Gas Liquid Chromatography
Gas-liquid chromatography (GC) was in invented by James and Martin and is a chromatography separation technique in which the mobile phase is a gas (usually helium or nitrogen) and the stationary phase is a liquid. In the original columns used by James and Martin, the liquid stationary phase was adsorbed on the surface of an inert support such as Celite (a diatomateous earth) or calcined Celite (a form of brick dust). The support was usually deactivated before use by acid treatment and subsequent reaction with hexamethyldisilazane. The technique was extensively used for the separation of a wide range of volatile substances. The packed column, however, had a high flow impedance, which limited the column length that could be used and, consequently, the column efficiency and the resolution that could be obtained. The packed columns were eventually replaced by the capillary columns in which the mobile phase was coated on the walls of an open tube. The tubes could be 50-500 micron ID and from 10 m to several 100 m long. Thus, very fast or very efficient columns could be employed. Gas liquid chromatography is now a very popular technique and used by almost every analytical laboratory.
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Introduction
phase,
the primary classification of 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:Gas Chromatography
Section:YES Preparative-Gas-Chromatography
Preparative Gas Chromatography
Gas
chromatography has not been used extensively for preparative work although its
counterpart, liquid chromatography, has been broadly used in the pharmaceutical
industry for the isolation and purification of physiologically active
substances. There are a number of unique problems associated with preparative
gas chromatography. Firstly, it is difficult to recycle the mobile phase and
thus large volume of gas are necessary. Secondly, the sample must be fully
vaporized onto the column to ensure radial distribution of the sample across
the column. Thirdly, the materials of interest are eluted largely in a very
dilute form from the column and therefore must be extracted or condensed from
the gas stream which is also difficult to achieve efficiently. Finally, the
efficient packing of large GC columns
YES Preparative-Gas-Chromatography
Author: RPW Scott
Book:Gas Chromatography
Section:YES Introduction
Introduction
Chromatography,
in one of its several forms, is the most commonly used procedure in
contemporary chemical analysis and the first configuration of chromatography
equipment to be produced in a single composite unit and made commercially
available was the gas chromatograph. Gas chromatography was invented by A. J.
P. Martin who, with R. L. M. Synge, suggested its possibility in a paper on
liquid chromatography published in 1941 (1). Martin and Synge recommended that
the liquid mobile phase used in liquid chromatography could be replaced by a
suitable gas. The basis for this recommendation was that, due to much higher
diffusivities of solutes in gases compared with liquids, the equilibrium
processes involved in a chromatographic process (see
Principles and Practice of Chromatography) would be much
faster and thus, the columns much more efficient and separation times much
shorter. So the concept of gas chromatography was envisioned more than
fifty years
YES Introduction
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Introduction
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 use of small
particles and high pressures in LC to improve the separation which proved to
the critical factors that initiated the development of high performance
liquid chromatography(HPLC).
"Thus, the smallest H.E.T.P. (the highest
efficiency) should be obtainable by using
HPLC Introduction
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Introduction
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
water, on silica in a packed bed and used it to separate some acetyl amino
acids. In their paper, they recommended replacing the liquid mobile phase by a
suitable gas, as the transfer of sample between the two phases would be faster,
and thus provide more efficient separations. In this manner, the concept of gas
chromatography was created but again, little notice was taken of the
suggestion and it was left to Martin himself and A. T. James to bring the concept
to practical reality nearly a decade later. In the same publication in 1941,
the essential requirements for HPLC (High Performance Liquid Chromatography
Principles Introduction
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Applications
indeed improve the performance of the TLC analysis but are also expensive 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
Principles Applications