Liquid-Liquid Chromatography
Liquid-liquid chromatography is a chromatography separation technique in which the mobile phase is a liquid (usually a solvent or a simple binary solvent mixture) and the stationary phase is also a liquid (which must be immiscible and insoluble in the liquid mobile phase). The liquid stationary phase is supported on some suitable material such as a diatomaceous earth or under certain circumstances silica gel. The system is inherently unstable, as the stationary phase will always have some solubility in mobile phase and, as a consequence, will eventually be stripped from the support. The first liquid-liquid system was reported by A. J. P. Martin who used water supported on silica gel as the stationary phase and n-heptane as the mobile phase. To avoid the instability of liquid-liquid systems, the bonded phases were developed which are strictly liquid-solid systems, but as the bonded moieties are very large, behave in a very similar manner to liquid-liquid systems. As the absorption isotherms of liquid-liquid systems are linear up to relatively high solute concentrations, relatively large loads can be applied to liquid-liquid columns. Liquid-liquid systems, as such, are not in common use in modern liquid chromatography.
main text
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Introduction
chromatography is based on the
physical nature of the mobile phase. The mobile phase can be
a gas or a liquid which gives rise to the two basic forms of
chromatography, namely, gas chromatography (GC) and liquid
chromatography (LC). The stationary phase can also take two forms,
solid and liquid, which provides two subgroups of GC and LC, namely;
gas–solid chromatography (GSC) and gas–liquid chromatography (GLC),
together with liquid solid chromatography (LSC) and liquid
chromatography (LLC). The different forms of chromatography are
summarized in Table 1. Most thin layer chromatography techniques are
considered liquid-solid systems although the solute normally
interacts with a liquid-like surface coating on the adsorbent or
support or, in some cases an actual liquid coating.
Table 1 The
Classification of Chromatography
chromatography systems
Principles Introduction
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Introduction
Stoll (2) in 1913. Willstatter
and Stoll repeated Tswett's experiments without heeding his warning not to use
too "aggressive " adsorbents as these would cause the chlorophylls to decompose.
As a consequence, the experiments of Willstatter et al. failed and their
published results, rejecting the work of Tswett, impeded the recognition of
chromatography as a useful separation technique for nearly 20 years.
In the late
1930s and early 1940s Martin and Synge introduced a form of liquid-liquid
chromatography by supporting the stationary phase, in this case water, on
silica gel in the form of a packed bed and used it to separate some acetyl
amino acids. They published their work in 1941 (3) and in their paper
recommended the replacement of the liquid mobile phase with a suitable gas
which would accelerate the transfer between the two phases and provide more
efficient separations. Thus, the concept of gas chromatography was born.
In the same paper in 1941, Martin and Synge suggested the
HPLC Introduction
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Introduction
costly and complex
instrument, and at the other, on a simple, inexpensive thin layer plate.
The first scientist to recognize chromatography as an efficient method
of separation was the Russian botanist Tswett (1), who used a simple form of
liquid-solid chromatography to separate a number of plant pigments. The colored
bands he produced on the adsorbent bed evoked the term chromatography for this
type of separation (color writing). Although color
has little to do
with modern chromatography, the name has persisted and, despite its
irrelevance, is still used for all separation techniques that employs the
essential requisites for a chromatographic separation,viz. a mobile
phase and a stationary phase.
The technique,
as described by Tswett was largely ignored for a along time and it was not
until the late 1930s and early 1940s that Martin and Synge(2) introduced
liquid-liquid chromatography by supporting the stationary phase, in this case
Principles Introduction
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Applications
and
in many cases tend to make the TLC system more like a liquid chromatograph. The
great advantage of TLC is its low cost and its relatively high separating
capability. If the required performance required is at the limit or beyond
the capability of the technique, there is no point in trying to stretch it. The
rational solution for the chemist or analyst would be to change to an alternative procedure such as
liquid chromatography or to some other technique if more appropriate.
Chromatography Applications
Gas
chromatography has an entirely different field of applications to that of liquid
chromatography. In general, gas chromatography is used for the separation of
volatile materials and liquid chromatography for the separation of involatile
liquids and solids. There are certain compounds, however, that can be separated
with either techniques, and more importantly, many involatile substances such
as amino acids, steroids and high molecular eight fatty acids
Principles Applications
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Introduction
the technique of chromatography,
originally invented by Tswett in the latter part of the nineteenth century, was
not initially developed for analytical purposes, but for the isolation of some
specific pigments from plant extracts. In fact, all the early applications of
chromatograph were exclusively for preparative purposes and it was not until
gas chromatography (GC) was introduced by Martin and Synge (1) was the
technique used for analytical purposes. Even after the introduction of GC,
liquid chromatography (then called column
chromatography) was still used largely for preparative work. Liquid
column chromatography evolved from a preparative procedure into an analytical
technique during the late nineteen sixties, largely provoked by the development
of high performance liquid chromatography
(HPLC), which, in turn, was largely
sparked off by the successful development of GC. Initially, column loads were
increased for preparative purposes by increasing the dimensions of the column
both
Preparative Introduction
Author: RPW Scott
Book:Plate Theory and Extensions
Section:Plate-Theory Derivation
the
value of (K), the more the solute will be distributed in the stationary
phase under equilibrium conditions. (K) is a dimensionless constant and, in
gas/liquid and liquid/liquid systems, (Xs) and (Xm) can
be measured as mass of solute per unit volume of phase. In gas/solid and
liquid/solid systems, (Xs) and (Xm) can be measured as mass
of solute per unit mass of phase.
Equation (1)
reiterates the general distribution law and presumes the adsorption isotherm as
linear. In both gas/solid chromatography (GSC) and liquid/solid chromatography
(LSC), virtually all the solutes exhibit Langmuir type isotherms between the
two phases which, over a wide concentration range, is certainly not
linear. However, at the extremely low solute concentrations employed in
chromatography, (i.e., that portion of the isotherm that is pertinent)
the isotherm can be considered as linear
Plate-Theory Derivation