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Fast and Easy Background Modeling For Practical Quantitative Analysis By John J. Donovan

Learn time-saving background modeling techniques for avoiding interferences, enhancing reproducibility, and achieving accurate quantitative imaging in spectroscopy.

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Fast and Easy Background Modeling For Practical Quantitative Analysis By John J. Donovan

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  1. Fast and Easy Background Modeling For Practical Quantitative Analysis By John J. Donovan University of Oregon, Department of Chemistry

  2. MAN versus Off-peak Background Measurements • What Is It Good For? Saving TIME! t = $

  3. How Much Time? 186 seconds w/ Off-Peaks 94 seconds w/ MAN

  4. What Else is it Good For? • Avoiding Off-Peak Interferences • Spectrometer Reproducibility Issues • Beam Sensitive Samples • Quantitative Imaging • Avoiding Wear and Tear on Spectrometers

  5. Avoiding Off-Peak Interferences By not measuring off-peak intensities in samples of unknown composition, one can eliminate even unforseen off-peak interferences.

  6. Spectrometer Reproducibility Issues • Handle spectrometer re-positioning problems for worn instruments • Ultra High Precision Measurements By reserving a spectrometer for a single MAN corrected element x-ray line (monochromator), one can obtain:

  7. Sensitive Samples Na/K Loss in glass (or Si/Al “grow-in”) • Everyone knows about sodium loss (and silica “gain”) over time in some glasses and especially, hydrous phases- but did you know that sodium can also “grow-in”?

  8. Sodium “grow-in” of Calcium Silicate (cement “gel”)

  9. Quantitative Imaging • Eliminate acquisition time for off-peak intensity images and still obtain background corrected quantitative images (512 x 2048 pixels @ .5 sec equals 6 days!)

  10. Equations for Calculation of Continuum Intensity Ic (l)~ iZmean[(l /lmin) - 1] Kramers (1923) Ic (l) = (W/4p) flPlkl iZmean [(l /lmin) - 1] Fiori et al. (1976) where : i is the absorbed electron current Zmean is the average atomic number (Z-bar) W is the detector solid angle flis the absorption factor for the continuum Plis the detector efficiency at wavelength l kl is Kramers’ constant

  11. But What Exactly Is The Average Atomic Number? Mass fraction weighting for continuum intensities in a compound, (Z-bar), is given by (Goldstein et. al., 1992) :

  12. No difference in continuum intensity due to mass

  13. It should be something like this: Where,

  14. But the difference is generally small compared to the uncertainty for continuum intensity measurements

  15. Therefore, let’s simply assume (for now), that :

  16. So how does it actually work in action?

  17. Acquire on-peak intensity data as a function of the approximate average atomic number range of the unknown samples.

  18. Correct the x-ray continuum (on-peak) intensities for absorption.

  19. Now fit the data to a 2nd order polynomial (or whatever).

  20. 1. Next, DE-CORRECT the interpolated continuum for absorption! 21 cps divided by 1.8778* = 11.2 cps to 2. Now, subtract the “raw” intensity from the “emitted” intensity! 313.5 cps minus 11.2 = 302.3 cps 3. Use this background corrected intensity in the matrix correction. 4. Iterate as necessary! *Na Ka at 15 keV in unknown Na-Al silicate

  21. Moderate Energy Region

  22. “Moderate” energy region Rule of Thumb: Background is (generally) the lowest thing one can measure! Delete the rest!

  23. “High” Energy Region

  24. “Typical” SilicateElement MAN Background Curves

  25. Typical “Sulfide” Element MAN Background Curves

  26. How Good Is It? • Major Elements • Minor Elements • Trace Elements • Comparison to Off-Peak Measurements • Matrix Issues (Low Z-bar vs High Z-bar) • Accuracy (reproducibility, drift, etc)

  27. Comparison with Off-peak Off-Peak 20 kev, 20 nA, 5 um, 20 sec on, 20 sec off St 305 Set 2 Labradorite (Lake Co.) ELEM: Ca K Fe Ti Na Al Mn Ni O H Si SUM AVER: 9.625 .102 .326 .023 2.841 16.529 .008 .003 46.823 .000 23.957 100.239 SDEV: .036 .008 .018 .014 .039 .032 .008 .005 .000 .000 .000 %RSD: .4 7.7 5.5 61.8 1.4 .2 89.4 165.7 .0 .0 .0 MAN St 305 Set 2 Labradorite (Lake Co.) ELEM: Ca K Fe Ti Na Al Mn Ni O H Si SUM AVER: 9.640 .100 .321 .023 2.864 16.543 .002 .004 46.823 .000 23.957 100.277 SDEV: .034 .007 .017 .012 .037 .033 .003 .005 .000 .000 .000 %RSD: .3 7.1 5.4 51.9 1.3 .2 140.3 126.3 .0 .0 .0 PUBL: 9.577 .100 .319 n.a. 2.841 16.359 .000 n.a. 46.823 n.a. 23.957 99.976

  28. High Z-bar Off Peak Comparison 20 kev, 20 nA, 5 um, 20 sec on, 20 sec off Off-Peak St 396 Set 2 Chromite (UC # 523-9) ELEM: Ca K Fe Ti Na Al Mn Ni O H Cr SUM AVER: .002 .004 20.392 .333 .006 8.004 .162 .087 33.042 .000 31.905 100.349 SDEV: .003 .005 .109 .021 .009 .036 .013 .014 .000 .000 .000 .000 %RSD: 114.0 129.1 .5 6.5 156.9 .5 8.0 15.9 .0 .0 .0 .0 MAN St 396 Set 2 Chromite (UC # 523-9) ELEM: Ca K Fe Ti Na Al Mn Ni O H Cr SUM AVER: .001 .001 20.441 .346 .007 7.976 .155 .087 33.042 .000 31.905 100.372 SDEV: .002 .002 .109 .016 .009 .036 .014 .008 .000 .000 .000 .000 %RSD: 316.2 223.9 .5 4.5 118.4 .5 9.1 9.0 .0 .0 .0 .0 PUBL: n.a. n.a. 20.692 .300 n.a. 7.690 .225 n.a. 33.042 n.a. 31.905 100.266

  29. Drift Issues in MAN Drift array background intensities for standards: ELMXRY: ca ka k ka fe ka ti ka na ka al ka mn ka ni ka MOTCRS: 2 PET 2 PET 4 LIF 3 LIF 1 TAP 1 TAP 3 LIF 4 LIF STDASS: 358 374 395 22 305 374 25 28 19.3 15.7 33.0 20.3 9.3 28.5 25.1 46.8 20.0 15.6 33.0 21.2 9.9 28.9 25.8 47.5 Drift array standard intensities (background corrected): ELMXRY: ca ka k ka fe ka ti ka na ka al ka mn ka ni ka MOTCRS: 2 PET 2 PET 4 LIF 3 LIF 1 TAP 1 TAP 3 LIF 4 LIF STDASS: 358 374 395 22 305 374 25 28 4564.9 2741.4 6926.4 2341.0 325.7 3296.5 6976.5 8176.6 4583.7 2745.9 6884.5 2305.0 327.5 3272.7 6960.2 8192.3 Note Fe drift in standard, but not background!

  30. Typical Sequence of MAN Fit

  31. Cr K-b interference removed

  32. Trace Ni “contamination” removed (natural chromite, 0.087 wt. % Ni)

  33. Conclusions 1. Absorption correction critical for low/moderate energies 2. Save time and money (especially quant imaging) 3. Improves accuracy 4. You gotta’ try it to believe it!

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