principles of extraction and the extraction of semivolatile organics ...

68 principles of extraction
Once the analyte has been extracted into phase B, it is usually desirable to
reduce the volume of the extracting solvent. This can be accomplished with
specialized glassware such as a Kuderna–Danish sample concentrator (Figure
2.13b), which is widely used for concentrating semivolatile compounds
dissolved in volatile solvents. The concentrator consists of three primary
components held together by hooks and/or clamps: a central flask with sufficient
capacity to hold the extracting solvent, a tapered receiving vessel to
contain the concentrated extract, and a distilling–condensing column that
allows the solvent vapor to pass while retaining the analyte. The apparatus
should be placed over a vigorously boiling water bath to bathe the central
flask in steam. The solvent should then be allowed to escape into a hood
or recovered via an additional solvent recovery system. Alternatively, a
mechanical rotary evaporator may be used to evaporate excess extracting
solvent, or other evaporating units that evaporate solvent with an inert gas
should be used.
Performing LLE of analytes from drinking water is relatively straightforward.
However, if your ‘‘aqueous’’ sample is blood, urine, or wastewater,
the extraction process can become more tedious. Quite often in such
samples, a scum forms at the layer interface, due to the presence of nonsoluble
debris and the formation of emulsions. Analysts overcome this di‰culty
using techniques such as adding salts, chilling the sample, or centrifugation.
Applying a continuous LLE technique can be useful also.
Continuous LLE is a variant of the extraction process that is particularly
applicable when the distribution coe‰cient of the analyte between phases A
and B is low. Additionally, the apparatus for conducting continuous LLE
(Figures 2.14 and 2.15) automates the process somewhat. The analyst is
freed from manually shaking the phases in a separatory funnel to e¤ect a
separation allowing multiple extractions to be performed simultaneously.
Since the phases are not shaken to mix them, this procedure also helps avoid
the formation of emulsions. The apparatus can be assembled to perform
extraction alone (Figure 2.14), or extraction and concentration (Figure
2.15). The extractor performs on the principle that organic solvent cycles
continuously through the aqueous phase, due to constant vaporization and
condensation of the extracting solvent. Continuous LLE apparatus designed
for heavier-than-water or lighter-than-water extracting solvents is available.
2.2.3. Procedures
A general extraction scheme (Figure 2.16) can be devised to extract semivolatile
organics from aqueous solution such that important categories of
organic compounds (i.e., bases, weak acids, strong acids, and neutrals) are
fractionated from each other and isolated in an organic solvent. Many