inconsistent CO2 / H2S area count

[chillipadi]

Hi,

I am a newbie in GCs. I have problems getting consistent area count for my compressed liquid calblend (in piston charged with helium) which will pass through a vaporiser. I have to run more than 10 times to get 3 consecutive consistent area count. Is it common for CO2 to be inconsistent in compressed liquid blend?

I also have problem having consistent H2S peak as well. Is there an issue with using coiled silicosteel tubing, coz I heard that the silicosteel tubing is not supposed to bend or coiled?

[aldehyde]

How inconsistent is it? What is the RSD you are getting now and what is the limit?

What instrument/column/inlet type/detector type/flow rate/carrier gas/temperatures?

[chromatographer1]

Since you have a vaporizer you may be losing the CO2 in the liquid portion which is near the heater and fresh liquid has to be flushed through it to get a consistent sample.

The tubing handles only a moderate amount of bending or the deactivation can break and allow active sites to be manifest. Think of it as 'ice' which if you bend the tubing too much causes the 'ice' to crack.

best wishes,

Rod

[chillipadi]

How inconsistent is it? What is the RSD you are getting now and what is the limit?

What instrument/column/inlet type/detector type/flow rate/carrier gas/temperatures?

Hi Tks for your prompt reply. I am using GC 6890A / Wasson Col (RGA) / 2 TCD + 1 FID / gas sampling valve (5ml/min) / He as carrier gas / temp ard 150 DegC

My RSD is around 40% for CO2 and around 20-30% for H2S. I am using compressed liq in piston for CO2 and Gas cal blend (balance - H2) in gas cylinder. My chemist is looking at <10% RSD,

Sorry, 1 more question: For gas cal blend store in gas cylinder, is it very difficult to get consistent area count compared to perm tube?

Really appreciate your help.

Thanks.

[chillipadi]

Since you have a vaporizer you may be losing the CO2 in the liquid portion which is near the heater and fresh liquid has to be flushed through it to get a consistent sample.

The tubing handles only a moderate amount of bending or the deactivation can break and allow active sites to be manifest. Think of it as 'ice' which if you bend the tubing too much causes the 'ice' to crack.

best wishes,

Rod

Hi Rod, thanks for your prompt reply.

How do I ensure I have fresh liquid to flush through to get consistent sample if I am using gas sampling valve & manual injection? Is there a spectial tactic of handling or injecting compressed liquid into the GC using gas sampling valve? Seems like I always have problem getting consistent area count for gas components like Methane, CO, CO2, H2S using GSV manual injections, is it coz of the way I inject (control purge flow @ 100ml/min for few mins, close the control valve then press START when the float in the flow meter drops to zero)? Oh forgot to mention, I have a vaporizer and methanizer for CO / CO2.

So if the deactivation in the tubing breaks, the H2S will stick onto the tubing and not flow to the GC u mean?

[chromatographer1]

Run your vaporizer a few minutes to clear out the non-representative sample of liquid before you sample the vaporized liquid.

From what I understand of your post, you seem to be sampling in a proper manner. You allow the vaporized sample to pass through the sampling system at 100mL/min for a few minutes, then stop the flow through the gas sampling valve so it reaches atmospheric pressure, then you inject onto the column.

That all seems right. But if you are not getting a representative sample then you need to mix the liquid sample better before you vaporize it. You may need to cool the cylinder and/or increase the pressure under which it is maintained as a liquid. Usually, these liquid samples containing extremely volatile components like CO2 are in a large syringe cylinder and the pressure is maintained not by pressurized gas but a physical force of a large syringe.

The H2S will adsorb and desorb at varying rates from any active sites (bare metal) along the flow path of the sample, causing lower or inconsistent results for your H2S.

Rod