ABSTRACT
An automated FTIR method for the determination of the base content (BC(pKa)) of oils at rates of > 120 samples/h has been developed. The method uses a 5% solution of trifluoroacetic acid in 1-propanol (TFA/P) added to heptane-diluted oil to react with the base present and measures the ν(COO(-)) absorption of the TFA anion produced, with calibrations devised by gravimetrically adding 1-methylimidazole to a heptane-TFA/P mixture. To minimize spectral interferences, all spectra are transformed to 2(nd) derivative spectra using a gap-segment algorithm. Any solvent displacement effects resulting from sample miscibility are spectrally accounted for by measurement of the changes in the 1-propanol overtone band at 1936 cm(-1). A variety of oils were analyzed for BC(0.5), expressed as mEq base/g oil as well as converted to base number (BN) units (mg KOH/g oil) to facilitate direct comparison with ASTM D2896 and ASTM D974 results for the same samples. Linear relationships were obtained between FTIR and D2896 and D974, with the ASTM methods producing higher BN values by factors of ~1.5 and ~1.3, respectively. Thus, the FTIR BC method correlates well with ASTM potentiometric procedures and, with its much higher throughput, promises to be a useful alternative means of rapidly determining reserve alkalinity in commercial oil condition monitoring laboratories.
Subject(s)
Automation, Laboratory , Lubricants/chemistry , Oils/chemistry , Spectroscopy, Fourier Transform Infrared/methods , Heptanes/chemistry , High-Throughput Screening Assays , Humans , Potentiometry/methods , Research Design , Trifluoroacetic Acid/chemistryABSTRACT
This paper described an FTIR spectrometer coupled to an auto-sampler and some attendant methodologies for a direct free fatty acid (FFA) method and triphenylphosphine (TPP)-based method for determination of FFA and peroxide value (PV) of edible oils by FTIR spectroscopy using spectral reconstitution. The viscosity of oil samples is reduced by mixing them with a diluent; such lowered viscosity allows them to be readily loaded into a flow-through transmission cell. The spectra of the neat oil samples are then reconstituted from those of the diluted samples by using the absorption of a spectral marker present in the diluent to determine the dilution ratio. For the direct FFA analysis, quantification is achieved using the (COO(-)) band at 1712 cm(-1), while PV is determined using triphenylphosphine oxide (TPPO) absorption bands at 542 cm(-1). Calibration procedures and data are presented. Validation and performance data obtained with this automated system demonstrate that it is capable of analyzing approximately 90 samples/h, a rate commensurate with the throughput required by commercial contract or high-volume process control laboratories.
ABSTRACT
A rapid, practical, and accurate Fourier transform infrared (FT-IR) method for the determination of moisture content in edible oils has been developed based on the extraction of water from oil samples into dry acetonitrile. A calibration curve covering a moisture content range of 0-2000 ppm was developed by recording the mid-infrared (MIR) spectra of moisture standards, prepared by gravimetric addition of water to acetonitrile that had been dried over molecular sieves, in a 500 microm ZnSe transmission flow cell and ratioing these spectra against that of the dry acetonitrile. Water was measured in the resulting differential spectra using either the OH stretching (3629 cm(-1) or bending (1631 cm(-1)) bands to produce linear standard curves having standard deviations (SDs) of approximately +/-20 ppm. For moisture analysis in oils, the oil sample was mixed with dry acetonitrile in a 1:1 w/v ratio, and after centrifugation to separate the phases, the spectrum of the upper acetonitrile layer was collected and ratioed against the spectrum of the dry acetonitrile used for extraction. The method was validated by standard addition experiments with samples of various oil types, as well as with oil samples deliberately contaminated with alcohols, hydroperoxides, and free fatty acids to investigate possible interferences from minor constituents that may be present in oils and are potentially extractable into acetonitrile. The results of these experiments confirmed that the moisture content of edible oils can be assessed with high accuracy (on the order of +/-10 ppm) by this method, thus providing an alternative to the conventional, but problematic, Karl Fischer method and facilitating the routine analysis of edible oils for moisture content.
