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Encyclopedia of Physical Science and Technology EN008B-382 June 30, 2001 18:58

688 Liquid Chromatography

ease of preparation, wide choice of liquid phases, and high
sample capacity. However, the lifetime and reproducibil-
ity of these columns were often poor due to the gradual
stripping of the stationary phase by the mobile phase. Sat-
uration of the mobile phase with the stationary phase only
partially alleviated the problem. Therefore, bonded-phase
packings that have the liquid phase covalently attached to
the solid support were developed and now are almost ex-
clusively used. Reversed-phase packings are synthesized
by reaction of the desired organochlorosilane with the hy-
droxyl groups of porous silica to form a siloxane bond as

O CH 3 O CH 3
dry
toluene
Si H Cl Si R Si O Si R HCl,
N 2
O CH 3 O CH 3

where R often = CH 3 (CH 2 ) 3 ,CH 3 (CH 2 ) 7 ,CH 3 (CH 2 ) 17 ,
or phenyl. An organic base such as pyridine is often
added to neutralize the HCl produced and drive the reac-
tion to the right. Recently, sonication during the bonding
reaction has improved coverage. Ligand loading for a
2
C-18 column is about 2–4 µmol/m silica. Specialty
normal phase bonded silica packings such as amino
or cyano functionalized materials can be made in an
FIGURE 16 LSC separation of nitroaniline isomers on 10-µm
alumina, 15 cm × 2.4 mm column, 40% CH 2 Cl 2 in hexane mobile analogous fashion using 4-aminobutyltriethoxysilane and
phase, flow rate 1.7 ml/min, 1 µg of each isomer. [From Majors, 3-cyanopropyltriethoxysilane. In these reactions, ethanol
R. E. (1973). Anal. Chem. 45, 757. Reprinted with permission by
is produced and the silane reagent can potentially bond to
the American Chemical Society.]
three silica sites. However, if incomplete bonding results,
the remaining Si OCH 2 CH 3 moieties will hydrolyze to
greater the number of hydrophobic groups (CH 3 or CH 2 ) deleterious Si OH groups. Unfortunately, reactions of the
or the lower the number of hydrophilic groups, the greater surface hydroxyls only proceed to an extent of about 50%
the expected retention. It has been shown that the log of and the residual Si OH groups can hydrogen bond with

k /k for molecules A and B which differ by the CH 2 polar groups of solute molecules, causing peak tailing.
B A
group in structure is proportional to the surface tension of To partially alleviate this problem, trimethylchlorosilane
the mobile phase. However, the partitioning mechanism (TMCS), because of its smaller size, is reacted to “end-
of retention is also important as indicated by linear plots cap” many of the remaining hydroxyls. Kirkland and co-
of log k versus number of carbons for a homologous se- workers have found it is important to fully hydroxylate the

ries of solutes. As shown by Dill and Dorsey, the density silica packing before silanization to minimize the number
of the stationary phase cannot be too high to permit entry of isolated acidic silanols, which, in particular, cause peak
of the solute between the C-18 chains. tailing for basic solutes. The C-18 packing is probably the
A quantitative relationship between log k and fraction most widely used reversed-phase packing. Gilpin as well

of the nonpolar solvent B (usually water) in the mobile as others have shown the orientation of these bonded hy-
phase (φ B )isgiven by drocarbon chains changes as a function of temperature and
solvent. The shorter chain hydrocarbon packings as well

log k = log k − Sφ B ,
w
as phenyl silica are used when lower retention is desired.
where k = retention factor for solute in pure water, and Since siloxane bands are cleaved by strong acid, mobile

w
S = solventstrengthparameter(ameasureofnonpolarity). phase pH constraints between 2 and 8 remain a limitation
The earliest LLC packings were simply a solid support of silica-bonded phase packings.
such as silica coated with the liquid of choice such as Toimprovethelifetimeofbondedphasesilicapackings,
oxypropionitrile. The advantages of these packings were reactions with di- or trichloroorganosilanes have been