After understanding the following you should know why Nitrogen, Helium and Hydrogen have different graphs when plot them according to Van Deemter equation.

We will discuss the different terms in the equation.We are looking for the gas which give lowest values for all terms “A,B,C” in the equation so we get lowest HETP which give best separation.

1- Term AIt represent the different paths in the column “ we have long and short paths ” Helium > Hydrogen > NitrogenThat is because the molecular weight of Nitrogen is larger than M.wt of Helium and Hydrogen so it is very slow in the column. Also the flow of Nitrogen at lowest HETP “which give best efficiency“ is very slow which mean that Different paths does not effect time for nitrogen to go out from the column.

If we apply the same principle on Hydrogen and Helium we can say the A term for Hydrogen is larger than A term for Helium. That is not true.

In fact A term for Helium is larger than A term for Hydrogen and that is because Hydrogen is very light and its flow at minimum HETP is large enough to delete the effect of different paths.

N2 give the lowest value for term A then H2 and at last He.

2- Term BIt represent solute diffusion in gas phaseHydrogen 0.082kg/m3 > Helium 0.164 kg/m3 > Nitrogen 1.153 kg/m3

As density of carrier gas increase as the diffusion of solute in gas decrease “ it can not find a space to fill in high density gas” so term B decrease.

N2 give the lowest value for term B then He and at last H2.

3- Term CIt represent resistance to mass transferNitrogen 1.153 kg/m3 > Helium 0.164 kg/m3 > Hydrogen 0.082kg/m3 C term depend on the following:Thickness of stationary phaseDiffusion of solute in liquid phaseFractional of cross section occupied by liquid phaseFractional of cross section occupied by gas phase --- This is the only parameter that changes when changing the mobile phase and it is directly proportional with C term

As density of carrier gas decrease as cross section occupied by the gas decrease so term C decrease.

H2 give the lowest value for term C then He and at last N2.