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UW-Madison G 777 - Light Element Analysis

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Electron probe microanalysis EPMAWhat’s the point?Which Elements?Issues for Low eV X-raysLong Wavelength SpectrometryFluorescence YieldsAbsorptionM.A.C.sPeak Shifts and ShapesDeconvolution of F peaksMeasuring Light Elements: peaks or integrals?Area Peak FactorsAPF compilationsCrystal types and peak shapesSlide 15But not always...Crystallographic effectsInterferencesInterferences (as shown by Virtual WDS)-1Interferences (as shown by Virtual WDS)-2PHA can helpSome (random) particular issuesBoronOxygenCarbon contaminationAnti-contaminationOther coatingsElectron probe microanalysisEPMA Light Element AnalysisWhat’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.K X-ray DataIssues for Low eV X-rays• Decreased X-ray generation (low fluorescence yields) • Absorption of these weak X-rays by matrix• 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 thin film effects magnifiedLong Wavelength SpectrometryTAP and stearate crystals, or the newer layered synthetic (“pseudo crystals”) are two options. 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.Fluorescence 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).AbsorptionX-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.Deconvolution 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 whole area (the integral) is a time-consuming task — particularly as you must measure from the peak maximum to minimum, and the count rate on the latter is low, so to achieve high precision, the count must be repeated many times. One does not want to do it very often. A time-saving alternative procedure is to first spend some time acquiring both peak counts and integrals (=wave scans) of the standard and each typical phase. A mathematical ratio, the area peak factor, APF, is then calculated which can then be invoked within the matrix correction. Then, for each unknown, only the peak (and background) needs to be measured.Area Peak FactorsAPF =IunkintegralIunkpeak⋅IstdpeakIstdintegralIf APFs are to be used, that option can easily be turned on in the Analytical Options menu at the very top of the PfW window. Prior to that, either APFs determined for element pairs (left) or for a specified phase (under Elements/Cations) must be determined and entered into the run.APF compilationsBastin and Heijligers, 1992, Present and future of light element analysis with electron beam instruments. Microbeam Analysis, 1, 61-73.Crystal types and peak shapesBastin and Heijligers, 1992, Present and future of light element analysis with electron beam instruments. Microbeam Analysis, 1, 61-73.In many cases,the synthetic crystals are ‘better’ than conventional stearate crystals: the count rates are much greater (compare top 2 spectra). And the synthetics greatly dampen out the higher order reflections (compare higher order Nd reflections, bottom left vs right).Crystal types and peak shapesBastin and Heijligers, 1992, Present and future of light element analysis with electron beam instruments. Microbeam Analysis, 1, 61-73.In many cases,the synthetic crystals are ‘better’ than conventional


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