Ion Exchange Chromatographic Columns
Ion
exchange columns are used to separate ions and molecules that can be
easily ionized. Separation of the ions depends on the ion's affinity for
the stationary phase, which creates an ion exchange system. The
electrostatic interactions between the analytes, moble phase, and the
stationary phase, contribute to the separation of ions in the sample.
Only positively or negatively charged complexes can interact with their
respective cation or anion exchangers. Common packing materials for ion
exchange columns are amines, sulfonic acid, diatomaceous earth,
styrene-divinylbenzene, and cross-linked polystyrene resins. Some of the
first ion exchangers used were inorganic and made from aluminosilicates
(zeolites). Although aluminosilicates are not widely used as ion
exchange resins used.
Size Exclusion Chromatographic Columns
Size Exclusion Chromatographic columns
separates molecules based upon their size, not molecular weight. A
common packing material for these columns is molecular sieves. Zeolites
are a common molecular sieve that is used. The molecular sieves have
pores that small molecules can go into, but large molecules cannot. This
allows the larger molecules to pass through the column faster than the
smaller ones. Other packing materials for size exclusion chromatographic
columns are polysaccharides and other polymers, and silica. The pore
size for size exclusion separations varies between 4 and 200 nm.
Figure 3: Schematic of a
size exclusion column. The larger particles will elute first because
they are too big to fit inside the pores. The smallest particles will
elute last because they fit very well inside the pores. This figure was created with Microsoft Paint.
Chiral Columns
Chiral columns are used to separate enantiomers. Separation
of chiral molecules is based upon steriochemistry. These columns have a
stationary phase that selectively interacts with one enantiomer over
the other. These types of columns are very useful for separating racemic
mixtures.
Some Stationary Phases Used to Separate Enantiomers| Stationary Phase | Method(s) Used |
| Metal Chelates | GC, LC |
| Amino Acid Derivatives | GC, LC |
| Proteins | LC |
| Helical Polymers | LC |
| Cyclodextrin Derivatives | GC, LC |
Column Efficiency
Peak or band broadening causes the
column to be less efficient. The ideal situation would to have sharp
peaks that are resolved. The longer a substance stays in the column it
will cause the peaks to widen. Lengthening the column is a way to
improve the separation of different species in the column. A column
usually needs to remain at a constant temperature to remain efficient.
Plate height and number of theoretical plates determines the efficiency
of the column. Improving the efficiency would be to increase the number
of plates and decrease the plate height.
The number of plates can be determined from the equation:
N=L/H
where L is the length of the column and H is the height of each plate. N can also be determined from the equation:
N=16(tR/W)2 or N=5.54(tR/W1/2)2
where tR is the retention time, W is the width of the peak and W1/2 is half the width of the peak.Height equivalent to a theoretical plate (HETP) is determined from the equation: ::
H=L/N
or HETP can also be determined by the equation:
H=A+B/u+Cu
where H equals HETP, A is the term for
eddy diffusion, B is the term for longitudinal diffusion, C is the
coefficient for mass-transfer between the stationary and mobile phases,
and u is the linear velocity. The equation for HETP is often used to
describe the efficiency of the column. An efficient column would have a
minimum HETP value.
Gas chromatographic columns have plate
heights that are at least one order of magnitude greater than liquid
chromatographic column plates. However GC columns are longer, which
causes them to be more efficient. LC columns have a maximum length of 25
cm whereas GC columns can be 100 metersRead more at :
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