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22-AUGUST-2008 06:13:22 - Absorbance In spectroscopy, the absorbance A is defined as A_\lambda = -\log_10I/I_0\, where I is the intensity of light at a specified wavelength λ that has passed through a sample transmitted light intensity and I0 is the intensity of the light before it enters the sample or incident light intensity. Absorbance measurements are often carried out in analytical chemistry, since the absorbance of a sample is proportional to the thickness of the sample and the concentration of the absorbing species in the sample, in contrast to the transmittance I / I0 of a sample, which varies logarithmically with thickness and concentration. Outside the field of analytical chemistry, e.g. when used with the Tunable Diode Laser Absorption Spectroscopy TDLAS technique, the absorbance is sometimes defined as the natural logarithm instead of the base-10 logarithm, i.e. as A_\lambda = -\lnI/I_0\, See the Beer-Lambert law for a more complete discussion. Contents 1 Explanation 2 Instrument measurement range 3 See also 4 References Explanation The term absorption refers to the physical process of absorbing light, while absorbance refers to the mathematical quantity. Also, absorbance does not always measure absorption: if a given sample is, for example, a dispersion, part of the incident light will in fact be scattered by the dispersed particles, and not really absorbed. However, in such cases, it is recommended to use the term attenuance formerly called extinction, which accounts for losses due to scattering and luminescence.1 Although absorbance does not have true units, it is quite often reported in Absorbance Units or AU. Instrument measurement range 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 absorbances below 10-4 with commercially available instruments for chemical analysis. In such cases, laser-based absorption techniques can be used, since they have demonstated detection limits that supersede those obtained by conventional non-laser-based instruments by many orders of magnitude detection have been demostated all the way down to 5 10-13. The theoretical best accuracy for most commercially available non-laser-based instruments is in the range near 1 AU. The path length or concentration should then, when possible, be adjusted to achieve readings near this range. Optical density, or OD, is the absorbance per unit length, i.e., the absorbance divided by the thickness of the sample, although it is sometimes used as a synonym for the absorbance with a base-10 logarithm. See also Beer-Lambert law Reflectivity Tunable Diode Laser Absorption Spectroscopy TDLAS References ^ International Union of Pure and Applied Chemistry IUPAC Glossary of terms used in photochemistry. Recommendations 1988 Braslavsky, S. E. Houk, K. N., eds Pure Appl. Chem. 60, 1055-1106 1988. An updated version, ed by J. W. Verhoeven, has appeared in Pure Appl. Chem. 68, 2223-2286 1996. Retrieved from http://en..org/wiki/Absorbance Categories: Spectroscopy Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Català Česky Deutsch Español Français Italiano Nederlands 日本語 Português SlovenÄ?ina This page was last modified on 3 August 2008, at 13:4
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