Problem with an ATD400, cannot see octadecane

[karipo]

Hello,
I am new on this forum and I write because I have big problems with an ATD400 connected to GC MS FID.
We use standards that contain toluen, an alcohol and octadecane.
Since I change the cold trap (same material as before) I see poorely octadecane and I don t understand why. Toluen and alcohol are as before.
Can someone help me ?

Thank you,
Karine

[MikeD]

Karine

Need a little more information. Why was it necessary to change the cold trap ? Is it the 3 mm inside diameter straight through, or low-flow narrow bore ? Did you pack it yourself ?

Assuming nothing else has changed and you are using the first type, the only thing I can think of is that it is possible for the trap material to move out of the heated zone if it is not retained firmly by the glass wool plugs. If you are using a low-flow trap this should not happen because there is a 1 mm restriction.

If the octadecane is 100% desorbed in the primary step it will be 100% adsorbed even on a weak sorbent like Tenax at say 30 C. It does not have to be cold. Toluene is the same. Because your toluene peak is OK that suggests you have no leaks. Therefore I believe your C18 is on the trap and the sorbent is not being heated to the temperature that the ATD is indicating, but heated to a temperature sufficient for toluene to come off.

Have you tried an n-alkane series to see where your apparent drop in recovery begins ?

If this does not explain your problem - sorry it's the best I can do. I have ignored many other possibilities because you say you have only changed the trap.

[karipo]

thank you for your answer.
I will give you more details because I try and try and still cannot fix this problem.
I have packed the cold trap myself and it is a low-flow narrow bore.
I have not tried other alcanes but I will.
And maybe I have to find a way to check the temperature.
What is very strange is that we have two exact systems (except that in one I have used tenax in the cold trap and in the other I have two adsorbents, one being carbosieve) and I get exactly the same problem juste after I change the cold trap.

I will be grateful for your help,
Thanks
Karine

[MikeD]

Carbosieve is not good for your application, but still if it was working before it should continue to work. I would prefer Tenax.

I assume your trap is backflushing when it fires. Early production models of the ATD-400 were forward-flushing by default. They could be re-configured for either flow direction. Carbosieve traps would not work for C18 in forward-flush configuration.