Movatterモバイル変換

[0]ホーム
URL:

Jump to content
WikipediaThe Free Encyclopedia
Search

Absorbance

From Wikipedia, the free encyclopedia
Logarithm of ratio of incident to transmitted radiant power through a sample
This article is about a quantitative expression. For the process itself, seeAbsorption (electromagnetic radiation).
"Optical density" redirects here. For other uses, seeRefractive index,Nucleic acid quantitation, andNeutral-density filter.

Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)".[1] Alternatively, for samples which scatter light, absorbance may be defined as "the negative logarithm of one minus absorptance, as measured on a uniform sample".[2] The term is used in many technical areas to quantify the results of an experimental measurement. While the term has its origin in quantifying the absorption of light, it is often entangled with quantification of light which is "lost" to a detector system through other mechanisms. What these uses of the term tend to have in common is that they refer to a logarithm of the ratio of a quantity of light incident on a sample or material to that which is detected after the light has interacted with the sample.

The termabsorption refers to the physical process of absorbing light, while absorbance does not always measure only absorption; it may measureattenuation (of transmitted radiant power) caused by absorption, as well as reflection, scattering, and other physical processes. Sometimes the term "attenuance" or "experimental absorbance" is used to emphasize that radiation is lost from the beam by processes other than absorption, with the term "internal absorbance" used to emphasize that the necessary corrections have been made to eliminate the effects of phenomena other than absorption.[3]

History and uses of the term absorbance

[edit]

Beer-Lambert law

[edit]

The roots of the term absorbance are in theBeer–Lambert law. As light moves through a medium, it will become dimmer as it is being "extinguished". Bouguer recognized that this extinction (now often called attenuation) was not linear with distance traveled through the medium, but related by what we now refer to as an exponential function.

IfI0{\displaystyle I_{0}} is the intensity of the light at the beginning of the travel andId{\displaystyle I_{d}} is the intensity of the light detected after travel of a distanced{\displaystyle d}, the fraction transmitted,T{\displaystyle T}, is given by

T=IdI0=exp⁡(−μd),{\displaystyle T={\frac {I_{d}}{I_{0}}}=\exp(-\mu d)\,,}

whereμ{\displaystyle \mu } is called anattenuation constant (a term used in various fields where a signal is transmitted though a medium) or coefficient. The amount of light transmitted is falling off exponentially with distance. Taking the natural logarithm in the above equation, we get

−ln⁡(T)=ln⁡I0Id=μd.{\displaystyle -\ln(T)=\ln {\frac {I_{0}}{I_{d}}}=\mu d\,.}

For scattering media, the constant is often divided into two parts,[4]μ=μs+μa{\displaystyle \mu =\mu _{s}+\mu _{a}}, separating it into a scattering coefficientμs{\displaystyle \mu _{s}} and an absorption coefficientμa{\displaystyle \mu _{a}}, obtaining

−ln⁡(T)=ln⁡I0Is=(μs+μa)d.{\displaystyle -\ln(T)=\ln {\frac {I_{0}}{I_{s}}}=(\mu _{s}+\mu _{a})d\,.}

If a size of a detector is very small compared to the distance traveled by the light, any light that is scattered by a particle, either in the forward or backward direction, will not strike the detector. (Bouguer was studying astronomical phenomena, so this condition was met.) In such case, a plot of−ln⁡(T){\displaystyle -\ln(T)} as a function of wavelength will yield a superposition of the effects of absorption and scatter. Because the absorption portion is more distinct and tends to ride on a background of the scatter portion, it is often used to identify and quantify the absorbing species. Consequently, this is often referred to asabsorption spectroscopy, and the plotted quantity is called "absorbance", symbolized asA{\displaystyle \mathrm {A} }. Some disciplines by convention use decadic (base 10) absorbance rather than Napierian (natural) absorbance, resulting in:A10=μ10d{\displaystyle \mathrm {A} _{10}=\mu _{10}d} (with the subscript 10 usually not shown).

Absorbance for non-scattering samples

[edit]

Within a homogeneous medium such as a solution, there is no scattering. For this case, researched extensively byAugust Beer, the concentration of the absorbing species follows the same linear contribution to absorbance as the path-length. Additionally, the contributions of individual absorbing species are additive. This is a very favorable situation, and made absorbance an absorption metric far preferable to absorption fraction (absorptance). This is the case for which the term "absorbance" was first used.

