Chromatography Forum

Dear all,
I am interested to analyze palmitic and 16-hydroxyhesadecanoic acid at the gas chromatograph using DB-WAX column with a FID detector. I have both the standards.

I am doing the esterification of the fatty acids to fatty acid methyl esters (FAMEs) using the alkylation derivatization reagent BCl3-Methanol. Unfortunately the esterification of the 16-hydroxyhesadecanoic can not be achieved. Can someone can give me an explanation?

Furthermore, which kind of derivatization for both the compounds can be used for their analysis at GC-FID (reagents, columns, protocols.…)

Thank you in advance for the help!!!!

I don't see an obvious reason why your procedure would not work for both acids. The hydroxyl group is way in the middle of the molecule, far away from the reactive carboxyl group so it doesn't make sense to me that it wouldn't work. How are you pulling your derivatives from the reaction medium?

I use MeOH:BF3 from Sigma-Aldrich. I doubt that the chlorine vs. fluorine will make a difference but you never know.

I don't see an obvious reason why your procedure would not work for both acids. The hydroxyl group is way in the middle of the molecule, far away from the reactive carboxyl group so it doesn't make sense to me that it wouldn't work. How are you pulling your derivatives from the reaction medium?

I use MeOH:BF3 from Sigma-Aldrich. I doubt that the chlorine vs. fluorine will make a difference but you never know.

Dear colleague, thank you for your replay.
You are right...it doesn't make sense...at the moment I have also tried to resuspend directly the omega hydroxy acid directly in the MeOH:BCl3...but I can not appreciate the methyl ester at the GC. On the contrary I didn't found any problems for the fatty acid either extracted from the reaction medium with hexane or resuspended directly in EtOH or in MeOH:BCl3.

We've done methyl esters of hydroxy fatty acids in the past, specifically ricinoleic acid from castor oil. I'm thinkin' that you did esterify the hydroxy fatty acid and just need to extend to relatively higher temperatures on the GC run, like higher temperatures that you might expect, try higher temperatures and longer run times until you get it. The hydroxy group adds mass and polarity.

Depending upon your system/solvent, if you can make trimethylsilyl derivative you will derivatize both the hydroxy group and the carboxylic acid group.

We've done methyl esters of hydroxy fatty acids in the past, specifically ricinoleic acid from castor oil. I'm thinkin' that you did esterify the hydroxy fatty acid and just need to extend to relatively higher temperatures on the GC run, like higher temperatures that you might expect, try higher temperatures and longer run times until you get it. The hydroxy group adds mass and polarity.

Depending upon your system/solvent, if you can make trimethylsilyl derivative you will derivatize both the hydroxy group and the carboxylic acid group.

Many thanks. I can not go further than 250° C because I am using a DB-Wax column. I will try to extend times. The simple fatty acid came out at around 190°C.

OH group increase the polarity. The effect into DBwax is to increase the elution temperature. You have two way to solve the question. One: As suggest try sililation. Is the best way because free hydroxyl group tends to be unstable into high gc temperature. With sililation you overcome the problem. TWO : try to use a UNPOLAR column such as DB5 or similiar. In this way the elution temperature (for hydroxy acids) DECREASE in comparison with Carbowax. Probabily the peak shape became unsimmetrical (tailing) but you must see anyway the peak.

Robertino Barcarolo, Italy