VOC/BTEX method on GC/MS/HS

[Charles_]

Hi everyone,
It's my fisrt post here but i come sometime to look for idea or some information.
But today I'm blocked and hope any kind of help.
We have bought a new GC/MS/HS system in replacement of an old VARIAN ion trap system.
The system is : GC 1300 - ISQ7000 - TRIPLUS500 from ThermoFisher.

We have started to transfer our method for VOC/BTEX analysis on the new device.
Thermo have provided the method they use in their laboratory.
We have try to use this one and adapt to our old method with a moderate success.
The VOC mix contain about 50 molecules. The method works pretty good for the first part of the chromatogram but in the second part who start with Toluene, xylenes elution time, problems appears :
Calibration becomes very quadratic and points are difficult to align on the calibration curve.
It's seems to be in relation with type of molecule, example no problem with bromoform or VOC but same situation with different degrees with benzene family.
Some information about the method :
It's ZB 624 PLUS column (20*0.18*1), same model we use with success on our old system.
Triplus500 is an headspace module with loop/valve. I've no experience with this system, the old system works with combipal and syringe.
I work in a SIM mode and use internal standard (in routine 1,2 dichloroethane D4 but some try with Toluene D8 or Chlorobenzene D5 have not correct the problem).
The calibration for each analytes is between 1ppb to 100 ppb.
If there is somebody with some kind of experience with this system and or analytes, I can detailed more the method.
At this moment i've try many changes but without success…

Charles

[Steve Reimer]

I have no experience with this headspace system but a couple of things come to mind.
Does Thermo specify the temperature for the loop/valve in addition to the headspace vial?
You say the problem compound responses are quadratic, are they decreasing with increased concentration?
What are your inlet conditions?

[Charles_]

Hi, thanks for your anwser !

The temperature they use is 70°C for vial and 80° for valve/loop.
I've tried to increase (as in the old varian system) to 80°/85°C but with out success.
For the problematics compound the response increase with concentration.
I've select some molecules here to show the response :





In this system, column is directly connected to the HS module, injector is partially removed.
Only the gas part keep in place. So, no liner are use. The method works with contant flow of 1 ml/min - split 15 ml/min.


Charles