Gas chromatographic analysis of fatty acid salts

Abstract

Generally, the analysis of fatty acid salts is based on their treatment with a strong acid to remove the cation, extraction of the liberated fatty acids and their methylation with an appropriate reagent for the gas chromatographic (GC) analysis of the fatty acid methyl esters. Surprisingly, it was found that fatty acid salts can be reacted directly with a methylating agent, e. g., boron trifluoride (BF 3)—methanol complex or dimethylformamide—dimethylacetal, without prior acidifacation and extaction. with BF 3—methanol, the fatty acid salt or salt mixture is reacted in a septum vial with excess of methanol and BF 3—methanol complex at 70°C for 30 min. After cooling, the reaction mixture can be injected directly into the gas chromatograph without any further preparation. By calibration, the contents of bonded and free fatty acids and the distribution of the individual fatty acids in the salt or salt mixture can be determined. The GC separation of the fatty acid methyl esters is carried out on a surface-bonded, cross-linked stationary phase (FS column) as usual. The analysis of metal stearate stabilizers and lubricants takes only 70 min and both the total and the distribution of the fatty acids, from lauric to behenic acid, can be calculated from a single analysis. For instance, the relative standard deviation was 2.66% for the content of free and bonded fatty acids in calcium stearate and 1.21% for the content of stearic acid.