Page 51 - Subyek Encyclopedia - Encyclopedia of Separation Science

46 I / CHROMATOGRAPHY/ Derivatization

sented by the stationary phase and the rest of the
system. In LC, Darcy’s law relates the properties of
the mobile phase, characteristic features of the col-
umn, and the external pressure required to obtain
a useful mobile-phase velocity. This law can be stated
as:

2
u" PK 0 d P / L [1]
where u is the mobile phase velocity, P is the pres-
sure drop across the column, K 0 is the column per-
meability, d P is the average particle diameter, is the
mobile phase viscosity, and L is the column length.
Since a minimum value for u is required for accept-
able column performance and separation times, and
Figure 6 Relationship between mobile-phase velocity and mi- the available column pressure drop is constrained to
gration distance for capillary-controlled and forced-flow develop- an upper limit by material and safety considerations,
ment in planar chromatography. (Reproduced with permission then there is a Rnite limit to the range of permissible
from Poole CF and Wilson ID (1997) Journal of Planar d P /L values that can be used. Thus a compromise
2
Chromatography 10: 332, copyright ^ Research Institute for
must be accepted between separation time and ef-
Medicinal Plants).
Rciency, which results in an upper limit to the number
of theoretical plates that can be obtained for fast
to minimize zone broadening. This has a number separations or the use of long separation times when
of consequences: zone brodening is largely domin- very large numbers of theoretical plates are required
ated by diffusion; the useful development length for a separation.
for PC is set by the range of acceptable mobile- Bulk liquid Sow under electrophoretic conditions
phase velocities; separation times are increased; occurs by electroosmosis. At the column wall or par-
and the separation potential of PC is less than that ticle surface (packed columns) an electrical double
predicted for a constant and optimum mobile-phase layer results from the adsorption of ions from the
velocity. mobile phase or dissociation of surface functional
Pneumatic transport of the mobile phase is com- groups. An excess of counterions is present in the
monly employed in column chromatography. The double layer in comparison with the bulk liquid and
mobile phase is pressurized externally to the column in the presence of an electric Reld shearing of the
(a simple high pressure cylinder with regulator in solution occurs only within the very thin diffuse
the case of a gas, or a mechanical pump for liquids). part of the double layer, transporting the mobile
The pressure gradient across the column provides the phase through the column with a nearly perfect plug
driving force to overcome the resistance to Sow pre- proRle (Figure 7). The velocity of the bulk liquid Sow

Figure 7 Flow profile for an open tube and a packed column using pneumatic and electroosmotic driving forces.