Page 48 - Subyek Encyclopedia - Encyclopedia of Separation Science

Sepsci*21*TSK*Venkatachala=BG
I / CHROMATOGRAPHY 43

being forced along by the displacer. At the end of the
separation the displacer must be stripped from the
column if the column is to be reused. Displacement
chromatography is used mainly in preparative and
process chromatography, where high throughputs of
pure compounds can be obtained (note that the con-
tact boundary between zones may not be discrete and
the collection of pure compounds may be restricted to
the central region of the displaced zones).
In elution chromatography the sample is applied to
the column as a discrete band and sample compo-
nents are successively eluted from the column diluted
by mobile phase. The stationary and mobile phases
are normally at equilibrium prior to sample introduc-
tion. The mobile phase must compete with the sta-
tionary phase for the sample components; separation
will only occur if the distribution constants for the
various components, resulting from the competition,
are different. Elution chromatography is the most
convenient method for analysis and is the most com-
mon method of separation in GC, SFC, LC and
MEKC. Development, a modiRcation of the elution
mode, is used in planar chromatography. Samples are
applied to the dry layer as compact spots or bands
Figure 3 Mode of zone displacement in chromatography.
and the layer subsequently contacted by the mobile
phase, which ascends and moves the sample compo-
nents to positions higher up the layer in the direction
In frontal analysis, the mobile phase introduces the of mobile-phase Sow. The separation is (usually)
sample continuously onto the column (or the sample stopped before the mobile phase reaches the opposite
is the mobile phase) until eventually the column is edge of the layer and neither the eluent nor the sample
saturated with the sample and the component with components exit the layer. The two processes can be
the lowest afRnity for the stationary phase is compared; all components travel the same distance
displaced from the column by sample components of and are separated in time using the elution mode in
greater afRnity. When the zone of pure compon- column chromatography, whereas all components
ent has completely exited the column it is followed by have the same separation time and are separated in
a mixture containing the next component, and so on. space (migration position) in planar chromatography
Frontal analysis can be used to obtain thermodyn- using the development mode.
amic data from chromatographic measurements and
to isolate a less strongly retained trace component Chromatogram
from a major component. However, quantitation for
each component in a mixture is difRcult, and at The information obtained from a chromatographic
the end of the experiment the column is contaminated experiment is contained in the chromatogram. When
by the sample so that reuse requires stripping the the elution mode is used this consists of a plot of the
sample from the column. concentration or mass proRle of the sample compo-
In displacement chromatography the sample is ap- nents as a function of the Sow of the mobile phase or
plied to the column as a discrete band and a substance as a function of time. Typically the y-axis will be
(or mobile-phase component) with a higher afRn- detector response and the x-axis time or volume of
ity for the stationary phase than any of the sample mobile phase in column chromatography or migra-
components is continuously passed through the col- tion distance in planar chromatography. The position
umn. The displacer pushes sample components down of each peak in the chromatogram is characteristic of
the column and, if the column is long enough, the identity of the compound and the area under the
a steady state is reached. A succession of rectangular peak is a function of the concentration or amount of
bands of pure components then exits the column. each compound. Peak widths in the chromatogram
Each component displaces the component ahead of it, are controlled by solute-dependent kinetic factors,
with the last and most strongly retained component which in turn can be used to deduce values for