EPARATION OF A
From General Chemistry review:
polarity, vapor pressure;
London dispersion forces;
dipole-dipole interactions; hydrogen bonding;
is a technique used to analyze, identify, and/or separate mixtures of compounds.
several types of chromatography techniques, for example, column, thin-layer, paper, gas, and liquid
All types of chromatography, however, have two common features: a mobile phase
gaseous or liquid and a stationary phase
which can be a solid or liquid adsorbent through which the mobile
The way chromatographic separation works is well illustrated by column chromatography
, a technique that
uses a solid adsorbent
as the stationary phase packed inside a glass or plastic column, and a liquid solvent
mobile phase called the eluent
In column chromatography a column is filled with small particles or beads of the stationary phase (insoluble
in the eluent) and the column is equilibrated with a small amount of the eluent.
Then a mixture of compounds,
, are dissolved in the solvent and applied to the top of the column as a narrow layer (part
of the diagram below).
If the stopcock of the column is carefully opened, a layer of the eluent solvent (mobile
phase) moves down the packed column by gravity.
As the eluent moves through the column, the compounds in
the mixture partition between the moving (mobile) phase and adsorbent (stationary) phase due to the differences
in the compounds’ physical properties (e.g., polarity, molecular weight, vapor pressure,
.) and therefore the
different interactions the compounds have with the stationary phase and mobile phase (e.g., London dispersion
forces, dipole-dipole interactions, hydrogen bonding,
A compound that is attracted less strongly to the
stationary phase will move through the column faster and elute, or come off, the column more rapidly in the
In contrast, a compound that is more strongly attracted to the stationary phase will move through the
column more slowly and elutes later.
Let us assume, for the purpose of this illustration, that component
more strongly to the stationary phase than component
does (see diagram below). Then, as the mixture travels
down the column, the molecules of
will be retarded with respect to the molecules of
and, in time, the two
components will separate into two bands and they will be eluted at different times.
That is, complete separation
of the initial mixture into its components is achieved.
The collected volumes of the eluent are called fractions.