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Optimization in liquid chromatography - Guiochon G.

Guiochon G. Optimization in liquid chromatography - Horvath, 1980. - 344 p.
Download (direct link): optimizationiquidchromatography1980.pdf
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high. Considerable saving can be achieved, therefore, by finding a
stationary-mobile phase system that has high selectivity. This is
illustrated by the relationship (29) between molar free energy changes,
AG, and AGt, associated with the equilibrium distribution of the solutes
1 and 2 between mobile and stationary phases [Eq. (6)]:
A(AN) = AGt ~ AGi = RT In a As this quantity is small we can write that
a - 1 - A(AG)//?7
(6)
(7)
where a Is the relative retention of the two compounds.
Optimization la Liquid Chromatography
5
* 0 30 60 t(min)
Fig. 1. Chromatogram of a complex mixture of
polychlorobiphenyls.
!
A small change in the free energies, which are usually of the order of
several kilocalories per mole, can result in a marked relative change in
A(AG). At ambient temperature (A =" 300K), a change, in A(Ao of 18
cal/mol from 18 to 36 cal/mol would convert a difficult analysis (N =
32,000; a = 1.03) into an easy separation problem (N = 8800; a = 1.06)
assuming that k' = 3 and R = 1.
The search for a "good" system is certainly worthwhile for simple
mixtures, such as those containing a few closely related isomers. For
complex mixtures, such as the one shown on Fig. 1, however, only an
improvement in column efficiency can effect a complete resolution of the
sample components.
Once the chromatographic system has been selected, the problem reduces
to the design of a column having the required resolution power while
minimizing analysis time and possibly inlet pressure. Before discussing
this problem we need to know the relationships between analysis time,
flow velocity, and pressure, and between resolving, power and flow
velocity.
6
Georges Guiochon
#
III. ANALYSIS TIME, FLOWRATli, AND I'KDSSUKL:
From the definition of k. the retention time can be explained hy
F.tj. (8)
tR = t0 (1 + *') (8)
where e is proportional to the thermodynamic equilibrium constant for
distribution of the solute between the stationary and the mobile phases.
Tho proportionality constant is a function of the amount of solvent in
the column, i.e., the (out porosity of the packing, and the amount or
stationary phase, i.e., the liquid loading of the support in liquid-
liquid chro-
ttwiogvaphy (LLC) or suite area of llte solid in liquid -solid cluoma
tography (LSC). Generally, e is defined once the chromatographic system
is chosen.
The transit time, r0, is given by Eq. (9)
to - L/u
(9)
where L is the column length and e the mobile phase velocity. As liquids
have a very low compressibility, for all practical purposes, the velocity
of the mobile phase is constant along the column in HPLC (30). It is
related to the column parameters by Eq. (10)
u = Mi .
(10)
1) L
where A? is the difference between inlet and outlet pressures-the latter
is practically always atmospheric pressure. As gauges measure pressures
by reference to the atmospheric pressure, AP is the reading on the
inlet
pressure gauge. In Eq. (10), tj is the solvent viscosity and d" is
the particle
diameter; ei is a dimensionless coefficient having a value usually about
I x 10~* (2 3,31). It is a function of the external porosity, e, given
by that fraction of the column volume which is not occupied by the
particles, and is therefore available to the solvent flowing around the
particles. It is noted that although the particles are porous, the
solvent does not flow through them, only around them. The value of ei can
be evaluated from the Kozeny-Carman equation given by Eq. (11):
ei - ?*/180(1 - ")*
(11)
Equation (11) shows that ei increases rapidly with increasing porosity
(31). For columns used in HPLC, e is usually about 0.40 and seems not to
depend on the material and the packing technique used for the preparation
of the column (22). For e = 0.40 the equation predicts that A0 - I x ?"3
in agreement with experimental observations. In order to achieve the very
homogeneous packing necessary for a good column efficiency, the
Optimization in Liquid Chromatography
7
methods used (end to give u very dense packing (23). The importance of
the parameter k0 has been stressed by Bristow and Knox (50), who prefer
to urc o - 1 /*". <
The product ktd% is the column permeability. It is in practice
entirely determined by the particle diameter. In reality, the packing
material is made of particles of different sizes. Sieving, or various
elutriation or sedimentation techniques, have a rather small efficiency
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