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New Mexico State University
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Department of Chemistry and Biochemistry
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 Beer-Lambert Law

 

Spectra

Introduction -- The Beer-Lambert law (or Beer's law) is the linear relationship between absorbance and concentration of an absorbing species.

The general Beer-Lambert law is usually written as: A = abc, where A is the measured absorbance, a is a wavelength-dependent absorptivity coefficient (sometimes called ε, also knows as molar absorptivity), b is the cell-path length, and c is the analyte concentration. When working in concentration units of molarity, the Beer-Lambert law is where is the wavelength-dependent molar absorptivity coefficient with units of M-1 cm-1.

Instrumentation -- Experimental measurements are usually made in termsBeer Law of transmittance (T), which is defined as: T = I / Io where I is the light intensity after it passes through the sample and Io is the initial light intensity. The relation between A and T is:

A = -log T = - log (I / Io)

Modern absorption instruments can usually display the data as either transmittance, %-transmittance, or absorbance. An unknown concentration of an analyte can be determined by measuring the amount of light that a sample absorbs and applying Beer's law. If the absorptivity coefficient is not known, the unknown concentration can be determined using a working curve of absorbance versus concentration derived from standards.


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