Wolff, John A. and Conrey, Richard M. GeoAnalytical Laboratory

1. Application of portable X-Ray Fluorescence to problems in volcanology Wolff, John A. and Conrey, Richard M. GeoAnalytical Laboratory School of Earth and Environmental Sciences Washington State University Pullman, WA 99164 USA

2. Portable (handheld) and mobile EDXRF bulk analysis Bruker handheld Tracer IV Innov-X 5000 mobile Moxtek 50 kV Au tube Recent advances in portability meet need for field measurement (e.g. customs, soil contamination, mine reclamation, scrap yards, mineral exploration etc) Employ miniature XRF tubes, microamp electronics, SDD detectors Bruker handheld benchtop setup

3. Portable XRF -- many people are disappointed with their pXRF on rock outcrops -- these are NOT tricorders, they are instruments that need consistent sample prepand analytical methods

4. Difficulties with portable XRF analysis • Analytical surface often not flat • Vacuum only inside instrument, not surrounding the sample • Sensitivity for lightest elements poor - down to Mg (Z = 12) only • Matrix (absorption and secondary enhancement) corrections must be approximated if all elements are not analyzed • Resolution (dispersion) is not as good as WDXRF, so overlaps and interferences may be problems • No pulse height discrimination so many spurious peaks (sum, escape, diffraction, tube, collimator) can be present in spectra • Manufactured software good for scrap yards, but not optimized for earth sciences

5. High silica rhyolite spectrum with a handheld EDXRF 1.2% Sensitivity improves dramatically with Z (due to increasing fluorescent yields of higher energy X-rays combined with their lower absorption coefficients) Fe K Ca 5% Si Ti 77% Mn Rh 0.33% Rb Zr Al 0.05% RhComp 0.09% 128 ppm 202 ppm 12%

6. What do you need for good pXRF analysis? • Consistent sample preparation, especially sample surface but grain size too if you can • Matrix correction for other elements present - even if only approximate • Calibration and validation of your method • Development of routines for single applications - one size does not fit all • A few examples, chiefly volcanologic….

7. - sample prep: mortar and pestle grind to sub-250 micron powder (loose powder in cups) -- high sensitivity trace elements critical to discrimination of pumice chemistries pXRF samples Fused bead WDXRF data Handheld XRF analyses of pumices • - samples from the Bandelier Tuff, all high-silica rhyolite • - analysis at 45 kV; Compton scatter and approximate matrix corrections employed (no matrix variation)

8. Uncertainties in pXRF analysis of high-silica rhyolite pumice Repeatability is very good, as is comparison with WDXRF analyses Average of 10 repeat pXRF analyses

9. pXRF analysis of pumices CRMs CRMs • Loose powder pXRF values are very comparable to same sample fused bead WDXRF values pXRF vs WDXRF (all values in ppm) • Loose powder certified reference material analyses (of silica-rich CRMs) agree well with certified values • pXRF data can be used to discriminate Bandelier pumices in the field with minimal sample prep and uncertainties for these elements approaching that of fused bead WDXRF CRMs

10. - sample prep: surface lapped flat on coarse diamond lap Handheld XRF analyses of thin section billets • - samples from diverse fresh, fine grained volcanic rocks • - analysis at 15, 30, and 45 kV; Compton scatter and approximate matrix corrections employed (wide range of matrix) -- altered rocks and drill core do not work so well, coarse grained rocks require multiple analyses

11. Billet analyses via pXRF Z < 16 • Light elements (Z <16) are best excited at low tube voltages • Mg, Al, Si, and P all have highest signal to background at 15 kV or lower • Uncertainties are from 1-2 wt% absolute for Mg, Al, and Si, approximately 0.05 wt% for P • Calibration is to billets of samples analyzed via fused bead WDXRF (there are no available CRMs for this use) units Wt% units Wt% HHXRF vs WDXRF

12. Billet analyses via pXRF Z = 19-28 • Z = 19-28 elements are best excited at 30 kV (no filters) • K, Ca, Ti, and Fe can be usefully analyzed at any voltage, but Cr and Ni appear best at 30 kV • Uncertainties are improved from light elements, but still no match for WDXRF Units ppm units Wt% HHXRF vs WDXRF

13. Billet analyses via pXRF Z = 29-56 • Z = 29-56 elements are best excited at 45 kV (no filters) units ppm • 45 kV allows excitation of Ba K lines (Ba L lines have severe interference) • Uncertainties are improved again but still no match for WDXRF • Powdered rock in a cup may provide better data, but we have not performed the experiments units ppm HHXRF vs WDXRF

14. Handheld XRF analyses of mudbricks • - two sample groups from Bolivia provided by WSU Anthropology Dept • - sample prep: grinding to fine powder in ring mill (loose powder in cups) • - analysis at 30 kV; Compton scatter and approximate matrix corrections employed (range of matrix) • - P2O5 data critical to assess presence of cow dung units Wt % units ppm acknowledgements: Melissa Goodman Elgar and Nichole Bettencourt HHXRF vs WDXRF

15. Summary and conclusions • The “tricorder” will be a practical X-ray laser, if it’s ever developed • But for now sample prep is critical to good analysis • Development of good pXRF analytical routines for problems in the Earth Sciences requires some fundamental knowledge of XRF analysis, can’t just rely on manufactured software • Practical methods for analysis of pumice are easy to develop, routines for analysis of a wide range of lithologies are more challenging • Thanks for your attention!