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

84 principles of extraction
physically adsorbed water, dehydrated oxide, geminal silanol, and bound
and reactive silanol. Porous silica consists of a directly accessible external
surface and internal pores accessible only to molecules approximately less
than 12,000 Da [86]. Pesek and Matyska [87] have reviewed the chemical
and physical properties of silica.
Silica particles used for SPE sorbents are typically irregularly shaped, 40
to 60 mm in diameter. Silica particles used for sorbents in high-performance
liquid chromatographic (HPLC) columns are generally spherical and 3 to
5 mm in diameter. Due to the di¤erences in size and shape, SPE sorbents
are less expensive than HPLC sorbents. Much greater pressures are required
to pump solvents through the smaller particle sizes used in HPLC.
Apolar Polymeric Resins
Synthetic styrene–divinylbenzene and other polymers, particularly the trademarked
XAD resins developed by Rohm & Haas, were used for SPE in the
late 1960s and early 1970s. However, the particle size of the XAD resins
is too large for e‰cient SPE applications, and therefore the resins require
additional grinding and sizing. Also, intensive purification procedures are
needed for XAD resins [73,75].
In the latter half of the 1990s, porous, highly cross-linked polystyrene–
divinylbenzene (PS-DVB) resins with smaller, spherical particle sizes more
suitable for SPE uses became available (Figure 2.23). The new generation of
apolar polymeric resins is produced in more purified form, reducing the level
of impurities extracted from the sorbent. Polymeric resins are discussed in
more detail by Huck and Bonn [69], Fritz [73], Thurman and Mills [75], and
Pesek and Matyska [87].
The enhanced performance of PS-DVB resins is due to their highly
hydrophobic character and greater surface area as compared to the bonded
silica sorbents, which are discussed in the following section. The strong
sorption properties of PS-DVB resins may arise from the aromatic, poly-
Figure 2.23. Cross-linked styrene–divinylbenzene
copolymer.
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