Quantitative analysis of quercetin using Raman spectroscopy

Raman spectroscopy was used for the quantitative analysis of quercetin. Raman spectra were measured with an Ar ion laser at 488 nm. A 20× objective lens was used for focusing the laser beam on the sample. The back scattering light was passed into the monochrometer and detected with a CCD detector. The Raman band of a solvent (ethanol and methanol) was used as the internal standard to remove the effect of factors such as laser power and other instrumental effect. The band ratio between the Raman intensity of the sample and that of the solvent had good linearity with the analyte concentration. The equations of the calibration curve were y = 9.005x with an R² of 0.9998 and y = 11.50x with R² of 0.9999 in ethanol and methanol, respectively. The limit of detection (LOD) was 5 × 10⁻⁵ mol/L. The quantity of quercetin extracted from onion peels was determined by Raman spectroscopy. Masses in quercetins of 73 mg/100 g and 70 mg/100 g were extracted from dried onion peels by using hot ethanol and hot methanol for three hours. These values are in good agreement with results obtained by HPLC and UV–vis spectroscopy.