Page 247 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

TYPES OF LlQUlO CHROMATOCRAPHV 8.2

3. Bonded-phase chromatography (BPC). To overcome some of the problems
associated with conventional LLC, such as loss of stationary phase from the
support material, the stationary phase may be chemically bonded to the support
material. This form of liquid chromatography, in which both monomeric and
polymeric phases have been bonded to a wide range of support materials, is
termed 'bonded-phase chromatography'.
Silylation reactions have been widely used to prepare bonded phases. The
\
silanol groups (b-OH) at the surface of silica gel are reacted with substituted
/
chlbrosilanes. ~'t~~ical example is the reaction of silica with a dimethylchlorosilane
which produces a monomeric bonded phase, since each molecule of the silylating
agent can react with only one silanol group:

The use of di- or tri-chlorosilanes in the presence of moisture can result in a
polymeric layer being formed at the silica surface, i.e. a polymeric bonded phase.
Monomeric bonded phases are, however, preferred since they are easier to
manufacture reproducibly than the polymeric type. The nature of the main
chromatographic interaction can be varied by changing the characteristics of
the functional group R; in analytical HPLC the most important bonded phase
is the non-polar C-18 type in which the modifying group R is an octadecyl
hydrocarbon chain. Unreacted silanol groups are capable of adsorbing polar
molecules and will therefore affect the chromatographic properties of the bonded
phase, sometimes producing undesirable effects such as tailing in RPC. Such
effects can be minimised by the process of 'end-capping' in which these silanol
groups are rendered inactive by reaction with trimethylchlorosilane:

- Si-OH + Cl-Si - CH3 - Si-O-Si-CH3
I I 1 I

An important property of these siloxane phases is their stability under the
conditions used in most chromatographic separations; the siloxane bonds are
attacked only in very acidic (pH < 2) or basic (pH > 9) conditions. A large
number of commercial bonded-phase packings are available in particle sizes
suitable for HPLC.48

4. Gel permeation (exclusion) chromatography (GPC). This form of liquid
chromatography permits the separation of substances largely according to their
molecular size and shape. The stationary phases used in GPC are porous
materials with a closely controlled pore size, the primary mechanism of retention
of solute molecules being the different penetration (or permeation) by each
solute molecule into the interior of the gel particles. Molecules whose size is
too great will be effectively barred from certain openings into the gel network