ISO 11352-2012 pdf free download.Water quality一Estimation of measurement uncertainty based on validation and quality control data.
5 PrincIple
A measurement result of a laboratory is an estimate of the value of the measurand. The quality of this estimate depends on the inevitable uncertainty that is inherent to the measurement result. In principle, the measurement uncertainty is a property of individual measurement results. The estimation of the measurement uncertainty for each individual measurement result is usually not necessary. if the measurement result originates from a controlled measurement process. In this International Standard, the measurement uncertainty is, therefore, determined for a set of similar measurement results. Generally, it is assumed that the set of measurement results obtained with a specific analytical method is obtained under controlled conditions. The estimation of the measurement uncertainty applies to all of the measurement results within the set, independently of, for example, sample matrix or analyst, provided that the measurement is carried out under a quality assurance programme.
This International Standard specifies procedures for the estimation of measurement uncertainty within the scope of the analytical method, and generally, random and systematic errors need to be considered. The estimation of measurement uncertainty is based on analytical quality control results and validation data which represent the within-laboratory reproducibility, and the method and laboratory bias.
6 Procedure
The procedure for the estimation of measurement uncertainty consists of the steps shown schematically in Figure 1. The figure gives references to appropriate clauses and sub-clauses within this International Standard.
In general, the method and laboratory bias (systematic error) and the within-laboratory reproducibility (random error) are determined independently using suitable data from method validation and analytical quality control results.
The combined measurement uncertainty. i.e. the root of the quadratic sum of the uncertainty component for the within-laboratory reproducibility and the uncertainty component associated with method and laboratory bias, is multiplied by a factor of 2 to obtain the expanded uncertainty at a confidence level of approximately 95 %.
If the measurement uncertainty varies significantly, depending on the matrix and/or concentration range, the uncertainty estimation shall be made separately for each matrix and/or concentration range.
7 Preparative considerations for the estimation of measurement uncertainty
7.1 Specification of the measurement
Before starting the estimation of measurement uncertainty, it is necessary that the analyst specify the analytical method under consideration, and the objectives and purposes of the measurement. The following list is a minimal checklist for this specification.
The specification comprises:
— the measurand;
— the measurement procedure;
— the field of application (matrices, concentration range).
7.2 Specification of the parametric form in which the measurement uncertainty is reported
The expanded uncertainty, U, is reported either as an absolute uncertainty value or as a relative uncertainty value. For results near the limit of quantification, the uncertainty is often found to be constant and can therefore be expressed as an absolute value. When results are well above the limit of quantification, the uncertainty is often proportional to the analyte concentration and can therefore be expressed as a relative value.
EXAMPLE Determination of a heavy metal (limit of quantification: 5 pg/I).ISO 11352 pdf download.