SF6 analysis with TCD - Baseline failure after O2 peak

[expendable]

Hi all

I'm a PhD student, analysing for SF6 using an old Shimadzu 8A TCD GC connected to a spectra-physics 4290 integrator. I'm running column at 55 degrees and N2 carrier gas at a flowrate of 15ml/min. I am using a K2CO3 trap to prevent against water incursion into the GC.

I have a problem with the O2 peaks when putting manual injections through the GC. When a pure standard (SF6 in N2) is used, this problem disappears due to low O2 contamination. However, when I inject a sample of SF6 in air, the baseline fails to return level after the (negative) O2 peak. As a result it swallows the SF6 peak and gives an erroneous area value.

On the next run the baseline will drift back down again slowly then climb rapidly again after the next O2 peak.

Any ideas?
Thanks

[AICMM]

expendable,

Interesting choice for a handle for a Ph.D student.

What column(s) are you using, how much are you shooting, and are you using a backflush?

I hate to say this but TCD is a terrible detector for SF6 unless your project is somehow generating huge quantities of the stuff.

Best regards,

AICMM

[chromatographer1]

You might change (lower) the temperature of the filament to see if that makes any difference.

But is this a detector issue or a chromatography one?

Is the oxygen tailing? Look at a chromatogram with the polarity reversed to see.

Perhaps your column is the cause of the tailing of the oxygen peak?

If it is not tailing, you might try replacing the filament with a new one or one that is gold coated.

Best wishes,

Rod

[expendable]

Yeah SF6 quantities are quite high, hence the use of the TCD. They will always be between ranges of 20,000 down to 100 ppm.

Columns are a mol sieve attached to a porapak, not sure on the type or lengths of them, will have to ask about that one. No backflush. Sample is in a 2ml fill loop.

The oxygen was tailing, but I may need to check to see if it still is.

It may well be an integrator issue. Unfortunately nobody around here has the manual for the integrator I'm using or any idea how to set the baseline/ask the integrator to draw a baseline.


...anybody on here have a manual for a SP4290 or any idea what the command is for baseline set?


Thanks