Spectroscopy

Nomenclature:

Much confusion exists in the use of terminology in the science of photometry, and several different symbols and names for the same concept or property can be found in the literature. The Joint Committee on Nomenclature in Applied Spectroscopy (SAS) was established by the American Society for Testing Materials (ASTM) to try to arrive at some standard. A summary can be found in every December's edition of Analytic Chemistry.

Absorbance, A (was Optical Density (OD), Extinction (E) Absorbancy) The Absorbance is the logarithm (base 10) of the reciprocal of the Transmittance.

A = log 1 / T

alternately the logarithm (base 10) of the ratio of the radiant power incident upon the sample to the radiant power transmitted by the sample.

A = log Po / P

Absorptivity, a (was Extinction coefficient(E) Absorbancy index, specific extinction) The Absorptivity equals the Absorbance divided by the product of the concentration (grams/liter) and the sample path length (cm) or :

a = A / b c    where A = Absorbance, b = sample path length (cm) and c = concentration (grams/liter)

The Absorptivity is sometime referred to as the Specific Absorbance since it is the absorbance per unit sample path length per unit concentration. Unless otherwise specified units are (grams/liter) and (cm).

Absorptivity, molar, small xi ( was molar Extinction coefficient(E), molar Absorbancy index, molar specific extinction) is the product of the absorptivity and the Molecular Weight or

= a MW

This is where concentration is expressed in mols/ liter not grams / liter.

Radiant Power, P ( was intensity I ) the rate at which energy is transmitted in a beam of radiant energy. It is the quantity measured by the photodetector. The Radiant Power incident on the cell is Po and that transmitted by the cell P.

Transmittance, T (was transmittancy) is the ratio of the Radiant Power transmitted by a sample to the Radiant Power of the incident beam upon the sample.

T = P / Po

It is assumed measurements are made at the same wavelength and at right angles to the sample.

Links :

http://www.spectroscopynow.com - Industry Journal

http://www.chm.davidson.edu/ChemistryApplets/spectrophotometry/Spectrophotometry.html