A common expression of theBeer's law relates the attenuation of light in a material as:A=εℓc{\displaystyle \mathrm {A} =\varepsilon \ell c}, whereA{\displaystyle \mathrm {A} } is theabsorbance;ε{\displaystyle \varepsilon } is themolar attenuation coefficient orabsorptivity of the attenuating species;ℓ{\displaystyle \ell } is the optical path length; andc{\displaystyle c} is the concentration of the attenuating species.

Absorbance for scattering samples

[edit]

For samples which scatter light, absorbance is defined as "the negative logarithm of one minus absorptance (absorption fraction:α{\displaystyle \alpha }) as measured on a uniform sample".[2] For decadic absorbance,[3] this may be symbolized asA10=−log10⁡(1−α){\displaystyle \mathrm {A} _{10}=-\log _{10}(1-\alpha )}. If a sample both transmits andremits light, and is not luminescent, the fraction of light absorbed(α{\displaystyle \alpha }), remitted(R{\displaystyle R}), and transmitted(T{\displaystyle T}) add to 1:α+R+T=1{\displaystyle \alpha +R+T=1}. Note that1−α=R+T{\displaystyle 1-\alpha =R+T}, and the formula may be written asA10=−log10⁡(R+T){\displaystyle \mathrm {A} _{10}=-\log _{10}(R+T)}. For a sample which does not scatter,R=0{\displaystyle R=0}, and1−α=T{\displaystyle 1-\alpha =T}, yielding the formula for absorbance of a material discussed below.

Even though this absorbance function is very useful with scattering samples, the function does not have the same desirable characteristics as it does for non-scattering samples. There is, however, a property calledabsorbing power which may be estimated for these samples. Theabsorbing power of a single unit thickness of material making up a scattering sample is the same as the absorbance of the same thickness of the material in the absence of scatter.[5]

Optics

[edit]

Inoptics,absorbance ordecadic absorbance is thecommon logarithm of the ratio of incident totransmittedradiant power through a material, andspectral absorbance orspectral decadic absorbance is the common logarithm of the ratio of incident totransmittedspectral radiant power through a material. Absorbance isdimensionless, and in particular is not a length, though it is a monotonically increasing function of path length, and approaches zero as the path length approaches zero.

Mathematical definitions

[edit]

Absorbance of a material

[edit]

Theabsorbance of a material, denotedA, is given by[1]

A=log10⁡ΦeiΦet=−log10⁡T,{\displaystyle A=\log _{10}{\frac {\Phi _{\text{e}}^{\text{i}}}{\Phi _{\text{e}}^{\text{t}}}}=-\log _{10}T,}

where

Absorbance is adimensionless quantity. Nevertheless, theabsorbance unit orAU is commonly used inultraviolet–visible spectroscopy and itshigh-performance liquid chromatography applications, often in derived units such as the milli-absorbance unit (mAU) or milli-absorbance unit-minutes (mAU×min), a unit of absorbance integrated over time.[6]

Absorbance is related tooptical depth by

A=τln⁡10=τlog10⁡e,{\displaystyle A={\frac {\tau }{\ln 10}}=\tau \log _{10}e\,,}

whereτ is the optical depth.

Spectral absorbance

[edit]

Spectral absorbance in frequency andspectral absorbance in wavelength of a material, denotedAν andAλ respectively, are given by[1]

Aν=log10⁡Φe,νiΦe,νt=−log10⁡Tν,Aλ=log10⁡Φe,λiΦe,λt=−log10⁡Tλ,{\displaystyle {\begin{aligned}A_{\nu }&=\log _{10}{\frac {\Phi _{{\text{e}},\nu }^{\text{i}}}{\Phi _{{\text{e}},\nu }^{\text{t}}}}=-\log _{10}T_{\nu }\,,\\A_{\lambda }&=\log _{10}{\frac {\Phi _{{\text{e}},\lambda }^{\text{i}}}{\Phi _{{\text{e}},\lambda }^{\text{t}}}}=-\log _{10}T_{\lambda }\,,\end{aligned}}}

where

Spectral absorbance is related to spectral optical depth by

Aν=τνln⁡10=τνlog10⁡e,Aλ=τλln⁡10=τλlog10⁡e,{\displaystyle {\begin{aligned}A_{\nu }&={\frac {\tau _{\nu }}{\ln 10}}=\tau _{\nu }\log _{10}e\,,\\A_{\lambda }&={\frac {\tau _{\lambda }}{\ln 10}}=\tau _{\lambda }\log _{10}e\,,\end{aligned}}}

where

  • τν is the spectral optical depth in frequency, and
  • τλ is the spectral optical depth in wavelength.

