Chromatography Forum

Hi All,

I have mistakenly configured the gas type from N2 to H2 into my 7890A (front inlet) and notice the first C1 peak appear after 8 minutes. after restoring the configuration, C1 peak appears at the expected retention time.

My question is, how does changing the configuration affect the retention time?
Thank you for assisting as i'm still new in GC.

Even though on most modern GCs, the flow is set in terms of a flow rate(usually mL/min) the GC actually can't directly control a flow. Instead, when you punch a flow rate into the GC, the instrument calculates what head pressure is needed to achieve that flow rate based on a couple of factors you supply-specifically the carrier gas and flow rate-as well as the oven temperature.

Smaller diameter columns have more resistance to flow, as do longer columns, so a higher head pressure is needed to get a given flow rate through the column.

In general, a gas's viscosity increases as the temperature increases, so if you are using a temperature program the instrument will actually vary the pressure through the run to keep the flow rate constant.

Then, to directly answer your question, different gases have different viscosity profiles.

To give some real world numbers-for a common 30mx.25mmx.25µm column at 200º vented to atmosphere, the pressure needed for nitrogen is 18.65psi, helium 19.86psi, and hydrogen 10.93 psi. If you are using nitrogen and the GC sets the inlet to 10.93 psi because you told it you are using hydrogen, the flow rate drops to .49mL/min. This lower flow rate will give you longer retention times.

All of these numbers came from Restek's flow calculator

https://www.restek.com/ezgc-mtfc

Hi Thank you so much for the reply.

It help me a lot especially in term of understanding.
Thank you again and stay safe.