background change (jumps and reverse peaks)

[kaydemir]

I want to ask a very recent problem just occurred in one consecutive run.
There are strong jumps in the backgroung even with only carrier gas Argon.

GC system is SRI 8610C with a TCD detector and two columns (silica gel and molecular sieve).

We are using only gas samples so there is not injection liner or so as
far as I know. We have just pipes connected to the gas `sample in` of
GC. So we did not change a piece so far.

I am using temperature program of 40-220 C. It is starting at 40 C for 5
min. Than with a ramp of 30 C/min and it is going upto 220 and waiting
there 3 min. Valve is changing at 4 min. Problem is coming even if I
pass only oxygen (or even nothing) from the system. We have two columns (silica gel and molecular sieves in the oven). I just adjust the flow of oxygen to 0.6 ml/min (20% oxygen 80% argon) with flowmeter and it enters GC after flowmeter.

I baked the columns at 300C for 18 hours but these jump numbers increased dramatically instead of solving the problem.

What can be the reason? Thanks a lot.

[AICMM]

I am more confused than clarified by your post. Are you trying to analyze 80/20 argon/oxygen? Or are you using that as the carrier, in which case, what are you trying to analyze? Are you supplying the carrier to both columns and both columns go to their own detector or are you doing a column switching application? You start at 40C and wait four minutes into the run before you make an injection using the valve?

Best regards.

[kaydemir]

I am more confused than clarified by your post. Are you trying to analyze 80/20 argon/oxygen? Or are you using that as the carrier, in which case, what are you trying to analyze? Are you supplying the carrier to both columns and both columns go to their own detector or are you doing a column switching application? You start at 40C and wait four minutes into the run before you make an injection using the valve?

Best regards.

Thank you for your answer.

Carrier is only argon. Oxygen is analyte but it is not important at this point. Because, even if I don`t pass anything to the system, background has jumps and steps. Columns are switching after 4 min but sample is entering the system at 0 min. There is one detector (TCD) and it detects the gases coming from columns. Since I am analyzing only gases there is no injection. Gas line is connected to the GC and at the beginning of each run GC is taking sample from this line. So there is no possibility as contamination of injection systems.

Thanks again.