Development of the method
Once the method has been chosen, it must then be developed to find the most suitable conditions (or even optimal conditions if an experimental design is implemented in this phase of development).
The number and nature of influencing factors varies according to the extraction technique used. However, some of these factors will always affect the result and must be taken into account, regardless of the technique: (i) extractant composition, (ii) duration of extraction, (iii) extraction temperature.
The development phase and the study of influencing factors are performed on a “spiked” matrix (with the considered solute) – the “non-spiked” matrix needs to be extracted under the same conditions to assess the possible presence of the solute(s) in question (unless the spiking is carried out with a molecule labelled with a stable isotope).
Once the method is fully developed, it has to be tested on a certified reference material if available – indeed the extraction of a spiked molecule is easier than that of a native molecule: the results obtained with a “spiked” matrix must therefore be confirmed on a real matrix. Otherwise, the matrix has to be extracted sequentially to assess the quality of the extracts and completeness of extraction. If the extraction appears to be less efficient on the real matrix than on the “spiked” one, the values of certain parameters must be adjusted (e.g. higher temperature to break solute-matrix interactions).
Spiking consists in adding to the sample the solute(s) to be extracted at a given concentration.
Certified Reference Material (CRM)
Matrix containing the native solute, the solute concentration of which has been precisely determined.
The same sample is successively extracted - each extract is recovered and analyzed (either separately or after the collection of all extracts).