whereais the absorptivity or absorption coefficient or
extinction coefficient.
Specific absorption coefficient—when path length is
expressed in cm and concentration in g/L, then the
absorption coefficient is termed as a specific absorp-
tion coefficient
.
Molar absorption coefficient—when path length is
expressed in cm and concentration in mol/L, then
the absorption coefficient is termed as the molar
absorption coefficient and is denoted asa
m.
am¼asMwmolecular weightðÞ
For a given compound, if the solvent and wavelength
are defined, then molar absorptivity is a physical con-
stant. Molar absorptivity of some compounds are listed
as below:
Compound Solvent λ max am×10
23
Adenine Water 260 13.3
NADH Water 340 6.22
ATP Water 260 15.4
FAD Water 445 11.3
When a sample is placed in a cuvette and light is passed
through it, we can calculate the concentration of the
sample. When path length is constant, then the optical
density is directly proportional to the concentration of
the sample, it can be written as follows:
OD1
C1
¼
OD2
C2
where
OD
1and OD2¼optical density of sample 1 and 2,
respectively
C
1and C2¼concentration of samples 1 and 2,
respectively
Deviations from Beer-Lambert law:
1. When the concentration of the reagent is high: It may
lead to dimerization or polymerization of the reagent.
OD of monomer differs from polymers. High concen-
tration may also lead to aggregation leading to the for-
mation of aggregates, which scatter light.
2. Temperature: On heating, the solvents may expand.
The change in the degree of solubility, dissociation/
association of solutes, and hydration of solutes may
vary according to the temperature. These changes can
lead to variation in absorbance.
3. Turbidity: A turbid solution absorbs more light.
4. Sample instability: Some colored complexes are
unstable and their intensity can increase or decrease
with time. For example, the ANSA method of phos-
phate determination.
5. Fluorescence: Some solutes, especially drugs or their
intermediates fluoresce, and their fluorescent inten-
sity are also detected by the spectrophotometer.
Absorption spectrum—it is the pattern of energy
absorption by a substance, when, the light of varying
wavelength passes through it. It is a unique characteristic
of the substance as every substance is made up of mol-
ecules and each element/ion of the molecule has unique
arrangements of an electron in their orbits/orbitals.
When light is passed through them, they absorb energy,
according to their electronic configuration and their
electrons get excited. These electrons get promoted to
a higher energy level. According to the quantum theory,
the electron gets excited only after accepting the radia-
tion, which has exact quantized energy that can push
the electron to a permitted energy level. The wavelength
at which it absorbs the maximum amount of light is
λ
max. When the concentration of any substance is
increased, the absorbance at all wavelength increases,
although the change in absorbance per unit change in
concentration is maximum atλ
max. Therefore, during a
quantitative experiment, absorbance of a compound
is measured atλ
max.
Factors that can affect the absorption spectra are
polarity (affect the transition states of the electrons)
and pH of the solvent (it can change the ionization state
of the molecules) as well as the relative orientation of
the neighboring absorbing groups.
Some isolated covalently bonded groups characteris-
tically absorb the light in esters, carbonyl, and nitrile
group of ethylenic or acetylenic groups. Similarly, there
are auxochromes that themselves don’t act as chromo-
phores, but their absence or presence shifts the absorp-
tion spectrum toward longer wavelengths. They are also
known as color enhancers, e.g., OH, OR, SH, NH
2
groups. They can share the nonbonding electrons by
extending the conjugation.
There are different conditions like change in the
polarity of the solvent or the presence of chromophore,
etc., which can shift the absorption spectrum in four dif-
ferent directions and result in the following shifts:
1. Bathochromic shift: Due to the presence of auxo-
chrome, the shift is toward the higher wavelength.
This is also known as the redshift.
2. Hypochromic shift: The shift is toward shorter wave-
length, due to the removal of conjugation or the
change in the polarity of the solvent. This is also
known as a blueshift.
3. Hyperchromic shift: This results in an increase in the
intensity of the absorption maximum. It results in a
higher extinction coefficient. This occurs mostly due
to the presence of auxochrome.
xiTECHNIQUES