Switching from He to N2 carrier gas

[VJP]

We are converting validated GC method with carrier gas He, make up gas He to carrier gas N2, make up gas N2. There are 2 peaks of interest (alcohol peaks) in the chromatograms.

During feasibility study, in some injections (2 to 4 injections out of total 45 injections) we are getting a nice taller and narrow peaks with clean baseline (it's not an electrical noise, no retention time shift). Normally, alcohol peaks we see are twice shorter in height and wider with Helium carrier gas and N2 carrier gas. With He carrier gas we never saw taller and narrow peaks. To rule out the instrument, we tried 2 GC instrument and on both instrument we saw 2 to 4 injections with narrow, sharp peaks with N2 as carrier gas. We changed the liner, syringe and septa for every run.

In the method, we did not change any parameters (liner, temp, syringe etc.) when switched from He to N2 carrier gas. To rule out the possible causes, we don't have the Universal Trap for N2 gas, while for He we have a Universal Trap that's the only difference. We are not sure why this is happening only in some injections with N2 as carrier gas? Any ideas or suggestions to fix the issue?

GC Parameters:
Agilent GC 6980
FID Detector with capillary column
Split injector
Carrier and Make up gas: N2

Thank you so much!

[James_Ball]

Column Flow?

Are you using same ml/minute or head pressure or linear velocity for the two gasses?

If same head pressure, the ml/minute will be different, if same ml/min the linear velocity will be different.

Unless you have calculated the linear velocity for each gas and keep that the same, then the peak shapes will change. Higher linear velocity usually gives sharper peaks.

[DR]

Agree - you need to use one of the carrier gas conversion tools available.
In theory, going to Nitrogen should mean longer run times and smaller peaks.
If you have to switch from He, I'd look at Hydrogen instead on Nitrogen. That change should benefit your separation - shorter run times, better peaks!

Either way, you have to do a little fiddling to find a sweet spot for your flow rate, and beware that different carriers will yield slightly different capacities too (if memory serves).

[VJP]

I have used the Big Universal N2 Trap and made some injections. If i overlay 6 injections from same vails, 2 injections has taller peaks (unusual) while other 4 injections has shorter and broader peaks. Not sure why getting taller peaks? I have used the same column flow rate when switched from He to N2. Do you think it could be due to LINEAR VELOCITY of N2 being 42 cm/sec which is higher than van Deemter Curve 10-20 cm/sec (FYI: The He velocity in original method is 41 cm/sec)? The run time for He method and N2 are same. It's isothermal method, column ID 0.53mm.