Chromatography Forum

Hello, I have an Agilent 6890 GC and an OI 4660 purge and trap device with an Archon autosampler. I am seeing an issue with my retention times as samples are analyzed. Basically what happens is the analytes elute slightly earlier and earlier with each successive run. I've contacted tech support for assistance and they seem to be stumped by this too. I've done 5 direct injections into the GC (basically removing the purge and trap from the equation) and I don't see this problem at all; the retention times are stable. The purge and trap also passes a leak test just fine. Any ideas would be greatly appreciated! Thanks!

Does the purge and trap trigger the GC to start when the desorb valve switches or when the desorb preheat begins?

If it triggers at the desorb preheat it could be that the trap is still a little warmer than it was on the first run and is preheating a little quicker each time.

Thanks for the reply, I don't actually use a desorb preheat, rather I just go straight into desorb and that's when it triggers the GC to start.

Do all the target compounds shift the same amount or proportionally? Or do more water soluble/polar compounds have a greater shift?

They all shift the same amount. I've used the AK101 surrogates (trifluorotoluene and bromofluorobenzene) as well as the BTEX (benzene, toluene, ethylbenzene and the xylenes) analytes and observed the same amount of shifting in each.

Is this a new problem, or did it just show up?
P$T collects quite a lot of water on the trap and this may be your issue, having a water liquid phase for the first few min. of the run.
Vocarb 3000 traps work well and are hydrophobic but should be used with a dry purge step.

As for your run start, you should get more consistent RT's if you put a pre heat step in your desorb, usually set 5C under desorb temp.

This issue has just started happening in the last few months. I'll try adding a desorb preheat step and see if that helps. Thanks for the replies! This problem has been bugging me for awhile now.