Question

-In gas chromatography, what is retention time? What affect would increasing the column temperature have on retention time?

-Why does the column length affect the ability to separate a mixture of compounds by gas chromatography?

-Why is the retention time for solvents like CH2Cl2 and pentane much shorter than the compounds we dissolve in these solvents to analyze using gas chromatography?

-A GC sample containing 0.5 mM cyclohexanol was analyzed by gas chromatography to give a peak for cyclohexanol in the gas chromatogram with a relative area of 23500. If another sample of cyclohexanol in the same solvent is analyzed using the same instrument with the same experiment setup to give a cyclohexanol peak in the gas chromatogram with a relative area of 47000, what is the approximate concentrate of cyclohexanol in that sample?

Answer 1

1)Retention time is the time the species spends in the column after the mixture of compounds that needs to be separated is injected. As temperature of the system increases, retention time decreases. This is due to the decrease in viscocity as temperature is increased.

2) As column length is increased, the distance travelled by the mixture is increased. This gives ample time for the mixture to separate out based on its interaction with the solid phase and mobile phase.

3) Boiling point of a substance depends on polarity. As polarity is increased, homo-intermolecular interactions (such as hydrogen bonding) are increased. This increases b.p of the substance. But CH₂Cl₂ and pentane have negligible polarity, resulting in low b.p. As b.p is decreased, higher is the vapour pressure of the compound, and lesser is the retention time.