The equilibrium−dispersive model of chromatography
and measurements of sorption isotherms were applied to
reversed-phase C18 systems of several solutes with
40:60 methanol/water mobile phases to calculate the band
shapes of eluites in the linear, analytical region of
chromatography. These predicted profiles were compared to
experimental profiles to determine the error associated
with the equilibrium−dispersive model. This error
is
associated with the assumption of a uniform plug flow
profile of mobile phase through the column. Analysis
of
the results of several systems and columns illustrates the
asymmetry that exists in the actual mobile-phase flow
profiles. The model error asymmetry decreases for
higher
efficiency columns as expected and increases in the form
of peak tailing for lower efficiency columns. This
behavior
suggests that portions of the column are not swept by the
mobile-phase flow at the same rate as the rest of the
column, indicating an inhomogeneous packing density.
The general method described is valuable for
determining
the relative contributions of thermodynamics and hydrodynamics to the peak asymmetry observed in liquid
chromatography.
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