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Revised labscript :
1. Introduction
Gas chromatography (GC) is one type of partition chromatography. It is similar in many ways to other techniques of this kind,
such as high -performance liquid chromatography (HPLC), paper chromatography etc. The distinguishing features are that the
mobile phase is a gas, and that the motion of the component bands, in the direction of "chromatographic development",
involves the forced diffusion of the respective substances in their vapor phases (1, 2).
The chromatographic technique requires that a solute undergo distribution between two phases, one of them fixed (stationary
phase) and the other moving (mobile phase). It is the mobile phase which transports the solute down the column, until it
eventually elutes from the end of the column, separated from the other solutes which elute earlier or later. If the liquid phase
does not preferentially dissolve molecules with certain functional groups, the order of elution is most volatile to least volatile
(order of increasing boiling point). One would expect this order to be always observed with molecules of a homologous series
and with structural isomers involving the same functional groups. They therefore display a relatively high selectivity toward
these molecules and are especially well suited for analysis of such mixtures.
The basic apparatus required to achieve gas chromatographic separations is quite simple, in sharp contrast to some of the
highly sophisticated commercial equipment (3, 4). A supply of carrier gas (helium or nitrogen) is usually available in
compressed form, in a cylinder fitted with a suitable pressure reducing valve connected to the sample injection port. Since
solutes to be chromatographed must be in the vapor phase, the injection port is heated to a temperature T1 which will ensure
rapid vaporization but not thermal degradation of the solute. Liquid samples are usually injected by syringe through a silicone
rubber diaphragm (septum) in the injection port. The solute vapor mixes nearly instantaneously with the flowing carrier gas and
is swept into the column. The column is the heart of the chromatograph. It is here that the different solutes in the vaporized
samples are separated from each other by virtue of their different interaction with the column packing. The column may be
maintained at a selected temperature, T2 (isothermal), which determines the time for passage of the solutes, and also
determines, to a degree, the resolution and efficiency obtained with the particular column.
However, the temperature of the column may also be increased at pre-determined rates (temperature-programmed), to improve
the separation of the solute molecules. As the solutes emerge individually from the column they enter the detector, a device
which generates a signal corresponding to the amount of solute leaving the column. The time at which the peak maximum is
detected is also measured (retention time), which is characteristic for the particular experimental conditions being employed.
The detector signal is amplified and fed to a suitable recording device, e.g. a recorder or integrator, which records a signal-time
plot, to identify and measure the various components and their respective concentrations.
2. Experimental Procedure
(A) Preparation of mixed alcohol standards:
Pipette 2.0mL each of methanol, ethanol, propan-1-ol, and butan-1-ol into the same 25 ml volumetric
flask and make up to the mark with distilled water
(B) Preparation of ethanol series:
(a) Pipette into four 25mL volumetric flasks, 1.0 ml of pure propan-1-ol.
(b) Into these flasks add 1.0, 2.0, 3.0, and 4.0 mL respectively of pure ethanol and make up to the mark
with distilled water.
(c) Prepare in duplicate a solution containing 2.5mL of an unknown spirit sample (provided by laboratory
technician) and 1.0mL of pure propan-1-ol in a 25mL volumetric flask, and make up to the mark with
distilled water.
Repeat (c) for a sample containing unknown ethanol content (provided by student).
(C) Gas Chromatography:
(a) Mixed alcohols:
(i) Inject 2uL of the mixed alcohols, prepared in (a) and obtain the chromatogram of peak areas and
retention times of the separated components.
(ii) Construct a graph of log10 of the relative retention (EtOH=1) vs carbon number. Comment on the
shape of this graph.
(b) Ethanol analysis:
(i) Injection 2uL each of the ethanol calibration series prepared in (b), followed by the unknowns.
(ii) For each ethanol standard, as well as the unknowns, calculate the peak ratio of
ethanol (analyte) : propan-1-ol (internal standard).
(iii) Construct a calibration curve of peak area ratios of ethanol : propan-1-ol vs ethanol concentrations.
(iv) Determine the ethanol concentration of the unknown samples by interpolation from the calibration
curve.
3. Exercises:
(a) Calculate the efficiency of the column, using the butan-1-ol peak and compare it with that of the ethanol peak.
(b) Calculate the relative retention of ethanol compared to butan-1-ol.
(c) Calculate the resolution using the methanol and ethanol peaks. Comment on the value obtained.
(d) Calculate the percentage ethyl alcohol by volume in both of the unknown spirit samples.
