Impurities: any component other than the pure active substance which is present in the active substance as manufactured (including non-active isomers) originating from the manufacturing process or from degradation during storage.
Relevant impurities: impurities of toxicological and/or ecotoxicological or environmental concern which are known, or can be expected, to occur in the active substance as manufactured. For preparations, impurities are defined as relevant, if the composition of the preparation is such that, on the basis of theoretical consideration, impurities of toxicological and/or ecotoxicological or environmental concern may be formed by the manufacturing process of the preparation or from degradation during storage.

STUDY DESIGN
Method validation, to determine the substance/s and relevant impurities content in formulated products and in technical materials, are performed according to SANCO/3030/99 rev. 4 guidance document.
Validation of the analytical method includes:
Linearity with at least 5 concentrations of the analytical standard or 3 concentrations of the analytical standard in the appropriate range of concentrations (R² > 0.98).
Precision with 5 sample solutions resulting from 5 independent weighings (RSD % < RSD % calculated by modified Horwitz equation).
Accuracy with at least 2 fortification levels on blank formulation and at least 1 blank sample for the interference check and specificity with the possible use of the standard addition method (range for mean recovery according to active substance/impurity nominal content and interferences < 3% of total peak area of the target analyte).
Confirmatory analyses/specificity  by high specific technique or other proper confirmatory technique.
LOQ determination (only for relevant impurities) as the lowest concentration tested, at which an acceptable mean (LOQ) recovery with an acceptable RSD, is obtained.
Study includes GLP managment and reporting.

REFERENCES AND GUIDELINES
SANCO/3030/99 rev. 4 (11/07/2000) – Technical Material and Preparations: Guidance for generating and reporting methods of analysis in support of pre- and post-registration data requirements for Annex II (part A, Section 4) and Annex III (part A, Section 5) of Directive 91/414.