Selection of proper internal standard for determining microwaveassisted interesterification yield of soybean oil

The objective of this study was selection of appropriate internal standard for determining the purity of soybean oil alkyl esters using gas chromatography. In this study, soybean oil fatty acid methyl esters were produced by microwave-assisted interesterification method. Margaric acid methyl ester and butyric acid were added as internal standards in the ratios of 10 and 30 percent (w/w) of soybean oil methyl esters. According to gas chromatography results, error percent at the concentrations of 10 and 30 percent were respectively 85.00 and 113.33 percent for butyric acid and respectively 0.10 and 6.67 percent for margaric acid methyl ester. Although, using butyric acid as an internal standard had some advantages such as preparation time saving and less raw materials consumption and final cost in comparison with margaric acid, but according to butyric acid polarity and lower molecular weight, it cannot be used as a good indicator for oils containing long-chain fatty acids such as soybean oil. Although, using margaric acid requires interesterification, but the results are more reliable. Therefore, using margaric acid methyl ester as an internal standard is proposed for determining the interesterification yield of soybean oil.