Question

10000 page 6 9.10 Two chromatograms are shown. Both chromatograms show the separation of the same mixture using the same chromatography column with the same flow rate, but under different conditions Chromatogram #1 Chromatogram #2 compound (a) If the chromatograms were obtained by gas chromatography, which one used the higher column temperature? Explain briefly, showing that you understand the effect of temperature on retention in gas chromatography (b) If the chromatograms were obtained by liquid chromatography, which one used the stronger ise? Explain briefly, showing that you know what a stronger mobile phase is in liquid chromatography

Answer 1

a)

Evaporation is the process in which a liquid slowly transfers from the condensed (liquid) phase to the gas phase—like the peaceful scene pictured above. If temperature is increased, heat transfers more energy to the liquid—giving molecules the power to escape from the liquid’s surface. This rise in temperature therefore increases the transfer from liquid into the vapor phase.

This equilibrium has a direct impact on temperatures in gas chromatography. If the oven temperature is too cool, a compound will spend most of its time condensed in the stationary phase. Only a very small amount that can evaporate will transfer down the column.

Conversely, if the temperature is very high then the equilibrium will shift in the opposite direction. The compound will spend all of its time in the vapor phase and not condense into the stationary phase.

Very volatile compounds will not interact with the column phase if the temperature is too high above the boiling point. Therefore, the temperature needs to be lowered to get favorable interactions with the phase. Obviously, too low a temperature will cause the entire amount of compound to be condensed. That doesn’t work well because it is the transition from the liquid phase to the gas phase and back which is responsible for the separation.

Chromatogram 1 used higher temperature as the retention time is less for all components in it.

b) Retention time (RT) is a measure of the time taken for a solute to pass through a chromatography column. It is calculated as the time from injection to detection. The RT for a compound is not fixed as many factors can influence it even if the same GC and column are used.

For polar supports (silica/alumina), the weak mobile phase is a non-polar solvent (hexane, benzene, etc.) and the strong mobile phase is a polar solvent (water, methanol, etc.) For non-polar supports (charcoal), the weak mobile phase is a polar solvent and the strong mobile phase is a non-polar solvent.

Chromatogram 2 used stronger mobile phase as the retention time is more for components in chromatogram 2.