4. References
1. Grob, R.L.Modern Practice of Gas Chromatography, N.Y. John Wiley & Sons Inc. (1977)
2. Krugers, J., Instrumentation in Gas Chromatography, Centrex Publishing Co. -Endhoven (1968)
3. The Practice of Gas Chromatography, Rowland, F. W., 2nd, Ed., Hewlett-Packard <1974)
4. High Resolution Gas Chromatography, Freeman, R.R. (Ed.) 2nd, Ed., Hewlett-Packard (1981)
anal-chem resources
Chem. Dept. UWI. St. Augustine Campus
Original labscript :
Determination of Alcohols : Gas Chromatography
Introduction
Gas chromatography (GC) is one type of partition chromatography; it is
similar in many ways to other techniques of this kind, such as HPLC
(high -performance liquid chromatography), paper chromatography etc. The
distinguishing features are that the mobile phase is a gas, and that
the motion of the component bands, in the direction of "chromatographic
development", involves the forced diffusion of the respected substances
in their vapor phases.
The chromatographic technique requires that a solute undergo
distribution between two phases, one of them fixed (stationary phase)
and the other moving (mobile phase). It is the mobile phase which
translates the solute down the column until it eventually elutes from the end
of the column separated from the other solutes which elute earlier or
later. If the liquid phase does not preferentially dissolve molecules
with certain functional groups, the order of elution is most volatile
to least volatile (order of increasing boiling point). One would expect
this order always to be observed with molecules of a homologous series
and with structural isomers involving the same functional groups. They
therefore display a relatively high selectivity toward these molecules
and are especially well suited for analysis of mixtures containing
them.Basic Apparatus
The basic apparatus required to achieve gas chromatographic separations is quite simple, in sharp contrast to some of the highly sophisticated commercial equipment. A supply of carrier gas (helium, nitrogen) usually available in compressed form in a cylinder fitted with a suitable pressure reducing valve, is conducted to the sample injection port. Since solutes to be chromatographed must be in the vapor phase, the injection port is heated to a temperature T1 which will ensure rapid vaporization but not thermal degradation of the solute.
Liquid and gas
samples are almost always injected by syringe through a silicone rubber
diaphragm (septum) in the injection port. The solute vapor mixes nearly
instantaneously with the flowing carrier gas and is swept into the
column. The column is the heart of the chromatograph. It is here that
the dfferent solutes in the vaporized samples are separated from each
other by virtue of their different interaction with the column packing.
The column must also be maintained at a selected temperature, T2, which
determines the time for passage of the solutes, and also determines, to
a degree, the resolution and efficiency obtained with the particular
column.
As the solute emerge individually from the column they enter the
detector; a device which supplies a signal corresponding to the amount
of solute leaving the column, as well as serving to indicate the time
or volume to the peak maximum which is characteristic for the
particular experimental conditions being employed. The detector signal
is supplied to a suitable recording device, e.g. a recorder or
intergrator, which records a signal-time plot to identify and evaluate
the various components and their concentrations.
Procedure
1. Pipette 5.0 ml each of methanol, ethanol, propan-1-ol, and butan-1-ol
into the same 100 ml volumetric flask and make up to the mark eith
distilled water
2. Prepare an ethanol series as follows:
(a) Pipette into four 50 ml volumetric flasks 2.0 ml of pure
propan-1-ol.
(b) To each flask add 1.0, 2.0, 3.0, and 4.0 mls of pure ethanol
respectively.
(c) Make up to the mark with distilled water.
3. Prepare in duplicate a solution containing 5.0 mls of an unknown
spirit sample (see lab Tech) and 2.0 mls of pure propan-1-ol in a 50 ml
volumetric flask, and make up to the mark with distilled water.
Repeat (3) above for a known spirit sample (one that you
brought).
4. Obtain chromatograms by injection of the calibration series
followed by the unknowns at optimum instrument settings.
For the chromatogram of the four alcohols obtained at optimum
conditions, construct a graph of log 10 of the relative
retention (EtOH=1) against carbon No. Comment on the shapes
obtained.
Exercises
Calculate the efficiency of the column using the butan-1-ol peak and
compare it wit that obtained for the ethanol peak.
Calculate the relative retention of ethanol compared to
butan-1-ol.
Calculate the resolution using the methanol and ethanol peaks. Comment
on the value obtained.
Calculate the percentage ethyl alcohol by volume in both the known and
unknown spirit samples using propan-1-ol as internal
standard.
References
Grob, R.L.Modern Practice of Gas Chromatography, N.Y. John Wiley &
Sons Inc. (1977)
Krugers, J., Instrumentation in Gas Chromatography, Centrex
Publishing Co. -Endhoven (1968)
The Practice of Gas Chromatography, Rowland, F. W., 2nd, Ed.,
Hewlett-Packard <1974)
High Resolution Gas Chromatography, Freeman, R.R. (Ed.) 2nd, Ed.,
Hewlett-Packard (1981)
anal-chem resources
Chem. Dept. UWI. St. Augustine Campus.
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