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

State the difference between the mobile phases used 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 stationary phase. Explain your reasoning.

State which compound in the mixture shown in Fig. 1.1 is the most abundant. Give a reason for your answer.

Suggest how gas chromatography could be used to confirm that oil pollution found on a beach originated from a specific oil tanker.

A mixture of volatile esters 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 volatile esters, methyl propanoate and propyl methanoate, is analysed by gas chromatography followed by mass spectrometry.

Explain why the mixture of esters 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

Deduce the compound that has the greatest affinity for the stationary phase. Justify and explain your answer.

Deduce 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 spectrometry 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) State the name of an appropriate mobile phase that could be used to form this gas chromatogram.

[1]

ii) State the number of compounds present in this mixture. 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) Deduce the molar mass and hence the molecular formula of the isomers.

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

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

[2]

ii) Deduce the identity of the fragments responsible for the following signals.

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

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

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

[3]

iii) Deduce the skeletal formula of the isomer responsible for this spectrum.

[1]

There is a small peak at m / e = 87 in the mass spectrum. This is known as the [M+1] peak.

i) State the identity of the isotope responsible for the [M+1] peak.

[1]

ii) Suggest the type of molecules for which this [M+1] peak 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)(iii).

[1]

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

[2]