Gas/Liquid Chromatography (Cambridge (CIE) A Level Chemistry): Exam Questions

Column chromatography is a variation of thin-layer chromatography. Column chromatography and gas chromatography work by the same principles of components travelling through the column at different rates. 

State the difference between the mobile phases involved in column chromatography and gas-liquid chromatography.

Three compounds, A, B and C, of similar volatility, are mixed together. The mixture is then analysed in a gas chromatograph.

The gas chromatogram produced is shown in Fig. 1.1.

Fig. 1.1 

State which compound, A, B or C, has the greatest affinity for the solid phase. Explain your reasoning. 

Use the gas chromatogram shown in Fig 1.1 to identify the most abundant compound in the mixture. Explain your answer.

An oil tanker is travelling through the English Channel. The tanker has a slight leak which is not large enough to result in an oil slick but some oil is noticed on a beach.

Suggest how gas chromatography could be used to identify the tanker as the source of crude oil pollution on the beach. 

A mixture of dipeptides can be analysed using gas chromatography, coupled with mass spectrometry.

Describe how each method contributes to the analysis.

Define the term retention time.

A mixture of two dipeptides, shown in Fig. 2.1, is analysed by gas chromatography followed by mass spectrometry.

Fig. 2.1

Explain why the mixture of dipeptides can be separated by gas chromatography.

A mixture is analysed using gas chromatography. The gas chromatogram is shown in Fig. 3.1.

Fig. 3.1

Identify the compound that has the greatest affinity for the solid phase. Justify and explain your answer.

Identify the most abundant compound in the sample. Justify your answer using the gas chromatogram in Fig. 3.1 in part (a).

An oil tanker is travelling through the English Channel. The tanker has a slight leak which is not large enough to result in an oil slick but some oil is noticed on a beach.

Suggest how gas chromatography could be used to identify the tanker as the source of crude oil pollution on the beach.

Gas chromatography is often connected to mass spectroscopy in order to analyse each compound as it exits the gas chromatography column. This combined technique is called GC-MS.

The gas chromatogram of an organic mixture is shown in Fig. 1.1. The stationary phase is a polar, high-boiling point liquid on a solid support.

Fig. 1.1

i) Name an appropriate mobile phase that could be used to form this gas chromatogram. 

[1]

 ii) Identify the number of compounds present in this mixture and explain how to identify the most polar compound present. 

[2]

Two of the compounds in the organic mixture are hydrocarbons with similar retention times.

The mass spectra of the two compounds revealed that they have the same molecular formula but with subtle differences indicating structural isomerism is present.

The mass spectrum of one of the isomers is shown in Fig. 1.2.

Both isomers are tested with bromine water, which remains orange.

Fig. 1.2

i) Identify the molar mass and hence the molecular formula of the isomers.

 molar mass ................................................................................

 molecular formula ................................................................................

 [2]

ii) Identify the fragments responsible for the following signals.

m / e = 29 ................................................................................

m / e = 43 ................................................................................

m / e = 57 ................................................................................

[3]

iii)

Suggest the most likely structure of the isomer responsible for this spectrum.

[1]

There is a small, additional peak on the far right of the mass spectrum. 

i) Explain the presence of the small peak at m / e = 87. 

[1]

 ii) Suggest the type of molecules for which this would become more pronounced. Explain your answer. 

[2]

One other isomer present in the original mixture was 2,3-dimethylbutane.

i) Suggest one way in which the mass spectrum of 2,3-dimethylbutane will differ from Fig. 1.2 in part (b).  

[1]

 ii) Suggest an alternative spectroscopic technique for distinguishing between 2,3-dimethylbutane and your answer to (b). 

[1]

 iii) Predict which of the two isomers would have the longest retention time. Explain your answer. 

[2]