X-Ray Fluorescence Analysis (XRF)
Also: XRF, X-ray fluorescence, RFA, XRF 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 regarded as 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 or melting the item.
Working 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 shell into an excited state; upon falling back to the 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 element concentrations from it.
Primary X-ray radiation → excitation of the electrons
→ emission of fluorescence radiation (element-specific)
→ detector measures energy & intensity
→ software calculates fineness in ‰ or carats
Typical measurement procedure
- Calibrate the device (the manufacturer provides certified reference standards).
- Place the test object over the measurement opening; the surface must be flat and clean.
- Start the measurement – duration 10 to 60 seconds depending on the device and desired accuracy.
- Read the result: main element (e.g. Au), alloying metals (Ag, Cu, Pd) and trace elements in weight percent or per mille.
Strengths and limits 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) |
| Device cost | high (from approx. €5,000) | low | low |
| On-site use | possible (handheld devices) | possible | possible |
Important limitation: XRF captures only 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, the combination of XRF and ultrasonic testing or density measurement by Archimedes is therefore recommended.
Fields of application
- Purchase of scrap gold and broken gold: Dealers determine the fineness during the customer conversation; 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 – complementary to coin authenticity checking.
- 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 travel traffic.
Device types
Stationary laboratory devices (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-operated and ready for use within a few seconds – ideal for mobile purchasing. Specialised coin testers such as the Sigma Metalytics, by contrast, use electromagnetic induction instead of X-ray radiation and are therefore not XRF devices, although they are often mentioned in the same breath.
In brief
XRF delivers a precise, non-destructive proof of fineness within seconds – but it does not detect a tungsten core beneath a thick gold layer. For maximum security with larger bars, combining it with a volumetric method such as density measurement or ultrasonic testing is recommended.