Loss of Resolution: Methane/Carbon Monoxide

[Spuzzin]

Hi.

We've recently acquired a Varian NatGasB instrument from another lab and are currently configuring it for permanent gas analysis.

The NatGasB instrument is a CP-3800 instrument purpose built for the analysis of natural gas. It's a fairly complicated instrument so I'm only using it in it's most basic form just now.

The separation of oxygen, nitrogen, methane and carbon monoxide is handled by a Molseive 13X 45/60 mesh column which is then backflushed.
The instrument runs isothermally at 120 Deg C.

Initially the separation was good
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I then switched all 3 250ul sample loops for 100ul sample loops to improve peak shape at higher concentrations and then got this.


Over the last few days the resolution has degraded further to this


No additional work has been done to the instrument during this time.

The plan for today is to reinstall the original sample loops and try and get it back to where it was when I first got it.

Regards

Rich

[larkl]

You might want to try baking the column out. Won't hurt anything and will help the separation if you've gotten a slug of something polar on the column. You wouldn't think something as simple as changing the loops would matter, but you don't know how much air was let into the system.

[GasMan]

Larkl is correct in saying that the Molecular Sieve needs to be conditioned. The separation of methane and carbon monoxide is very dependent on the amount of water in the column. With a water content of about 9%, CO will elute before methane. With 4% water, they will elute together, and with 2% water the CO will elute after the methane.

If you have moisture traps in the carrier gas line, they need to be replaced or regenerated.

Gasman

[AICMM]

Spuzzin,

Very likely a sieve needs baking. Bad news is, it might be hard to do easily in place because the column is probably resident with other lower max temp columns. If you do bake, be aware of what other columns are nearby and keep your bake temp below the maximum of the lowest temp column. Better yet, if possible, remove the sieve column and bake in another GC nice and hot. No mean feat in a natural gas instrument, I know, but still the best option if you can do it.

Best regards.

[Spuzzin]

Morning.

Thanks for the responses.

I tried baking out at 170 Deg C last night with no improvement. As AICMM has mentioned there are 5 columns in the instrument, two of which max out at 200 Deg C so I guess I'll just have to remove the column and bake it out elsewhere. C'est la vie.

Rich

[Spuzzin]

Update.

In the end it was more practical to remove the more delicate columns than it was to remove the mol sieves.

Baked at 340 Deg C for about 4 hours and now have baseline resolution for carbon monoxide/methane.

Thanks again for the advice.

Rich