Although absorbance is properly unitless, it is sometimes reported in "absorbance units", or AU. Many people, including scientific researchers, wrongly state the results from absorbance measurement experiments in terms of these made-up units.[7]

Relationship with attenuation

[edit]

Attenuance

[edit]

Absorbance is a number that measures theattenuation of the transmitted radiant power in a material. Attenuation can be caused by the physical process of "absorption", but also reflection, scattering, and other physical processes. Absorbance of a material is approximately equal to its attenuance[clarification needed] when both the absorbance is much less than 1 and the emittance of that material (not to be confused withradiant exitance oremissivity) is much less than the absorbance. Indeed,

Φet+Φeatt=Φei+Φee,{\displaystyle \Phi _{\mathrm {e} }^{\mathrm {t} }+\Phi _{\mathrm {e} }^{\mathrm {att} }=\Phi _{\mathrm {e} }^{\mathrm {i} }+\Phi _{\mathrm {e} }^{\mathrm {e} }\,,}

where

This is equivalent to

T+ATT=1+E,{\displaystyle T+\mathrm {ATT} =1+E\,,}

where

According to theBeer–Lambert law,T = 10−A, so

and finally

Attenuation coefficient

[edit]

Absorbance of a material is also related to itsdecadic attenuation coefficient by

A=∫0la(z)dz,{\displaystyle A=\int _{0}^{l}a(z)\,\mathrm {d} z\,,}

where

  • l is the thickness of that material through which the light travels, and
  • a(z) is thedecadic attenuation coefficient of that material atz.

Ifa(z) is uniform along the path, the attenuation is said to be alinear attenuation, and the relation becomesA=al.{\displaystyle A=al.}

Sometimes the relation is given using themolar attenuation coefficient of the material, that is its attenuation coefficient divided by itsmolar concentration:

A=∫0lεc(z)dz,{\displaystyle A=\int _{0}^{l}\varepsilon c(z)\,\mathrm {d} z\,,}

where

  • ε is themolar attenuation coefficient of that material, and
  • c(z) is the molar concentration of that material atz.

Ifc(z) is uniform along the path, the relation becomes

A=εcl.{\displaystyle A=\varepsilon cl\,.}

The use of the term "molar absorptivity" for molar attenuation coefficient is discouraged.[1]

Measurements

[edit]

Logarithmic vs. directly proportional measurements

[edit]

The amount of light transmitted through a material diminishesexponentially as it travels through the material, according to the Beer–Lambert law (A = (ε)(l)). Since the absorbance of a sample is measured as a logarithm, it is directly proportional to the thickness of the sample and to the concentration of the absorbing material in the sample. Some other measures related to absorption, such as transmittance, are measured as a simple ratio so they vary exponentially with the thickness and concentration of the material.

Absorbances and equivalent transmittances
Absorbance:−log10⁡(Φet/Φei){\textstyle -\log _{10}\left(\Phi _{\mathrm {e} }^{\mathrm {t} }/\Phi _{\mathrm {e} }^{\mathrm {i} }\right)}Transmittance:Φet/Φei{\textstyle \Phi _{\mathrm {e} }^{\mathrm {t} }/\Phi _{\mathrm {e} }^{\mathrm {i} }}
01
0.10.79
0.250.56
0.50.32
0.750.18
0.90.13
10.1
20.01
30.001

Instrument measurement range

[edit]

Any real measuring instrument has a limited range over which it can accurately measure absorbance. An instrument must be calibrated and checked against known standards if the readings are to be trusted. Many instruments will become non-linear (fail to follow the Beer–Lambert law) starting at approximately 2 AU (~1% transmission). It is also difficult to accurately measure very small absorbance values (below10−4) with commercially available instruments for chemical analysis. In such cases,laser-based absorption techniques can be used, since they have demonstrated detection limits that supersede those obtained by conventional non-laser-based instruments by many orders of magnitude (detection has been demonstrated all the way down to5×10−13). The theoretical best accuracy for most commercially available non-laser-based instruments is attained in the range near 1 AU. The path length or concentration should then, when possible, be adjusted to achieve readings near this range.

