How rhodium plating can affect gold XRF results

Significance of the topic

Accurate determination of gold content in jewelry is critical for valuation, trading, and quality control in the precious‑metals market. Surface treatments such as rhodium (Rh) plating are commonly applied to white gold to improve appearance, but because X‑ray fluorescence (XRF) measures surface and near‑surface composition, rhodium coatings can bias reported gold (Au) concentrations. Understanding how surface plating affects XRF results prevents misclassification, costly disputes, unnecessary destructive testing, and incorrect pricing.

Goals and overview of the note

This application note explains how rhodium plating influences gold measurements by XRF, describes how Thermo Scientific Niton XRF analyzers help detect Rh plating, and provides practical guidance for interpreting Au results on plated jewelry. The aim is to enable analysts, buyers, sellers, and refiners to recognize plating effects and make informed decisions when XRF results appear inconsistent with expected karat values.

Methodology and analytical principles

XRF is a surface‑sensitive technique: incident X‑rays excite atoms in the top micrometers of a sample and the emitted characteristic X‑rays are used to infer composition. A rhodium layer on the surface contributes its own X‑ray signal and attenuates—and in some thickness regimes significantly obscures—the signal from the underlying gold alloy. The net effect on the reported Au concentration depends primarily on plating thickness, uniformity, and whether worn or unplated areas are present.

Used instrumentation

• Thermo Scientific Niton handheld XRF analyzers (model family referenced in the source material).
• Capabilities relevant to this application: rapid, non‑destructive surface and near‑surface alloy analysis; identification of surface Rh presence; capability to measure multiple spots and detect variations across an item.

Main results and discussion

Key observations summarized from the note:

• Rhodium plating can cause a correctly karated white gold item to report a lower Au concentration by XRF. The thicker or heavier the plating, the larger the potential reduction in reported Au.
• Very thin Rh coatings may change the measured Au by less than about 1%, often negligible for routine screening. Repeated or thick platings produce a more noticeable negative bias in Au readings.
• Niton XRF analyzers can detect the presence of Rh at the surface. Detecting Rh provides important context that a low Au reading may reflect surface plating rather than a lower‑karat substrate.
• Testing multiple locations—including edges, inner surfaces, hallmarks, and areas showing wear—reduces the risk of misinterpretation because plating thickness and coverage often vary across an item.

Best practices and practical recommendations

• Always check for a Rh signal when an Au result is lower than expected for white gold.
• Measure several spots on each item, prioritizing worn or unplated regions and locations where plating may be thin or missing (e.g., inside bands, under settings).
• Use the presence of Rh as a flag to interpret Au results cautiously; do not assume a low Au result automatically indicates a lower karat alloy.
• When necessary and feasible, remove the rhodium plating (mechanically or chemically) and reanalyze to reveal the true substrate composition; consider destructive testing only when justified by value or verification requirements.
• Communicate findings clearly to customers and trading partners, explaining that surface finishes can influence XRF measurements and describing any additional steps taken to verify alloy composition.

Benefits and practical use of the method

Handheld XRF instruments provide fast, non‑destructive screening that detects surface plating and supports rapid decision‑making in buying, selling, and refining contexts. Recognizing Rh plating reduces unnecessary retesting and prevents disputes over reported gold content. The ability to test multiple spots and identify surface elements helps labs and field operators distinguish between surface treatments and the underlying alloy, improving confidence in routine precious‑metals transactions.

Future trends and potential applications

• Improved spectral deconvolution and software algorithms to better separate coating signals from substrate composition, reducing the need for destructive confirmation.
• Advances in detector sensitivity and lower limits of detection for thin coatings, enabling quantification of very thin platings and more accurate correction factors.
• Integration of mapping or micro‑XRF techniques for spatially resolved analysis of plating thickness variations across complex jewelry geometries.
• Use of complementary techniques (chemical stripping followed by XRF, ICP‑MS, or fire assay) in cases where definitive substrate composition is required for legal or commercial purposes.
• Application of machine learning models trained on plated and unplated spectra to predict underlying alloy composition and suggest when plating removal is advisable.

Conclusion

Rhodium plating can materially influence gold readings obtained by XRF because the technique probes the sample surface and near surface. Thermo Scientific Niton XRF analyzers can detect surface Rh, enabling operators to recognize when a low Au result may be due to plating rather than a lower‑karat alloy. Best practice is to test multiple locations, check for Rh, inspect worn or unplated areas, and remove plating when definitive substrate analysis is required. Applying these precautions reduces trading risk and avoids unnecessary destructive testing.

References

• Thermo Scientific SmartNote SN1862, How rhodium plating can affect gold XRF results, Thermo Fisher Scientific, 2026.