Absorbance Measurement

Absorbance is the most widely used spectroscopic technique for studying liquids and gases. It is simple, accurate and easy to use. Absorbance measurements can take many forms: it, for instance, it can identify fingerprint substances or measures the concentration of a molecule in solution. The method works for gases as well as for liquids to analyze consumer products. It is also used in many as industrial applications as well.

A classic introductory chemistry lab experiment concerning absorbance measurement is a solution in a cuvette., This can be measured in transmission with a dual-beam spectrometer. However, modern modular spectroscopy is much more flexible in choosing the wavelength range and resolution needed. With this method, it is possible to move between sampling optics quickly and easily for measurements in the lab or field. The measurement can be repeated without altering or destroying the sample and can quantify absorbance to within 0.001 absorbance units.

The science

The transmission of material with a thickness d supplies information on the spectral dependence of the absorption constant α (λ) (I0: incident intensity, l(d): transmitted intensity). Immersion probes connected to a light source and a spectrometer module using fibers is the simplest system for measuring the concentration c of liquids. The concentration is related to the absorption constant via the absorbance coefficient ε. In other cases, it is advisable to set up a collimated beam path. Measurements where the input of the cross-section converter is in direct contact with the object to be measured can also be performed. Very high spectral resolution is less important than a very good wavelength accuracy and high dynamic resolution – as provided by the spectrometer modules.

Examples of use

  • Measurement of filters (color or interference filters)
  • Measurement of the concentrations of liquids
  • Thickness measurement if absorption coefficient is constant
  • Determination of the sugar and alcohol contents in beverages
  • Quality control in the petrochemical industry
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