characteristic flavonoid fluorescence signals we first measured the absorption characteristics of representative compounds crystal powder samples with highest possible purity were obtained from sigma aldrich inc they included quercetin dihydrate and kaempferol for flavonol examples apigenin luteolin and diosmin rutinoside for flavone examples naringenin and hesperidin rhamnoglucoside of hesperitin for flavanones the catechins gallocatechin and epicatechin the isoflavone genistein and the anthocyanidin compound pelargonidin chloride the manufacturer lists the following synonyms for some of the measured flavonoid compounds for each compound we determined the absorption characteristics of samples in crystal powder form using white light reflection spectroscopy experimental details of the method can be found in pending patent application ser no 12 134 667 the entire content of which is incorporated herein by reference for solutions of the compounds we used a perlin elmer uv vis nir absorption spectrophotometer in fig4 and 5 we show the absorption results for the flavonol compounds quercetin and kaempferol the absorption bands are very similar and feature prominent broad bands peaking in the 400 to 430 nm range importantly they also feature a weak but clearly noticeable long wavelength absorption tail in the visible far red wavelength region extending from about 500 to 800 nm this weak long wavelength tail is not only visible for these flavonol compounds in powder form which could point to a scattering effect but also for methanol solutions the absorption spectra obtained for perfectly clear methanol solutions of quercetin and.