• XRF Gold Analysis Penetration Depth and Surface Considerations
  • 1. It Varies by Gold Purity (Karat)
  • 2. The "Info Depth" for Different Elements
  • 3. Practical Implications for Testing Gold
  • Summary

XRF Gold Analysis Penetration Depth and Surface Considerations

The penetration depth of X-Ray Fluorescence (XRF) when analyzing gold depends on the energy of the X-rays and the composition of the gold alloy, but generally, it ranges from a few microns to about 1 millimeter.

Here is a more detailed breakdown of how deep XRF actually “sees” when testing gold:

1. It Varies by Gold Purity (Karat)

The depth is determined by how far the incoming X-rays can travel into the sample and, more importantly, how far the resulting fluorescent X-rays (the “signal”) can travel back out to the detector.

• High Purity Gold (24K): Gold is a very dense metal (19.3 g/cm³). Because of this density, X-rays have a hard time penetrating it. In pure gold, the analysis depth is very shallow, typically around 1 to 5 microns (µm). This is less than the thickness of a human hair.

• Lower Purity Gold (10K, 14K, 18K): These alloys contain significant amounts of other metals like silver, copper, or zinc, which are less dense than gold. Therefore, the X-rays penetrate slightly deeper. In 14K gold, the depth might reach up to 10 to 20 microns.

2. The “Info Depth” for Different Elements

It is important to understand that an XRF instrument does not see all elements at the same depth.

• Light elements (like copper, nickel, or zinc) emit lower-energy X-rays. These low-energy signals can be absorbed by the sample itself. Therefore, the measurement for these metals comes only from the very top surface (often less than 1 micron).

• Heavy elements (like gold, silver, or platinum) emit higher-energy X-rays that can escape from deeper within the sample.

3. Practical Implications for Testing Gold

Because XRF is a surface-level technique, the shallow penetration depth has important consequences:

• Plating and Filled Gold: XRF is excellent for detecting gold-plated items. If the gold layer is thicker than the penetration depth (e.g., a 10-micron plate on 14K gold), the machine might read it as solid gold because it cannot see the base metal underneath. If the plate is thinner, the machine will detect both the gold layer and the underlying metal (like tungsten or copper).

• Surface Contamination: Dirt, oil from fingers, or polishing compounds on the surface will be included in the analysis because the beam does not penetrate past them. This is why samples must be thoroughly cleaned before testing.

• Surface Depletion: In ancient or jewelry gold, sometimes the surface is richer in gold than the interior because the other metals at the surface have corroded or been worn away. An XRF reading might show a higher purity than the item truly is.

Summary

When you put a gold ring in an XRF machine, you are analyzing a volume of material that is roughly the size of the beam (usually 1–3 mm wide) but only about 0.001 mm to 0.020 mm deep. It is a very precise surface chemistry test, not a test of the bulk material’s interior.