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Chemical testing laboratories with a quality system accredited to ISO/IEC 17025 need to have established quality systems covering the management and technical issues specified in the standard. Technical issues include estimation of uncertainty of the test results and validation of test methods which implies the confirmation by examination and provision of objective evidence that the particular requirements for a specific intended use are fulfilled. Validation studies for quantitative analytical methods include the following parameters or technical characteristics: accuracy, scope, specificity, sensitivity, precision (repeatability and reproducibility), bias, linearity, detection limit, robustness, selectivity and uncertainty of the test results. This work describes the design and application of methodologies based in the use of nested experimental designs and control samples to obtain the information necessary to validate analytical methods after evaluation of their technical characteristics while performing a minimum amount of measurements. Different analytical methods for the examination of wastewater and continental water have been selected for validation: (i) determination of dissolved anions by ion chromatography, (ii) determination of total phenols by UV-vis spectrometry and (iii) determination of total grease and oil by solid phase extraction and gravimetry. Prior to method validation, calibration and/or verification of the analytical instruments was carried out to assess they were operative and in compliance with specific requirements for their application in the analytical methods of interest. Simultaneously, a series of experiments were performed to establish some experimental conditions such as linearity, instrumental requirements, or maximum period of conservation of samples and standard. Results from the nested experimental designs have allowed the estimation of the validation parameters of the three methods. From the Analysis of Variance, ANOVA, of these results different contributions to total variance that likely affect the result of measurement such as change of analyst and time elapsed from sample collection to measurement, have been evaluated and quantified as repeatability and intermediate precisions. Measurement uncertainty was estimated from the data collected during the validation experiments as a potential function, I=k3 x, of analyte concentration, x. From this function, it is possible to estimate the uncertainty associated to the measured concentration for a given sample. Finally, the model proposed in this work for estimation of measurement uncertainty was compared with other available methods: (i) the classical bottom-up model which identifies, quantifies and combines the sources of uncertainty acting on each step of the analytical process, and (ii) the top-down method, which uses the results from interlaboratory trials (proficiency testing, method performance studies...) to estimate measurement uncertainty, model that was extrapolated to results from intralaboratory trials in this study.

Validación de métodos de ensayo y estimación de la incertidumbre de medida conforme a la norma ISO/IEC 17025. Aplicación al análisis de aguas residuales