Presentation on theme: "Fast and Easy Background Modeling For Practical Quantitative Analysis By John J. Donovan University of Oregon, Department of Chemistry."— Presentation transcript:
Fast and Easy Background Modeling For Practical Quantitative Analysis By John J. Donovan University of Oregon, Department of Chemistry
MAN versus Off-peak Background Measurements What Is It Good For? Saving TIME! t = $
How Much Time? 186 seconds w/ Off-Peaks 94 seconds w/ MAN
What Else is it Good For? Avoiding Off-Peak Interferences Spectrometer Reproducibility Issues Beam Sensitive Samples Quantitative Imaging Avoiding Wear and Tear on Spectrometers
Avoiding Off-Peak Interferences By not measuring off- peak intensities in samples of unknown composition, one can eliminate even unforseen off-peak interferences.
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:
Sensitive Samples 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? Na/K Loss in glass (or Si/Al grow-in)
Sodium grow-in of Calcium Silicate (cement gel)
Quantitative Imaging Eliminate acquisition time for off-peak intensity images and still obtain background corrected quantitative images (512 x 2048 sec equals 6 days!)
I c ( ~ iZ mean [( / min ) - 1] Kramers (1923) I c ( = ( /4 ) f P k iZ mean [( / min ) - 1]Fiori et al. (1976) where : i is the absorbed electron current Z mean is the average atomic number (Z-bar) is the detector solid angle f is the absorption factor for the continuum P is the detector efficiency at wavelength k is Kramers constant Equations for Calculation of Continuum Intensity
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) :
No difference in continuum intensity due to mass
It should be something like this: Where,
But the difference is generally small compared to the uncertainty for continuum intensity measurements
Therefore, lets simply assume (for now), that :
So how does it actually work in action?
Acquire on-peak intensity data as a function of the approximate average atomic number range of the unknown samples.
Correct the x-ray continuum (on-peak) intensities for absorption.
Now fit the data to a 2nd order polynomial (or whatever).
1. Next, DE-CORRECT the interpolated continuum for absorption! 21 cps divided by * = 11.2 cps *Na Ka at 15 keV in unknown Na-Al silicate 2. Now, subtract the raw intensity from the emitted intensity! cps minus 11.2 = cps 3. Use this background corrected intensity in the matrix correction. 4. Iterate as necessary! to
Moderate Energy Region
Moderate energy region Rule of Thumb: Background is (generally) the lowest thing one can measure! Delete the rest!
High Energy Region
Typical Silicate Element MAN Background Curves
Typical Sulfide Element MAN Background Curves
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)
Comparison with Off-peak St 305 Set 2 Labradorite (Lake Co.) ELEM: Ca K Fe Ti Na Al Mn Ni O H Si SUM AVER: SDEV: %RSD: Off-Peak MAN St 305 Set 2 Labradorite (Lake Co.) ELEM: Ca K Fe Ti Na Al Mn Ni O H Si SUM AVER: SDEV: %RSD: kev, 20 nA, 5 um, 20 sec on, 20 sec off PUBL: n.a n.a n.a
High Z-bar Off Peak Comparison St 396 Set 2 Chromite (UC # 523-9) ELEM: Ca K Fe Ti Na Al Mn Ni O H Cr SUM AVER: SDEV: %RSD: Off-Peak MAN St 396 Set 2 Chromite (UC # 523-9) ELEM: Ca K Fe Ti Na Al Mn Ni O H Cr SUM AVER: SDEV: %RSD: kev, 20 nA, 5 um, 20 sec on, 20 sec off PUBL: n.a. n.a n.a n.a n.a
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: 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: Note Fe drift in standard, but not background!
Typical Sequence of MAN Fit
Cr K- interference removed
Trace Ni contamination removed (natural chromite, wt. % Ni )
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!