This paper summarizes the application of stoichiometric analytical approaches to quantitative IR analysis and describes the development of a rapid and sensitive Fourier transform infrared (FTIR) method using such an approach for the determination of low levels (<0.005%) of free fatty acids (FFA) in refined edible oils. The method simply involves mixing the sample with methanol containing 2 g/L sodium carbodiimide (NaHNCN) on a vortex mixer for 30s to convert the FFA to their salts, centrifuging the sample to separate the methanol phase containing the FFA salts from the oil, recording the FTIR spectrum of the upper methanol layer in a 100-μm CaF2 transmission flow cell, and ratioing this spectrum against that of the NaHNCN/methanol solution. The concentration of FFA salts is determined from the resulting differential spectrum by measurement of the v(COO-) absorbance at 1573 cm -1 relative to a reference wavelength of 1820 cm-1. A calibration spanning the range 0-0.1% FFA (expressed as oleic acid) was devised by gravimetric addition of a defined, pure fatty acid to an acid-free oil. Validation of the method by standard addition of palmitic acid to a variety of oils yielded an overall standard error of <±0.001% FFA. Comparison of triplicate FTIR and IUPAC titrimetric analyses of oils spiked with palmitic acid demonstrated that this FTIR method was more sensitive, accurate, and reproducible than the titration procedure, the latter having a significant positive bias of ∼0.02%. Solvent/oil consumption in the FTIR method is 2 mL/10 g versus 150 mL/20 g for the titrimetric procedure. The FTIR method developed is particularly well suited for the determination of the low levels of FFA in refined oils but can readily be adapted with a simple adjustment of the oil/methanol ratio to cover FFA levels of up to 4.0%.
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