Introduction to Soil Chemistry

128 extraction
Organic compounds, including pollutants, can be desorbed from soil by
heating using a procedure called headspace analysis. Heat-desorbed compounds
can then be determined by gas chromatography. A soil sample that
does not fill the container is sealed inside the container using a cap containing
a septum. The container is brought to a predetermined temperature and
allowed to equilibrate. A syringe is inserted into the sample container, and a
sample of the headspace gas is removed and injected into a gas chromatograph
(GC) or a gas chromatograph–mass spectrometer (GC/MS) for separation and
identification of the components released from the soil.
There are other ways of removing components of interest from the soil
matrix without using a solvent per se. Sparging, solid phase and solid-phase
microextraction (SPME) are three of the most common of the nonsolvent
methods.
In sparging, a soil sample is placed in a container and a flow of gas, usually
pure helium or nitrogen, is passed through it to remove components of interest.
The soil may also be heated to remove components with lower vapor pressures.
Gas exiting the container flows over a sorbant that removes or traps the
components of interest. After a preset period of time at a prescribed temperature
and flow rate, the flow is stopped and sorbed components are extracted
from the sorbant and analyzed by GC or GC/MS.
Alternatively, the sorbant used in sparging can be placed directly in soil to
sorb components of interest. After a prescribed period of time, the sorbant is
removed from soil, extracted, and analyzed by GC or GC/MS. In direct sorption
there is always the question of contact between the sorbant and soil components.
Therefore these procedures must be investigated to ascertain and
validate their precision and accuracy.
Direct heating of soil in a gas chromatograph inlet and gas chromatographic
analysis of released compounds from soil has also been used to analyze for
absorbed compounds [1–9].
7.2. EXTRACTION
As a first step in any soil extraction, a decision between an aqueous and nonaqueous
extracting solvent or solution is made.Typically inorganic compounds
are extracted using either one of two types of aqueous solutions or in the simplest
cases water. One type of aqueous solution containing a cation is designed
to extract cations attached to exchange sites in the soil. Another is an aqueous
solution containing ligands designed to form a strong attachment to the compound
of interest. For simple, highly soluble compounds, a simple water extraction
may be sufficient [10].
In extracting cations and, in some cases, anions, the extracting solution must
contain ions appropriate for accomplishing two tasks; they must (1) be capable
of exchanging with the ion of interest and (2) not interfere with subsequent
analytical procedures or analysis. Most commonly in soil the clay and organic
matter carry a net negative charge and thus attract cations, resulting in soils