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C30F Analytical Chemistry Labscript

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Determination of Alcohols by Gas Chromatography

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 Resourcea
Chem. Dept. UWI. St. Augustine Campus