Determination of free fatty acids in edible oils with the use of a variable filter array IR spectrometer

The feasibility of employing a portable variable filter array (VFA) IR spectrometer equipped with a transmission flow cell to quantitatively analyze edible oils or biodiesel feedstocks for free fatty acids (FFA) was evaluated. The approach to FFA determination employed was based on a previously reported FTIR method that involves the extraction of FFAs into methanol containing the base NaHNCN, which converts the FFAs to their salts, followed by measurement of the carboxylate absorbance at similar to 1,573 cm(-1) in the spectrum of the methanol phase. When this methodology was implemented on the low-resolution VFA-IR spectrometer, the analytical performance was comparable to that of conventional FTIR instrumentation at FFA concentrations of < 1%. However, at higher FFA levels, the relatively weak pulsed IR source of the VFA-IR spectrometer was found to provide insufficient energy for accurate measurement of the carboxylate absorption superimposed on the strong methanol absorption at similar to 1,450 cm(-1). By changing the extraction solvent to ethanol (EtOH), good spectra and calibrations could be obtained over an FFA range of 0-5%, having an overall SD of +/- 0.07% FFA. Based on this assessment, a VFA-IR spectrometer provides an economical instrumental means for at-line monitoring of FFA levels in crude and refined edible oils and biodiesel feedstocks, capable of analyzing similar to 20-30 prepared samples per hour.