Question

The samples of cyclohexane and toluene you collected during distillation were kept in a fridge for a week before being analyzed by GC. Explain what would be different about the GC analysis results if they were kept out on a shelf in lab instead. Refer to each fraction collected

Answer 1

GAS CHROMATOGRAPHY :

• GC is an analytical tool used to separate and identify the different chemical components in a given mixture.
• Like any chromatography techniques, GC also has a stationary phase and a mobile phase. Its stationary phase is a liquid polymer which is applied to the walls of the column used in the instrument. Its mobile phase is an inert gas, usually helium or hydrogen.
• The samples to be analyzed are in gaseous form. Different chemical components interact with the stationary phase differently (like dissolves like) and hence are eluted from the column at different times. The component having greater interaction with the stationary phase stays in the column for a longer time.
• The time during which the gaseous component stays in the column is known as retention time.
• Also, the separation of sample components or elution of a sample can be controlled by controlling the temperature of the column. Samples are eluted through the column based on their boiling points too. The chemical component with lower boiling point will get eluted first and will have lesser retention time than the sample with higher boiling point.
• When the gaseous sample comes out of the column, it is detected by the detector and electronically a peak, corresponding to the compound, appears on the GC curve.

In the above question we will use the criteria of boiling points for eluting samples through the column.

• Boiling point of Cyclohexane : 80.74 °C
• Boiling point of Toluene : 110.6 °C

Cyclohexane has lower boiling point than toluene.

• Samples placed on the lab shelf - By controlling the temperature of the GC, boiling point of cyclohexane can be attained first as it has lower boiling point. So, cyclohexane gets eluted first from the column. In this case, the retention time of cyclohexane is less, where as toluene has higher retention time in the column. A peak or for cyclohexane in the GC graph is obtained at lesser time than toluene.
• Samples placed in the fridge for one week : The temperature of the samples of both toluene and cyclohexane will be quite low. When they are injected in the GC column, it will take a while to attain the boiling points of the two chemicals. So, this increases their retention time in the column. But the relative retention times remain same. That is, the retention time of toluene will be more that that of cyclohexane. So, there is a shift in the peak or signal of the two compounds in the GC graph. The time axis shows greater retention time for both the compounds ( GC graph is drawn for signal on the y axis and time on the x axis).