An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 265

Specific methods of separation based upon particle charge depend on the differences in
the migration of free charged particles while in an electrical field. Differences in the
particle mass, size, and shape also make it possible to separate substances by such
methods as gel filtration or centrifugation. Separations can also be based on vapor
pressure differences between compounds using distillation, condensation, sublimation,
and crystallization.
For extraction of solids, extraction desorption concepts apply and may be of several
different types. An aqueous solution may be used to extract inorganic components and
ions from a solid material for subsequent analysis, or an organic solvent may be added to
a soil or solid matrix either before or after the sample has been dried with a chemical
agent such as anhydrous sodium sulfate. To ensure good chemical mixing and extraction
the sample/solvent combination is mixed for a period of time either on a mixer or using
an ultrasonic device. Ultrasonic mixing is performed using either an ultrasonic bath or
horn. During this procedure solvent aliquots are alternately added to and decanted from
the sample after each period of sonication, and placed into a single vessel for future
concentration.
Aqueous extracts are often used as is, while organic extracts are often concentrated.
Organic sample extracts are concentrated using a “blow down” device where a regulated
stream of dry, pure gas is passed over the vessel top in such a way as to promote
evaporation of the solvent without disturbing the liquid surface. The gas blow down
method of concentration is better than the older methods of “cooking off” the excess
solvent using special glassware and a distillation column on a warm water bath. Boiling
methods are time- and labor-intensive, and even when used with great care, are prone to
catastrophic loss through “bumping” or explosive boiling and leaking or broken
glassware. After the sample extract has been concentrated to a set volume (usually 1 ml),
it is stored in a small vial sealed with a Teflon-faced septum or cap at 4°C until
instrument analysis takes place.
Another general form of extraction is what is termed gas/liquid or gas/ solid extraction.
These are both variations upon the principle of headspace analysis. Headspace analysis is
the analysis of a gas that has been in contact with a liquid or solid sample and is used to
draw conclusions about the nature and/or composition of the underlying sample. There
are two general variations of headspace extraction; the first and most frequently used
form is where an inert gas, such as helium, is bubbled through a liquid sample, thereby
stripping volatile analytes from it. This is often referred to as purge and trap (P&T) or
dynamic headspace analysis. The stripped gas sample is passed through an absorbent
trap, which retains the volatile analytes. The collected analytes are analyzed by rapidly
releasing them from the absorbent trap (usually by heating and back flushing) and
introducing them as a “band or slug” into a gas chromatograph.
Gas chromatography, discussed below, is particularly well suited for this analytical
method because the extracted sample is already in a vapor or gas phase and can be
directly introduced into the GC's carrier gas stream for the separation.
The other form of headspace extraction is where the sample—be it liquid or solid—is
placed in a sealed container with a gas volume above it. The container is then heated to a
set temperature and allowed to come to equilibrium between the two phases (i.e., solid
and gas). After a prescribed period of time a fixed volume of the container's gas phase is