Pushing the Envelope for Beam Sensitive Samples and Trace Element Sensitivity and Accuracy John Donovan (541)

Every sample is beam sensitive at a sufficiently high beam current... Usually thermally insulating samples (e.g., non conductors…) Classical beam sensitive samples (e.g., alkali, hydrous glasses) Orientation dependent intensity changes over time (e.g., apatites) Trace element measurements (high beam currents, long counting) Use alternating on and off-peak measurements (constant delta) Extrapolate to zero time intensities Use a “blank” correction to apply a systematic error offset SiO2 GlassSiO2 Quartz

Fe K  and Si k  in VG2 Glass Alternating On and Off peak Acquisition To Document Sample Stability 15 keV, 100nA, 20um

F Ka in VG2 Glass (1800 secs total count time)

Water By Difference From 100% Effects from Water By Difference Effects from Na, Si, etc “migration” Measure all Cations Calculate Oxygen (assume stoichiometry for Fe, etc.) Subtract from 100 to Obtain H2O Matrix Effects: Time Dependent Intensity (TDI) Corrections Intensity Changes over Time Na, K, Si, Al, etc. Improved Accuracy of Measured Elements

Correcting for Intensity Loss (and Gain) Results in Oxide Weight Percents ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM AVER: SDEV: SERR: %RSD: VOL%: DEV%: VOLF: LINEAR LINEAR LINEAR LINEAR LINEAR

But Not Always What You Expect!

Hyper-exponential Loss Results in Oxide Weight Percents ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM AVER: SDEV: SERR: %RSD: VOL%: DEV%: VOLF: QUADRA LINEAR LINEAR LINEAR LINEAR

Can We Test This Extrapolation? Results in Oxide Weight Percents (20 nA) ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM AVER: VOL%: DEV%: Results in Oxide Weight Percents (15 nA) ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM AVER: VOL%: DEV%: Results in Oxide Weight Percents (10 nA) ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM AVER: VOL%: DEV%: Results in Oxide Weight Percents (5 nA) ELEM: Na2O SiO2 Al2O3 MgO TiO2 MnO P2O5 Cl FeO K2O CaO O H2O SUM AVER: VOL%: DEV%:

Matrix Effects from “missing” elements Results in Oxide Weight Percents ELEM: CaO K2O FeO SiO2 MgO Na2O Al2O3 TiO2 P2O5 O H2O SUM AVER: SDEV: %RSD: ZCOR: KRAW: PKBG: No Volatile Correction, No Water By Difference Excel Spreadsheet Subtraction Gives ~ 4.7% H2O

Volatile Correction Matrix Effects Results in Oxide Weight Percents ELEM: CaO K2O FeO SiO2 MgO Na2O Al2O3 TiO2 P2O5 O H2O SUM AVER: SDEV: %RSD: ZCOR: KRAW: PKBG: VOL%: DEV%: With Volatile Correction, No Water By Difference Na, Si, K Intensity Over Time Iteration Gives ~ 4% H2O

Water By Difference Matrix Effects Results in Oxide Weight Percents ELEM: CaO K2O FeO SiO2 MgO Na2O Al2O3 TiO2 P2O5 O H2O SUM AVER: SDEV: %RSD: ZCOR: KRAW: PKBG: VOL%: DEV%: With Volatile Correction, Water By Difference Both Na, Si, K Change and Water By Difference Gives 3.4% H2O

Water By Difference vs. FTIR Comparison of melt inclusion water contents measured by FTIR in sub-micron melt inclusion water contents estimated as the difference between 100% and microprobe analysis totals or “water by difference” (WBD). Solid diagonal line indicates 1:1 correspondence, dashed line is a linear regression fixed at the origin. Gray symbols show WBD water content before matrix correction of the analysis and black symbols show matrix-corrected WBD estimates. From Roman, 2003

The “Blank” Correction

Accuracy at the 400 PPM Level? Note: Blank level (C level ) can be non-zero

PET LPET 5 Spectrometers

(normal PET crystal) 5 Spectrometers 1 ppm

Water By Measured Excess Oxygen (Nash, 1992) Wither’s Synthetic Rhyolite Glasses Measure All Cations Measure Oxygen (easier said than done) Calculate Stoichiometric Oxygen From Cations Subtract Stoichiometric Oxygen From Measured Oxygen Calculate Water By Stoichiometry To Hydrogen (OH, H2O) Where do we stand today if we do not use an oxygen standard similar to our unknowns? (Combined TDI and “Blank” Corrections)

Accuracy Issues for Oxygen Analysis Sample/Standard Roughness and Sample/Standard Carbon Coat Thickness - Use 0.05 um final polish - Coat standards and unknowns simultaneously Sample Conditions - Lower voltages (for oxygen), defocus beam (reduce damage) PHA adjustments - Na Ka II on O ka (use differential mode) Missing elements and cation valences - Li, Be, B, C (and Ba, Sr, etc) - Fe2+, Fe3+ Matrix Corrections - Take your pick! Mass Absorption Coefficients and Area Peak Factors (APFs) - Use modern MAC database (FFAST), empirical MACs

Wither’s Glass Analyses (using MgO std) Armstrong phi-rho-z, FFAST MACS, no APFs, no volatile corr. 3.9 wt% Na2O4.5 wt % Na2O

Hyper-exponential Loss Two exponential processes with different decay constants overlapping in time(?)

Add Volatile Corrections Intensity changes for Na, K, Si and O (ignoring Al!) Si Ka, -0.73% relativeO Ka, -2.9% relativeO Ka, -3.7% relative Linear ExtrapolationQuadratic Extrapolation

Add APFs and Empirical MACs FFAST vs. Empirical MACs Area Peak Factors (APFs) MgO Al2O3 SiO2

“Blank” Correction… Continuum artifacts

Accuracy Calibration ELEM: Na2O K2O Cl BaO F TiO2 FeO MnO CaO SiO2 Al2O3 MgO O SUM AVER: SDEV: SERR: %RSD: NBS K-411 Mineral Glass, (same conditions as unknown) C.M. Taylor: FeO 4.39, Fe2O Total as FeO 14.49, Excess O 1.12

Wither’s Glasses without “Blank” Correction Wither’s Glasses with “Blank” Correction

Accuracy of Oxygen Using Glass Standards Blank: NBS, K-411, no corrections NBS, K-411, with all corrections (excess oxygen should be 1.12 wt%) NBS, K-412, with all corrections (excess oxygen should be 0.82 wt%)

Suggestions for Improved Beam Sensitive Sample Accuracy Standards (correct for TDI effects here too!) Matrix effects of H2O by difference (iterated re-calculation) Acquisition parameters (keV, beam focus, current: linear regime) MAN backgrounds for speed (minimize damage) Intensity changes over time (“volatile” or TDI corrections) Oxygen analysis accuracy issues (sample prep, TDI corrections, MACs, APFs, “blank” accuracy correction) When Analyzing Water by Measuring Excess Oxygen:

Julien Allaz, University of Massachusetts, Tomorrow at 5:30 PM "Optimizing Background Measurement Accuracy Using Multi-Point Background Positions"