= trans-Resveratrol
\b[<trans-Resveratrol>] is the <resveratrol> <stereoisomer> with the two aryl groups on opposite sides of the central <double bond>, corresponding to the $E$ configuration. It retains the three <hydroxy groups> and neutral <molecular formula> $\mathrm{C_{14}H_{12}O_3}$ of <resveratrol>. ChEBI specifies the double-bond geometry explicitly, unlike the geometry-unspecified parent record.<Resveratrol in ChEBI>[CHEBI:45713, definition, formula and stereochemical structure; CHEBI:27881, parent]{ref}
The geometry affects the optical measurement. <Brent C. Trela> and <Andrew L. Waterhouse> reported an ethanol absorption maximum at $308\,\mathrm{nm}$ and a <molar absorption coefficient> of $30000\,\mathrm{L\,mol^{-1}\,cm^{-1}}$ for this isomer. They tested chemical stability under different light and pH conditions.<Resveratrol: Isomeric Molar Absorptivities and Stability by Brent C. Trela and Andrew L. Waterhouse>[1996, Abstract, trans-isomer maximum and stability observations]{ref}
For an original <Beer-Lambert law> calculation under those stated spectral conditions, $10\,\mathrm{\mu mol\,L^{-1}}$ in a $1\,\mathrm{cm}$ path gives
$$
A=\epsilon\ell c=(30000)(1)(10\times10^{-6})=0.300.
$$
The calculation assumes a suitable homogeneous solution of the specified isomer, correctly subtracted background and the stated wavelength and solvent. A different solvent, wavelength or isomer mixture requires the corresponding optical model.
The trans and cis forms have the same <molar mass>, so weighing alone cannot determine their proportions. Illumination can also change a preparation after it has been weighed. Separating the species by <chromatography> and using appropriate wavelength-dependent response factors addresses a different question from merely measuring total formula-based mass. This chemical identity does not, by itself, establish a biological benefit.
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