Metrological Traceability of Analytical Results

Significance of the topic

A clear and documented metrological traceability of analytical results is essential for comparability, reproducibility and regulatory compliance. Traceability links measured values to agreed references, typically SI units or certified reference materials, and is an explicit requirement of ISO/IEC 17025. In routine chemical analysis this ensures that results from different laboratories, instruments and times can be meaningfully compared and used for decision making in quality assurance, environmental monitoring, food safety and research.

Objectives and overview of the document

The source leaflet explains the concept of metrological traceability in analytical chemistry and provides practical guidance for laboratories to demonstrate traceability of their results. It defines metrological traceability (as in the International Vocabulary of Metrology), illustrates traceability chains with simple examples (temperature and chemical standards), and gives a concrete case study for mercury determination in tuna. The leaflet also outlines the evidence a laboratory should hold and how method validation, calibration and quality control contribute to measurement quality.

Methodology and key concepts

Metrological traceability is the documented, unbroken chain of calibrations that relate a measurement result to a reference standard, with each calibration step contributing to the combined measurement uncertainty. In practice this involves:

• Selection of appropriate primary or secondary references (SI realizations, national standards, certified reference materials).
• Establishing calibration chains for instruments (balances, thermometers, volumetric equipment, analytical instruments).
• Documenting calibrations, certificates and uncertainties at each step.

The leaflet emphasizes that traceability applies to quantities such as mass, volume, temperature and time, and in chemistry to concentrations and mass fractions via CRMs and pure-substance standards.

Illustrative example: mercury in tuna

The leaflet presents a worked example to show how traceability is documented in a typical analytical workflow. Key elements of the example include:

• Reported result: total mercury 4.03 ± 0.11 mg/kg (dry weight), uncertainty at ~95 % confidence (k = 2).
• Sample pretreatment: drying at 105 °C for 12 h (dry weight basis) and microwave digestion prior to analysis.
• Analytical technique: cold vapour atomic absorption/atomic spectroscopy (mercury analyser).
• Calibration: external calibration prepared from a certified reference material mercury solution specified as 0.998 ± 0.005 mg/L (k = 2) with traceability to pure mercury.

The leaflet lists the specific documentary evidence required to demonstrate traceability for this result:

1. Certificate for the mercury calibration solution (CRM) showing the certified concentration and stated traceability.
2. Calibration certificate for the balance used to weigh the sample relating mass to the SI unit (kg).
3. Manufacturer or calibration certificate for the volumetric flasks used for dilutions linking volume to a national standard.
4. Calibration records for the drying oven temperature.
5. Calibration or validation records for digestion conditions (e.g. digestion system temperature control).
6. Evidence for timing devices where relevant (clock or stopwatch traceability as appropriate).

The example clarifies that matrix CRMs used for method validation verify method performance but are only part of the traceability chain if they are used for calibration or for recovery correction that directly affects the reported value.

Main results and discussion

The leaflet draws several practical conclusions: establishing traceability is straightforward when laboratories follow good practice in selection of standards, calibration of equipment and documentation. For physical quantities (temperature, mass, time) traceability chains are well established. In chemical analysis, traceability depends heavily on the availability and correct use of CRMs and well-characterized calibration solutions. Method validation and regular quality control are necessary to ensure that the demonstrated traceability at validation is maintained in routine operation. Measurement uncertainty should be assessed from both the traceability chain and method validation data.

Benefits and practical applications

Documented metrological traceability provides multiple practical benefits:

• Enables comparability of results between laboratories and over time.
• Supports regulatory compliance and accreditation to ISO/IEC 17025.
• Informs realistic and defensible uncertainty budgets for decisions based on analytical data.
• Improves confidence in method performance through use of CRMs and validated calibration chains.

Typical applications include environmental contaminant monitoring, food safety testing, clinical assays, industrial quality control and research measurements where harmonized results are required.

Used instrumentation

The leaflet mentions the kinds of instrumentation and equipment that must be maintained and calibrated to support traceability in the mercury example and in general laboratory practice:

• Analytical instrument: mercury analyser employing cold vapour atomic spectroscopy or AAS.
• Sample preparation: microwave digestion system, drying oven with temperature control.
• Mass and volume metrology: analytical balance with calibration certificate, volumetric flasks with documented calibration or manufacturer traceability statements.
• Timing devices: clocks or stopwatches for procedural timing.
• Reference materials: certified reference materials for mercury solutions and matrix CRMs for validation.

Measurement quality: validation, calibration and QC

Key components to assure measurement quality are:

• Method validation to confirm the method is fit-for-purpose under laboratory-specific conditions and to identify significant effects.
• Calibration of critical equipment to close the traceability chains to standards with known uncertainties.
• Estimation of measurement uncertainty from both validation data and contributions from the traceability chain.
• Ongoing internal and external quality control to ensure results (and stated uncertainties) remain consistent with the validated state.

Future trends and potential applications

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

• Wider availability and improved characterization of matrix-specific CRMs to support complex analyses and low-level determinations.
• Increased use of primary and higher-order reference materials traceable to SI for chemical quantities.
• Enhanced digital documentation and electronic calibration records to simplify traceability chain management and auditability.
• Standardized approaches to uncertainty evaluation that integrate validation data and calibration contributions more systematically.
• Automation and connectivity of laboratory instruments enabling continuous monitoring of calibration status and environmental effects that impact traceability.

Conclusion

Metrological traceability is a foundational element for reliable and comparable analytical results. Laboratories can achieve traceability by using suitable reference materials, performing documented calibration chains to SI or agreed standards, validating methods and maintaining robust quality control. Proper documentation of each link in the chain and an uncertainty assessment that includes calibration contributions are essential for accreditation and for confident interpretation of analytical data.

References

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