X-Ray Fluorescence Analysis (XRF)
Also: XRF, X-ray fluorescence, RFA
A non-destructive analytical method that uses X-rays to determine the elemental composition of precious metals and alloys with high precision.
X-ray fluorescence analysis (XRF) is today regarded as the gold standard for non-destructive fineness testing. Dealers, refiners and customs authorities use it to check bars and coins for their true elemental composition within seconds, without damaging or melting the piece.
How it works
An XRF instrument irradiates the sample surface with high-energy X-rays (primary radiation). This energy raises electrons in the inner atomic shell into an excited state; as they fall back to the ground state, the atoms emit characteristic secondary or fluorescence radiation. Every element has a unique energy signature, and the instrument's detector measures the intensity and energy of this radiation to calculate the element concentrations.
Primary X-rays -> excitation of the electrons
-> emission of fluorescence radiation (element-specific)
-> detector measures energy & intensity
-> software calculates fineness in per mille or carat
A typical measurement sequence
- Calibrate the instrument (the manufacturer supplies certified reference standards).
- Place the sample on the measurement aperture; the surface must be flat and clean.
- Start the measurement, which takes 10 to 60 seconds depending on the instrument and the accuracy required.
- Read the result: the main element (e.g. Au), alloying metals (Ag, Cu, Pd) and trace elements in weight percent or per mille.
Strengths and limits compared
| Criterion | XRF | Acid test | Density test |
|---|---|---|---|
| Non-destructive | Yes | No (scratch) | Yes |
| Measurement depth | Surface (10-50 microns) | Surface | Whole volume |
| Fineness accuracy | plus/minus 0.1-0.5 wt% | rough | indirect |
| Detects tungsten core | No | No | Yes (density) |
| Instrument cost | high (from around EUR 5,000) | low | low |
| On-site use | possible (handheld units) | possible | possible |
Important limitation: XRF only captures the near-surface layer, 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 the Archimedes density test.
Fields of use
- Buying scrap and broken gold: dealers determine fineness during the customer conversation; the melt value calculator turns the value into a figure immediately.
- Coin checking: bullion coins such as the Krugerrand or Maple Leaf can be checked for fineness and alloy composition, complementing the coin authenticity check.
- Industrial incoming-goods inspection: refiners test doré bars and granules against the LBMA Good Delivery standard.
- Customs control: authorities use mobile handheld units to verify declared precious metals quickly 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 units (gun-shaped, also called gun XRF) run on battery power and are ready to use within seconds, ideal for mobile buying. Specialist coin testers such as the Sigma Metalytics use electromagnetic induction rather than X-rays and are therefore not XRF instruments, although they are often mentioned in the same breath.
In a nutshell
XRF delivers a precise, non-destructive proof of fineness in seconds, yet it cannot detect a tungsten core beneath a thick gold layer. For maximum certainty with larger bars, it is advisable to combine it with a volumetric method such as the density test or ultrasound testing.