What is Beer Lambert's Law

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Area of ‚Äč‚ÄčExpertise - Analytical chemistry, spectroscopy

The Lambert-Beer law is the fundamental law of absorptiometry, i.e. it applies to all optical methods of analytical chemistry that are based on the measurement of the absorption of radiation in the ultraviolet and visible range of the spectrum. It combines Beer's and Lambert's law. According to Beer's law, the light absorption of a colored solution is proportional to the concentration of a substance that is dissolved in a colorless solvent. According to Lambert's law, the light absorption of a solution at constant concentration of the dissolved substance is proportional to the length of the path that the light travels through the sample. According to IUPAC recommendations1) and if scattering and luminescence are neglected, the following formula notation results for the Lambert-Beer law:

-(also) decadic absorbance (outdated: extinction) []
-Intensity of the incident light []
-(according to IUPAC) Intensity of the attenuated light []
-Transmittance (outdated: transmission) []
-molar (decadic) absorption coefficient []
-Concentration of the sample []
-(according to IUPAC) Path length of the light beam through the sample []

In most cases, the scattering and luminescence of substances in a sample container are neglected in spectroscopy, the optical properties of substances in a sample container are given without the influence of the degree of reflection. Therefore it makes sense to speak of absorbance instead of extinction. The term Absorbance should be reserved for the sum of the effects of absorption, scattering and luminescence.

Learning units in which the term is dealt with

Spectrum editing30 min.

chemistryAnalytical chemistryIR / Raman spectroscopy

In the chapter on spectra processing, the basics of important evaluation routines for infrared and Raman spectra are presented. The conversion of transmittance spectra into absorbance spectra and vice versa is dealt with especially for infrared spectra. The spectrum subtraction and the smoothing of the spectrum are accompanied by theoretical details with practical exercises. Baseline correction, derivative spectroscopy, deconvolution and curve fitting are also considered.

Interaction between radiation and matter45 min.

chemistryPhysical chemistrySpectroscopy

The interaction of radiation and matter, as it is fundamental for the spectroscopic methods, is illustrated using animations and applets.