Chromatography Forum

Dear All,
I am investigating whether I can derivatize glycerine to something more volatile.
The trouble is that my samples contain mainly ethanol and water.
Please can anyone point me in the right direction?
Best Regards
WK

What levels of glycerin are in this ethanol water solution? Trimethylsilyl derivatives of glycerine can be detected at low levels by GC-FID.

We routinely assay for glycerin in liquid soap type products, which can be up to 85% water, by dissolving the sample in DMF and derivatizing an aliquot. Excess derivatizing agent is added to react with the water (and alcohol, in your case). Levels of glycerin in the 0.1% range are readily quantified, been doing this for decades....

TMS derivitives are easily made. It requires an excess of the best reagent. You should go to sites that sell these reagents and determine for yourself the best solution.

I do mean an EXCESS of reagent. You generally require a 5 to 10 fold excess of the molar amounts of content that will react with the reagent for best results. Using 10mg of water may require 1mL of reagent for example (not a good one, but hopefully illustrative)

best wishes,

Rod

Thank you for the replies !
I have had a brief go this morning with an ethanol solution of glycerine.
Would you expect to see 2 peaks for the glycerin derivatives?
Regards
WK

Would you expect to see 2 peaks for the glycerin derivatives?
Regards
WK

No, WK, have never seen that. You must make sure that you have excess derivatizing agent: you're reacting water, ethanol, glycerin, and whatever else has active hydrogens in your sample. Add additional derivatizing chemical (e.g., BSTFA + 1% TCMS), make the overall concentrations of sample solution the same, and see if the peak gets larger or if one peak disappears. And, of course, run a blank, but you already know that. I've been derivatizing glycerin this way for 3 decades, there were publications out there for this.

If there is insufficient derivatizing agent, I could see where one could (for example) derivatize one or two of glycerin's hydroxy functions but not the third, which could result in two peaks. Do you have GCMS available? Glycerin-3TMS has distinct EI mass spectrum.

Hi CPG,
I will get back on to it tomorrow but I will need to get some reagent in anyway.
I ran a solution of glycerine & ethanol under same conditions and there were no peaks
near glycerin RT in the derivatized glycerine.But then of course response for glycerine is low (Homer moment....). I am using 20-40mg with 2ml BSTFA/TMCS solution.
These 2 peaks also occurred when I used glycerine only with reagent. Both were not in the reagent blank.
I will check the solutions tomorrow after they have sat overnight.
Would you expect glycerine to react instantly at room temperature?
Thnaks again
WK

It should be completely reacted in less than 1 minute.

I used to use Sweeley reagent, but it is hard to get in high purity.

TMS-imidazole is another reagent used in samples with high water content.

Rod

Would you expect glycerine to react instantly at room temperature?

Yes.