What is “molar absorptivity”?

So, what about ξ (sometimes designated a) in the Beer’s law equation? ξ is the molar absorptivity, also known as the extinction coefficient of the sample. It is a unique physical constant of the chemistry of the sample that relates to the sample’s ability to absorb light at a given wavelength. Like path length (b) and sample concentration (c), ξ is also directly proportional to Absorbance.

To begin we will rearrange the equation:

A = ξbc

to

ξ = A / bc

2) Quant Mode - Quantization is usually performed on a major peak of the sample. Data can be collected as either peak height or area. Beer’s law states that the sample absorbance is directly proportional to concentration. Standards are measured first and can consist of either one or many different concentrations. Concentrations of the standard are then analyzed and graphed using a least squares statistical analysis (seen above). Unknown samples can be calculated from the line fitting equation. If only a single standard is used, linearity is assumed.

In words, this relationship can be stated as “ξ” is a measure of the amount of light absorbed per unit of concentration” at a defined wavelength. Molar absorptivity is a constant for a particular substance, so if the concentration of the solution is halved so is the absorbance, which is exactly what you would expect. A compound with a high molar absorptivity is very effective at absorbing light at the stated wavelength, and hence low concentrations of a compound with a high molar absorptivity can be detected at lower concentrations. In addition, the absorbance value at a given wavelength can be calculated if you know the molar absorptivity, path length, and concentration.