Overview
ISO 5724:2023 - "Jewellery and precious metals - Determination of very high purity gold - Difference method using ICP‑MS" defines an analytical procedure for measuring very high purity gold (≥ 999,99 ‰). The method uses dissolution in diluted aqua regia followed by inductively coupled plasma mass spectrometry (ICP‑MS) to quantify trace impurities and derive gold fineness by difference (1 000 ‰ minus summed impurities).
Key topics and technical requirements
- Scope: Applicable to gold samples with nominal content of and above 999,99 ‰.
- Principle: Dissolve a weighed sample, measure trace elements by ICP‑MS using internal standards, correct for blank contributions and spectral interferences, then calculate gold fineness by subtracting impurities from 1 000 ‰.
- Sample preparation: Surface etching, wash and dry; typical analytical aliquot is (100 ± 1) mg dissolved and diluted to 50 ml (i.e., ~2 g/l). Two independent sample solutions are prepared for each analysis.
- Reagents and standards: Uses ultrapure water and high‑grade acids (HCl, HNO3, HF, H2SO4), aqua regia and prepared monoelemental stock solutions. A gold standard stock solution (20 g/l) and multi‑element stock and internal standard solutions (e.g., Sc, Cs, Re) are specified.
- Calibration: Standard addition approach with both blank and sample calibration curves to account for reagent and procedural impurities. Calibration levels and dilution steps are defined.
- ICP‑MS measurement: Requires clean fluoropolymer labware, instrument stabilization, sufficient rinse times, and at least three integrations per measurement. Collision‑cell technology or mathematical correction must be used to correct spectral interferences (examples given for As and Se corrections).
- Calculations: Convert intensity ratios (element/internal standard) to mass concentrations, compute mg/kg of gold, sum impurities above detection limits and calculate precious metal fineness (‰) by difference.
- Quality practices: Emphasis on cleanliness, use of blanks, and validating stability of prepared calibration solutions.
Applications and users
ISO 5724:2023 is intended for:
- Assay and analytical laboratories performing trace impurity analysis in high‑purity gold.
- Precious metal refiners, hallmarking and quality control units in the jewellery industry.
- Commodity traders, certification bodies and regulatory laboratories requiring reproducible, standardized fineness determinations.
Benefits include accurate trace‑level impurity control, consistent fineness reporting and improved comparability of assay results across labs.
Related standards
- ISO 11596 - Jewellery and precious metals: Sampling of precious metals and precious metal alloys (referenced for sampling procedures).
Keywords: ISO 5724:2023, very high purity gold, ICP‑MS, difference method, jewellery, precious metals, gold fineness, trace impurities, aqua regia, internal standards, spectral interference correction.
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