Question

Explain how liquids can be analyzed by gas chromatography.

Briefly define or describe the function, in gas chromatography, of the

carrier gas:

stationary liquid phase:

solid support:

If ethyl acetate and n-butyl acetate are analyzed by gas chromatography, which of these esters will generally produce a peak with the shorter retention time?

What operating variables determine retention time?

Explain why is it important to inject the sample quickly.

Answer 1

1) Gas chromatography is the technique used to separate a mixture of volatile liquids by vaporizing a mixture of volatile liquids and injecting onto a column. The column contains a stationary liquid phase, usually a polymer, on a solid support. The liquid is packed in a glass or stainless steel tube called a column. The mixture of liquids traverses through the liquid stationary phase. A mobile gas is used to carry these liquids through the stationary phase.

It must be remembered that the liquids in the mixture must have different boiling points; else the technique doesn’t provide satisfactory results.

The liquids in the mixture are separated based on the differences in their volatility (i.e, depends on the boiling points) and are analysed by a detector.

2) Carrier Gas: A mixture of two or more volatile liquids can be easily separated by gas chromatography. In gas chromatography, the liquid mixture is injected on to a liquid stationary phase and a carrier gas carries the liquids through the column. The liquids in the mixture interact with the particles in the stationary phase. The carrier gas is the mobile phase and “pulls” the liquid molecules from the stationary phase and transports them to the detector. The stronger the interactions between the liquid molecules and the particles of the stationary phase, the more is the time taken by the carrier gas to transport the molecules and thus, longer is the retention time.

The carrier gas used in gas chromatography must be cheap and easily available. Moreover, the carrier gas must be thermally stable and high temperatures and should be inert, i.e, the carrier gas doesn’t interact with the liquids in the mixture. Common carrier gases are helium, nitrogen, argon, etc.

Stationary Liquid Phase: The stationary liquid phase is an important component in gas chromatography. The liquids interact with the particles in the stationary phase. Depending on the strength of the interactions between the particles of the liquid and the stationary phase, the liquid molecules are partitioned and separated.

Gas chromatography works on the basis of the differences in volatility of compounds in the mixture. The volatility of the compounds in the mixture depends on the boiling points and hence, the polarities of the compounds. Thus, the polarity of the stationary phase is an important criterion in gas chromatography. Usually, the liquid stationary phase is so chosen that the polarity of the stationary phase is similar to the polarity of the compounds being analysed (“like dissolves like” rule). Common stationary phase liquids are polymers like dimethyl siloxanes, polyethylene glycols, etc.

Solid Support: The stationary liquid phase in gas chromatography is placed into a solid support. This solid support is usually a glass or steel tube, mostly coiled and having a small internal diameter. There are two types of columns. Packed columns have immobilized liquid on the solid support. Capillary columns are more common and contain long hollow tubes with the liquid immobilized on the surface of the support.

3) Ethyl acetate and n-butyl acetate can be separated by gas chromatography due to differences in their volatilities. The volatility of a compound depends on its boiling point. The higher the boiling point of a compound, the less volatile is the compound.

The boiling point of a compound depends on the molecular weight of the compound. The higher the molecular weight of a compound, the higher is the boiling point. n-butyl acetate has a higher molecular mass than ethyl acetate and thus, n-butyl acetate will have a higher boiling point. Consequently, n-butyl acetate is less volatile and will be retained more strongly by the liquid stationary phase. Thus, ethyl acetate will be eluted first, i.e, have a shorter retention time and n-butyl acetate will have a longer retention time.

4) The sample must be injected onto the injector quickly so that the sample band is small, i.e, a broad dispersed sample band is not observed. Moreover, solvent effects are minimized when the sample is injected onto the column quickly.