Gas chromatography of alcohols

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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 integrator, 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

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)

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