Question

A few basic gas chromatography questions:

In general, how would you determine which of several peaks in a chromatogram obtained for a mouthwash sample might correspond to ethanol? (Is your suggested method 100% reliable?) How would you use that peak to determine how much ethanol is present in the mouthwash?

In general, will an increase in operating temperature cause a given peak to have a longer or a shorter retention time? Why?

In general, will a decrease in mobile phase flow rate cause a given peak to have a longer or a shorter retention time? Why?

Suppose that you need to “speed up” a chromatogram (i.e., decrease the retention times of all the peaks). Suggest three specific GC conditions that you could change in order to accomplish this.

Answer 1

- We will run gas chromatography (GC), for mouthwash sample and ethanol under identical condition (temperature ramp, gas flow and etc). Both the ethanol in mouthwash sample and pure ethanol will give a peak at same retention time.

- Increase in operating temperature cause a given peak to have a shorter retention time, since increasing the temperature the compound will vaporize to gas at fater rate and retention time will decrease.

- A decrease in mobile phase flow rate cause a given peak to have a longer retention time. Because the flow rate of the mobile phase (which carries the compound will decrease) and hence the retention time will increase.

- Increase the gas flow rate.

Increase the temperature ramp.