In
chemistry, the molar absorption coefficient or molar attenuation coefficient
is a measurement of how strongly a
chemical species
A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the wa ...
absorbs, and thereby
attenuate
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at vari ...
s, light at a given
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
. It is an
intrinsic property
In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass ...
of the species. The
SI unit
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. ...
of molar absorption coefficient is the square metre per
mole
Mole (or Molé) may refer to:
Animals
* Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America
* Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
(), but in practice, quantities are usually expressed in terms of
M−1⋅cm
−1 or L⋅mol
−1⋅cm
−1 (the latter two units are both equal to ). In older literature, the cm
2/mol is sometimes used; 1 M
−1⋅cm
−1 equals 1000 cm
2/mol. The molar absorption coefficient is also known as the molar extinction coefficient and molar absorptivity, but the use of these alternative terms has been discouraged by the
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
.
Beer–Lambert law
The
absorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
of a material that has only one absorbing species also depends on the pathlength and the concentration of the species, according to the
Beer–Lambert law
The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling. The law is commonly applied t ...
:
where
*''ε'' is the ''molar absorption coefficient'' of that material;
*''c'' is the
molar concentration
Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of sol ...
of those species;
*''ℓ'' is the pathlength.
Different disciplines have different conventions as to whether
absorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
is decadic (10-based) or Napierian (e-based), i.e., defined with respect to the transmission via
common logarithm (log
10) or a
natural logarithm (ln). The molar absorption coefficient is usually decadic.
[ When ambiguity exists, it is best to indicate which one applies.
When there are ''N'' absorbing species in a solution, the overall absorbance is the sum of the absorbances for each individual species ''i'':
:
The composition of a mixture of ''N'' absorbing species can be found by measuring the absorbance at ''N'' ]wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
s (the values of the molar absorption coefficient for each species at these wavelengths must also be known). The wavelengths chosen are usually the wavelengths of maximum absorption (absorbance maxima) for the individual species. None of the wavelengths may be an ''isosbestic point
In spectroscopy, an isosbestic point is a specific wavelength, wavenumber or frequency at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. The word derives from two Greek words ...
'' for a pair of species. The set of the following simultaneous equations
In mathematics, a set of simultaneous equations, also known as a system of equations or an equation system, is a finite set of equations for which common solutions are sought. An equation system is usually classified in the same manner as single e ...
can be solved to find the concentrations of each absorbing species:
:
The molar absorption coefficient (in units of cm2) is directly related to the attenuation cross section via the Avogadro constant
The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. It is an SI defining c ...
''N''A:
:
Mass absorption coefficient
The mass absorption coefficient The mass attenuation coefficient, or mass narrow beam attenuation coefficient of a material is the attenuation coefficient normalized by the density of the material; that is, the attenuation per unit mass (rather than per unit of distance). Thus, i ...
is equal to the molar absorption coefficient divided by the molar mass
In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, ...
of the absorbing species.
Proteins
In biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
, the molar absorption coefficient of a protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
at depends almost exclusively on the number of aromatic residues, particularly tryptophan
Tryptophan (symbol Trp or W)
is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...
, and can be predicted from the sequence of amino acids. Similarly, the molar absorption coefficient of nucleic acids
Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cl ...
at can be predicted given the nucleotide sequence.
If the molar absorption coefficient is known, it can be used to determine the concentration of a protein in solution.
References
External links
Nikon MicroscopyU: Introduction to Fluorescent Proteins
includes a table of molar attenuation coefficient of fluorescent protein Fluorescent proteins include:
* Green fluorescent protein (GFP)
* Yellow fluorescent protein
Yellow fluorescent protein (YFP) is a genetic mutant of green fluorescent protein (GFP) originally derived from the jellyfish '' Aequorea victoria''. Its ...
s.
{{BranchesofSpectroscopy
Analytical chemistry
Absorption spectroscopy