Question

1) Order the following compounds in order of elution in gas chromatography.  Explain your reasoning. (Label “1^stEluted”, 2^ndEluted, and so on).  

Mobile Phase – Air (Flow Rate = 25 mL/min), Stationary Phase – poly(dimethylsiloxane),

Oven Temperature – 90 ^oC.

1-butanol, 1-pentanol, 1-hexanol, pentane

b) How would a gradient elution (55 ^oC – 120 ^oC) impact the chromatogram?

Answer 1

1)In gas chromatography (GC) which component elute first depends on the boiling point. In GC the mixture is first converted to its gaseous states and then passes through the column. The component which has lowest boiling point elutes first and the component which has highest boiling point elutes last.

Here there are four components. Boiling points are following

Pentane – 36⁰C

1-butanol – 117⁰C

1-pentanol – 138⁰C

1-hexanol - 157⁰C

Thus pentane will be eluted first followed by 1-butanol, 1-pentanol and 1-hexanol at last.

2)In gradient gas chromatography temperature of the column increases gradually. If the components are having close boiling points gradient gas chromatography will give better separation. In this case lowest boiling point component is pentane. Thus at 55⁰C it all will be eluted. After that 1-butanol has 117⁰C boiling point. In between there is nothing. Increasing temperature from 55⁰C to 120⁰C will not affect the separation.