Page 173 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN006F-275 June 29, 2001 21:12

464 Gas Chromatography

of a greater surface area of such columns and, conse-
quently, a relatively larger amount of stationary phase.
A proper choice of the liquid stationary phase is ex-
ceedingly important to a successful chromatographic sep-
aration. A great number of chemically different stationary
phases have been described in the scientific and commer-
cial literature. Several requirements govern the choice of
a chemical substance as a GC stationary phase. First of
all, it should have adequate selectivity for the substances
to be resolved. It must be chemically stable at the column
temperatures used in a given separation problem. The sta-
tionary phase must easily adhere as a uniform film to the
column support without running off the column; if such
mechanical instability occurs, the phase contaminates the
detector and, naturally, the columns function properly for
only a limited time. Finally, the stationary phase should
be a well-defined chemical compound, so that the column
preparation as well as the chromatographic process itself
are reproducible.
In spite of the above strict requirements, many chemical
substances can adequately perform as the stationary phase.
The thermal stability requirement has made various syn-
thetic polymers (silicones, polyglycols, polyesters, poly-
imides, etc.) most popular. Since the stabilities vary ac-
cording to chemical stucture, nonpolar polymers are more
stable than the polar column substrates. Column tempera-
tures above 300 C are seldom used in the practice of GC.
◦
The general solution rules roughly determine the suit-
FIGURE 9 Column drawing apparatus for the preparation of ability of a stationary phase for a given separation task:
fused silica capillaries. [From Lipsky, S. R., McMurray, W. J., Polar substances (solutes) are readily dissolved and chro-
Hernandez, M., Purcell, J. E., and Billeb, K. A. (1980). J. Chro- matographically retained by the polar stationary phases,
matogr. Sci. 18, 1.]
while the nonpolar column materials retain the chro-
matographed sample components according to their boil-
To secure a uniform film deposition from the solution ing points, without any particular regard to the presence
of a stationary phase, the inner column’s surface is first of unique functional groups in the sample molecules.
treated by an organic compound, the task of which is to Although the rules appear relatively straightforward, the
improve the surface wettability and to mask potentially stationary phases for many practical separations are still
adsorptive column sites. A variety of stationary phases selected empirically.
can now be successfully coated for capillary GC with a The main solute–column interactions can be classified
controlled film thickness. Refined procedures now exist as dispersion forces and dipole–dipole interactions. The
even for the chemical immobilization of some stationary dispersion forces are present in any solute–solvent system,
liquids. a hydrocarbon solute interacting with a nonpolar paraf-
The third type of a GC column (Fig. 7) is a porous- fin being often shown as an example. The polar solute
layer, open tubular column. While such a column has an molecules have permanent dipoles that can interact with
inner diameter and a length comparable to the wall-coated those of the polar phases; on occasions, the dipole mo-
columns, its inner wall is modified through a chemical ments can also be induced in certain solute molecules in
treatment or deposition of finely dispersed particles. The the presence of highly polar column materials. Dipole–
porouslayercanbeeitheranadsorbentorathinlayerofthe dipole interactions are clearly evident in the separations
solid support impregnated with a liquid stationary phase. of alcohols, esters, amines, aldehydes, and so on, on the
Efficiencies of the porous-layer, open tubular columns are polyglycol, polyamide, polyester, or cyanoalkylsilicone
not as high as those of “true” capillary columns, but their stationary phases.
greater sample capacity is an advantage to some separa- Some extremely selective GC separations have been ac-
tions. The enhanced sample capacity is obtained because complished. In a number of cases, the hydrogen-bonding