Metrological Traceability of Analytical Results

Importance of the topic

The leaflet explains metrological traceability for analytical chemistry and its central role in ensuring results are comparable, reliable and acceptable for regulatory, commercial and scientific use. Traceability links laboratory measurement results to agreed references (ideally SI units) through documented calibration chains that quantify and propagate uncertainty. Compliance with this principle is a requirement of ISO/IEC 17025 and underpins quality assurance, method validation and inter-laboratory comparability.

Objectives and scope of the document

The primary aim is to define metrological traceability in practical terms for analytical laboratories and to illustrate, with concrete examples, how a laboratory can demonstrate traceability of routine measurement results. The leaflet sets out the elements forming a traceability chain, highlights typical critical points in chemical analysis and connects traceability to method validation, uncertainty estimation and quality control.

Methodology and traceability principles

Traceability is defined following VIM: a property of a measurement result allowing it to be related to a reference via a documented, unbroken chain of calibrations, each contributing to the overall measurement uncertainty. In chemistry the reference values typically come from national or international standards or from certified reference materials (CRMs) with values expressed in SI units where possible. The leaflet emphasizes that traceability is implemented by:

• Establishing calibration chains for instruments and reference standards.
• Documenting calibration certificates and their uncertainty contributions.
• Including physical quantities (mass, volume, temperature, time) and chemical quantities (concentration of calibration solutions, purity of standards) in the chain.

Illustrative examples include temperature traceability back to the 0 °C reference and an extended case study for mercury determination in tuna that traces each contributor to the final reported mass fraction and uncertainty.

Used instrumentation

The leaflet lists typical laboratory items and analytical equipment that must be calibrated and documented for traceability. For the mercury-in-tuna example these include:

• Analytical balance with calibration certificate traceable to the SI kilogram.
• Volumetric vessels (e.g., flasks) with manufacturer or calibration certificates relating their volume to national standards.
• Oven used for drying (temperature calibration required).
• Microwave digestion system (temperature/condition calibration where applicable).
• Mercury analyzer employing cold-vapour atomic absorption/spectroscopy.
• Timing device (clock or stopwatch) for drying time records.
• Certified Reference Materials (aqueous mercury CRM for calibration and matrix CRM for validation).

Key results and discussion

The leaflet conveys several practical conclusions through the mercury example and general guidance:

• An example measurement: total mercury in tuna reported as 4.03 ± 0.11 mg/kg (dry weight), uncertainty given at ~95 % confidence (k = 2).
• Traceability contributors identified explicitly: CRM concentration, sample mass, volumetric dilution, temperatures and times used during sample preparation and digestion, and calibration of the analytical instrument.
• Some items are straightforward to trace with commercial equipment and certificates (balances, volumetric glassware, CRM certificates). Other elements require more attention and documentation (oven and digestion temperature control/calibration, and any process steps that could impart significant uncertainty).
• Matrix CRMs are essential for method validation to demonstrate method performance (bias, recovery, repeatability) but are not part of the calibration chain unless used directly for calibration or for recovery correction—in which case they must be included in the traceability documentation.
• Measurement uncertainty should be estimated by combining the contributions from the traceability chain and from method validation; continuous quality control ensures the laboratory remains in the validated state.

Benefits and practical applications

Implementing metrological traceability provides concrete benefits:

• Enables comparison of results between laboratories and over time.
• Supports compliance with regulatory limits and contractual specifications by providing documented uncertainty and reference to standards.
• Improves confidence in decision-making based on analytical data (e.g., food safety, environmental monitoring, industrial QC).
• Facilitates accreditation under ISO/IEC 17025 by demonstrating calibration chains and uncertainty evaluation.

Future trends and potential applications

Expected developments and opportunities include:

• Broader availability and improved documentation of matrix-specific CRMs to support traceability in complex matrices.
• Increased integration of digital calibration records and metadata to make traceability chains more accessible and auditable.
• Enhanced methods for uncertainty evaluation that combine validation data with traceability contributions in a transparent way.
• Growth of reference measurement networks and harmonized protocols across regions to strengthen SI-based traceability in chemical analysis.

Conclusion

Metrological traceability is essential, practical and achievable for analytical laboratories when good laboratory practice is combined with appropriate calibration, documented reference materials and rigorous method validation. Laboratories can demonstrate traceability by assembling and maintaining documented calibration chains, estimating and reporting measurement uncertainty, and applying internal and external quality control to ensure continued performance consistent with validation.

References

1. Eurachem/CITAC Guide on Traceability.
2. Barwick V., Wood S. (Eds.), Meeting the Traceability Requirements of ISO/IEC 17025, 3rd Ed., LGC, 2005.