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

82 principles of extraction
a colored extract from leaves using column chromatography on a polar
calcium carbonate column [78,79]. The alternate system, in which the sorbent
is nonpolar while a polar solvent is used, was not used in chromatography
until the late 1940s to early 1950s [80–83]. Howard and Martin [83]
introduced the term reversed-phase to describe separation of fatty acids using
solid-supported liquid para‰n or n-octane as nonpolar stationary phases
that were eluted with polar aqueous solvents. At that time, these systems
appeared to be ‘‘reversed’’ to the ‘‘normal’’ arrangement of polar stationary
phases used with less polar eluents. Although reversed-phase applications
outnumber normal-phase chromatographic applications today, the nomenclature
still applies.
The most common polar sorbents used for normal-phase SPE are silica
(SiO2)x, alumina (Al2O3), magnesium silicate (MgSiO3 or Florisil), and the
bonded silica sorbents in which silica is reacted with highly polar functional
groups to produce aminopropyl [(SiO2)xa(CH2)3NH2]-, cyanopropyl
[(SiO2)xa(CH2)3CN]-, and diol [(SiO2)xa(CH2)3OCH2CH(OH)CH2(OH)]modified
silica sorbents (Figure 2.21). Polar SPE sorbents are often used to
S i l i c a
C
N
O
H
(a) cyanopropyl-modified silica sorbent (b) silica sorbent
S i l i c a
O
OH
S i l i c a
O
O
(c) diol-modified silica sorbent
Figure 2.21. Interactions between analytes and polar sorbents via dipolar attraction or hydrogen
bonding.
H
H
H
O
H
N
H