Chemical Analysis Proficiency Testing
Chemical analysis is one of the most critical outputs produced by a laboratory, because a single number often translates into a conformity decision, product release, environmental reporting, regulatory limit assessment, or customer acceptance. However, confidence in chemical analysis is built not only on the instrument operating properly, but on the correct, consistent management of sample preparation, method execution, calibration strategy, matrix effects, measurement uncertainty, and reporting discipline.
For this reason, Proficiency Testing (PT) is more than “external quality control” for laboratories performing chemical analysis; it is a fundamental tool that regularly measures performance, makes deviations visible, and produces demonstrable evidence that can be used in accreditation and audit processes.
Why Is PT More Challenging in Chemical Analysis?
In chemical analysis, inter-laboratory differences often arise not from a single instrument, but from the cumulative effect of many small variables:
- Matrix effects: salinity, organic load, particulates, oils, complexing agents, and inhibitors can directly influence results.
- Sample preparation steps: extraction, precipitation, digestion (microwave/open-vessel), dilution, filtration, and derivatization carry high risks of loss or contamination.
- Calibration and standards: choices such as standard preparation, traceability, calibration range, selection of the linear region, and use of internal standards can introduce bias.
- Method parameters: “small” settings—such as column selection, mobile phase, oven program, nebulizer settings, RF power, gas flows, and reaction cell parameters—can create major differences.
- Calculations and reporting: dry/wet basis, unit conversions, blank corrections, recovery corrections, rounding, and LOQ/LOD practices are prone to critical errors.
PT makes this multi-variable structure visible “within a single round,” ensuring performance is based on measurable data, not intent.
What Does Chemical Analysis PT Verify?
A well-designed chemical analysis PT program verifies laboratory performance in a multi-dimensional way:
- Accuracy (bias) and precision: closeness to the assigned value and repeatability
- Method compliance: standard-/method-compliant execution, correct units, and correct calculations
- Sample preparation competence: extraction/digestion efficiency, contamination control, recovery management
- Calibration and traceability: reference standards, CRM use, instrument verification checks
- Result integrity: the relationship between raw data and reported results, reporting consistency, uncertainty approach (where applicable)
- Decision quality: correct statements of conformity and application of decision rules against specification/regulatory limits
Typical Matrices and Parameter Groups (Examples)
Chemical analysis PT schemes can be designed for different matrices and analyte groups depending on the laboratory scope, such as:
- Water / wastewater / environmental samples: metals (Pb, Cd, Cr, Ni, etc.), anions/cations, nutrients, organics (PAH, VOC, pesticides)
- Food and feed: pesticide residues, mycotoxins, additives, heavy metals, nutritional components
- Pharmaceuticals and cosmetics: purity/impurity profile, residual solvents, elemental impurities, active ingredient assays
- Petrochemicals / energy: sulfur species, BTEX, fuel properties, trace metals
- Mining / metallurgy: ore/product composition, trace elements, purity assessment
Evaluation Approach: Fit-for-Purpose and Audit-Ready
In chemical analysis PT, performance evaluation may be carried out using approaches such as z-score, z′-score, zeta-score, or Eₙ, depending on the program’s objective and design. The assigned value may be determined via reference measurement, formulation, expert laboratory consensus, or participant consensus.
Considering matrix complexity and the number of participants, statistical reliability is maintained in line with ISO 13528 principles, while program management and reporting are structured to be auditable according to ISO/IEC 17043 requirements.
What Does It Deliver to Your Laboratory?
- Demonstrable PT evidence for accreditation and customer audits
- Clear performance indicators that help distinguish deviations related to method, instrument, operator, and sample preparation
- Improved decision quality (statements of conformity), especially near regulatory/specification limits
- An early-warning approach through trend monitoring—detecting issues before they become “unsatisfactory” results
- Targeted technical feedback that can be used in CAPA processes