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0.5 g solid
sample
Pressing to
create a
compact column
bed
SPE-
column
1.5 g
sorbent
material
PE-frit
Homogenization
Nonpolar
solvent
Polar
solvent
Elution of the
concentrated
and
clean analytes
Elution of
polar
impurities
FIGURE 4.1
Extraction technique MSPD. (From Albert, K.,
On-Line LC-NMR and Related Techniques
,
John Wiley & Sons Ltd., 131, 2002. With permission.)
compounds such as carotenoids can be performed with the help of HPLC employing “reversed
phase” stationary phases. These materials are composed of
n
-alkylsilyl ligands covalently bound via
a Si-O-Si bonds to silica particles (diameter 3-5 mm, pore size 100-300 Å). Conventional reversed-
phase materials have an
n
-alkyl chain length of 18 carbons. These C
18
phases are not efi cient for
separating structural and stereo isomers of the many different carotenoids. Lane Sander developed a
tailor-made C
30
stationary phase where the separation of shape-constrained isomers can be achieved
(Sander et al. 1994). This C
30
phase exhibits a unique shape selectivity behavior due to a sophisticated
alkyl chain organization, Figure 4.2. Here, tight clusters of alkyl chains, extended in more crystalline
all-“
trans
-like” conformations alternate with more l uid clusters of alkyl chains exhibiting l exible
gauche conformations (Albert et al. 1998, Raitza et al. 2000). This “slot model,” outlined in Figure 4.3,
a
a
b
43 Å
a
≈ 32 Å
b
≈ 112 Å
FIGURE 4.2 (See color insert following page 336.)
Alkyl chain organization of a C
30
phase. (From
Raitza, M. et al.,
Investigating the Surface Morphology of Triacontyl Phases with Spin-Diffusion Solid-State
NMR Spectroscopy
, John Wiley & Sons Ltd., 3489, 2000. With permission.)
FIGURE 4.3 (See color insert following page 336.)
Slot model. (From Meyer, C. et al.,
Nuclear Magnetic
Resonance and High Performance Liquid Chromatography Evaluation of Polymer Based Stationary Phases
Immobilized on Silica
, Springer-Verlag GmbH, 686, 2005. With permission.)
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