Hi,

I am a new GC user. I was trying this HP-1 capillary column (25m) and noticed something which was puzzling to me. I made several injections of pure methanol on this column and noticed that the results were the opposite of what I expected them to be. I started out with using 200C for oven temperature. In the succedding injections, I used 150C followed by 100C. What I noticed was that as you lower the oven temperature, the retention time of methanol became shorter which was the exact opposite of the results that I expected to see. Can anybody out there explain to me why this is so?

Hi,
it seems you set flow rate by setting head pressure. Lowering the oven temperature you should correct a flow rate of carrier gas, because of increasing its linear velocity. Methanol shows a very little retention on HP-1 and elutes nearly in dead time. When linear velocity of your carrier gas increases, methanol elutes earlier.

If you go to Agilent's site you can download a nice little program which will calculate flow at different temperatures or pressures.

You can note your inlet pressure gauge readings at different temperatures and put them into the program to see the effect on flow rate and linear velocity.

Regards,

Ralph