Quantitative characteristics of absorption

1. Transmittance -the ratio of the radiant power passing through a sample to that from the radiation’s source (T).

The attenuation of electromagnetic radiation as it passes through a sample is described quantitatively by two separate, but related terms: transmittance and absorbance. Transmittanceis defined as the ratio of the electromagnetic radiation’s power exiting the sample, P_T, to that incident on the sample from the source, P₀,

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption).

All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation.

Attenuation of radiation as it passes through the sample leads to a transmittance of less than 1. As described, equation 10.1 does not distinguish between the different ways in which the attenuation of radiation occurs. Besides absorption by the analyte, several additional phenomena contribute to the net attenuation of radiation, including reflection and absorption by the sample container, absorption by components of the sample matrix other than the analyte, and the scattering of radiation. To compensate for this loss of the electromagnetic radiation’s power, we use a method blank. The radiation’s power exiting from the method blank is taken to be P₀.

Diagram of Beer–Lambert absorption of a beam of light as it travels through a cuvette of width ℓ.

Optical density А (Absorbance)

An alternative method for expressing the attenuation of electromagnetic radiation is absorbance, A, which is defined as

An alternative method for expressing the attenuation of electromagnetic radiation is absorbance, A, which is defined as

Absorbance is the more common unit for expressing the attenuation of radiation because, as shown in the next section, it is a linear function of the analyte’s concentration.