Page 51 - Tandem Techniques

Page 33

Thus equation (9) can be expanded to the form:-

Unfortunately equation (1.10) is not precise as it assumes a constant linear mobile phase velocity
throughout the column. Due to the fact that in gas chromatography the mobile phase is compressible,
the mean mobile phase velocity can not be employed, unless the pressure drop across the column is
extremely small. Ogan et al. [24] extended the Van Deemter equation further, to take into account the
compressibility of the carrier gas and produced the following equation:

where (G) is the inlet/outlet pressure ratio of the column,
(uo) is the mobile phase velocity at the column exit,
and (Do) is the diffusivity of the solute in the mobile phase

at atmospheric pressure.
An explicit equation for the variance per unit length of a chromatographic column is important in the
design of interfaces for tandem systems, as it allows the factors that control the dispersion to be
identified, and the actual dispersion to be calculated if required. Equation (1.11) is the first example and
others will now be considered for capillary columns and LC columns. In 1958 Golay [25] derived the
following equation for the variance per unit length of a capillary column:

where (r) is the radius of the capillary column.