Solid-phase extraction (SPE) 

Solid-phase extraction is a technique that comes from liquid chromatography, which deals with liquid matrices. It takes place in several successive steps (see diagram).

1^st step

Rinsing and conditioning of the solid phase (important step to enable the solid phase to interact with the solutes to be retained).

2^nd step

Loading of the solid phase: the liquid sample is percolated through a controlled flow.

3^rd step (optional)

Possible rinsing of the solid phase to eliminate poorly retained interfering compounds.

4^th step

Elution of the compounds of interest with an adapted solvent (fractionating the extract with several solvents is also possible).

There are several advantages to this technique compared to liquid-liquid extraction: lower consumption of solvent; large variety of solid phases; pre-concentration of solutes on the phase; high selectivity.

Several parameters influence the extraction efficiency and must be optimized, including: the nature and mass of the solid phase; the sample flow rate and the percolated volume; the flow rate, nature and volume of the elution solvent; and the nature and volume of the rinsing solvent, when this step is carried out.

The solid phase must be chosen so as to ensure good retention of the solutes to be extracted; it should not interact too much with the sample solvent. In practice, hydrophobic phases are used for aqueous samples (“reversed phase” mode) and hydrophilic phases for hydrophobic organic samples (“normal phase” mode).