Figure 12.1

A I Figure 12.1

0.14 0.22 Separate

Progress of a liquid–liquid extraction using two identical extractions of a sample (initial 0.03 0.45 phase) with fresh portions of the extracting phase. All numbers are fractions of solute in A I the phases; A = analyte, I = interferent.

A I Extracting

phase

Initial phase

Extract A I

0.83 0.33 Separate

0.17 0.67 Add new initial phase

A I Progress of a liquid–liquid extraction in

0.69 0.11 Separate

which the solutes are first extracted into the extracting phase and then extracted back 0.14 0.22 into a fresh portion of the initial phase. All numbers are fractions of solute in the A I phases; A = analyte, I = interferent.

We can improve the separation by first extracting the solutes into the extracting phase, and then extracting them back into a fresh portion of the initial phase (Fig- ure 12.2). Because solute A has the larger distribution ratio, it is extracted to a greater extent during the first extraction and to a lesser extent during the second ex- traction. In this case the final concentration ratio of

[ A]

546 Modern Analytical Chemistry

in the extracting phase is significantly greater. The process of extracting the solutes

countercurrent extraction

back and forth between fresh portions of the two phases, which is called a counter-

A liquid–liquid extraction in which

current extraction, was developed by Craig in the 1940s. 1* The same phenomenon

solutes are extracted back and forth between fresh portions of two extracting

forms the basis of modern chromatography.

Chromatographic separations are accomplished by continuously passing one sample-free phase, called a mobile phase, over a second sample-free phase that re-

phases.

mobile phase

mains fixed, or stationary. The sample is injected, or placed, into the mobile phase.

In chromatography, the extracting phase

As it moves with the mobile phase, the sample’s components partition themselves

that moves through the system.

between the mobile and stationary phases. Those components whose distribution

stationary phase

ratio favors the stationary phase require a longer time to pass through the system.

In chromatography, the extracting phase

Given sufficient time, and sufficient stationary and mobile phase, solutes with simi-

that remains in a fixed position.

lar distribution ratios can be separated.

The history of modern chromatography can be traced to the turn of the cen- tury when the Russian botanist Mikhail Tswett (1872–1919) used a column packed with a stationary phase of calcium carbonate to separate colored pigments from plant extracts. The sample was placed at the top of the column and carried through the stationary phase using a mobile phase of petroleum ether. As the sample moved through the column, the pigments in the plant extract separated into individual col- ored bands. Once the pigments were adequately separated, the calcium carbonate was removed from the column, sectioned, and the pigments recovered by extrac-

chromatography

tion. Tswett named the technique chromatography, combining the Greek words

A separation in which solutes partition

for “color” and “to write.” There was little interest in Tswett’s technique until 1931

between a mobile and stationary phase.

when chromatography was reintroduced as an analytical technique for biochemical separations. Pioneering work by Martin and Synge in 1941 2 established the impor- tance of liquid–liquid partition chromatography and led to the development of a theory for chromatographic separations; they were awarded the 1952 Nobel Prize in chemistry for this work. Since then, chromatography in its many forms has become the most important and widely used separation technique. Other separation meth- ods, such as electrophoresis, effect a separation without the use of a stationary phase.