Question

If a GC separation is performed on a nonpolar and slightly more polar column (same length), which of the following sentences is most correct. Please explain your answer.

a) On a more polar column, all molecules will have longer retention times.

b) On a more polar column, retention times will be unchaged.

c) On a more polar column, molecules that are more polar will have longer retention times.

d) On a more polar column, nonpolar compounds will have shorter retention times.

PLEASE EXPLAIN WHY.thank you!

Answer 1

c) On a more polar column, molecules that are more polar will have longer retention times.

c) On a more polar column, molecules that are more polar will have longer retention times.

A longer column generally improves the separation. The trade-off is the retention time increases propotionally to the column length and a significant peak broadening will be observed as well because of increased longituinal diffusion inside the column. One has to keep in mind that the gas molecules are not only traveling in one direction but also sideways and backwards. This broadening is inversely proportional to the flow rate. Broadening is also

observed because of the finite rate of mass transfer between the phases and because the molecules are taking

differnt paths through the column