Chapter 20, Problem 1Q

Interpretation Introduction

Interpretation:

The reason for the better efficiency of GC than fractional distillation should be stated.

Concept introduction:

Gas chromatography is efficiently used for the compounds that have high vapor pressures which allow them to pass through a GC column.

A gas chromatography does not identify compounds if unknown samples are loaded into the column. GC is one of type of partition chromatography, where the sample that has to be analyzed gets adsorbed on the stationary phase.

The stationary phase is formed by high boiling nonvolatile liquid, usually a polymer. An inert gas like helium or nitrogen is utilized to form mobile phase. In GC, no interaction of compound with mobile phase occurs, unlike LC and TLC. The inert gas that makes the mobile phase carries the compound down the column when it is in vapor state. The compounds present in the mixture divide themselves between the gas phase and the liquid phase in the column in equilibrium.

The components of gas-liquid chromatography are as follows:

• High-pressure pure carrier gas source
• Flow controller
• Heated injection port
• Column and column oven
• Detector
• Recording device or data station

Explanation of Solution

The separation of mixtures during gas chromatography is due to the interaction of compounds present in the mixture in different ways with the liquid stationary phase.

A GC column is composed of thousands of theoretical plates which result in a huge surface area on which the gas and liquid phases can interact. The partition of a substance between the liquid and gas phases is based on its relative affinity to the liquid phase and its vapor pressure. The higher is the vapor pressure, the greater is its tendency to go from the liquid stationary phase into the mobile gas phase.

As the substances rise up in the GC column, thousands of liquid-gas equilibria take place. A low boiling compound remains in gas phase for longer time than a high boiling compound.

Generally, compounds that have low boiling points have high vapor pressures. So they rise through a GC column faster than compounds that have high boiling points.

In the case of fractional distillation, the number of theoretical plates is less than 10 so the surface is in comparison to GC is less for interaction between the gas and liquid phases. This leads to poor separation of compounds.