Carrier gas
The term carrier gas was introduced by A. J. P .Martin, the inventor of gas chromatography who used it as an alternative term for the mobile phase; obviously, the term could only be used as such in gas chromatography. The term has persisted and is still used synonymously for a gaseous mobile phase. The carrier gas can be any inert gas (a gas that can not react with either the solutes or the stationary phase) such as helium, argon, nitrogen, and under certain circumstances hydrogen. The gas that is used is sometimes dictated by the detector, e.g., the argon detector requires argon to be used as the carrier gas. With no other restrictions, helium is the most popular carrier gas used in gas chromatography although it is the most expensive. Helium has a low density and solutes have a high diffusivity in it. Thus, high gas velocities can be used providing faster analyses without seriously denigrating the performance of the column.
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors General-Properties
Flow programming, attempts to
achieve the same result as temperature programming which is to accelerate the
strongly retained peaks through the column (see
Gas Chromatography). Some detectors require
no other gas than that used as the carrier gas, other require specific gases to
be added to the columns eluent for them to function. In some cases the detector
prescribes a certain gas to be used as the carrier gas (e.g., the sensitivity of the katharometer is greater when helium is
used as the carrier gas). In addition, if the gas chromatograph is being used
for permanent-gas analysis, then helium must be used to differentiate the
carrier gas from the other gases being analyzed.
All gas chromatographs are designed to
operate over relatively wide ranges of temperature (e.g., -20oC
to 400oC). Consequently, to avoid solute condensation in the
detector or detector-connecting tubes, the detector should be capable of
operating at least 20oC higher than the maximum column
GC-Detectors General-Properties
Author: RPW Scott
Book:Gas Chromatography
Section:YES Preparative-Gas-Chromatography Moving-Bed-System
relative rates of upward carrier gas flow and downward
stationary phase flow (contained on the falling support) some components were
arranged to move upward with the carrier gas, and others move downwards with
the stationary phase. Referring to figure 39, if the ordinary chromatogram of
the mixture is that depicted at (A), the relative speed of the carrier gas and
the stationary phase defines an imaginary line on the chromatogram. Those
components to the left of the line, move up with the carrier gas (B) and those
components to the right of the line, move down with the stationary phase (C).
The components that move down in the stationary phase are stripped out by
arranging a portion of the column to be heated and a second stream of gas
elutes them through a second port (D). Scott and Maggs designed a three stage
moving bed system to extract pure benzene from coal gas. Coal gas contains a
range of saturated aliphatic hydrocarbons, alkenes, naphthenes and aromatics
(benzene
YES Preparative-Gas-Chromatography Moving-Bed-System
Author: RPW Scott
Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem GC-IR
Subsequent to the IR spectrum being obtained, a small sample of the vapor was drawn from the IR cell into a low-resolution mass spectrometer and the mass spectrum was also taken.
This system was not a tandem system but, in fact, the first triplet instrument to be reported (GC/IR/MS). The layout of the pneumatic system of the triplet instrument is shown in figure 30. The procedure for analyzing a peak was as follows. As the peak started to elute it was sensed by the detector and the exit carrier gas diverted through the IR cell into a packed trap which concentrated the peak onto the front of the trap packing. After peak elution was complete, the flow of carrier gas was stopped and the solute regenerated back into the IR cell by heating the trap in a secondary stream of nitrogen.
Figure 30. Diagram of an Automatic GC/IR Tandem System
GC-Tandem GC-IR
Author: RPW Scott
Book:Gas Chromatography Detectors
Section:GC-Detectors Simple-Gas-Density-Balance
the
katharometer can be used in most GC analyses that utilize packed columns and
where there is no limitation in sample availability. The device is simple,
reliable, rugged and relatively inexpensive. An example of the use of a
katharometer to monitor the separation of various compounds of hydrogen, deuterium and tritium, employinggas solid chromatography is shown in figure 14. The stationary
phase was activated alumina [treated with Fe(OH)2], and the column
was 3 m long and 4 mm I.D. The carrier gas was neon, the flow rate 200 ml/min
(at atmospheric pressure) and the column temperature was -196oC.
The Simple Gas Density Balance
The original gas density balance has already
been described. It was complicated, difficult to fabricate and its manufacture
was notacommercialsuccess. Intheearlydays of chromatography
GOW-MAC developed some elegantly designed filaments for use in the construction
of katharometers, which, in due course, were used in many other manufacturer's
GC-Detectors Simple-Gas-Density-Balance
Author: RPW Scott
Book:Gas Chromatography
Section:YES Preparative-Gas-Chromatography
or condensed from
the gas stream which is also difficult to achieve efficiently. Finally, the
efficient packing of large GC columns is difficult. Nevertheless, preparative
GC has been successfully used in a number of rather special applications; for
example the isolation of significant quantities of the trace components of
essential oils for organoleptic assessment.
The layout of a
preparative gas chromatograph is shown in figure38
Figure 38 Layout of
Preparative Gas Chromatograph
Air can not
normally be used as the mobile phase due to likely oxidation and so either a
gas tank or a gas (e.g., nitrogen) generator must be used. As the flow
rates can be large, more than one generator operating in parallel will often be
necessary. The sample is usually placed on to the column with a syringe pump
and rapidly vaporized in a suitable heater. Passing the gas in vapor form onto
the column helps evenly distribute the sample radially across the column
YES Preparative-Gas-Chromatography
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Column-Switching-Techniques Back-flushing-Techniques
After the solutes of interest have been eluted, the valve is rotated, and the connection of the column inlet to the sampling system is now directed to waste. At the same time the column exit is disconnected from the detector and connected to a separate carrier gas supply that forces a backward flow of carrier gas through the column and the strongly retained solutes are eluted to waste. As suggested above, to accelerate the purging process, the column temperature can be raised. When back flushing is complete, the valve is returned to the sampling position and the column temperature brought back to the initial conditions for analysis.
The sampling stage of the back flushing technique shown in figure 20 and depicts the sample passing through the valve to the column and from the column back to
Capillary Column-Switching-Techniques Back-flushing-Techniques