Deviations from Beer- Lambert Spectroscopic Absorbance Principle

Spectroscopic Absorbance measurements are based on the famous Beer Lambert law which states that at selected wavelength the absorbance by absorbing species is in direct proportion to its concentration and path length of the sample cell.

Deviation from Beer-Lambert Law at Higher Concentrations

Deviation from Beer-Lambert Law at Higher Concentrations

The deviations from the Beer Lambert law are classified under three main categories:

  • Limitations of the law
  • Deviations resulting from chemical changes
  • Stray radiation

Beer Lambert Law Limitations

Beer Lambert law applies to dilute solutions typically below 10mM concentrations. At higher concentrations physical interactions such as solute– solvent, solute – solute and hydrogen bonding can come into play and influence absorption measurements. As such the concentration versus absorbance plot begins to show deviations from linear behaviour.

Chemical Changes

The absorption behaviour of solutes changes due to chemical interactions such as associations and dissociations and from increase in concentration or changes in pH of the solvent. Due to electronic transitions with changing pH the colour of absorbing species can undergo distinct changes. Substitution of chromophore groups in a molecule can result in shifts of wavelengths of maximum absorption.

Stray Radiation

Stray radiation is radiation reaching the detector whose wavelength is outside the range selected by the monochromator. It results from reflection and scattering of light by surfaces of optical components of the system.Ideally monochromator should produce a single wavelength from a polychromatic beam but this seldom happens and instead a band of wavelengths gets isolated. Absorption by analyte can also take place at stray radiation wavelengths and result in deviations from Beer Lambert law.

A subsequent article will describe how instrumental parameters influence affect spectral observations.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

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