Things You'll Need:
- Calculator
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Step 1
Define the variables needed to calculate the molar absorptivity. The absorbance A is the amount of light with a given wavelength that is absorbed by the solution. The concentration c of the absorbing species is the amount of absorbing species per unit volume. The path length l is the distance that the light travels through the solution. We will call the molar absorptivity e.
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Step 2
Use the Beer-Lambert Law to calculate the molar absorptivity of a single absorbing species. This is given as A = ecl, so we can express the molar absorptivity as e = A/cl.
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Step 3
Calculate the total absorbance of a solution that contains more tha one absorbing species. We can expand the Beer-Lamber law to A = (e1c1 + e2c2 + ...)l where ei is the molar absorptivity of species i and ci is the concentration of species i in the solution.
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Step 4
Calculate the molar absorptivity from the absorption cross section and Avogadro's Number (approximately 6.022 x 10^23). We have d = (2.303/N) e where d is the absorption cross section and N is Avogadro's Number. Therefore, d = (2.303/(6.022 x 10^23)) e = 3.82 x 10^(-21) e, so e = (2.62*10^20)d.
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Step 5
Predict the molar absorptivity of light at 280 nm by a protein. The molar absorptivity under these conditions depends almost entirely on the number of aromatic residues that the protein has, especially tryptophan.
