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Study Notes: Two Measures Used with UV-Vis Spectra

Absorption bands in UV-Vis spectra, whether they are relatively narrow or broad, reach a peak where the absorbance is maximum. There may be more than one peak in a spectrum.

The absorption peak is the basis of two key measures often quoted to characterise a spectrum. These measures are:

  1. the wavelength of an absorption maximum ie. the wavelength at the point where the absorbance is greatest (ie the peak) - this measure is known as lambdamax (lambda max) and is commonly expressed in nanometres (nm).

  2. the intensity of an absorption maximum - measured as the molar absorptivity as defined in Beer’s Law where the absorbance is that for lambdamax. This measure is known as epsilonmax and has units of L cm-1 mol-1 (but is often expressed without units) and is a constant at that particular wavelength.

Absorption spectra with one peak

To calculate epsilonmax, transpose Beer’s Law, (A=epsilonlC refer to Study Notes: Beer's Law), to give:

   
A
epsilonmax
=

   
lC

  where C = concentration (mol L-1)
    l = path length (cm)
    A = absorbance at lambdamax

 

 

 

The point to note about epsilonmax is that it is independent of concentration and optical path length. It therefore is an absolute measure that enables direct comparison of peak intensity from one spectra to another or against a reference standard taken from a spectroscopic database for instance.

Both measures are used in qualitative analysis for the identification of the chemical structure within a compound. Also, lambdamax is the standard analytical wavelength used in quantitative work (ie lambdamax is lambda and epsilonmax is epsilon in the Beer’s Law equation).

Note that sensitivity (limit of detection of an analyte) increases with intensity of absorption. That is, at stronger absorption (increasing epsilonmax), lower concentrations of analyte can be measured.

Good sensitivity for quantitative analysis is associated with epsilonmax values of the order of 104 whereas values of less than 102 are of limited usefulness for quantitative work.

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