Detailed description of (A)SE

Typically, soil samples are dried before extraction in order to allow the solvent to get into direct contact with all particles. In dry soils, however, a considerable amount of the analyte may be sorbed not only in humic matter but also on mineral surfaces. It is therefore advisable to use a strong mono-polar solvent such as acetone for extraction. Such a mono-polar solvent will facilitate the replacement of the analyte from the mineral surfaces because this solvent can form H-bond interactions with the bipolar hydroxyl groups on the surface of mineral oxides. While bipolar solvents share this advantage, they are less suitable because of their rather high cohesive energy that is generally unfavorable for the partitioning of analytes into the solvent.

The equilibration process between a soil sample and a solvent requires the diffusion of the analyte out of the soil particles into the solvent. In order to keep the diffusion path length* small it helps if the soil is milled before extraction. It also helps if the solvent and the sample are hot because this increases the diffusion coefficient.

Because most solvents are quite volatile they can only be heated under pressure so that their boiling point is not reached. This procedure (called Accelerated Solvent Extraction ) requires special equipment.

*The required time increases proportional to the square of the diffusion path length.