Predict qualitatively how the retention times in gas chromatography will vary fo

Question

Predict qualitatively how the retention times in gas chromatography will vary for: heptane, octane, nonane, and decane. benzene, toluene, ethyl benzene, and xylene. 2-pentanol (an alcohol) and 3-hexanone (a ketone) have nearly identical boiling points. Does this mean that these two compounds cannot be separated by GC? Suppose you find that two compounds have identical retention times. What can you do to try to get a separation? On a carbowax column such as that used in lab, would you expect to get separation of the two isomers shown below?

Explanation / Answer

1. Retention time depend upon boiling point of sample.

Higher bolining point compund will have a high retention time.

a retention time: decane (174)> nonane (151)> octane (125)> heptane(98)

b. retention time: xylene (140)>ethylbenzene (132)>toluene(110)>Benzene(80)

2.yes, they can be seprated based on attraction force b/w column and compund.

3.  gas chromatography uses 2 methods of separation. The first is the separation by boiling point, as you noted. Secondly there is a column that the gas goes through before it reaches the detector. Different substances have different amount of attraction to the inside of this column. If a substance has a high amount of attraction to the column, it will take longer to reach the end of the column where the detector is. This dual separation allows separations beyond boiling point differences.

4. both Isomer have same mass w.r.t. to different geometry so we can not seperate it by GC-MS.

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