An Evaluation of Portable X-Ray Fluorescencefor Artifact Sourcing in the Field:

Can a Handheld Device DifferentiateAnatolian Obsidian Sources?

Ellery FrahmDepartments of Anthropology and Geology & Geophysics

University of Minnesota - Twin Cities

Geological Society of America2007 Annual Meeting, Denver

Session: Sourcing Techniques in Archaeology

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

Ultra-Portable XRF

Handheld X-ray fluorescence (HH-XRF) isone of a few techniques showing promisePictured: Thermo Fisher Scientific NITON analyzers

XLt seriesXL3t series

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

AA

BB

CC

Typical UsesA: Pb Testing

Surveying houses, paint

EU’s RoHS compliance

B: Soil SurveyingMapping toxic metals

EPA-regulated surveys

C: Metals & AlloysMaterials identification

Scrap yards, recycling

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?

ArchaeologyNITON advertises

Conservation/RestorationReconnaissance surveysResource management NAGPRA compliance

[Relative] Dating AuthenticationProvenance

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

From NITON Literature

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?

Ni Fe

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.

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

Standards: NITON vs NIST• SRM 1412 - 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 610 - Trace-element glass:

• Min %RSD = 4% for Ca; Max = 230% for K

• Data often off by factor of 2-3; more at times

Sample Size Effect

Same thickness: 3 mm

Only diameter changed:30 mm (“big”) vs 5 mm

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

Clusters “smeared” dueto confounding effect?Size/shape? Density?

Calibration error?

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

• 99.999% Cu reported as almost 600% Cu

• Definite size effect; shape effect possible

Use SuggestionsMost 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

NITON Recommends…• From their literature: “The quality of data produced by

field XRF varies with site conditions, soil composition,and sample preparation. Quality assurance protocolsfor the field method usually require that a number offield samples be split and sent to a laboratory forconfirmatory analysis.”

• “Although portable XRF analysis was once thought tobe less reliable than traditional lab-based techniques,the US EPA now acknowledges that inspectors shoulduse the data to ‘adjust their testing strategy for theproperty’ in real-time to investigate unusual readings.”

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?

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