Boron trifluoride (BF3) -methanol complex is widely used for the preparation of methyl esters as the sample for gas chromatography. However, it is feared that short-chain fatty acids are lost, when esters are prepared from fats and oils containing these acids by the conventional BF3-methanol method.
Collaborative studies were carried out for the purpose of establishing a standard method for the quantitative preparation of short-chain esters.
Soybean oils containing definite amounts of glycerol trihexanoate and glycerol trioctanoate were prepared and used as the samples for the collaborative studies. GC analysis of the esters prepared from these samples by the conventional method revealed the appreciable loss of hexanoic- and octanoic acids through the procedure. Partial evaporation of the short chain esters and partial passage of them into aqueous sodium chloride layer were supposed to be responsible for the low recovery of these esters.
Further collaborative studies were performed to examine the conditions for the prevention of the loss of short chain esters.
In conclusion, the collaborative studies suggested that the recovery of the esters of short-chain acids (C6 and C8) could be improved by the following modification of the conventional method.
a) Shaking the mixture after the addition of aqueous sodium chloride solution.
b) Reextraction of esters from the aqueous layer after the separation of hexane (or heptane) layer.
c) Immediate GC analysis after the esterification.
It was concluded that 7 ml of 7%-BF3-methanol reagent was enough to esterify 300 mg of oil sample.
