Chromatography Forum

Hi !

I was working on a analysis of ethanol's volatile impurities by GC.
At first, I had a method from the USP, but it didnt give me a good resolution.

So I changed some parameters:

velocity (35cm/s to 28cm/s)
Starting temperature of oven (40°C to 30 °C)
Oven warming (10°C/min to 5°C/min)
And split ratio (20:1 to 50:1)

Now I obtained a good resolution, but in a sample with 0.01% of 4-methylpentan-2-ol in alcool, the peak of this compound just disapeared, while it was clearly here in the first USP method.

I can't find why, does someone have an idea ?

So, 0.01% is nominally 100 ug/ml (mas o' menos) and at 20:1 that means you are putting about 5 ng on column. At 50:1 only 2ng which is getting very near the detection limit of the FID. Also, is the compound reactive at all since lower concentrations of reactives can disappear. Plus, if you broaden the peak....

Best regards,

AICMM

Thank you for your answers AICMM.

About the split ratio, the thing is that in a other sample I have acetaldehyde and methanol 0.001% in ethanol, and it was detected even with a split ratio of 50:1.

A new test is running right now with fresh mixture. If I still have this problem, I will notwithstanding try to lower the split ratio, to 35:1 maybe...

Finally it's ok, I found it. It were just far far after the original retention time and I forget to check there.

Sorry for that !