Retention time change with Gas Saver time

[ahmed ismail]

Dear Colleagues,

I have GC-FID 8890 and I'm using the Gas saver feature but I'm observing when I change the time of the gas saver , the retention time is changed.

In my case, the gas saver was at 20 ml/min after 2 min then injected methanol, and the RT was 4.74 min but when I changed the time to 15 min, the RT of methanol became 7.9 min!
since this problem has appeared, the response of methanol has jumped from 6000 to 16000pA
I'm perplexed what's the relation between gas saver and retention time ??.

The column flow is 4ml/min.
the injection technique is manual by syringe.
split ratio 100:1

[Peter Apps]

Can you reproduce this, or did it only happen once ?

[ahmed ismail]

Can you reproduce this, or did it only happen once ?

it always happens.

[James_Ball]

Have you measured the flow at the vent to see if it actually gives the proper flow at each split ratio?

It could be a clogged line there. Does the inlet pressure change when the flow changes?

[ahmed ismail]

Have you measured the flow at the vent to see if it actually gives the proper flow at each split ratio?

It could be a clogged line there. Does the inlet pressure change when the flow changes?

Yes, the flow at the vent is equal and I made a split vent restriction test consequently it passed.

[James_Ball]

Thinking about this.

If the instrument was struggling to maintain adequate head pressure with the 400ml/min flow out the split vent, then what happens is when you move to 20ml/min at 2 minutes it can then supply the pressure to maintain the column flow. If you wait until 15 minutes to switch to 20ml/min then it is not getting enough pressure in the inlet to maintain the proper column flow so the retention time is later.

If it is set to Constant Flow for the column and the oven is ramping temperature then the head pressure will need to increase with increasing oven temperature to maintain the set flow rate, if the temperature is sufficiently higher after 15 minutes then it just is not able to increase the pressure enough to get the flow through the column.

If you currently use Constant Flow mode, try this.

Set to Constant Pressure mode, then make an injection with the gas saver set to 2 minutes then set to 15 minutes and see if the retention time changes. It will be interesting to see what happens.