Page 246 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

8 COLUMN AN0 THIN4AVER LlilUlO CHROMATOCRAPHV

Table 81 Typical stationary and mobile phases for normal and reverse phase chromatography
Stationary phases Mobile phases
-
Normal
j, j'-Oxydipropionitrile Saturated hydrocarbons, e.g. hexane, heptane; aromatic solvents,
Carbowax (400, 600, 750, etc.) e.g. benzene, xylene; saturated hydrocarbons mixed with up to
Glycols (ethylene, diethylene) * 10 per cent dioxan, methanol, ethanol, chloroform, methylene
Cyanoethylsilicone chloride (dichloromethane)
Reverse-phase
Squalane Water and alcohol-water mixtures; acetonitrile and
Zipax-HCP acetonitrile-water mixtures
Cyanoethylsilicone

Although the stationary and mobile phases in LLC are chosen to have as
little solubility in one another as possible, even slight solubility of the stationary
phase in the mobile phase may result in the slow removal of the former as the
mobile phase flows over the column support. For this reason the mobile phase
must be pre-saturated with stationary phase before entering the column. This
is conveniently done by using a pre-column before the chromatographic column;
the pre-column should contain a large-particle packing (e.g. 30-60 mesh silica
gel) coated with a high percentage (30-40 percent) of the stationary phase to
be used in the chromatographic column. As the mobile phase passes through
the pre-column it becomes saturated with stationary phase before entering the
chromatographic column.
The support materials for the stationary phase can be relatively inactive
supports, e.g. glass beads, or adsorbents similar to those used in LSC. It is
important, however, that the support surface should not interact with the solute,
as this can result in a mixed mechanism (partition and adsorption) rather than
true partition. This complicates the chromatographic process and may give
non-reproducible separations. For this reason, high loadings of liquid phase are
required to cover the active sites when using high surface area porous adsorbents.
It is appropriate here to refer to ion-pair chromatography (IPC) which is
essentially a partition-type process analogous to the ion-association systems
used in solvent extraction (see Section 6.5). In this process the species of interest
associates with a counter ion of opposite charge, the latter being selected to
confer solubility in an organic solvent on the resulting ion pair. The technique
can be used for a wide variety of ionisable compounds but particularly for those
yielding large aprotic ions, e.g. quaternary ammonium compounds, and for
compounds such as amino acids which are difficult to extract in the uncharged
form. The stationary phase consists of an aqueous medium containing a high
concentration of a counter ion and at an appropriate pH, typical support
materials being cellulose, diatomaceous earth and silica gel. The mobile phase
is generally an organic medium having low to moderate solvating power. The
application of IPC is well illustrated by the separation of sulpha drugs on a
microparticulate ~ilica;~~ the stationary phase contains 0.1 M tetrabutylammonium
sulphate and is buffered at pH 9.2, and a butanol-hexane (25:75) mobile phase
is used. A useful advantage of the ion-pair technique is the possibility of selecting
counter ions which have a high response to specific detectors, e.g. counter ions
of high molar absorptivity like bromothymol blue, or highly fluorescent
anions such as anthracene sulphonate.