The relationship between absorbance and concentration is defined by Beer-Lambert Law (or Beer’s Law). Beer’s Law states that the absorbance of light absorbing matter in water is directly proportional to its concentration, expressed by the following equation:
A = ε•b•c
where ε is the molar absorbtivity of the particular type of matter in the water sample, b is the path length of the water sample, and c is the concentration of matter in the water sample. Therefore, as an example, if the concentration of organic material in the water were to double, then the UVA would also double. An important note to make is that UV transmittance (UVT) does not have a linear relationship with concentration, therefore if the purpose of monitoring is to determine concentration, absorbance would be a more direct parameter to record.
Because of this relationship, UVA instrumentation can be calibrated to output concentration values, in mg/L or ppm, using a site-specific correlation. This enables real-time measurement of light absorbing matter such as nitrate, nitrate and organic matter (BOD, COD, TOC, DOC).