Quantification of coumarin
Legally, a small amount of coumarin (determined by country) must be added to kerosene to distinguish it from diesel oil. To prevent the production of illegal diesel oil, a standardized analytical method for the quantitative determination of the coumarin content using fluorescence spectroscopy has been established (Japan Petroleum Institute Standard: JPI-5S-71-2010 ). The coumarin is first hydrolyzed in an alkaline solution and is then isomerized to a “trans” form by UV irradiation (Fig. 11).
Fig. 11 Hydrolysis and photoisomerization of coumarin.
The resulting samples were then analyzed using fluorescence spectroscopy. Calibration curves obtained from samples containing mixtures of coumarin-free and coumarin-containing oil with mixing ratios of 0, 20, 40, and 80% exhibited a correlation coefficient of 0.9996 (Fig. 12). The standard error (σ) between the measured mixing ratio and the actual mixing ratio was 0.65%. The detection limit for the mixing ratio was 2.1% and the lower quantification limit was 6.5%.
Fig. 12 Fluorescence spectra (left) and calibration curve for mixing ratio based on coumarin detection (right)
Reflectance measurements using spectrofluorometer
Fig. 13 Reflectance measurement of fluorescent red card (Blue: Reflection spectrum obtained using UV/Vis spectrometer, Red: Reflection spectrum obtained using spectrofluorometer)
Fig. 14 Phosphorescence and fluorescence spectra of benzene