= cis-Resveratrol
\b[<cis-Resveratrol>] is the <resveratrol> <stereoisomer> with the two aryl groups on the same side of the central <double bond>, corresponding to the $Z$ configuration. It has the same neutral <molecular formula>, $\mathrm{C_{14}H_{12}O_3}$, and formula-based <molar mass> as <trans-Resveratrol>, but a different spatial arrangement.<Resveratrol in ChEBI>[CHEBI:36002, cis definition, formula and stereochemical structure]{ref}
The isomers need different optical response factors. <Brent C. Trela> and <Andrew L. Waterhouse> measured an ethanol absorption maximum at $288\,\mathrm{nm}$ with <molar absorption coefficient> $12600\,\mathrm{L\,mol^{-1}\,cm^{-1}}$ for the cis form; their stability tests also depended on light exposure and pH.<Resveratrol: Isomeric Molar Absorptivities and Stability by Brent C. Trela and Andrew L. Waterhouse>[1996, Abstract, cis-isomer maximum and stability]{ref}
At that maximum, an original $10\,\mathrm{\mu mol\,L^{-1}}$ solution in a $1\,\mathrm{cm}$ path has ideal <absorbance>
$$
A=(12600)(1)(10\times10^{-6})=0.126.
$$
This is lower than the trans-isomer's corresponding example at its own maximum, but the two maxima occur at different wavelengths. Dividing their peak coefficients does not supply the cis/trans response ratio at one common detector wavelength.
For a mixture at a chosen wavelength $\lambda$, an additive <Beer-Lambert law> model is
$$
A_\lambda=\ell\{\epsilon_{c,\lambda}c_c+\epsilon_{t,\lambda}c_t\}.
$$
One reading generally cannot identify both <concentrations>. Two wavelengths help only if their response rows are independent and the coefficients are appropriate to the solution. <Multivariate calibration> makes that requirement explicit; physical separation by <chromatography> supplies another route. Neither equal molecular formulas nor a single colour measurement establishes isomeric purity.
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