[browniee]

A certain sample is a mixture of 4 organic compounds and these compounds exhibits the following retention times in a GC experiment ->
 
Compound P = 1.6min
Compound Q= 2.2 min
Compound R = 4.7 min
Compound S = 9.8 min

Some known compounds were injected into the GC and the following data were discovered-
Benzene - 0.5 min
Methylbenzene - 1.6 min
Ethylbenzene - 3.4 min
n-propylbezenze - 4.7 min
Isopropylbenzene - 5.8 min

I am trying to tell what compounds from these 5 are definitely not present and which compounds are they. As well as which compounds are possibly present.

Am I correct to use the method of “similar retention time” means they are present and too far off retention time means they are definitely not present ?

For example, my answers are-

Definitely not present -> benzene, ethylbenzene and isopropylbenzene
Possibly present -> methylbenzene and n-propylbenzene.

Is this the correct way to attempt this question ?

[DrCMS]

Not having a matching retention times rules a material out but having a matching retention times only make it a possibility not a certainty.

[browniee]

Does this mean that for example, ethyl benzene is being ruled out ? Doesn't ethyl benzene could be a mixture of compound Q and R ? and therefore could be present ?

[DrCMS]

Does this mean that for example, ethyl benzene is being ruled out ?

Yes

Doesn't ethyl benzene could be a mixture of compound Q and R ? and therefore could be present ?

NO! GC does not work like that. 

• Ethylbenzene is a single substance not a mixture and in this system has a rentention time of 3.4mins.
• Mixing Q and R and injecting them into the GC would give two peaks one for Q at 2.2mins and one for R at 4.7mins.