XRF testing for precious metals
How X-ray fluorescence spectrometry works and why it's replacing acid tests.
At a glanceX-ray fluorescence spectrometry (XRF) is the cornerstone testing technology in modern Swiss precious metal counters, refineries, and assay laboratories. Capable of delivering a full quantitative elemental assay in under 30 seconds without removing a single milligram of metal, XRF has transformed scrap buying. However, it harbors a physical blind spot that every bullion professional must master. This guide explores the science and limits of XRF analyzers.
Physics of X-Ray Fluorescence Analysis
The XRF instrument (handheld gun or benchtop spectrometer) fires a primary X-ray beam at the test piece:
1. Excitation: Primary radiation knocks inner-shell electrons (K and L shells) out of sample atoms.
2. Fluorescent emission: Higher-energy outer-shell electrons drop down to fill vacancies. This quantum transition releases secondary fluorescent X-ray photons with energies unique to each element (Gold: 9.71 keV; Silver: 22.16 keV; Platinum: 9.44 keV).
3. Detection and calculation: A Silicon Drift Detector (SDD) counts photons and measures spectral energies, calculating alloy composition in seconds with precision down to 0.01%.
The Critical Blind Spot: Shallow Penetration Depth
Despite extraordinary speed and precision, XRF has one major limitation:
X-rays penetrate pure gold only about 15 to 45 micrometers (0.015 to 0.045 mm) deep!
XRF only analyzes the extreme surface skin. If a counterfeiter manufactures a tungsten or brass bar clad in a 100-micrometer (0.1 mm) layer of gold, an XRF scanner will confidently report: *'99.99% Pure Gold'*, while the core contains no precious metal.
Assay Methods Comparison in Switzerland
| Assay Method | Typical Precision (‰) | Destructive? | Depth Analyzed | Test Speed | Equipment Cost in Switzerland (CHF) |
|---|---|---|---|---|---|
| XRF Spectrometry (Benchtop) | ± 0.5‰ to 1‰ | 100% Non-destructive | Surface skin (20–40 µm) | 10 to 30 seconds | CHF 22,000 – 45,000 |
| Fire Assay / Cupellation | ± 0.1‰ (Legal Standard) | Destructive (sample cut) | Core material | 2 to 4 hours | CHF 50,000+ (Chemical lab) |
| Touchstone Acid Test | ± 10‰ to 20‰ | Near non-destructive | Surface (with file mark) | 1 to 2 minutes | CHF 80 – 250 |
| Hydrostatic Balance | ± 5‰ to 15‰ | 100% Non-destructive | 100% Bulk volume | 2 to 3 minutes | CHF 150 – 500 |
| Ultrasonic Velocity | Core authenticity check | 100% Non-destructive | Full bar thickness | 5 seconds | CHF 1,500 – 3,500 |
The Swiss Triple-Verification Protocol
Reputable bullion counters in Zurich and Geneva apply a three-tier verification protocol for high-value transactions:
1. Precision weighing & dimensions: Confirming weight and geometry match mint specifications.
2. XRF spectrometry: Instant verification of surface fineness and detection of alloy elements (Pd, Pt, Cu, Ni).
3. Volumetric test (Ultrasound or Archimedes density): Verifying the core has the exact same density as the surface.
Frequently Asked Questions (FAQ): XRF Testing
Is XRF testing legally binding in Swiss disputes?
The Swiss Central Precious Metals Control Office (BCMP) uses XRF for screening, but the only legally binding referee method in disputes remains fire assay cupellation (ISO 11426).
Can XRF detect gold plating?
Yes. Modern instruments with multilayer calibration algorithms measure plating thickness and identify substrate base metals underneath.
Buy Investment Gold in Switzerland (VAT-Exempt)
Buy and store physical gold in secure Zurich or Geneva vaults at direct live LBMA spot rates.