principles of extraction and the extraction of semivolatile organics ...

86 principles of extraction
S i l i c a
sorbents available at the time. Reversed-phase bonded silica sorbents having
alkyl groups covalently bonded to the silica gel backbone interact primarily
with analytes via van der Waals forces (Figure 2.24).
Bonded-phase sorbents are stable to aqueous solvents over a pH range of
1 to 8.5, above which the silica backbone itself begins to dissolve and below
which the SiaC bond is attacked. Manufacturers have continued to extend
these ranges through improved products, and researchers have stretched
the limits of these restrictions. The development of bonded silica sorbents
led to a proliferation of pharmaceutical and environmental applications for
extracting semivolatile organics from aqueous solution.
The bonded phases produced by manufacturers vary according to the
nature of the silica used to prepare the bonded phase and in the reactants
and reaction conditions used. The variations are closely guarded, proprietary
manufacturing processes. However, it is generally known that the most
common commercially manufactured bonded-phase sorbents are based on
chemical reaction between silica and organosilanes via the silanol groups on
the silica surface to produce chemically stable SiaOaSiaC covalent linkages
to the silica backbone [75,87]. Nonpolar, polar, or ionic bonded phases can
be prepared by varying the nature of the organic moiety bonded to the silica
surface.
Bonded phases can be obtained as monomeric or polymeric coverage of
an organic ligand group, R, on the silica surface depending on whether a
monofunctional (R3SiX) or a trifunctional (RSiX3) reactant is used, respec-
NH 2
reversed-phase octadecyl (C 18 ) modified silica sorbent
Figure 2.24. Interactions between analytes and nonpolar bonded silica sorbents via van der
Waals forces.