Proficiency testing schemes for sampling

Importance of the topic

Proficiency testing (PT) schemes that focus on sampling address a critical and often underappreciated source of measurement error in analytical chemistry: the sampling step. Properly designed sampling PTs help laboratories and field teams identify procedural weaknesses, quantify the contribution of sampling to overall measurement uncertainty, and promote harmonised, reproducible practices across organisations. They also provide valuable educational feedback and support competency development through exercises and workshops.

Objectives and overview

The leaflet outlines how ISO/IEC 17043 can be applied to PT schemes where sampling is the principal focus. It classifies sampling PTs into three types with distinct evaluation goals and describes the particular requirements of ISO/IEC 17043 that need adaptation for sampling exercises. The aim is to ensure that providers design PT schemes that attribute observed variability correctly (to sampling, testing, or both), and that performance assessment is meaningful and reproducible.

Methodology and practical considerations

Key methodological elements and practical adaptations needed when implementing ISO/IEC 17043 for sampling PTs are summarised below.

• Types of PT schemes:
  
  • Type 1 — Assessment of sampling procedure only: performance evaluated against predefined criteria or by expert audit; scoring systems convert qualitative observations into performance outcomes.
  • Type 2 — Participants collect samples which are analysed centrally: a single, accredited laboratory performs tests using validated low-variability methods so that resulting variability is attributable primarily to sampling.
  • Type 3 — Combined assessment of sampling and analytical methods: participant testing (on-site or in-house) is evaluated; use of an appropriate reference material (ideally a certified reference material) enables estimation of analytical bias and separation of sampling vs analytical contributions.
• Personnel: Providers must demonstrate competence in planning and organising sampling PTs, including knowledge of sampling strategies, preparation of sampling sites, and evaluation of participant performance.
• Equipment, accommodation and environment: Environmental factors (weather, ambient conditions, site disturbances) must be considered either by controlling/minimising them or by explicitly including them in performance criteria.
• Planning and logistics: Production, quality control, storage and distribution concepts translate to preparation and handling of the sampling site and of collected samples (container selection, preservation, transport).
• Preparation of PT items (sampling sites): Sites should be arranged so that each participant faces an equivalent challenge; potential perturbations (e.g. previous sampling impacts such as drill holes) must be minimised.
• Homogeneity and stability: Where possible, the sampled item/environment should be sufficiently homogeneous and stable during the exercise; dynamic systems (rivers, air plumes) pose special challenges and may inherently lack homogeneity.
• Statistical design: Use ISO 13528 guidance when designing the statistical treatment of results. The design should distinguish variability from sampling versus analytical methods and consider transport-induced effects.
• Assigned value and evaluation criteria: Determination of assigned values depends on PT type. Evaluation criteria should address pre-sampling decisions (containers, preservatives) and post-sampling handling (storage, shipping).
• PT item handling, packaging and labelling: Instructions for participant handling, packaging and labelling should be clear when relevant. For direct on-site measurements some packaging requirements may be irrelevant.
• Data analysis and records: Performance based on reference procedures can be qualitative. Observed deviations can be converted into numeric scores (for example negligible/minor/major mapped to 0/1/2) to support statistical analysis.
• Confidentiality and collusion: When participants perform sampling simultaneously at the same site, confidentiality cannot be fully guaranteed; participants must be informed and reasonable measures taken to limit collusion.

Main findings and discussion

The guidance emphasises that the nature of the PT (Type 1–3) determines how variability is attributed and how assigned values are established. Centralised analysis (Type 2) isolates sampling variability, whereas combined schemes (Type 3) require reference materials or robust comparative approaches to separate sampling and analytical contributions. Practical implementation requires careful site preparation to ensure equivalence, rigorous planning to control environmental and logistical confounders, and an appropriate statistical framework to interpret results. The leaflet stresses that dynamic systems are intrinsically difficult to use as PT items because of poor homogeneity and temporal instability.

Benefits and practical applications

Implementing sampling-specific PT schemes yields several concrete benefits:

• Improves field sampling protocols and operator competence through direct feedback and training opportunities.
• Quantifies the contribution of sampling to total measurement uncertainty, which supports risk-based decision making and improves data comparability across organisations.
• Provides objective performance metrics for accreditation, regulatory compliance and internal quality assurance.
• Facilitates harmonisation of sampling practices across laboratories, contract organisations and regulatory bodies.

Future trends and opportunities

Emerging directions that can enhance sampling PT schemes include:

• Advanced statistical methods and simulation (Monte Carlo approaches, hierarchical models) to better partition sources of variability and to design robust PTs for dynamic environments.
• Digitalisation and remote monitoring (e.g. time-stamped photos, GPS, sensor metadata) to strengthen chain-of-custody, document sample context, and reduce uncertainty related to environmental variation.
• Greater use of well characterised reference materials and matrix-matched standards to support bias estimation and method validation within PTs.
• Distributed and decentralised data platforms (secure logging, blockchain concepts) to improve transparency of sample handling and avoid collusion or tampering.
• Integration of training workshops and blended assessment formats to combine hands-on skill evaluation with analytical result comparison.

Conclusion

PT schemes that focus on sampling are essential to robust analytical practice because sampling often dominates uncertainty in many measurement contexts. Applying ISO/IEC 17043 to sampling PTs is feasible when providers adapt requirements for personnel competence, site preparation, environmental control, statistical design and data handling. Well-designed sampling PTs support competency development, make the impact of sampling measurable, and contribute to improved measurement quality and traceability.

References

• ISO/IEC 17043:2010 Conformity assessment — General requirements for proficiency testing.
• ISO 13528:2015 Statistical methods for use in proficiency testing by interlaboratory comparison.
• AMC Technical Brief 78, 2017. Proficiency testing of sampling.
• Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing Schemes by Laboratories, 2nd edition, 2011.
• Eurachem Guide: Measurement uncertainty arising from sampling, 2nd edition, 2019.
• Eurachem Leaflet on Pre- and post-analytical proficiency testing, 1st edition, 2009.