Fatty acids analysis - in the presence of 90% water.

[magda roth]

I am trying to analyze C6-C10 aliphatic carboxylic acid in a water solution (yes, it contains surfactants to have a stable solution!). Is there any derviatization method I can use in the presence of plenty of water (more than 90%)? Anybody has experience in derivatization in the injector port with m-trifuoromethylphenylammonium hydroxide? Or other derivatization technique? I have a basic HP5890, everything very simple and cheap and a DB5 megabore column, FID. The lab also has HPLC available, I can switch the analysis there (did not se separation for C6-C8 acids...)

Thank you

Magda

[chromatographer1]

I have not heard of anyone making an ester from those acids in 90% water. I would acidify with sulfuric acid and extract into hexane or ether. Dry with sulfate salt.

Then I would make a TMS ester or methyl ester and do GC or do ion exchange column via LC.

Good luck,

Rod

[magda roth]

Thank you Rod. Unfortunately, it will not work in my environment: there is a lot of other crap in the product. I would love to be able to go in dry conditions and there are plenty of derivatization techniques. There is some literature about on-column derivatization and I was wondering if that would work (not pyrolisys, but regular injector temperature). The reagent is sold by Sigma Aldrich Catalog # 1899-02-01, references : B.S. Middleditch and D.M. Desiderio in Anal. Lett. 5, 605 (1972). I do not have the original article. Was wondering if anybody is using this procedure and what are the pitfalls.

Yeah, I need some luck...

Magda

[chromatographer1]

I doubt in an aqueous solution these would work.

I had inconsistent results with dry samples years ago.

Trace amounts of acid in samples, perhaps, followed by a thorough cleaning of the injector.

But my results may not be yours (and again they may be)

best wishes,

Rod

[Consumer Products Guy]

OK, suppose one had a dilute liquid soap solution. Add methanolic sulfuric acid, cook on steam bath 10 minutes in a flask, cool, add a small amount of hexane, add some NaCl solution, draw off the fatty acid methyl esters. Inject the hexane layer of fatty acid methyl esters.

[magda roth]

Thnaks, will try. Unfortunately have some other URGENT issues (there is always paper work more important then lab work! , ) but will report back.

[Peter Apps]

OK, suppose one had a dilute liquid soap solution. Add methanolic sulfuric acid, cook on steam bath 10 minutes in a flask, cool, add a small amount of hexane, add some NaCl solution, draw off the fatty acid methyl esters. Inject the hexane layer of fatty acid methyl esters.

Wouldn't you need to get rid the water to drive the equilibrium towards the esters - A Dean and Starke separator would do it if the samples were large enough.

Peter

[Consumer Products Guy]

OK, suppose one had a dilute liquid soap solution. Add methanolic sulfuric acid, cook on steam bath 10 minutes in a flask, cool, add a small amount of hexane, add some NaCl solution, draw off the fatty acid methyl esters. Inject the hexane layer of fatty acid methyl esters.

Wouldn't you need to get rid the water to drive the equilibrium towards the esters - A Dean and Starke separator would do it if the samples were large enough.

Peter

No, there'd be excess of methanol. If you'd rather, add excess of sodium hydroxide, dry down to remove the water or most of it, then take up in sulfuric-acid methanol directly, take to steam bath, etc.

[Johnny Rod]

Sounds like a nightmare for GC. Can't you use the LC with RI or ELS detector, if UV isn't so good?

[Yama001]

Ion chromatograhy on the HPLC with UV works well, although the surfactants give our analysts fits.

[magda roth]

Thank all. As soon as I will be allowed to even do some chemistry not just useless paper work will try to have some fun.

I like the most the CSI comment : Unfortunately, I have people who believe that is for real...Is difficult to explain chemistry for guys (and gals) who consider "cool" to be ignorant.

[magda roth]

Going back to the same issue: derivatization of carboxylic acid in the presence of large amount of water, surfactants and glycols. Is there any derivatization agent/method to enhance UV absorption (and change the wavelenght?).

Thanks

[chromatographer1]

You might try a negative absorption UV detection. Put a small amount of UV absorbing component into the mobile phase and then see if you can detect peaks (your acids) which lower the absorption of the UV light.

Good luck, you will need it.

Rod

[magda roth]

I am not familiar with the negative absorption technique. So to understand : I will spike the mobile phase assuring an absorption of let's say 100 mAU. When the carboxylic acid is eluting, by its presence the absorption will be lower. Interesting! Question: the spike has to be neutral, not reacting with the carboxylic acid. If I need to have gradient, what is the proper technique: Do I need to spike both solvents? In practical termss; Maybe I can do a blank run and substract the background...
Thanks for the suggestion.

[chromatographer1]

There is not a simple answer to your questions, but yes, this 'spike' should not react with anything in your sample or with your column packing. Did not claim it was simple or easy. I think it would behove you to perform ion chromatography, perhaps with a sample prep to remove non-acids from your sample.

But money time hardware and abilities all have bearing on your solution which I certainly can't state with confidence with what I know about your situation.

best wishes,

Rod