helium and methane detection in the same sample

[emma2009]

Hi, I am trying to use GC-TCD and a CTR-1 column to detect methane and helium in the same vapor-phase sample. My questions are:
1. Using the method, is it possible to seprate helium and methane?


Also, we are using a potable hemliun detector based on thermal conductivity. It seems methane is a interfence. If methane is co-exisitng in the sample, what is the effect of methane on my actual helium concentration? Is the helium concentration goign to higher than its actual or lower?

Thank you!

Emma

[Johnny Rod]

What levels are you looking for? I'm not familiar with a CTR-1 column, has this got another name?

Thermal conductivities:
http://www.engineersedge.com/heat_trans ... -gases.htm
Helium has a high conductivity compared to many other gases e.g. oxygen/nitrogen and also methane, so I'm not sure it would interfere a great deal, but it depends if you're looking for a leak or trying to measure accurately.

[GasMan]

The CTR-1 column is supplied I believe by Alltech. It is basically two columns in one, consisting of a 1/4 column with a 1/8 column inside. The 1/8 column has one phase and the space between the two columns contains the second phase.It would be interesting to hear from users how these columns perform, especially over time.

Gasman

[chromatographer1]

Emma,

Your detector is a TCD?

You are using nitrogen for the carrier or helium?

If you are using helium carrier then you will not detect helium in the sample.

If the Porapak Q in the outer shell is in good condition then you should have a small separation of helium and methane. With nitrogen as the carrier your response for methane should be quite small. Helium should have excellent response. If your carrier is helium then methane will have a high response and helium will not be detected.

Helium and methane should separate by a large amount of the molecular sieve inner column.

Unfortuntately, at least to me, your sample matrix and the carrier you are using are not clearly defined. This effects any answer I or anyone else may try to offer you.

Please restate your method, conditions, detector, temperature, sample size injected, and the amount of analytes in your sample.

best wishes,

Rod

[emma2009]

Hi All,

Thank you all for the reply.

Rod, here are the parameters:

I am using GC-TCD that has a 6-ft CTR-1 column by Alltech. Temperature is kept at 40 oC all the time. the whole run is about 12 min. There are O2, N2, He, CH4 in my vapor sample. I am using hydrogen as my carrier gas to get both helium and methane detection. Helium repsonse is not very good which is understandable. Is it possible to seprate helium and methane? Theroetically methane and helium should come out very fast at the very beginning.

Additional question: Using a CTR-1 column, methane has two peaks, one at the very beginning from the inner column, and one at almost the end from the outer column. I am wondering which one I should use.

[chromatographer1]

You forgot to mention the concentrations of the gases in your sample, and the amount you are injecting. I think at this point using hydrogen instead of argon as a carrier you are simply not seeing the helium peak. At the same concentration it will be orders of magnitude smaller than the methane peak.

And it is confusing using this column because you will see two of every peak which can cause peaks from the two columns to elute at the same time hiding peaks you wish to see. You should use a Mole Sieve column or a Carboxen 1000 column. I have posted chromatograms on this forum previously. Do a search.

best wishes,

Rod