Headspace cycle time

[mojo]

We're transferring a method for residual solvents using an Agilent 6890 with headspace sampler. As the run progresses we are seeing a drop in the areas of our standard solutions and internal std. My colleague thinks this may be due to the increasing time each vial spends in the oven (time increases as GC oven cools down, knock-on effect for vials further down the sequence), and that we may have too short a cycle time. Does this sound reasonable, or may a leak between the headspace and column be more likely?

[zokitano]

Mojo,

If you're suspecting on leak in your HS, try to run the leak test on your HS (if there is any). You can detect any leakage in your GC if there is a unstable pressure on your column or if the pressure that you have set can't be established. Try to put a soap foam on the connecting parts (between injector and column and column and detector) and observe if there is any leakage from these connections.

But I think that decrease of the peaks in your subsequent injections arises due to the different thermostating time of your HS vials (and this is a case expecially when the vial septum is already bored/perforated by earlier injection). If this is the main problem, I suggest to set the proper cycle time (the exact GC run in minutes, of your analysis) to avoid extra thermostating of your vials and to ensure equal thermostating time for every vial.

Best regards

[mojo]

Thankyou

[Peter Apps]

WHOA, STOP, HOLD UP !!

Do not put soap solution anywhere near any connection on a GC system, ever. The soap will contaminate the connection and lead to intractable contamination of the whole system. If you want to localise a leak to a particular connection then use a leak seeker. No GC lab should be without one.

In equilibrium HS analysis it is important that all the samples equilibriate for the same time, so change your cycle time to allow the GC to run, process its data and cool down again before the next sample is ready to inject. If you are getting "waiting for GC ready" or similar messages from the headspacer your cycle time is too short.

Peter

[mojo]

Thankyou too

[AICMM]

I am not a headspace expert but I do know that if you are double punching vials, you are asking for headaches. Second, someone correct me if I am wrong, but I always thought the longer time at temp would simply ensure you would reach equilibrium rather than decrease the headspace concentration. So your response being lower is a bit of a puzzle to me, I would have thought higher results would be the problem.

[leadazide]

I know this is a very simple answer and that you may already have checked this or ruled it out as a problem:

We had similar problems a few years back.. It turned out to be the crimpcap wasn't crimped tight enough so when pressure increased inside the vail it sample escaped.. Turned out to be the crimper had been "played" with by someone so it didn't crimp hard enough!! On the other hand you don't want it to crimp to tight as the septa in the crimpcap can be deformed.. This we learn when trying to fix the crimper..

[randytoad]

I am not sure how the Agilent HS sampler works (we use a Tekmar 7000/7050), but if it works by pressurizing your vial and then allowing the contents of the vial to expand into the sample loop (as most do), double puncturing would almost certainly be a problem. When the vial is pressurized the first time, it is essentially dilluting your headspace with carrier gas, so when you do the second pressurization it is from a dilluted sample. It will ALWAYS be lower than the first time. You can also get lower results if your vial caps are not crimped tightly enough. Believe me it is very easy to improperly crimp a headspace vial. We have our techs crimp once, turn the crimper 90 degrees and crimp again just to be sure. An non-gastight vial will not fully pressurize and consequently the headspace will not expand into the sample loop. We had so much trouble with this that we developed kind of a neat method to check for whether our samples had leaky vials. We analyze with parallel ECD and FID. ECD will detect the oxygen in air (it's not terribly sensitive to it, but at the concentrations of O2 in normal air its enough). When we see a chromatogram from a headspace sample that has an ECD "air (unretained) peak" that is significantly smaller than normal, it almost always means that we have a leaking vial. You could probably do the same with FID and atmospheric methane. Maybe also with an internal standard.