancel out’)Fig 1: from Bastin and Heijligers, 1992, Present and future of light element analysis with electron beam instruments. Microbeam Analysis, 1, 61-73.M.A.C.sAn additional complication is that many mass absorption coefficients for use with light elements are not known with great accuracy, as shown in the adjacent table of m.a.c.s for O Ka X-rays, which show the ‘best’ determinations by Bastin and Heijligers (1992), two pre-eminent researchers in this field.Bastin and Heijligers, 1992, Present and future of light element analysis with electron beam instruments. Microbeam Analysis, 1, 61-73.Peak Shifts and ShapesThe electrons involved with the transitions yielding the C k X-ray are valence electrons. Differences in bonding are reflected in differences in the shapes of the peaks (including shifts of the maxima).Fig 18.1, from Reed 1993, p. 275(From Meeker and Lowers, Standards for the analysis of geological and ceramic materials, Slide 813, NIST-MAS Workshop, April 2002.)Here the Fe L and L peak shapes and intensities are functions of the bonding (valence states). Attempts have been made to utilize this for determining valences of Fe and Mn compounds; results are complicated. C KaDeconvolution of F peaksFig 4, Fialin et, 1994, Microbeam AnalysisThis pair of figures provides some explanation why these low energy peaks behave as they do, by deconvolution into the (apparent) related peaks that the spectrometer does not have enough resolution to separate out. Fluorite (left) and F-topaz (right).Measuring Light Elements: peaks or integrals?The previous slide demonstrates a complication for light element analysis: a simple measurement of intensity at the nominal (=standard’s) peak position, may result in an incorrect quantitative analysis. Clearly, the whole area under the peak is the true representation of the X-ray’s intensity. However, to measure the
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