Chemistry Reference
In-Depth Information
2.6 REVERSE PHASE RESINS
Normal phase chromatography is used to separate fairly nonpolar compounds.
Generally, the order of elution is that nonpolar compounds elute first and oth-
ers follow based on increasing order of polarity. Conversely, reverse phase chro-
matography can be used to separate compounds that are much more polar. The
more polar compounds elute first. The resins used for reverse phase are often
macroporous beads. These are in the form of polymeric material, with high sur-
face area and porosity, offering increased solute retention, selectivity, and supe-
rior loading capacity. An example is a highly cross-linked polystyrene resin ~50 Å
pore diameter, such as XAD resins (see “Glossary”).
2.7 HIGH- PERFORMANCE LIQUID CHROMATOGRAPHY
For high-performance liquid chromatography (HPLC) using modern macroporous
resins as the stationary phase, the beads can be as small as 2.5 to 3.0 microns.
Popular resin-based packing material is made from the copolymerization of polysty-
rene and divinylbenzene. The degree of cross-linking determines the rigidity of the
resin. The advantages of this system include fast separations using high pressure and
small volumes of solvent. Good resolution can be achieved with baseline separation
using ultra high-performance technology.
C18 reverse phase packing materials have served as the workhorse resins of liquid
chromatography for many years. There are now many different packing materials
to choose from, ranging from C2 to C30. One manufacturer provides a C30 alkyl
silane coated on spherical, porous silica gel. This LC column is designed to provide
high shape selectivity for separation of hydrophobic structurally related isomers and
compatible with highly aqueous mobile phases. Typically, reverse phase columns are
available in 3 and 5 μm particle sizes and 4.6, 3.0, and 2.1 mm diameters, with an
average pore diameter of 200 Å, ideal for separating hydrophobic structurally related
isomers. Applications include separations of the long chain, structurally related iso-
mers (e.g., carotenoids, steroids, etc.).
2.8 CAPILLARY ELECTROPHORESIS
Capillary electrophoresis (CE), also called capillary zone electrophoresis (CZE), is
used to separate ionic species by charge, frictional forces, and hydrodynamic radius.
In traditional electrophoresis, electrically charged analytes move in a conductive
liquid medium under the influence of an electric field. CE separates species based
on the size:charge ratio in the interior of a small capillary, filled with an electrolyte
and shown in Figure 2.4.
2.9 POLYAMIDE GEL CHROMATOGRAPHY
The porous polyamide resin possesses hydrogen bond acceptor properties and is a
very good medium for the separation of polyphenolic solutes such as phenolic acids,
flavanols, and flavonoids. Typical solvent mixtures include aqueous acetic acid and
ethanolic solutions.
Search WWH ::




Custom Search