Page 172 - 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

Gas Chromatography 463

FIGURE 8 The resolution advantage of a capillary column (a) over a packed column (b), in Calmus oil analysis.
[From Grob, K., and Grob, G. (1979). J. High Resolut. Chromatogr. 3, 109.]

to theoretical description. An example is a description of packed columns, various (less accurate) empirical con-
the physical processes that occur inside such capillary stants must be used.
columns. Equation (10), the van Deemter equation, can Equation (11) is strictly valid only for the cases where
be translated, for the capillary column into the stationary phase film thickness amounts to no more
than a few tenths of a micrometer (thin-film columns).
2
2D G (1 + 6k + 11k ) r 2 Columns with film thicknesses up to several microme-
H = + u, (11)
u (1 + k) 2 24D G ters can also be prepared. Although their efficiencies are
lower than those obtained for the thin-film columns, due to
where D G is the solute diffusion coefficient in the mobile the impaired solute mass transfer, they can tolerate larger
(gas) phase and r the capillary inner radius. The equa- sample amounts without signs of overloading.
tion shows explicitly how the plate height is dependent on Refined aspects of column technology have been cru-
the diffusion processes and the column radius. At low gas cial to the success of GC capillary columns. Early in the
velocities, molecular diffusion significantly increases the development of such columns, metal or plastic tubes were
plate height. At higher velocities, the opposite is true (D G used exclusively. Highly efficient glass capillary columns
is in the denominator), reflecting the fact that the solute were developed at a later stage, and the problems of glass
mass transfer from one phase to another is primarily diffu- fragility were successfully overcome through the technol-
sion controlled. Reducing the column radius is a powerful ogy of fused-silica flexible tubes. Production of fused-
way to increase the column performance. silica capillaries is reminiscent of the fabrication of optical
Note that Eq. (11) is an accurate description of the col- fibers: thin-walled sillica tubes, drawn from a hot zone are
umn processes because the column geometry is well de- immediately protected by an overcoat of a stable organic
fined. Although somewhat similar equations exist for the polymer (Fig. 9).