Frederik R. van de Voort

An automated FTIR method for the routine quantitative determination of moisture in lubricants: An alternative to Karl Fischer titration.

An accurate primary Fourier transform infrared (FTIR) method for the determination of moisture in mineral and ester based lubricants has been developed based on the extraction of moisture into dry acetonitrile. FTIR evaluation of acetonitrile extracts from new and used lubricants as well as common lubricant additives and contaminants which might co-extract indicated that phenolic constituents interfered significantly with moisture measurements. By measuring moisture at 3676cm(-1) on the shoulder of the asymmetric OH stretching band, spectral interferences from extracted phenolic constituents were minimized. The spectra of calibration standards (0-2100ppm), prepared by gravimetric addition of water to dry acetonitrile, were recorded in a 1000-mum CaF(2) transmission flow cell and produced linear standard curves having an S.D. of approximately +/-20ppm. Lubricant sample preparation involved the vigorous shaking (20min) of a 1:1.5 (w/v) mixture of lubricant and dry acetonitrile, centrifugation to separate the phases, acquisition of the FTIR spectrum of the upper acetonitrile layer, and subtraction of the spectrum of the dry acetonitrile used for extraction. A Continuous Oil Analyzer and Treatment (COAT((R))) FTIR system was programmed to allow the automated analysis of acetonitrile extracts, and the methodology was validated by analyzing 58 new and used oils, independently analyzed by the Karl Fischer (KF) method. Linear regression of FTIR versus KF results for these oils produced a linear plot with a between-method S.D. of +/-80ppm. As implemented on the COAT((R)) system, this FTIR method is capable of analyzing 72 acetonitrile extracts/h and provides a high-speed alternative to the KF titrimetric procedures for the determination of water in lubricants.

A New FTIR Method for the Analysis of Low Levels of FFA in Refined Edible Oils

Abstract 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 30 s 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%.

A novel wire mesh "cell" for studying lipid oxidative processes by fourier transform infrared spectroscopy.

A novel infrared (IR) sample handling accessory has been developed to monitor and study oxidation processes of edible oils under moderate temperature conditions by Fourier transform infrared (FT-IR) spectroscopy. A reusable stainless steel mesh IR “cell” was designed and evaluated from the standpoint of mesh size, transmission characteristics, its ability to entrap oil, and techniques to apply sample and normalize path length so as to obtain good quality, reproducible spectra. The concept is to entrap oil within the mesh by means of its inherent surface tension and to take advantage of the high surface area provided by the mesh to facilitate rapid oxidation of the oil by air at ambient or slightly elevated temperatures without having to resort to more extreme temperature conditions to track oxidative changes in real time. Changes taking place in canola oil at room temperature, in the dark and exposed to light, as well as at 50 °C are presented to illustrate the performance of the cell in monitoring oxidative changes in real time (e.g., formation of hydroperoxides, loss of cis and formation of trans double bonds). The mesh cell should be useful for comparing the relative performance of antioxidants as well as evaluating the oxidative stability of oils, among other applications.

Principles, Performance, and Applications of Spectral Reconstitution (SR) in Quantitative Analysis of Oils by Fourier Transform Infrared Spectroscopy (FT-IR)

Spectral reconstitution (SR) is a dilution technique developed to facilitate the rapid, automated, and quantitative analysis of viscous oil samples by Fourier transform infrared spectroscopy (FT-IR). This technique involves determining the dilution factor through measurement of an absorption band of a suitable spectral marker added to the diluent, and then spectrally removing the diluent from the sample and multiplying the resulting spectrum to compensate for the effect of dilution on the band intensities. The facsimile spectrum of the neat oil thus obtained can then be qualitatively or quantitatively analyzed for the parameter(s) of interest. The quantitative performance of the SR technique was examined with two transition-metal carbonyl complexes as spectral markers, chromium hexacarbonyl and methylcyclopentadienyl manganese tricarbonyl. The estimation of the volume fraction (VF) of the diluent in a model system, consisting of canola oil diluted to various extents with odorless mineral spirits, served as the basis for assessment of these markers. The relationship between the VF estimates and the true volume fraction (VFt) was found to be strongly dependent on the dilution ratio and also depended, to a lesser extent, on the spectral resolution. These dependences are attributable to the effect of changes in matrix polarity on the bandwidth of the v(CO) marker bands. Excellent VFt estimates were obtained by making a polarity correction devised with a variance-spectrum-delineated correction equation. In the absence of such a correction, SR was shown to introduce only a minor and constant bias, provided that polarity differences among all the diluted samples analyzed were minimal. This bias can be built into the calibration of a quantitative FT-IR analytical method by subjecting appropriate calibration standards to the same SR procedure as the samples to be analyzed. The primary purpose of the SR technique is to simplify preparation of diluted samples such that only approximate proportions need to be adhered to, rather than using exact weights or volumes, the marker accounting for minor variations. Additional applications discussed include the use of the SR technique in extraction-based, quantitative, automated FT-IR methods for the determination of moisture, acid number, and base number in lubricating oils, as well as of moisture content in edible oils.