Chromatography Forum

Hi,
I am trying to profile Volatile organic compounds in healthy subject alveolar breath sample using two dimensional gas chromatography from leco (GC GC TOF MS). I am using multibed Thermal desorption tube with carbopack B, Carbopack C and carbosive S III as adsorption matrix purchased from Gerstel. The TDSA2 with cool injecting system (CIS4) and liner filled with carbotrap B inside the CIS 4 is being used for injecting sample into GC. Every time I desorb a conditioned (heated at 350C for 2 hr with constant flow of ultrapure (Purity 99.9%) Helium gas) blank inert glass tube (without any adsorption matrix inside) which ideally should give me a blank TIC, I am getting TIC with good number of VOCs of different classes. Initially, I suspect it to be the contribution of few mililitre of room air inside the blank tube. So I run autosampler without putting any tube inside the thermal desorption system however it give TIC with similar VOCs profile. This result lead me to conclude that these VOCs could have been released from carbotrap B liner in the CIS4 itself as a result of incomplete desorption. I conditioned the CIS liner at 330C for 30 min with constant flow of helium gas and again run a blank run. Even after repeated conditioning at high temperature (330C), I am not able to get a clean blank TIC using blank tube and carbotrap B liner in CIS 4. I will really appreciate any expert suggestions.
Auto sampler method used for sample injection:
TDS initial temp: 25C, final temp: 300C @ 100C/min hold for 5 min
CIS initial temp: -35C, Final temp: 300C @ 12C/sec hold for 2 min
Transfer line temp: 310
Thanks
Iswar

have you try to make run with desorb at -25°C (like no desorb)
If you have yet peaks in chro so you need to think about your helium
Next step i would take is liner exchange with brand new (you may try different type: in the past i worked well with tenax/carbosieve sIII liner self packed)
You can try with lower desorption temp, because at high temp some artefacts can come from adsorbent material
hope this helps
Sorry for english

You probably have contaminants in the helium carrier gas - 99.9% is nowhere near ultrahigh purity, you need to be using 99.999%, often called instrument grade, and then install a specialised helium gas scrubber immediately upstream of the GC. You will not see an immediate improvement because all the plumbing in your GC has been contaminated by the current low purity helium.

Peter

Thanks a lot danicrd and peter for your valuables suggestions....
I have replaced the liner with brand new conditioned liners....one with Tenax TA packed, and other with baffled glass liner without any matrix inside. However, the TICs gave me the same profile...mostly rich in hydrocarbons...like decane, dodecane, tridecane etc. Does the impurity in helium gas include these hydrocarbons.......will it be helpful to put a hydrocarbon trap just before GC helium input line....? Is there any way to assess the purity level of helium gases....the supplier say its zero grade helium ...?

"Zero" grade should be at least 99.999% pure.

Adding a hydrocarbon trap cannot do any harm.

As I said before, if the carrier gas was contaminated you will not see an immediate improvement when you change to clean gas because the contaminants have to be flushed out of the internal plumbing of the GC.

Peter

Hi Peter,
Thanks a lot for your suggestions. I am worried about the purity of helium gas. would you please tell something more about helium gas scrubber?

Ther is not much to tell; you can buy gas scrubbers from the major GC-MS companies and from independents like Restek or Supelco. I much prefer those with metal bodies over those with glass and or polymer bodies. You can get purifiers that are specific to each gas.

Peter

I know this is an old post, but curious to know if Iswar was able to clean up the 2D separation background.
Thanks
Jon