Selectivity
In chromatography, the selectivity of a phase system refers to its capacity to retain certain types of solutes to a significantly greater extent than others. For example, a gas chromatographic system, employing a long chain hydrocarbon as the stationary phase, would retain hydrocarbons very strongly, due to their mutual, strong dispersive interactions. In contrast, short chain alcohols would elute very rapidly due to the relatively small dispersive interactions between the alcohols and the hydrocarbon stationary phase. The phase system would be said to ‘selectively’ retain hydrocarbons relative to short chained alcohols. However, if the stationary phase was polyethylene glycol, the short chained alcohols would be retained very strongly due to the strong polar interactions between the hydroxy groups on the solute and those of the stationary phase. In contrast, due to the small dispersive capability of the polyethylene glycol, hydrocarbons (including long chained hydrocarbons) would interact very weakly with the stationary phase and would only be slightly retained. Thus, the stationary phase would be said to selectively retain alcohols relative to hydrocarbons. The cases sited are extreme and chosen to illustrate selectivity, but, in fact, selectivity can be far more subtle when separating similar types of compounds.
Author: RPW Scott
Book:The Thermodynamics of Chromatography
Section:Thermodynamics Other-Methods Chiral-Separations
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It is clear
that there is yet another limitation of which to be aware when exploring the effect
of solvent composition on retention and selectivity. It is important to examine
the effect of solvent composition over a range of
temperatures, to ensure that the true effect of solvent composition
on selectivity is disclosed. If the distribution system is evaluated at, or
close to that temperature where the separation ratio remains constant and
independent of solvent composition, then the potential advantages that can be
gained from an optimized solvent mixture will never be disclosed. Any evaluation
of either a particular stationary phase, or solvent mixture, for the separation
of closely eluting solutes must be carried out over a
range of temperatures.
 
Optimum
Thermodynamics Other-Methods Chiral-Separations
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Applications Chiral-Separations
is distinctly more selective than the b material. It has been employed in the separation of a very wide range of compound classes, and from very small to very large molecules. Yet another stationary phase has been synthesized by substituting the cyclodextrin hydroxyl groups with pure the 'S' hydroxypropyl groups followed by permethylation. As a result, the size selectivity of the material is reduced but more polar (hydrophilic) groups are introduced. The b material has a greater chiral selectivity than the a or g phases. This material provides a good general purpose column. It is clear that there are many possibilities for derivatizing the cyclodextrins to provide unique interactive character; there are a large number commercially available and many more are likely to be synthesized in the future.
A number of examples of some specifically derivatized cyclodextrin stationary phases have been reported. In one example, the positions 2 and 6 are alkylated (pentylated) thus
Capillary Applications Chiral-Separations
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Criteria-for-Successful-Operation
or mixtures of both) so that polar
selectivity remains dominant in the stationary phase. Unfortunately, all
substances are not simply polar or dispersive or ionic but can exhibit various
combinations of all three interactive characteristics. It follows, that the
stationary phase will also need to have the appropriate mixture of interactive
properties to maximize selectivity and in contrast the interactive character of
the mobile phase will need to oppositely balanced to ensure maximum selectivity
still resides in the stationary phase. Under some circumstances the selectivity
of the mobile phase can also be exploited to interact more strongly with the
neighboring substance and thus elute it more quickly. It is clear, that some
considerable effort must be made to identify the bests phase system for the isolation
of a substance from a hitherto unknown mixture. This work can be carried out on
an analytical column and its success will determine the maximum load that can
be used,
Preparative Criteria-for-Successful-Operation
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Chiral-Stationary-Phases
in competing for the
benzene against the dispersive interactions of the n-hexadecane.
Chiral Stationary Phases
There are
basically five general types of chiral stationary phase in common use in LC.
The first is the protein based stationary phase. These stationary phases
usually take the form of natural proteins bonded to a silica matrix. As they
are proteins, they contain a large number of chiral centers and are known to
interact strongly with small analytes exhibiting strong chiral selectivity.
There are specific interactive sites that provide chiral selectivity, but there
are many more sites that only contribute to general retention. These other
sites can be deactivated by mobile phase additives (e.g. octylamine) which
reduces the overall retention and increases the chiral selectivity. The second
type consists of relatively small molecular weight chiral substances bonded to
silica 9 Pirkle (37). Each bonded group has a limited number of chiral centers
available but, due
HPLC Chiral-Stationary-Phases
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Applications Chiral-Separations
small molecules might be separated on the derivatized a-cyclodextrin whereas, in contrast, larger molecules might be better separated on the derivatized g-cyclodextrin. It follows, that the derivatized b-cyclodextrin might be selected for separating the isomers of solute molecules of intermediate size. In contrast, if the 3-position hydroxyl group is derivatized all the cyclodextrins completely loose their size selectivity. Derivatizing the 6-hydroxyl position has little or no effect on chiral selectivity but does appear to enhance the loading capacity of the stationary phase. This position is also used to anchor the cyclodextrins to the surface of silica gel in the preparation of LC stationary phases. After permethylation the a-, b- or g- cyclodextrins must be dissolved directly in appropriate polysiloxanes mixtures, and the coated on the walls of the capillary column (glass or fused quartz) using the techniques already described. The underivatized cyclodextrins, and those that have
Capillary Applications Chiral-Separations
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Applications Gas-Chromatography Chiral-Separations
character, but it is interesting to note that the propane diols are
eluted between the C10 and C11 hydrocarbons. This indicates that the C3 chain
alcohol is retained by polar forces to an almost equivalent extent as the C11
hydrocarbon retained by dispersion forces only. This implies a fairly strong
overall polar character to the stationary phase. It is also seen that the m
and p xylenes are well separated showing good spatial selectivity and
the separation of diol enantiomers good chiral selectivity.
Courtesy of Supelco
The
a-DEX™ column was 30 m long, 0.25 mm I.D., carrying a
film of stationary phase 0.25 mm thick. The temperature was
90˚C and the helium flow velocity was 30 cm/s.
Figure 41
Chromatogram of a Test Mixture for a Permethylated a-Cyclodextrin Stationary Phase
As the two
enantiomers are strongly polar it would also appear that the greater retention
of the (–)-1,2-propane diol was due to polar interactions between the OH
Principles Applications Gas-Chromatography Chiral-Separations