Chromatography Forum

hi all (this is my first topic!),

during the sample preparation of GC analysis of some fatty acids
I perform esterification (by BF3/Methanol) of:

caprilic acid
capric acid
myristic acid
palmitic acid
stearic acid

in the chromatograms I obtain these peaks:

caprilic acid methyl ester
capric acid methyl ester
myristic acid methyl ester
palmitic acid methyl ester
stearic acid methyl ester
linoleic acid methyl ester

the question is:
which fatty acid gives the linoleic acid methyl ester?
It is an esterification isomer, I suppose, right?

Thanks in advance for your help!

Linoleic acid methyl ester comes from linoleic acid.

Which one of the acids you esterified contained the linoleic acid is the next question.

I assume you esterified the acids with a reaction that did not exceed 150°C and for a period of time under 20 minutes.

Is this correct?

best wishes,

Rod

the fatty acids I esterified are 99%+ pure, and the linoleic acid methyl ester peak area is too big to think that linoleic acid was an impurity, IMHO

esterification run about 30 minutes

T was not under strict control, I controlled only the rate of drops falling down during reflux (not too fast) and that no boiling occurred

thank you Rod

With fatty acids, you only need to heat with BF3-methanol a few minutes at most, like five minutes. We've done this in 100ml volumetric flasks (maybe 15 ml BF3-methanol) and used a steam bath for 30 years. You need to esterify your standards separately to determine which contains the linoleic acid. I'm surprised you find linoleic acid methyl ester but no oleic or linolenic. When we assay hydrogenated soaps or fatty acids here, that's all we get: saturated methyl esters.

Is your detector MS and that's how you identified as linoleic acid methyl ester, or fID using retention times, where maybe it could be C20? On a nonpolar capillary linoleic and oleic methyl esters will elute before stearic acid methyl ester, but after it if on a polar column like SP-2330.

CP Guy is quite correct about the speed of BF3 in methanol and his suggestions are sound.

In fact if you heat linoleic acid for more than about 10 minutes, and at temperatures higher than 100°C you can easily degrade it if you have oxygen (air) in the reaction vessel.

I suspect you have contamination of the glassware, like a residual soap product, or an earlier sample which contaminated your analysis.

You don't make linoleic acid ME from saturated FA MEs

Either your standards are contaminated or your equipment was.

I hope you find out the problem.

Rod

I assume you performed a full blank ( rinsed similar vials/glassware, used same dispensing devices, mixed reagents, refluxed, treated blank solution same as for samples ), and injected that.

If not, you should perform that first. You should then confirm that the peak is linoleic ( MS or use a different polarity column )

As noted by those above, Linoleic is a contaminant, and you have to identify the source. You will not form it from the listed compounds and conditions.

I don't think it will be from your standards ( it is a C18:2 ), more likely from contaminated reagents or glassware ( especialy vial caps, and a precusor can be present in some cleaning agents ) - hence the need for a good blank.

If you blank is clean, then you have to look to the standards as the source - test them individually. If the blank is positive, inject a solvent blank using the injector to separate injector / instrument contamination from sample contamination.

You have to be systematic to identify and eliminate the contamination.

Bruce Hamilton