Method issues - optimal flow rate

[ganna]

Hi all,

I running at method on our GC and its giving me a few probs; my suspicion is the flow rate but I'd appreciate opinions!!

The parameters are

flow rate: 10 cm/s
split flow: 2 cm/s
carrier gas: Helium
column dims: 30m x 0.53mm x 5 microns
oven temperatures: 40C hold for 8mins, ramp to 200C at rate of 30C/min
run time: ~ 19mins

I'm expecting 5 standard peaks; at that flow rate I'm seeing 3. I changed the flow rate to 33 cm/s and I managed to see 5 peaks. Unfortunately the method is validated at 10 cm/s!

Any views?

Thanks,
Jane

[chromatographer1]

I believe a typo was made or a flow was mismeasured originally.

Review the orginal chromatograms and compare with yours now.

Might be a time for method revision/correction.

best wishes,

Rod

[ntruong]

Hi Jane,
Have you tried it with different column lot? You could also see less peaks if there is a leakage in the system, too. I would pay attention to the inlet first if I were you.
Good luck,

[Peter Apps]

Hi Jane

I'll agree with Rod on a misrecorded parameter here.

10 cm/s is way too slow for a linear flow velocity, 33 cm/s is slightly slower than optimal, but close enough. The volume flow in a 530 micron column is then around 10 ml/min.

You give your split flow as 2 cm/s, which is a velocity, but split flow is always measured as a volume - usually ml/min. Perhaps it should be 2 cm cubed/s which is 30 ml/min. It could also be that you have to set a split ratio of 2:1.

I would go back and carefully check the documentation, even as far back as the original method record for the validation.

Peter

[chromatographer1]

Thanks for the confirmation Peter,

What if the flow was not 10cm/sec but 10cc/sec?

Big difference, heh?

best wishes,

Rod