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

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

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

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

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.

Related Concepts

• Transmittance
• Reflectance
• Fluorescence
• Phosphorescence
• Absorption spectrum
• Spectrophotometry

See Also

• Beer-Lambert law
• Spectroscopy
• Spectrophotometer
• Quantitative analysis
• Molar absorptivity

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