Page 49 - Tandem Techniques

Page 31

phase system and the linear velocity of the mobile phase. The dispersive capacity of a column is
measured as the variance per unit length of the column, (H), which can be calculated from the ratio of
the column length, (1), to the number of theoretical plates in the column, (n).

Thus

Due to the nature of equation (1.8), (H) has also been given the term 'height of the theoretical plate'. The
first equation for (H) was derived for packed columns in gas chromatography by Van Deemter [23], and
took the following form:-

where (A),(B),(C) are constants for a given system,

and (u) is the linear mobile phase velocity.

Each expression in equation (1.9) pertains to a specific dispersion process each of which will now be
described.

The Multipath Process

During progress along a packed column, the solute molecules describe a tortuous path through the
interstices between the particles of the stationary phase support. It is fairly obvious that some molecules
will randomly travel shorter paths than others. Those that on the average, pass along

shorter paths will move ahead of the maximum of the concentration profile, while those molecules that
pass along paths of greater length will lag behind the maximum of the concentration profile. This will
result in dispersion of the solute band and its contribution to (H) was deduced by Van Deemter as
independent of the linear velocity and equal to a constant (A), where,

A = 2ldp

where (l) is a constant,
and (dp) is the particle diameter.