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

Abstract

Tetramethylammonium acetate (TMAAc) was found to be a selective reagent for the methylation of free acids. It was shown that free fatty acids and their salts could be fully derivatised by TMAAc in the presence of esterified fatty acids. This was performed on extracts containing mixtures of wood extractives and directly on pulps containing extractives. The alkaline reagent tetramethylammonium hydroxide (TMAH) was efficient in hydrolysing and derivatising all fatty acids present, both as free acids and in different esters. Even steryl esters, which are difficult to hydrolyse, were completely cleaved and the fatty acid part was obtained as the corresponding methyl ester. The results from the pyrolytic technique were in agreement with the results 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 conditions when TMAH was used. When the samples containing polyunsaturated fatty acids, 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 standard in the analytical pyrolysis experiments. The use of different ammonium salts in combination with Py-gas chromatography (GC) offered a rapid method for the estimation of free, esterified and total acids.