Choosing and developing an extraction method
Choice of method
Once the technique has been chosen, the method must then be selected – by method is meant the implementation of an extraction technique under specific conditions (see illustrative example).
When choosing the extraction conditions, several questions must be raised.
Nature of the matrix?
It is important to take into consideration the type of matrix it is (e.g. environmental, food, pharmaceutical) and its composition. Thorough knowledge of the matrix, its components and the possible location of the solute molecules often guarantees the success of the extraction method (in terms of selectivity and completeness). In particular, we can distinguish between fatty and non-fatty matrices in order to develop methods adapted to each case.
Coumpounds to be extracted?
The chemical structure of the molecule (cf. structural formula) and the presence of certain groups (e.g. alcohol, amino or carboxylic acid functions, aromatic rings) are useful to anticipate potential molecular interactions of the solute (with the extractant or with the matrix). They can also help to inform the choice of the extractant (polarity and selectivity of interactions).
Knowing the values of certain physicochemical parameters (Sw, Kow, Pv and KH) is also very useful in the choice of operating conditions. In particular, the extracting agent should be chosen on the basis of the principle “like dissolves like”.
It is also useful to know the stability of the solute and especially its sensitivity to hydrolysis, oxidation, photolysis or temperature.
Finally, an a priori estimate of the expected concentration of the solute in the matrix is desirable.
What becomes of the extract?
The recovered extract is usually analyzed by chromatographic techniques – in this case, the extraction solvent can be chosen according to the type of chromatography (GC or HPLC) in order to be compatible and to avoid an evaporation step and solvent change before analysis.
However, the extract can also be dedicated to other characterizations (e.g. biological effect, sensory test). The choice of the method can therefore be guided by specific requirements (e.g. non-toxicity of the extraction solvent).