Page 63 - Subyek Encyclopedia - Encyclopedia of Separation Science

58 I / CHROMATOGRAPHY/ Derivatization

powerful separation method, but more frequently individual substituents can be varied to create the
than not, solvents of different composition are desired variation in selectivity (R"methyl, phenyl,
used for the two developments employing retention 3,3,3-triSuoropropyl, cyanoethyl, Suorine-contain-
mechanisms that differ in intensity rather than ing alcohol, etc.) Special phases in which R contains
kind, and the zones are only dispersed around the a chiral centre or a liquid-crystalline unit are used to
diagonal between the two development directions separate enantiomers and geometric isomers. Other
and not uniformly over the whole layer. common stationary phase include hydrocarbons,
poly(phenyl ethers), poly(esters) and poly(ethylene
glycols), although many of these phases are restricted
Mode Selection
to packed column applications because of difR-
Chromatography provides many different ap- culties in either coating or immobilizing them on the
proaches for the separation of mixtures. There are walls of fused-silica capillaries, favoured for the
many instances where the same mixture can be ad- manufacture of open-tubular columns. The solvation
equately separated by more than one approach. In parameter model provides a reliable systematized ap-
this section we will take a mechanistic look at how proach for selectivity optimization and the prediction
solutes are separated by the common chromato- of retention in GLC. For GSC the stationary phase is
graphic techniques to provide some guidelines for usually silica, alumina, graphitized carbon, organic
method suitability. polymer or zeolite porous particles (packed columns);
If the only consideration were efRciency and or a thin layer dispersed over the inner surface of
speed, then GC would be the preferred technique. In a capillary column with an open passageway down
practice, GC is restricted to thermally stable com- the centre (porous layer open-tubular column, or
pounds with a signiRcant vapour pressure at the tem- PLOT column). These materials are used to separate
perature required for their separation. The upper inorganic gases, volatile halocarbon compounds, low
temperature limit for common GC stationary phases molecular weight hydrocarbons and, in particular,
is 200}4003C. Few compounds with a molecular geometric and isotopic isomers.
weight greater than 1000 Da have sufRcient va- LC and GC should be considered as complement-
pour pressure to be separated in this temperature ary techniques. Since the only sample requirement for
range, and many low molecular weight compounds LC is that the sample has reasonable solubility in
are known to be labile at temperatures required for some solvent suitable for the separation, and since
their vaporization. Derivatization techniques extend separations by LC are commonly carried out close to
the scope of GC to otherwise labile compounds by room temperature, thermal stability is not generally
forming thermally stable derivatives, often with in- an issue. The large number of separation mechanisms
creased volatility, and by tagging compounds with easily exploited in the liquid phase provides a high
speciRc groups that simplify trace analysis using one level of Sexibility for selectivity optimization. In gen-
of the selective and sensitive group or element-selec- eral, many applications of LC can be categorized as
tive detectors available for GC. those for which GC is unsuited and includes applica-
Under typical conditions the mobile phase in GC tions to high molecular weight synthetic polymers,
behaves essentially as an ideal gas and does not con- biopolymers, ionic compounds and many thermally
tribute to selectivity. To vary selectivity either the labile compounds of chemical interest.
temperature is changed or a new stationary phase Mode selection within LC is quite complicated
(column) is employed for the separation. Temper- because of the number of possible separation mech-
ature and separation time are closely connected in anisms that can be exploited, as illustrated in Fig-
GC. The range over which temperature can be varied ure 22. Preliminary information on the molecular
is usually short and will likely provide only a small weight range of the sample, relative solubility in or-
change in selectivity, but because of the large number ganic solvents and water, and whether or not the
of theoretical plates available for a separation in GC, sample is ionic, can be used as a starting point to
this may be sufRcient to provide adequate resolu- arrive at a suitable retention mechanism for a separ-
tion. Provided that stationary phases that differ ation. The molecular weight cutoff at 2000 in-
in their relative capacity for intermolecular interac- dicated in Figure 22 is quite arbitrary and reSects the
tions are selected, then larger changes in selectivity fact that size exclusion packings are readily available
can be anticipated by stationary-phase optimization. for the separation of higher molecular weight solutes,
In modern column technology the most versatile although size exclusion is not used exclusively to
group of stationary phases are the poly(siloxanes), separate high molecular weight compounds because
which can be represented by the basic structure of its limited peak capacity. Wide-pore packing
}(R 2 SiO) n }, in which the type and relative amount of materials allow polymers with a molecular weight