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

Portable (handheld) and mobile EDXRF bulk analysis Bruker handheld Tracer IV Innov-X 5000 mobile 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 Moxtek 50 kV Au tube Bruker handheld benchtop setup

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

Difficulties with portable XRF analysis 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 Analytical surface often not flat

12% 77% 5% 0.33% 0.09% 0.05% 1.2% 128 ppm202 ppm High silica rhyolite spectrum with a handheld EDXRF Sensitivity improves dramatically with Z (due to increasing fluorescent yields of higher energy X-rays combined with their lower absorption coefficients) ZrRb Fe Mn Ti Ca K Si Al Rh RhComp

What do you need for good pXRF analysis? 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…. Consistent sample preparation, especially sample surface but grain size too if you can

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

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

pXRF vs WDXRF (all values in ppm) CRMs 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 Loose powder pXRF values are very comparable to same sample fused bead WDXRF values pXRF analysis of pumices

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

units Wt% HHXRF vs WDXRF 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) Billet analyses via pXRF Light elements (Z <16) are best excited at low tube voltages Z < 16

HHXRF vs WDXRF Units ppm units Wt% 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 Billet analyses via pXRF Z = elements are best excited at 30 kV (no filters) Z = 19-28

units ppm HHXRF vs WDXRF 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 Billet analyses via pXRF Z = elements are best excited at 45 kV (no filters) Z = 29-56

Handheld XRF analyses of mudbricks -- 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) -- P 2 O 5 data critical to assess presence of cow dung HHXRF vs WDXRF acknowledgements: Melissa Goodman Elgar and Nichole Bettencourt units ppm units Wt % -- two sample groups from Bolivia provided by WSU Anthropology Dept

Summary and conclusions 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! The “tricorder” will be a practical X-ray laser, if it’s ever developed