X-Ray Fluorescence Analysis (XRF)
Also: XRF, X-ray fluorescence, RFA, X-ray fluorescence spectrometry
A non-destructive analytical method that uses X-ray radiation to precisely determine the elemental composition of precious metals and alloys.
X-ray fluorescence analysis (XRF) is today considered the gold standard of non-destructive fineness determination. Dealers, refineries, and customs authorities use it to check bars and coins for their actual elemental composition within seconds — without damaging the piece or melting it down.
Operating Principle
An XRF device irradiates the sample surface with high-energy X-ray radiation (primary X-rays). This energy excites electrons in the inner atomic shells; as they return to their ground state, the atoms emit characteristic secondary or fluorescence radiation. Each element has a unique energy signature — the device's detector measures the intensity and energy of this radiation and calculates the elemental concentrations from it.
Primary X-ray radiation → Excitation of electrons
→ Emission of fluorescence radiation (element-specific)
→ Detector measures energy & intensity
→ Software calculates fineness in ‰ or karat
Typical Measurement Procedure
- Calibrate the device (manufacturer provides certified reference standards).
- Place the sample on the measurement aperture; the surface must be flat and clean.
- Start measurement — duration 10 to 60 seconds depending on device and desired accuracy.
- Read the result: primary element (e.g. Au), alloying metals (Ag, Cu, Pd), and trace elements in weight percent or per mille.
Strengths and Limitations Compared
| Criterion | XRF | Acid Test | Density Test |
|---|---|---|---|
| Non-destructive | Yes | No (scratches) | Yes |
| Measurement depth | Surface (10–50 µm) | Surface | Full volume |
| Fineness accuracy | ± 0.1–0.5 wt.% | rough | indirect |
| Detects tungsten core | No | No | Yes (density) |
| Equipment cost | high (from approx. €5,000) | low | low |
| On-site use | possible (handheld devices) | possible | possible |
Important limitation: XRF only captures the near-surface layer of typically 10 to 50 micrometres. A tungsten forgery — a gold bar with a tungsten core — can therefore go undetected if the gold coating is thicker than the measurement depth. For bars above a certain weight, it is therefore advisable to combine XRF with ultrasound testing or density determination by Archimedes.
Fields of Application
- Purchasing scrap gold and broken gold: Dealers determine the fineness during customer consultations; the melt value calculator converts the value instantly.
- Coin testing: Bullion coins such as the Krugerrand or Maple Leaf can be tested for fineness and alloy composition — as a complement to coin authenticity checking.
- Industrial goods-in inspection: Refineries test doré bars and granules against the LBMA Good Delivery standard.
- Customs inspection: Authorities use portable handheld devices to quickly verify precious metals declared by travellers.
Device Types
Stationary laboratory devices (tube power up to 50 W, liquid nitrogen or Peltier cooling) achieve the highest accuracy. Compact handheld devices (pistol form factor, also known as gun-XRF) are battery-powered and ready for use within seconds — ideal for mobile purchasing. Specialised coin-testing devices such as the Sigma Metalytics use electromagnetic induction rather than X-ray radiation and are therefore not XRF devices, though they are frequently mentioned in the same context.
In Brief
XRF delivers a precise, non-destructive determination of fineness within seconds — however, it cannot detect a tungsten core beneath a thick gold coating. For maximum security with larger bars, combining it with a volumetric method such as density determination or ultrasound testing is recommended.