Determination of Extinction Coefficients
Introduction
User Defined Reagents
UniCel DxC Systems Instructions For Use A13914
Page 9-20
October 2005
Determination of Extinction Coefficients
Introduction
In general, there are two ways to determine the molar absorptivity or extinction
coefficient "e" for a given chemistry at each of the wavelengths required for analysis.
Option 1
Prepare a stock, standard solution of the chromophore. From this stock solution,
prepare a working standard solution at the same molar concentration recommended by
the reagent manufacturer. Obtain absorbance values on this working standard solution
within a narrow-bandpass, manual spectrophotometer which uses 1 cm pathlength
cuvettes. Calculate the extinction coefficient for the appropriate wavelength as
follows:
This method may only be used when the chromophore is measurable in a stable form.
When the chromophore is not attainable in a stable form and may only be measured in
a dynamic state, the second method of determining extinction coefficients is
suggested. For additional information, refer to Textbook of Clinical Chemistry,
Norbert W. Tietz, 1999.
Option 2
Prepare the reagent according to the manufacturer instructions. With a manual
spectrophotometer or automated instrument, obtain the delta absorbance per minute on
at least ten replicates of a normal or high normal sample at each of the appropriate
wavelengths selected. Average the delta absorbance per minute obtained at each
wavelength. The extinction coefficient at the secondary wavelength may be calculated
by use of a factor or correction coefficient (R) derived from the ratio of the delta
absorbance per minute at the secondary and primary wavelengths. The following
formula may be used:
E014427L.EPS
e =
Concentration of standard solution (mmol / L)
×
pathlength
Absorbance of chromophore
e
2
= e
1
× R
where:
e
2
= Extinction coefficient at the secondary wavelength
e
1
= Extinction coefficient at the primary wavelength
a
a
The extinction coefficient of the chromophore at the primary wavelength is usually specified by the reagent manufacturer.
E014428L.EPS
R =
Delta absorbance / minute at primary wavelength
Delta absorbance / minute at secondary wavelength