Introduction to Soil Chemistry

140 extraction
serious interferences during analytical procedures. The standard method of
separation is filtration, which can be accomplished using standard laboratory
paper filter papers. Often laboratory personnel will crease filter paper with
such force that small tears or holes are torn in the paper, allowing soil to pass
though during filtration. In other situations the porosity of the paper may be
sufficient to allow clay particles and organic matter to pass through the paper.
In this case refiltering through the same paper will sometimes be sufficient to
remove all particles. Sometimes two layers of the same filter paper will be sufficient.
Some combination of these approaches used together should remove
all particles.
Sometimes, however, none of the above mentioned procedures will be
sufficient, and other filtering methods may be required. Small filtering disks
that fit on a plastic syringe and have porosities of 0.22 or 0.45mm will
effectively remove particles from soil extract. These disks clog rapidly with
soil, and so it is essential that the suspension be filtered through filter
paper before using filtering disk, particularly if significant quantities of filtrate
are needed. Either the suspension to be filtered can be pulled into the
syringe, the filter disk added, and the suspension expressed out of the syringe;
or the disk can be put on the syringe, the plunger removed, suspension
poured into the syringe, the plunger inserted, and the suspension expressed,
thereby filtering it.
An alternate procedure is to use a centrifuge to remove suspended material
from soil extracts. Centrifugation is very effective in removing small particles;
however, caution must be exercised when removing centrifuged
suspensions from the centrifuge and centrifuge tube because any motion may
resuspend particles, thus undoing the centrifugation process. The supernatant,
when carefully removed, can be used directly for analysis. In many cases this
is faster than filtration, especially when a large quantity of solution free of suspended
material is required.
7.5.2. Sorption Cleanup Methods
There are a myriad of sorption cleanup procedures available. Some are simple,
requiring little sample preparation and manipulation; others are more
complex. Common cleanup materials along with their advantages and disadvantages
are listed in Table 7.3. These sorption materials can be used in glass
columns similar to chromatographic columns, or they may be purchased as
prepared columns, termed solid-phase extraction columns, the use of which is
simply called solid-phase extraction (SPE) (see discussion above).
The basic method involves passing the extract through a column containing
an appropriate solid sorbing material. One of two cleanup processes can
occur.The component of interest can be sorbed and thus separated from other
components. It is later released by extraction and analyzed. The other process
involves the component passing through the column while unwanted components
are retained on the column. Both of these can be effective cleanup