Solid Phase Extraction (SPE) is a chromatographic technique which allows the purification of compounds using differences in polarity and the interactions between compounds of interest and the stationary phase. Solid phase extraction cartridges are usually based on syringe barrels which have a solid phase sandwiched between two frits and can either be purchased commercially or constructed in the laboratory.

SPE packing materials

Solid phase extraction can be performed with normal, reversed phase or ion exchange packing materials. As with flash column chromatography, modified silica gels are the most common material with polar and non polar variants. The choice of material will depend on the nature of both the compound of interest and any contaminants present in the mixture.

Normal phase (polar)

• Silica gels with polar functional groups eg cyano, amino, alcohol, carboxylic acids

• Alumina

• Magnesium silicate (Florisil)

Reversed phase (non-polar)

• Silica gels with non-polar functional groups eg alkyl side chains (C8, C18), cyclics, aromatics.

Ion exchange

• Polymer bound carboxylic or sulfonic acids or quaternary amines

Cartridge size

SPE cartridges are typically labelled with the amount of material they can purify. It is important to select a cartridge of sufficient size to purify the amout required and not overload the solid phase. SPE cartridges are usually used for small scale preparative work. Larger scale purifications are usually acheived with alternative methods.

Performing solid phase extraction

SPE can be carried out in a laboaratory either using reduced pressure (vacuum) or positive pressure (compressed gases). The same principles apply regardless of the method used to draw the solvent through the cartridge. A step by step procedure using positive pressure (typically compressed air or nitrogen) is given in the procedure tab.