Cinnamic acid absorption maxima

Measuring absorption at 280 nm seems preferable to the Folin-Ciocalteu test, as it presents a number of advantages, including speed and reproducibility. However, certain molecules, such as cinnamic acids and chalcones, have no absorption maximum at this wavelength. However, as they are present in wine at very low concentrations, any error in the value will be very small. 

Regarding phenolic acids, most compounds are commercially available and can thus be positively identified by comparison of their retention time and UV spectra with those of reference compounds. Hydroxy cinnamic acids absorb at two regions of the UV spectrum, presenting a maximum at 225-235 nm and two other, very near from each other, by 290-330 nm. The double absorption in this region arises from the presence of cis and trans isomers, and the ratio between these two forms contributes to the final spectrum. 

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