Study on the origin of linear deviation with the Beer-Lambert law in absorption spectroscopy by measuring sulfur dioxide

Abstract

In accordance with the Beer-Lambert law, absorbance is proportional to concentration and optical path length of the absorbers in the sample, and in a linear relationship with total column concentration (product of concentration and optical path length) at a single wavelength. However, limitation of spectral resolution will result in linear deviation with the Beer-Lambert law in actual measurement. Regarding additivity of polychromatic light intensity as the theoretical basis, this paper attributed linear deviation with the Beer-Lambert law to spectral resolution, concentration and light intensity, and verified this explanation by measuring sulfur dioxide at various total column concentrations using spectrometers with different spectral resolutions in the waveband range of 216-230 nm. It was found that linear deviation with the Beer-Lambert law was in negative correlation with spectral resolution, and in positive correlation with total column concentration, and absorbance could be considered to be linear with total column concentration (below 171.4 mg/m²) of sulfur dioxide in the wavelength range of 216-230 nm. In addition, it was also proved that linear deviation increases with decreasing light intensity at a fixed sulfur dioxide column concentration.

Keywords: Absorption spectroscopy; Beer-Lambert law; Light intensity; Linear deviation; Spectral resolution; Sulfur dioxide.