Chromatography Forum

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

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

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,

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

Thanks for the confirmation Peter,

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

Big difference, heh?

best wishes,

Rod