Molar Extinction Coefficient can be defined as the Coefficient is extinction coefficient of a solution containing a one-gram molecule of solute per Liter of solution. The extinction coefficient is a measure of how strongly a compound in solution absorbs light at a particular wavelength.
Molar Extinction Coefficient Definition and calculation
According to Beer-Lamberts law,
A = ecl —— I
e = A/cl —— II
Where,
A = absorbance (no units)
c = concentration in moles per Litre
l = thickness of the path cell in cm
e = molar extinction coefficient Litre moles-1 cm-1
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Ultraviolet Visible Spectroscopy Fundamentals 2023
Determination of Molar Extinction Coefficient
Preparation of stock solution: Take 10 mg of the drug in a 100 ml volumetric flask and dissolve the minimum amount of solvent and made up to 100 ml with the same solvent.
Preparation of sample solution: 1.0 ml of the stock solution is to be diluted to 10 ml in a volumetric flask by the solvent of the stock solution.
Scan the sample solution against the blank in the wavelength range of 200-400nm. Based on the result obtained select the wavelength for further analysis.
To minimize the error in the result, perform the precision and linearity studies to get nearer to the true value.
Precision: precision of the method can be checked by analyzing the sample six times at a selected concentration and calculating %RSD.
Linearity: prepare a minimum of five concentrations 20% above and below the selected Concentration and check the linearity by plotting absorbance vs concentration (at the selected Wavelength).
Then calculate the molar extinction coefficient by using the equation given in equation II.
Conclusion: Report the results in “molar extinction coefficient” or “molar absorption coefficient”.
Note: The “Molar extension coefficient” is wavelength specific.
Finalize the method, based on the satisfactory results and justification.