Method of measurement

[edit]

Typically, absorbance of a dissolved substance is measured usingabsorption spectroscopy. This involves shining a light through a solution and recording how much light and what wavelengths were transmitted onto a detector. Using this information, the wavelengths that were absorbed can be determined.[8] First, measurements on a "blank" are taken using just the solvent for reference purposes. This is so that the absorbance of the solvent is known, and then any change in absorbance when measuring the whole solution is made by just the solute of interest. Then measurements of the solution are taken. The transmitted spectral radiant flux that makes it through the solution sample is measured and compared to the incident spectral radiant flux. As stated above, the spectral absorbance at a given wavelength is

Aλ=log10(Φe,λiΦe,λt).{\displaystyle A_{\lambda }=\log _{10}\!\left({\frac {\Phi _{\mathrm {e} ,\lambda }^{\mathrm {i} }}{\Phi _{\mathrm {e} ,\lambda }^{\mathrm {t} }}}\right)\!.}

The absorbance spectrum is plotted on a graph of absorbance vs. wavelength.[9]

AnUltraviolet-visible spectroscopy#Ultraviolet–visible spectrophotometer will do all this automatically. To use this machine, solutions are placed in a smallcuvette and inserted into the holder. The machine is controlled through a computer and, once it has been "blanked", automatically displays the absorbance plotted against wavelength. Getting the absorbance spectrum of a solution is useful for determining the concentration of that solution using the Beer–Lambert law and is used inHPLC.

Shade number

[edit]

Some filters, notablywelding glass, are rated by shade number (SN), which is 7/3 times the absorbance plus one:[10]

SN=73A+1=73(−log10⁡T)+1.{\displaystyle {\begin{aligned}\mathrm {SN} &={\frac {7}{3}}A+1\\&={\frac {7}{3}}(-\log _{10}T)+1\,.\end{aligned}}}

For example, if the filter has 0.1% transmittance (0.001 transmittance, which is 3 absorbance units), its shade number would be 8.

See also

[edit]

References

[edit]
  1. ^abcdIUPAC,Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorbance".doi:10.1351/goldbook.A00028
  2. ^abIUPAC,Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "decadic absorbance".doi:10.1351/goldbook.D01536
  3. ^abBertie, John E. (2006). "Glossary of Terms used in Vibrational Spectroscopy". In Griffiths, Peter R (ed.).Handbook of Vibrational Spectroscopy.doi:10.1002/0470027320.s8401.ISBN 0471988472.
  4. ^Van de Hulst, H. C. (1957).Light Scattering by Small Particles. New York: John Wiley and Sons.ISBN 9780486642284.
  5. ^Dahm, Donald; Dahm, Kevin (2007).Interpreting Diffuse Reflectance and Transmittance: A Theoretical Introduction to Absorption Spectroscopy of Scattering Materials.doi:10.1255/978-1-901019-05-6.ISBN 9781901019056.
  6. ^GE Health Care (2015)."ÄKTA Laboratory-Scale Chromatography Systems - Instrument Management Handbook". Uppsala: GE Healthcare Bio-Sciences AB. Archived fromthe original on 2020-03-15.
  7. ^Kamat, Prashant; Schatz, George C. (2013)."How to Make Your Next Paper Scientifically Effective".J. Phys. Chem. Lett.4 (9):1578–1581.doi:10.1021/jz4006916.PMID 26282316.
  8. ^Reusch, William."Visible and Ultraviolet Spectroscopy". Retrieved2014-10-29.
  9. ^Reusch, William."Empirical Rules for Absorption Wavelengths of Conjugated Systems". Retrieved2014-10-29.
  10. ^Russ Rowlett (2004-09-01)."How Many? A Dictionary of Units of Measurement". Unc.edu. Archived fromthe original on 1998-12-03. Retrieved2010-09-20.
High resolution
Medium resolution
Spectroscopic
Translational diffusion
Rotational diffusion
Chemical
Thermodynamic
Computational
Authority control databases: NationalEdit this at Wikidata
Retrieved from "https://en.wikipedia.org/w/index.php?title=Absorbance&oldid=1275030379"
Categories:
Hidden categories:
[8]ページ先頭
©2009-2025 Movatter.jp