Comparison of three spectrophotometric methods for the determination of gamma-oryzanol in rice bran oil

A comparison of the results obtained by applying three spectrophotometric methods (at fixed wavelength, second-derivative and multicomponent analysis) to the determination of n-oryzanol in rice bran oil is reported. At fixed wavelength the results are more accurate when using isopropyl alcohol, rather than n-heptane, to dilute the oil samples, because the absorption bands of n-oryzanol are red-shifted and the absorbance, measured at umax=327 nm, is less affected by the interference of the oil "matrix" (umax=314 nm in n-heptane). However, to obtain accurate results also in oils with a low content of n-oryzanol, it is necessary to perform the analysis using second-derivative (2D330.365) or multicomponent (u=310-360 nm) methods. The first one fully removes the interference of oil matrix whilst the second, which needs a specific computational program to process the spectrophotometric data, furnishes evidence the presence of some unexpected interference in the analysis and/or standards which are not representative of the analysed samples, from the square root of the sum of the squared differences at each point between the linear combination of the standards and the unknown spectra (RMS error). Finally, some aspects of the chemical, spectroscopic (UV, IR) and thermoanalytical (TG, DSC) behaviour of n-oryzanol and the values of the parameters which enable "computation" of its UV spectra are reported.