Extraction from solid matrices
Extracting from solid matrices is often delicate: to be solubilized by the extractant, the solute must be accessible and diffuse through the matrix – this step is often limiting in the extraction process. Another problem is the adsorption of solute molecules on active energetic sites in the matrix (e.g. soils, sediments): the interaction must be broken to free these molecules and extract them. Finally, solute molecules may become trapped inside the matrix (e.g. within cells or organs in plants). Extraction conditions therefore need to allow for their release if total extraction of the solute is to be achieved.
Thus, the process of extracting a solute from a solid matrix particle can be divided into 5 steps, as illustrated below.
A pre-treatment of the sample is recommended: a grinding step reduces the particle size, thus making the solute to be extracted more easily accessible.
Stirring during extraction helps the diffusion through the extractant.
Temperature is also an important factor: an increase in temperature can promote diffusion (especially in the matrix), if the energy input is sufficient to break the solute-matrix interactions.
Several techniques can be used for solid matrices. The most basic one is extraction by simply stirring the sample with the liquid extractant, but only easily accessible compounds will be extracted, and the extractant can be quickly saturated resulting in non-quantitative extractions. An alternative is Soxhlet extraction, which recycles the extracting solvent and improves the extraction recovery; but this is a time-consuming technique (several hours) and therefore not very selective. A recent alternative is pressurized liquid extraction (PLE).