Volatile free fatty acids, FAPP liquid injection

[Karen01]

I am going to have to quantitate SCFA from an alcohol or an alcohol water solution that could have other solvents in it too, which I would have to quantitate as well.

I need to keep sample prep quick, simple and easy, so no derivatization.
Because of equipment availability it would be best for me to do this by GC rather than LC.
Because of the very variable composition of the matrix, I think headspace is out even if there were no other issues.
That leaves liquid injection using a FFAP column.

I have done SCFA by headspace,but never using liquid injection. For headspace I acidify samples to about pH 2 to ensure the SCFA are protonated... do I need to do that for liquid injections? If so, what acid should I use?

I would think I would need to acidify, but searching on-line I see methods where the SCFA are just in water.

Thanks,
- Karen

[chromatographer1]

SCFA is what?

Rod

[Karen01]

SCFA is what?

Rod

Short Chain Fatty Acids

- Karen

[chromatographer1]

There is a history of FA analysis in which I have participated since 1980, and which dates back even earlier.
Recoveries can vary according to matrix and can vary greatly from sample to sample, even with the same matrix.

With water present in the matrix, Free acids are difficult to measure, as different solvents and other dissolved materials in the matrix can also make 100% recovery difficult.

Your suggestion about acidifying the solution is valid and necessary. It will also require repeated replacement of injection liners and quite possibly the use of a disposable retention gap at the inlet.

You will also have to validate the entire range of matrix composition to have an appropriate method.

Remember, Karen, that using standard addition, the method is self validating for recovery over the range of the additions. This is true for liquid injection as well as HeadSpace.

Your endeavor is daunting and I wish you success.

Rod

[Peter Apps]

The first imponderable is the likely concentration of the acids; more information about the problem gets you more useful answers.

Also a lot hangs on how much water there is - if very little then simply injecting the sample might work, but adding acid might catalyse the formation of esters - which you could use as the basis of the analysis to make chromatographically tractable esters out of adsorptively active acids. If there are several tens of percent of water repeatable injections are likely to be difficult.

Peter

[chromatographer1]

I agree with Peter.

If the alcohol is dry then esters are likely.
As Peter said, the water content amount is critical.

Good luck,

Rod

[jdezeeuw]

There are many groups out there that do alcoholic-beverage analysis and inject alcohol water mixtures in the the range of 3-50%.
Biggest challenge is to get a good evaporation in the injection system and you need an inert column to give good acid peaks. Derivatized PEG (like FFAP)is one option which works pretty good for FFA.

evaporation is key here and thats where we need to fight our battle, Glasswool would be great for evaporation, but will hydrolyse and triggers discrimination. So the liner must be offer large heat capacity and relative low surface. One of our customer's favorite is a double gooseneck, but repeatability was about 8%. for him it was OK. I think it can be better using a cyclosplitter type.

Other then then that: I would also use Hot Needle injection. This should make the evaporation smoothly and liner type becomes less critical.

good luck

jaap
Restek corporation