Chromatography Forum

hello,
I have some strange problems with peak areas. The system is a GCMS with two different injection systems, pyrolysis and standards split injection. Depending on the analysis I change the column fron one to the other. Normally I make screening runs, therefore I do not know how long the problem exists. It came up on a pyrolysis method, but now I am using the other injector and it is still there, so the injector should not be the reason.
The peak area as well as the retention time stay stable during several runs, but from one day to th other, or from the morning to the afternoon there are huge changes in peak area (20-50%) and retention time. I cannot see a correlation between increase or decrease of rt and area. I am now measering easy alkan mixtures to find out, if it only happens after a pause or also within long sequence.
The system has no leak I have a very good vacuum. I can exclude the syringe as I do not use one in the pyrolysis, split problems? But on both injectors?
I have no idea, any good advice?
Thanks
Beate

When you move the column from one inlet to the other do you change the ferrule on the end of the column? Reseating a ferrule a few times should not be a problem, but reseating it many times could be.

Allright, I did not change it for some time, but I do not have any problems with leaks. Depending on the area-changes there should be big leaks. I compared the chromatograms and watched the mz 32 to detect changes of the air in the system - nothing.

Beate

beate,

Does the pyrolysis system get it's flow from a different flow controller than the splitless/split inlet? Also, electronic split ratio or manual (although this does not explain the rt issue....)

Best regards.

I suspect you may have a partially restricted split flow line. I have seen this on pyrolysis systems, there is a lot of material passed out through the split line and deposits build up there. Put a flow meter on the split vent and see if the flow remains constant or if it drops when the peaks get bigger and the retention time shifts.