Metrological Traceability of Analytical Results

Significance of the topic

The leaflet explains metrological traceability as the foundation for comparability and reliability of analytical measurement results. Traceability links laboratory results to agreed references (ideally SI units) via documented, unbroken calibration chains. For chemical analysis this ensures results are comparable across laboratories, supports regulatory compliance (ISO/IEC 17025), and underpins credible uncertainty statements that are essential in quality control, environmental monitoring, food safety and forensic applications.

Objectives and overview of the document

The document aims to clarify the concept of metrological traceability, illustrate how traceability is established in practice, and show how a laboratory can demonstrate traceability of analytical results. It provides definitions from the International Vocabulary of Metrology (VIM), practical examples (temperature and a mercury-in-tuna case study), and guidance on evidence required to support traceability claims.

Methodology and core concepts

Traceability is defined as the property of a measurement result to be related to a reference through a documented, unbroken chain of calibrations, each contributing to the overall measurement uncertainty. Key methodological elements described include:

• Use of SI-traceable references or agreed standards (national/international standards) where possible.
• Construction of traceability chains through sequential calibrations of instruments and reference materials.
• Application of Certified Reference Materials (CRMs) and primary standards for calibration of chemical measurement systems.
• Role of method validation to demonstrate fitness for purpose and to identify significant effects on measurements.
• Estimation of measurement uncertainty by combining contributions from traceability elements and validation data.

Used instrumentation

The leaflet and worked example refer to typical laboratory equipment and calibrated items that form parts of traceability chains, including:

• Analytical instrument: mercury analyzer using cold vapour atomic absorption spectroscopy (CV-AAS) after microwave-assisted digestion.
• Balances with calibration certificates traceable to the SI unit of mass (kg).
• Volumetric glassware (volumetric flasks) with manufacturer-documented traceability for nominal volume.
• Ovens and digestion systems with temperature calibration.
• Timing devices such as clocks or stopwatches for controlled drying times.
• Certified Reference Materials (CRMs) for solution standards and matrix CRMs for method validation.

Main results and discussion (illustrative example)

The leaflet presents a concrete example: determination of total mercury in tuna reported as 4.03 ± 0.11 mg/kg (dry weight, k=2, 95% confidence). The example dissects the traceability contributions and documents evidence needed to support the reported value. Key points:

• Calibration standard: CRM for mercury solution with certificate stating concentration and uncertainty, traceable to pure mercury.
• Mass measurement: sample weighed on a balance with SI-traceable calibration certificate.
• Volume measurement: dilutions made in volumetric flasks whose volumes are traceable to national standards as provided by the manufacturer.
• Process parameters: oven/digestion temperatures and drying times must be calibrated or controlled; these contribute to uncertainty.
• Method validation: use of a matrix CRM (e.g., certified mercury in fish) is essential to demonstrate method performance and recovery; when used only for validation it is not a calibration link in the traceability chain but must be included if applied for recovery correction.

The leaflet emphasizes that some traceability elements (balances, volumetric glassware, certified solutions) are straightforward to document, whereas aspects like digestion conditions and oven temperature require focused calibration and control to avoid underestimated uncertainties.

Benefits and practical applications of the approach

Adopting a metrological traceability approach provides several practical benefits:

• Comparable and defensible measurement results across time and laboratories, enabling regulatory compliance and mutual recognition.
• Transparent uncertainty budgets that combine contributions from calibration, validation and routine control.
• Improved method reliability through targeted calibration of critical equipment and use of CRMs in validation.
• Clear documentation that supports accreditation (ISO/IEC 17025) and stakeholder confidence in analytical data.

Future trends and potential applications

Emerging and continuing developments that influence traceability in analytical chemistry include:

• Broader availability and improved characterization of matrix-matched CRMs to support complex analyses.
• Advances in digital calibration certificates and metadata standards to streamline traceability documentation and chain-of-custody for reference materials and instruments.
• Integration of uncertainty propagation tools and automated data management to maintain real-time traceability and QC records.
• Greater emphasis on inter-laboratory comparability for non-traditional measurands (e.g., biomarkers, speciation) and complex matrices, requiring novel reference materials and validated protocols.

Conclusion

Metrological traceability is the cornerstone for producing comparable, reliable analytical results. It requires documented calibration chains to SI or agreed references, method validation to demonstrate fitness for purpose, calibrated critical equipment, and clear estimation of measurement uncertainty. Following good laboratory practice and maintaining appropriate documentation makes traceability achievable and supports accreditation and confidence in analytical data.

Reference

• Eurachem/CITAC guide on Traceability.
• Meeting the Traceability Requirements of ISO17025, 3rd Ed, V. Barwick, S. Wood (Eds), 2005, LGC.
• International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM), JCGM 200:2012 (VIM 3rd edition).