Absorption spectroscopyAbsorption Spectroscopy is an analytical tool used by chemists. It is based on the absorption of quanta of light by a substance, due to the promotion of electrons from one orbital to another in that substance.
The absorption spectrum is characteristic for a particular compound, and does not change with varying concentration. It is usually measured on a sample of the substance in solution, although reflectance spectroscopy can also be used.
Absoprtion is measured as A = log(Io/It) where Io is the incident light, It the transmitted light.
The measured absorbance has been shown to be proportional to the molar concentration of the absorbing species and the thickness of the sample the light passes through. This is known as Beer's Law, and is expressed: A = (molar absorption coefficent) * (concentration) * thickness.
The plot of Absorption versus wavelength for a particular compound is referred to as the absorption spectrum.
The absorption spectrum is measured using a spectrophotometer, where a light source coupled with a diffraction grating allows Absorption to be measured at various wavelengths.
Optical Absorption spectra are measured from the near UltraViolet to near Infrared, usually with samples in silica or pyrex sample cells.
Infrared absorption spectra are typically carried out with a ground up sample held in place between sodium chloride sample plates.