How to Calculate Extinction Coefficients
- 1
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Run an ultraviolet-visible spectrum of the sample under investigation if it is not already available. - 2). Locate the wavelength of maximum absorbance, commonly referred to as "lambda max," on the spectrum by identifying the wavelength at which the absorbance reaches a maximum and forms a peak.
- 3). Calculate the concentration of the solution if it is not already known. By convention, chemists normally use units of molarity, or moles per liter, for concentration units in ultraviolet-visible spectroscopy. Calculate molarity by dividing the moles of substance by the total liters of solution. Moles of substance is given by dividing the grams of substance by its formula weight or molecular weight. For example, sucrose, C12H22O11, exhibits a molecular weight of 342 grams per mole, and a sample containing 0.100 grams of sucrose dissolved and diluted to 100 milliliters (0.100 liters) would contain (0.100 / 342) = 0.000292 moles. The molarity of the solution would then be 0.000292 moles / 0.100 liters = 0.000292 mol/L.
- 4). Calculate the molar extinction coefficient according to e = A / cl using the absorbance at the wavelength of maximum absorbance. Continuing the example, if a sample with a concentration of 0.000292 moles/L exhibited an absorbance of 0.65 at 550 nanometers in a cell with a 1.0-cm path length, then e = 0.75 / (0.000292 * 1.00) = 2568.
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