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Chapter 7 Obtaining and Preparing Samples for Analysis 215

Continuous extractions also can be accomplished with supercritical fluids. 19
supercritical fluid
When a substance is heated above its critical temperature and pressure, it forms a A state of matter where a substance is
supercritical fluid whose properties are between those of a gas and a liquid. Super- held at a temperature and pressure that
critical fluids are better solvents than gases, making them a better reagent for ex- exceeds its critical temperature and
tractions. In addition, the viscosity of a supercritical fluid is significantly less than pressure.
that of a liquid solvent, allowing it to pass more readily through particulate samples.
One example of a supercritical extraction is the determination of total petroleum
hydrocarbons (TPHs) in soils, sediments, and sludges with supercritical CO 2 . Ap-
proximately 3 g of sample is placed in a 10-mL stainless steel cartridge, and super-
critical CO 2 , at a pressure of 340 atm and a temperature of 80 °C, is passed through
the cartridge for 30 min at flow rate of 1–2 mL/min. The petroleum hydrocarbons
are collected by passing the effluent from the cartridge through 3 mL of tetra-
chloroethylene at room temperature. At this temperature the CO 2 reverts to the gas
phase and is released to the atmosphere. 20

Chromatographic Separations In an extraction, the sample is initially present in
one phase, and the component of interest is extracted into a second phase. Separa-
tions can also be accomplished by continuously passing one sample-free phase,
called the mobile phase, over a second sample-free phase that remains fixed or sta-
tionary. The sample is then injected or placed into the mobile phase. As the sam-
ple’s components move with the mobile phase, they partition themselves between
the mobile and stationary phases. Those components having the largest partition
coefficients are more likely to move into the stationary phase, taking longer to pass
through the system. This is the basis of all chromatographic separation techniques.
As currently practiced, modern chromatography provides a means both of separat-
ing analytes and interferents and of performing a qualitative or quantitative analysis
of the analyte. For this reason a more thorough treatment of chromatography is
found in Chapter 12.

7 G Liquid–Liquid Extractions

A liquid–liquid extraction is one of the most important separation techniques used
in environmental, clinical, and industrial laboratories. Two examples from envi-
ronmental analysis serve to illustrate its importance. Public drinking water supplies
are routinely monitored for trihalomethanes (CHCl 3 , CHBrCl 2 , CHBr 2 Cl, and
CHBr 3 ) because of their known or suspected carcinogeneity. Before their analysis
by gas chromatography, trihalomethanes are separated from their aqueous matrix
by a liquid–liquid extraction using pentane. 21 A liquid–liquid extraction is also
used in screening orange juice for the presence of organophosphorous pesticides. A
sample of orange juice is mixed with acetonitrite and filtered. Any organophospho-
rous pesticides that might be present in the filtrate are extracted with petroleum
ether before a gas chromatographic analysis. 22
In a simple liquid–liquid extraction the solute is partitioned between two im-
miscible phases. In most cases one of the phases is aqueous, and the other phase is
an organic solvent such as diethyl ether or chloroform. Because the phases are im-
miscible, they form two layers, with the denser phase on the bottom. The solute is
initially present in one phase, but after extraction it is present in both phases. The
efficiency of a liquid–liquid extraction is determined by the equilibrium constant
for the solute’s partitioning between the two phases. Extraction efficiency is also in-
fluenced by any secondary reactions involving the solute. Examples of secondary re-
actions include acid–base and complexation equilibria.