Chromatography Forum

Good Day,
I am trying to quantify methanol in my sample using Headspace GC/FID .
I have used an external calibration using methanol.
The problem is that the peak area of methanol in my unknown sample is bigger than the peak area in pure methanol , this can not be possible !!!
Can you please help Plz ??

Hello!

This is really weird...
I also analyse methanol... my samples are usually wine... What kind of sample do you have?

Maybe you have some co-eluting peaks... Ethyl acetate may be a coeluting peak if you use a Wax column, for example...it dependes the temperature gradient, of course... acetaldehyde and ethanol have retention time next to methanol too.

Besides that... What is the higher concentration which you use in the calibration curve? And what is the methanol concentration you hope to find in your sample?

I hope that is helpful

There simply has to be a peak that is coeluting with methanol; acetone, ethyl acetate...

If you are doing dilutions for your standard and sample prep, are they correct?

first check your sample preparation and injection
maybe it is acetone with methanol

Good Day,
I want to check the purity of methanol regenerated after transesterification reaction.
I am using a pure methanol 3 standards for external calibration (100%), 50% methanol-water and 5% methanol-water.
Is this correct ?

Good Day,
I want to check the purity of methanol regenerated after transesterification reaction.
I am using a pure methanol 3 standards for external calibration (100%), 50% methanol-water and 5% methanol-water.
Is this correct ?

I don't like this: your standards are in differing amounts of water, and expansion of water will affect your results.

If you want to use water as solvent, keep injection volume to 0.5 microliters or less and dissolve your standards and samples a lot with water.

A higher-boiling solvent could also be used in place of water.

yea there are a ton of high boiling solvents you can use. Xylenes would be my choice but you could go environmentally friendly and use limonene or pinene.

Good Morning ,
Thanks all !!
So i need to use xylene as solvent for my external calibration curve.
Also do you i need to add an internal standard for this type of method ?
Does anyone have another method for the methanol purity ?
i don't care what are the impurities i just need to know how much is the mass concentration of methanol in my non pure samples !!!

I do ethanol a lot but I use total evaporative headspace sampling with acetonitrile as my internal standard. for direct inject if it is just dilute and shoot and your ALS is very precise you could probably get away without an internal standard.

Dozens of things could be wrong. This will go a lot quicker if you tell us your complete method, including what the samples are and how you prepare them, the GC setup etc etc, the more we know about what you are doing the more we can help.

Peter

Buenas tardes, la curva de calibracion esta muy concentrada, dudo mucho que el biodiesel tenga concentraciones del 100% de metanol. Si debes utilizar un estandar interno porque pueden llegar a indicar algo mas o algo menos y por ese tu señal en la muestra te sale mayor que la curva. En mi caso, ese metodo lo monte en una columna innowax de 30 metros, utilizo isopropanol como estandar y aplico la norma EN 14110 que es para concentraciones menores a 1% en biodiesel. Lo que hacemos es lavar el biodiesel con agua y despues lo dejamos a 110°C por dos horas y con esta matriz preparamos el estandar. Espero te sea util