Vector representation of spectrophotometric measurements of medium composition.

A rigorous theoretical description of the standard spectrophotometric method for determining medium composition is given using a vector representation of measured signals. Further modifications, methods of orthogonal optical absorption spectroscopy (OOAS) and differential optical absorption spectroscopy (DOAS), are also substantiated. It is shown that the conventional criterion, the difference between the measured spectrum and the spectrum simulated on the basis of calculations, does not serve as an indicator of quality of the latter. The main problem of metrological assurance of the methods-estimating uncertainties of calculation results-is discussed in detail. Inability to quantify the quality of approximation of some contributions to measured signals appears to be a significant disadvantage of the DOAS procedure.