I'm studying gas-chromatography but I don't undertand one crucial point.
When I'm in liquid chromatography if I use a polar compound (-SiOH) as stationary phase the most polar solute will eluite first.
When I use some apolar phase, the less polar solte will eluite first.
I wonder if this difference will occour also in GC...or is the eluition order always defined by the relative vapor pressure (volatility) of the different solutes
i.e: the most volatile solute will always elute first
I wouldn't normally respond to a thread that is nearly 3 months old, but as the author appears to be a regular/frequent poster, I figured why not as I had some data to hand that might help...
Like Mravche, I think there is something wrong with the author's comments regarding LC...
With normal phase HPLC (i.e. Polar stationary phase and non-polar mobile phase) polar analytes will be retained for longer on the column than less polar analytes and hence elute later.
With reverse phase HPLC (i.e. Non-polar (like C18) stationary phase and polar mobile phase) non-polar analytes will be retained for longer than polar analytes and hence elute later. The classic 'like attracts like' really helps getting to grips with this.
GC is (in my view) a bit simpler than HPLC in some ways. Volatility does play an important part in elution order but other factors (such as the choice of column) are as, if not more important. The following samples were run on a simple GC-FID using an isothermal temperature programme and most importantly using a highly polar ZB-WAX+ column.
Reference Sample RT (mins) VP (kPa)
TMP Liquid 1.410 5.1
Octane Liquid 1.502 1.33
Methanol Liquid 1.647 12.9
Ethanol Liquid 1.743 5.8
Propanol Liquid 2.241 2
You can probably see that Octane (which has the lowest VP) comes off the column fairly rapidly, whereas methanol (which has the highest VP) comes off after
Octane. Using a different column (such as a ZB-1) and similar conditions the elution order and retention times would be quite different, although you would expect that VP would play a significant part in defining order of elution of 'similar' compounds like these simple alcohols.
Hope that Helps