I try to run 1,3-butadiene on a PE-GC using a ATD 400 for thermal desorption. The thing is that depending on if I use a PE-tub packed whith carbopack or a empty tub which I put a diffussion sampler in. The retention time for Butadiene is altering whith several minutes? The onley thin that should be different after the coldtrap is heated is the amount of other component entering the GC-column?

We analyse 1,3-butadiene among other things on a P-E Autosystem/ATD-400 and have not seen the r.t. shift you describe. We routinely use empty tubes in a sequence to demonstrate minimal carry over and have never seen times change by more than a fraction of a second. Given information about column inlet pressure, split flows, column, sampling conditions, I might be able to help, but it looks very strange. I assume you are using Carbopack B so the water content of the sample should be small. Large amounts of water can affect r.t. If you used flow-controlled carrier then there might be a difference between packed and empty tubes (but only a few seconds). You must use pressure-controlled carrier with the ATD-400, even with a packed column.

I am curious about the sampling. If you are using Carbopack B the safe sampling volume must be less than 0.2L (active sampling). Are you doing P-E diffusive sampling on B? The isotherm is very weak. Have you measured the uptake rate? I would be surprised if it does not fall to <50% of theoretical over a few hours. At the moment I would only recommend Molecular Sieve 13X for thermal desorption, but new types of carbon stronger than B are under consideration.

Are you sure that you are measuring the same compound? I don't see how you could possibly have a retention time shift of several minutes if the system is at all functional. I am assuming that you are using an FID as the detector, so you can't tell what compound you are really detecting as you could by mass spec.

I cant see what component it is but I put a standard on the tub containing carbon and the same standard containing the diffusion sampler. and compare whith the same samples whitout standard. I have a old Chrompac AlO2/KCl column which is to be exchanged but I dont know what to change to? Is there any alternativ to separate diens?

I use J&W/Agilent GasPro. Water does not build up on the column and the retention times are stable.