A rapid method to discriminate between free and esterified fatty acids by pyrolytic methylation using tetramethylammonium acetate or hydroxide

Tetramethylammonium acetate (TMAAc) was found to be a selective reagent for the methylation of free acids. It was shown that free fatty acids and thei r salts could be fully derivatised by TMAAc in the presence of esterified f atty acids. This was performed on extracts containing mixtures of wood extr actives and directly on pulps containing extractives. The alkaline reagent tetramethylammonium hydroxide (TMAH) was efficient in hydrolysing and deriv atising all fatty acids present, both as free acids and in different esters . Even steryl esters, which are difficult to hydrolyse, were completely cle aved and the fatty acid part was obtained as the corresponding methyl ester . The results from the pyrolytic technique were in agreement with the resul ts of other more laborious techniques. Polyunsaturated fatty acids, such as linoleic acid and linolenic acid, were isomerised to a number of different octadecadienoic and -trienoic acids, due to the strongly alkaline conditio ns when TMAH was used. When the samples containing polyunsaturated fatty ac ids, were pyrolysed in the presence of TMAAc, no isomerisation was observed . The resin acids can be considered to be always non-esterified. Thus, when they are present, they can be used as a reference similar to an internal s tandard in the analytical pyrolysis experiments. The use of different ammon ium salts in combination with Py-gas chromatography (GC) offered a rapid me thod for the estimation of free, esterified and total acids. (C) 1999 Elsev ier Science B.V. All rights reserved.