First draft :D

GC column efficiency and carrier gas

In an open tube capillary column

a) The separation of the components of a mixture relies on their differences in partitioning between the stationary phase and the mobile carrier gas phase (jumping in and out of the two phases).

b) Most importantly. all components spend the same amount of time in the mobile carrier gas phase - it is the amount of time spent in the stationary phase that determines the separation.

c) If the linear velocity is too slow then the separation efficiency is reduced by diffusion of the separated components in the stationary phase and mobile phase. For a packed column Eddy currents also have an effect.

d) If the linear velocity is too fast then the components don’t spend enough time in the stationary phase to separate properly.

e) The best separation of a mixture is governed by the carrier gas linear velocity, not the volume flow!

These concepts are summed up in the van Deemter equation

See http://www.chromatographytoday.com/news ... ion/31273/

So, there is a sweet spot for the best efficiency. This is different for different carrier gases. This is best shown by a graphical interpretation of the equation – see for example

Image


You can see that Hydrogen is the most forgiving and Nitrogen the least forgiving.

A useful carrier gas flow/linear velocity guide can be found here (there are many others)
https://www.agilent.com/en-us/support/g ... alculators

To get the best separation we usually go for 1.5x the optimum linear velocity - near the lowest point in the curve to allow for changes in temperature and to keep on the best side of the curve.

This region equals the thinnest theoretical separation plates and therefore more separation plates per column .

See also the Jaap de Zeeuw article on the forum site under "GC Articles. Impact of GC Parameters on The Separation"
http://www.sepscience.com/Information/A ... l-Diameter

Plates - As an aside. the HETP (height equivalent theoretical plate number) in the graph refers back to older industrial distillation, which used cups – the more cups in the column (literally an industrial stack or column) the better the separation. The cups are now called theoretical plates. So the narrower the plates, the smaller the HETP, and the more can be fitted into the column and thus the more separation we can get.

Regards

Ralph