Incertidumbre en métodos analíticos de rutina

Zusammenfassung

According to the ISO Norm 17025, the accredited analytical laboratories should obtain their results accompanied by two basic quality parameters: traceability and uncertainty. Therefore, nowadays the analysts are increasingly urged to assess the traceability of their results and to estimate the uncertainty of these results. In this thesis, we have proposed different methodologies to calculate uncertainty in analytical methods that are used routinely in the laboratory. These methodologies calculate uncertainty as a whole (i.e. by grouping terms of uncertainty whenever possible) and use the information generated during the validation process and, specially, the information obtained in the assessment of traceability. Moreover, the uncertainty can also be calculated using the information generated during precision studies, during robustness studies and during the internal quality control. The advantage of the proposed methodologies is that they require few additional work to calculate uncertainty because the analytical methods should always be validated before applying them to the analysis of routine samples. We have developed the calculation of uncertainty at one concentration level when traceability is assessed with several types of references such as certified reference materials (CRM), reference methods and interlaboratory studies. This uncertainty is calculated using the information obtained in the assessment of traceability. Moreover, this uncertainty can be calculated by using the information generated from control charts, precision studies and robustness studies. The calculation of uncertainty at one concentration level should only be applied in a restricted concentration range because it is assumed that both the method bias and precision are the same in the whole concentration range. However, many analytical methods are applied in wide concentration ranges and to several kind of matrices. In these cases, we have developed methodologies to calculate uncertainty at several concentration levels and taking into account the matrix variability in the uncertainty budget. To calculate this uncertainty, it is necessary to verify traceability with several reference samples whose concentrations are comprised within the concentration range of the routine samples. Moreover, the absence of two types of biases should be verified: proportional bias (i.e. that varies with concentration and is expressed in terms of recovery) and constant bias. We have developed methodologies for calculating uncertainty when traceability is assessed using spiked samples. However, these methodologies can also be applied when traceability is assessed using other type of references such as certified reference materials. Finally, we have studied if the uncertainty of results is underestimated when it is wrongly concluded in the assessment of traceability that the method bias is not significant. We have observed that uncertainty can be very underestimated when the uncertainty of bias represents more than 30 % of the final uncertainty. Therefore, it is necessary to have contributions of the uncertainty of bias lower than 30 % so that the uncertainty of results is not underestimated. This can be achieved by analysing the reference samples at least ten times and by using references with lower uncertainty.