Chromatography Forum

HI, I want to analyse short chain fatty acids, including formic acid, acetic acid, proponic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, captanoic acid, heptanoic acid, lactic acid and succinic acis, on the GC of Agilent 6890N without derivation in a water solution. the method is for oven is 100degree for 0.5min, then 8degrees/min to 180degrees, hold for 1min,and 20degrees/min to 200degree, then hold for 5mins, with a inject temperature of 180degrees,.however,I failed to found the peak of Formic acid, and the lactic always has a very small peak, it is very difficult to identify it from the noise.
the column is DB-FFAP 125-3237,30m long, from agilent
so ,anyone could advise me to increase the response factor of lactor and how to find the peak of formic acid?

Zgh,
Yuo didn't mention the detector you were using byt from the poor response I would assume it is an FID. These small molecules will always be difficult because as a first approximation the response on an FID is based on the number of carbons and these compounds just don't have enough to be very sensitive. Derivatization is really the best method to analyze these compounds at low levels, derivatives such as the methyl esters (or even higher esters) are one good ways to analyze these.

Regards,
Mark

Mark
first, i would like to extend my thanks to you.
you are right, the detector of my method is FID. from the point of you, I know the reason for the disappear of formic acid. But, I am still puzzled with the poor response factor of lactic acid and succnic acid, what is the reasone for these two, and how to improved the response factor without derivatization?
regards

zgh,

Lactic acid still has one three carbons and between the COOH and the OH these reduce the response also. Would not expect a great response for this compound either. Also with the OH I would think tailing could possibly be a factor sometimes which would aslo reduce the appearant response. The succinic acid being a diacid would have sort of the same problem, the response will be reduced by the second COOH and even with 4 carbons will still be somewhat reduced. There was an article published many years ago where the relative response of compounds in the FID was estimated and the effects of various groups on the response was summarized. I no longer have the article I'm afraid, but it was very good. Maybe someone esle has a copy or the reference they could share.

Regards,
Mark

Formic acid has very little response on a FID. It will elute after the acetic acid on a PEG-modified FFAP phase capillary as a fairly sharp peak. It may be under your propionic or isobutyric acid peak. I hate to bear bad news but I suggest that you should use a pDID or TCD for the analysis of this analyte. I have measured small amounts of formic in acetic acid. The presence of water will also make the analysis difficult. Let us know if you have any success.

PS you should be doing HPLC for this analysis. It is documented and straightforward with that technique.

Good luck

An FID responds best to unoxidized hydrocarbons. Formic acid is about as oxided as you can get carbon without forming carbon dioxide, so its response will be minimal on an FID. Lactic acid is also highly oxidized and will have a poor response. For a good analysis of these compounds you will probably have to form a derivative or use a different detector.