Page 220 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: ZCK/MBQ P2: GQT Final Pages
Encyclopedia of Physical Science and Technology EN008B-382 June 30, 2001 18:58

Liquid Chromatography 687

TABLE III Properties of Solvents for HPLC a The usual order or elution of organic solutes is depen-
dent on the type of polar functional groups, number of
Viscosity at 25 C
◦
Solvent ε ob UV cutof (nm) [centipoise (cP)] groups, and orientation. A listing of functional groups

from low k (capacity factor which is proportional to re-

Pentane 0.0 195 0.22 tention) to high k follows: Alkane < olefins < aromatic
Isooctane 0.01 197 0.47 ≈ organic halides < sulfides < ethers < nitro compounds
Cyclohexane 0.04 200 0.90 < esters ≈ aldehydes ≈ ketones < alcohols ≈ amines
Carbon tetrachloride 0.18 265 0.90 < sulfones < sulfoxides < amides < carboxylic acids.
p-Xylene 0.26 290 0.60 As expected, this order roughly reflects the eluotropic se-
Toluene 0.29 285 0.55 ries in Table I. A greater number of polar groups will
Benzene 0.32 280 0.60 promote retention unless their close proximity permits in-
Ethyl ether 0.38 218 0.24
tramolecular hydrogen bonding.
Chloroform 0.40 245 0.53
One of the strengths of LSC is its ability to separate
Methylene chloride 0.42 233 0.41
isomers, particularly aromatics functionalized with polar
Tetrahydrofuran 0.45 212 0.46
groups, in the retention order ortho < meta < para. The or-
Acetone 0.56 330 0.30
tho compound is retained the least due to intramolecular
Ethyl acetate 0.58 256 0.43
hydrogen bonding. The meta functional groups can inde-
Aniline 0.62 310 3.8
pendently interact with the stationary adsorption sites but
Acetonitrile 0.65 190 0.34
not often at the same time. The para isomer is retained
Dimethylsulfoxide 0.75 268 2.0
longest because the two opposite functional groups can
Isopropanol 0.82 205 1.9
“sit down” on the adsorption surface and both simultane-
Ethanol 0.88 210 1.1 ously interact with the active sites. A chromatogram of
Methanol 0.95 205 0.54 nitroaniline isomers is shown in Fig. 16. As required for
Water large 191 0.90 all types of liquid chromatography, the sample must be
a Most of this data was taken with permission from a similar table in soluble in the mobile phase. Therefore, LSC is generally
Snyder, L. R., and Kirkland, J. J. (1979). “Introduction to Modern Liquid used for organic solvent extracts of solid or aqueous sam-
Chromatography,” Wiley, New York, p. 248. ples as well as characterization of product solutions from
b Eluotropic series for alumina (similar rank for silica). organic synthesis.
A classification of solvents to their ability to adsorb
B. Partition LC
on the stationary phase is called an eluotropic series
(Table III). Retention of solute is reduced with mobile Partition LC or liquid–liquid chromatography (LLC) in-
0
phase solvents of higher solvent strength parameter, ε . volves solvation of the solute molecules in the stationary
0
Solvent polarity parameters (φ) are similar to ε values. phase held by the packing or solid support. The versatility
Theycanbeusedtoestimatetheoverallpolarityofabinary of partition LC is due to the wide variety of possible sta-

solvent as P = φ A P + φ B P . For example, φ values for tionary phases. Partition LC, like adsorption LC, can be

AB A B
hexane, diethyl ether, tetrahydrofuran, ethyl acetate, ace- used in the normal phase mode but is more commonly em-
tonitrile, and water are respectively 0.1, 2.8, 4.0, 4.4, 5.8, ployed for reversed-phase LC which uses a nonpolar sta-
and 10.2. The solvent polarity P required for a desired k
tionary phase and a polar mobile phase. The retention be-

2 2

can be predicted from k and that solvent polarity P using tween solute and stationary phases can be due to hydrogen
1 1

the equation k /k = 10 (P 1−P 2)/2 . A two-fold change in bonding, dipole–dipole, and/or Van der Waal forces. Hy-

2
1

P results in a ten-fold change in k . One of the problems drogen bonding forces have been previously described for

of adsorption LC is that solvent impurities (particularly LSC. Dipole–dipole interactions are electrostatic in nature
water) in organic solvents can markedly affect solute re- due to the charge asymmetry of the solute and stationary
tention and cause nonreproducible chromatograms. To al- phases. Van der Waal forces, which dominate in reversed-
leviate this problem and also help reduce peak tailing, the phase HPLC, are interactions between hydrophobic or
mobile phase can be intentionally saturated with water. nonpolar groups of the solute and the liquid phase. Es-
Alternatively, addition of a polar organic solvent at less sentially, the water or miscible organic solvent molecules
than 1% will also work. Generally, alkanes with either exist in a high-energy state when adsorbed to the nonpolar
chlorinated, ether, or ester solvents as modifiers are used (C-18) derivatized silica surface. A larger aromatic solute
as mobile phases for LSC. Hexane modified with 50% molecule will preferentially displace many adsorbed sol-
methylene chloride and 0.1% isopropanol or acetonitrile vent molecules in an entropy driven process, resulting
is considered a good mobile phase to start with. in a lower energy state. A general rule of thumb is the