Dual beam spectrophotometer

Spectrophotometer quantitative basis

The Lambert-Beer law is the basic principle behind the quantitative analysis of spectrophotometry. When a bundle of monochromatic light (referring to light of one color) passes through a uniform solution, a portion is absorbed and a portion is transmitted. Let the intensity of the incident light be I0 and the transmitted light intensity be I, then I/I0 is the transmittance, which is represented by T. Percent transmittance is T% = (I/I0)x100%

Studies have shown that the absorption of light by solution, ie absorbance (A) (also known as extinction E, or optical density D), is negatively logarithmically related to transmittance (T), ie: A = - lgT

Lambert's law: The degree of absorbance (A) of a colored solution is proportional to its thickness (light path) b. When the concentration of the solution is constant, the thickness of the liquid layer of the solution is larger, and the greater the value of the absorption A of light, the smaller the transmittance.

Ie: A = ab

Beer's law: The degree of absorbance (A) of a colored solution is proportional to its concentration (absorption number of dots) C. That is, when the liquid layer thickness of the solution is constant, the concentration of the solution is larger, and the greater the degree of absorption of light, the smaller the transmittance.

Ie: A = ac

Combining the above two formulas can be expressed by the following formula: A = -lgT=abc

That is, A = abc.

The above formula is Lambert Beer's law, which means that when a bundle of monochromatic light passes through a homogeneous solution, the absorbance of the solution to monochromatic light is proportional to the product of the solution concentration and the thickness of the liquid layer.

In A = abc, the absorption coefficient a, which characterizes the sensitivity of the light absorbing material. The greater the value of a, the higher the sensitivity. If the unit of concentration is the amount concentration of the substance (unit: mol/L), the absorption coefficient a can be written as ε, and ε is called the molar absorption coefficient.

It can be seen from the above formula that when the solution layer thickness b and the absorption coefficient a are fixed, the absorbance A is linearly related to the concentration of the solution. In the quantitative analysis, it is first necessary to determine the absorption of the different wavelengths of light (absorption spectrum), determine the maximum absorption wavelength, and then use the light of this wavelength as the light source (monochromatic light) to measure the absorbance A. Berber's law is the primary condition for quantitative analysis.

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