X-Ray Fluorescence Analysis (XRF)
Also: XRF, X-ray fluorescence, RFA
A non-destructive analytical method that uses X-rays to precisely determine the elemental composition of precious metals and alloys.
X-ray fluorescence analysis (XRF) is today regarded as the gold standard for 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 or melting down the item.
Operating Principle
An XRF instrument irradiates the sample surface with high-energy X-rays (primary X-radiation). This energy raises electrons in the inner atomic shell into an excited state; when they fall back to the ground state, the atoms emit characteristic secondary or fluorescence radiation. Each element has a unique energy signature — the instrument's detector measures the intensity and energy of this radiation and calculates the element concentrations from it.
Primary X-radiation → excitation of electrons
→ emission of fluorescence radiation (element-specific)
→ detector measures energy & intensity
→ software calculates fineness in ‰ or carat
Typical Measurement Procedure
- Calibrate the instrument (the manufacturer provides certified reference standards).
- Place the sample over the measurement aperture; the surface must be flat and clean.
- Start the measurement — duration 10 to 60 seconds depending on the instrument and desired accuracy.
- Read the result: main element (e.g. Au), alloying metals (Ag, Cu, Pd), and trace elements in weight per cent or per mille.
Strengths and Limitations in Comparison
| Criterion | XRF | Acid test | Density measurement |
|---|---|---|---|
| Non-destructive | Yes | No (scratch) | 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) |
| Instrument cost | high (from approx. £5,000) | low | low |
| On-site use | possible (handheld devices) | possible | possible |
Important limitation: XRF only detects the near-surface layer of typically 10 to 50 micrometres. A tungsten counterfeit — 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, a combination of XRF and ultrasonic testing or density measurement (Archimedes) is therefore recommended.
Areas of Application
- Purchasing scrap gold and broken gold: Dealers determine the fineness during the customer meeting; the melt value calculator converts the value immediately.
- Coin testing: Bullion coins such as the Krugerrand or Maple Leaf can be checked for fineness and alloy composition — as a supplement to the coin authenticity check.
- Industrial incoming goods inspection: Refineries check doré bars and granules on the basis of the LBMA Good Delivery standard.
- Customs control: Authorities use mobile handheld devices to quickly verify declared precious metals in passenger traffic.
Instrument Types
Stationary laboratory instruments (tube power up to 50 W, liquid nitrogen or Peltier cooling) achieve the highest accuracy. Compact handheld devices (pistol design, also called gun XRF) are battery-powered and ready to use within a few seconds — ideal for mobile purchasing. Specialised coin testers such as the Sigma Metalytics, by contrast, use electromagnetic induction rather than X-rays and are therefore not XRF devices, though they are often mentioned in the same breath.
In Brief
XRF provides a precise, non-destructive verification of the fineness in seconds — but it does not detect a tungsten core beneath a thick gold layer. For maximum security with larger bars, combination with a volumetric method such as density measurement or ultrasonic testing is recommended.