Division 2.3.1 Common performance criteria and requirements

Confirmatory methods shall provide information on the chemical structure of the analyte. When more than one compound gives the same response, then the method cannot discriminate between these compounds. Methods based only on chromatographic analysis without the use of spectrometric detection are not suitable on their own for use as confirmatory methods.

Where used in the method, a suitable internal standard shall be added to the test portion at the beginning of the extraction procedure. Depending on availability, either stable isotope-labelled forms of the analyte, which are particularly suited for mass-spectrometric detection, or compounds that are structurally related to the analyte shall be used.

When no suitable internal standard can be used, the identification of the analyte shall be confirmed by co-chromatography. In this case only one peak shall be obtained, the enhanced peak height (or area) being equivalent to the amount of added analyte. With gas chromatography (GC) or liquid chromatography (LC), the peak width at half-maximum height shall be within the 90-110 % range of the original width, and the retention times shall be identical within a margin of 5 %. For thin layer chromatography (TLC) methods, only the spot presumed to be due to the analyte shall be intensified; a new spot shall not appear and the visual appearance shall not change.

Reference or fortified material containing known amounts of analyte, at or near either the permitted limit or the decision limit (non-compliant control sample) as well as compliant control materials and reagent blanks should preferably be carried through the entire procedure simultaneously with each batch of test samples analysed. The order for injecting the extracts into the analytical instrument is as follows: reagent blank, compliant control sample, sample(s) to be confirmed, compliant control sample again and finally non-compliant control sample. Any variation from this sequence shall be justified.