Chromatography Forum

i use a 5A-molecular sieve column with nitrogen as carrier gas for detection of hydrogen from the headspace of an anaerobic digester. while standardization, i get the hydrogen peak at 11secs (flow rate of carrier gas is approximately 60ml min-1). when an air sample is injected, under the same conditions, i get a small peak at the same retention time with an area that is one -eighth that of pure hydrogen peak.
is this possible ???
moreover when a 50:50 mixture of CH4 and CO2 was injected, again only one peak at the same retention time was observed.
what is the reason for such an occurence? cud it be the column packing?
the temperature conditions used are oven: 50, injecter:90,detector:90 degree celsius.
attenuation : x4
need a reply ASAP.
thnks

With such a high flow rate the peak you see may be a flow upset, low pressure sample is inserted into a high pressure area. (TCDs are flow sensitive) This is caused by the injection of the sample. I would slow the flow rate to half of the present flow.

best wishes,

Rod

Thnks for the quick reply.
Well the reason why i am using this flowrate is because the minimum pressure that is set in the particular GC that i use is 1.5kgf cm-2. So the corresponding flowrate happens to be so much. The same GC is used without any problem for methane and carbon dioxide detection using a Porapak Q column and hydrogen as carrier gas with the same flow rate.
so let me try out whether i can set a lower min pressure and use a lower flow rate.
Thanks once again for the reply.

You can also try changing the way you are injecting. Do it differently from how you are presently doing it. Slow if fast, fast if slow, etc.

best wishes,

Rod

Thnks a lot for the suggestion !
i shall try it and post the results soon .