## 4 Methods Validation  
Analytical methods should be well characterized to define their application area and reliability, by meeting the specifications related to the intended use of the analytical results, according to current international recommendations [228]. This process, by which the method proves its \"fitness for purpose\", is known as validation [229]. The validation should be started only after an initial period of familiarization with the method to be validated. Operators acquire knowledge regarding the conditions of handling and storing samples, preparation of reagents, carrying out tests, quality control procedures and interpretation of obtained results. The methods can be standard or internal. Regarding standardized methods, it is assumed that they have been adequately validated, are subject to periodic updating, and are recognized by the national or international authority [230]. Relative to an investigation, new methods are implemented or developed. Therefore, it is necessary to define which assays and statistical tests to apply, in order to assess whether the method applies to a given matrix. The quality and credibility of the results depend on the results of these tests. Any method is a process that involves manipulations that are likely to accumulate errors (systematic or random) and, in some situations, can significantly change the result. The errors that are associated with analytical measurements can have different origins [230; 231]: (i) random measurement errors (affect repeatability); (ii) effect of the run-in (considered as resulting from the bias of a run and the random variation of several runs); (iii) effect of the laboratory (viewed as the bias of a single laboratory); (iv) bias of the method; and, (v) matrix effect. The error component that is associated with repeatability includes contributions of volumetric, gravimetric, and heterogeneous errors of the test material, and they are evaluated based on the dispersion of the results from the replicate measurement. The running effect contributes with an additional effect of variation between days, associated, for example, with the change of the operator, change of reagent lots, recalibration of the equipment, and environmental modifications (for example, change of temperature). The laboratory's impact result from factors that are related to variations in standards, reagents, equipment, and environmental conditions. Differences between laboratories are assessed through collaborative studies between laboratories or participation in interlaboratory trials [230; 231]. The validation requirements should be specified in directives, standards, or guides, which should be followed whenever possible. For example, the validation of methods for water analysis must follow a strategy in line with the ones that are used by International Standard Organization (ISO) [232; 233; 234], International Union of Pure and Applied Chemistry (IUPAC) [230], or EURACHEM [231], which ensure that the terminology applied to the validation, together with the statistical treatment, is interpreted in agreement with the concerning sector [229]. The minimum requirements for a new or developed method include the study and knowledge of the following parameters: (i) linearity/working range; (ii) analytical thresholds (detection and quantification limits); (iii) repeatability; (iv) reproducibility; (v) accuracy; and, vi) uncertainty [232]. It is necessary to define the acceptance criteria for each of these parameters, which varies with the matrix under analysis, the target compound's concentration, or the developed method. Usually, these acceptance criteria are, in national or community legislation, applicable to that matrix and analyte. In the absence of such standards, it is usual to adopt others by analogy. For example, if the monitoring of PhACs in water is not part of the legislation, the acceptance criteria of other organic compounds analyzed by the same type of methods and whose concentration in the samples is in the same order of magnitude is assumed.