Introduction to Soil Chemistry

138 extraction
Figure 7.5. A microwave-assisted instrument showing extraction vessels and attachment for
sensors (courtesy of Dr. Courso and Dr. Conklin, University of Cincinnati).
to the liquid CO 2 before extraction to change the extraction conditions. A
diagram of a liquid CO 2 extraction setup is shown in Figure 7.6.
An advantage of supercritical extraction is that when the extract is brought
to room temperature and pressure, the carbon dioxide is released as a gas,
leaving the extract free of extraction solvent (assuming that no additives have
been used) and thus in a concentrated state ready for further cleanup or direct
analysis [21,25,26].
7.4.6. Accelerated Solvent Extraction
Physically, accelerated solvent extraction (ASE) is similar to supercritical
extraction, except that in this case the extractant is normally a liquid at room
temperature and pressure. However, it is heated to above its boiling point
but kept in its liquid state by keeping it under pressure. The advantage of
this process is that it allows for a greater range of extraction parameters in
that solvents of varying functional groups, polarities, and so on can be used
to effect the extraction. The extraction process can be more selective in the
type of material extracted, and the speed of extraction can be increased
[9,27,28].
7.4.7. Solid Phase and Solid-Phase Microextraction
In solid phase and solid-phase microextraction a solid adsorbant is placed
directly in soil for a period of time. The solid adsorbant adsorbs compounds