Absorbance
(Redirected from Transmission density)
Absorbance
Absorbance, also known as optical density, is a measure of the amount of light absorbed by a sample as it passes through a medium. It is a dimensionless quantity and is commonly used in spectroscopy to quantify the concentration of a solute in a solution. Absorbance is defined by the Beer-Lambert law, which relates the absorbance to the concentration of the absorbing species and the path length of the sample.
Principle[edit | edit source]
The principle of absorbance is based on the interaction of light with matter. When light passes through a sample, certain wavelengths are absorbed by the molecules in the sample, while others are transmitted. The absorbance (A) is calculated using the formula:
- A = -\log_{10}(T)
where T is the transmittance, defined as the ratio of the transmitted light intensity (I) to the incident light intensity (I_0):
- T = \frac{I}{I_0}
Beer-Lambert Law[edit | edit source]
The Beer-Lambert law states that absorbance is directly proportional to the concentration (c) of the absorbing species, the path length (l) of the sample, and the molar absorptivity (ε), a constant that depends on the nature of the absorbing species and the wavelength of light:
- A = \epsilon \cdot c \cdot l
This law is fundamental in quantitative analysis using spectroscopic techniques.
Applications[edit | edit source]
Absorbance measurements are widely used in various fields, including:
- Biochemistry: To determine the concentration of proteins, nucleic acids, and other biomolecules.
- Environmental science: To monitor pollutants and contaminants in water and air.
- Clinical chemistry: To measure the levels of glucose, cholesterol, and other substances in blood samples.
- Pharmacology: To study drug interactions and concentrations.
Instrumentation[edit | edit source]
The primary instrument used to measure absorbance is the spectrophotometer. A spectrophotometer consists of a light source, a monochromator to select the desired wavelength, a sample holder, and a detector to measure the intensity of transmitted light. Modern spectrophotometers can measure absorbance across a wide range of wavelengths, from the ultraviolet (UV) to the visible (Vis) and infrared (IR) regions of the electromagnetic spectrum.
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