Chromatography Forum

Hi,

I'm using GC to analyze a biodiesel sample.
I chose internal standard method, and C17:0 is the internal standard.
Today I injected two kinds of samples both of which I know the exact concentrations.

The problem I have is that the relative response factor is varied by concentration. The specs of my samples and calculated relative response factors of C18:3 are like this:


Sample1
C17:0 (Internal standard), Conc. = 0.5, Relative RF = 1
C18:3, Conc.=0.249, Relative RF = 0.993

Sample2
C17:0 (Internal standard), Conc = 0.251 mg/ml, Relative RF = 1
C18:3, Conc. = 0.5 mg/ml, Relative RF = 0.958


With another sample, I got the relative RF of C18:3 as around 0.78.


I don't know why internal standard method doesn't work for me.
It worked before when I was using a different column (non-polar).
But I needed the current polar column for better analysis...

The internal standard method should work no matter what type of GC column that I'm using, shouldn't it?

Does anyone has comment?

Your linolenic acid is probably degraded - contained peroxides of the acid so the purity of the acid was off by 20% (not unusual) .

I am making a lot of assumptions about your stds and what you are doing.

Your FID response of Linolenic acid and C:17FAME should not vary more than 5% or so unless one acid is impure (more than one possible cause).

Your first sample's RRF seems reasonable.

Your internal standard method should work as long as you do NOT have random co-elutions AND you do not degrade the linolenic acid 18:3 during sample preparation.

best wishes,

Rodney George

What about overloading or splitting problems?