An Evaluation of Portable X-Ray Fluorescence for Artifact Sourcing in the Field: Can a Handheld Device Differentiate Anatolian Obsidian Sources?

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1 An Evaluation of Portable X-Ray Fluorescence for Artifact Sourcing in the Field: Can a Handheld Device Differentiate Anatolian Obsidian Sources? Ellery Frahm Departments of Anthropology and Geology & Geophysics University of Minnesota - Twin Cities Geological Society of America 2007 Annual Meeting, Denver Session: Sourcing Techniques in Archaeology

2 The Problem Analytical instruments not portable Heavy, electricity and water, dust-free, etc. Must obtain export permissions for analysis Most are destructive in some way Powdered, polished, radioactive, pitted, etc. Ideal: accurate, nondestructive, fast, field Very few field-capable instruments

3 Ultra-Portable XRF XL3t series XLt series Handheld X-ray fluorescence (HH-XRF) is one of a few techniques showing promise Pictured: Thermo Fisher Scientific NITON analyzers

4 Disclaimers HH-XRF sold by different companies Innov-X, AppliTek, Oxford Instruments, etc. Likely differ in features, performance, etc. Only NITON analyzer evaluated Has about 80% of HH-XRF market share Lead-laden toys delayed further tests Second round of testing planned, delayed

5 Typical Uses A B C A: Pb Testing Surveying houses, paint EU s RoHS compliance B: Soil Surveying Mapping toxic metals EPA-regulated surveys C: Metals & Alloys Materials identification Scrap yards, recycling

6 Results of Its Pedigree Rugged, easy to use: point and shoot Used on receiving dock, in scrap yard, etc. Fast, low cost/sample, no sample prep 1000s of analyses in one eight-hour shift Nondestructive analyses* *Produces heat in samples Often used in pass/fail mode Does a product meet RoHS standards?

7 Archaeology NITON advertises Conservation/Restoration Reconnaissance surveys Resource management NAGPRA compliance [Relative] Dating Authentication Provenance

8 HH-XRF Weaknesses Low-resolution spectrometer: 300 ev SEM-EDS: 150 ev, EPMA-WDS: 5 ev Peak overlaps, element misidentifications Low-energy X-rays absorbed by air No Si, Al, Mg w/o extra equipment; no < Mg Radioisotope models heavily restricted 25 elements; limited by model, source

9 From NITON Literature Ni Fe Ni & Fe in rock: Ni low by about 3%, Fe high by 14%* *Error fits what is expected due to matrix effects; under-corrected data? Is this good enough for sourcing purposes?

10 Testing Details Evaluated off-the-shelf model XLt series analyzer, X-ray tube source 5-min analyses, portable analyzer stand 21 elements were factory-set Factory-set calibration: bulk/soil mode Claims to correct for geometry, matrix, etc.

11 Samples Analyzed NIST standards: three glass SRMs Included standards of different diameters 15 obsidian samples analyzed w/ INAA Comparison of INAA and NITON analyses 600 obsidian samples, 90 collection areas Are sources/regions clearly differentiated? 11 of 21 pre-set elements consistently detected

12 Standards: NITON vs NIST SRM Multi-component glass: Min %RSD = 22% for K; Max = 80% for Sr SRM 93a - Borosilicate glass: Min %RSD = 43% for Zr; Max = 310% for K SRM Trace-element glass: Min %RSD = 4% for Ca; Max = 230% for K Data often off by factor of 2-3; more at times

13 Sample Size Effect Same thickness: 3 mm Only diameter changed: 30 mm ( big ) vs 5 mm

14 Obsidian: NITON vs INAA 15 sub-samples from same collection set Error varies wildly by element and sample: K: Min %RSD = 3%, Max %RSD = 170% Zn: Min %RSD = 1%, Max %RSD = 120% As: Min %RSD = 9%, Max %RSD = 86% Zr: Min %RSD = 14%, Max %RSD = 120% INAA databases likely incompatible with NITON

15 Clusters smeared due to confounding effect? Size/shape? Density? Calibration error?

16 Problems Identified Deviations from NIST & INAA analyses RSD sometimes < 10%; other times > 300% Incorrect element identifications 100% Au misidentified as 80% Nb & 20% W Density/matrix estimates imperfect % Cu reported as almost 600% Cu Definite size effect; shape effect possible

17 Use Suggestions Most useful as a first sort tool: Analyze 100s or 1000s of artifacts on-site Explore data for initial chemical clusters Identify representative samples Request fewest samples possible for export Supply HH-XRF analyses in paperwork Conduct subsequent lab-based analyses

18 NITON Recommends From their literature: The quality of data produced by field XRF varies with site conditions, soil composition, and sample preparation. Quality assurance protocols for the field method usually require that a number of field samples be split and sent to a laboratory for confirmatory analysis. Although portable XRF analysis was once thought to be less reliable than traditional lab-based techniques, the US EPA now acknowledges that inspectors should use the data to adjust their testing strategy for the property in real-time to investigate unusual readings.

19 Future Work Further testing with custom calibrations Better accuracy, precision for sourcing? More compatible with existing databases? Analyze set of known rhyolitic glasses Evaluate factory calibrations, seek errors How can errors be minimized by users? Establish adequate sample size, shape?

20 Conclusions Not a replacement for lab-based analysis Likely incompatible with extant databases Separated 2/3rds of sources; others overlap Overlaps have serious archaeological implications Uses must play to its strength: portability Most useful as first sort field tool HH-XRF shows potential for the future but be wary of claims in the sales literature