= Molar absorption coefficient
{title2=$\varepsilon=A/(lc)$}
= Molar attenuation coefficient
{synonym}
The decadic <molar absorption coefficient> specifies <absorbance> per geometric <length> and <molar concentration>. With $l$ in centimetres and $c$ in $\mathrm{mol\,L^{-1}}$, $\varepsilon$ has units $\mathrm{L\,mol^{-1}\,cm^{-1}}$; its <SI> unit is $\mathrm{m^2\,mol^{-1}}$.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/Beer–Lambert law>[IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry; Beer–Lambert law, entry B00626: coefficient name and conventional and SI units]{ref}
The units cancel in $\varepsilon lc$. Since $1\,\mathrm L=10^{-3}\,\mathrm{m^3}$ and $1\,\mathrm{cm}=10^{-2}\,\mathrm m$, one $\mathrm{L\,mol^{-1}\,cm^{-1}}$ equals $0.1\,\mathrm{m^2\,mol^{-1}}$. Specify the <wavelength> and <chemical solution> conditions when giving a <molar absorption coefficient>: changing the absorbing <chemical species> can change it even at fixed total <analytical concentration>.
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