Determining the Total Content of Similar Substances Using Integrated Indices or Multivariate Calibrations at a High Intragroup Selectivity of Signals

The total content c_Σ of similar organic compounds is conventionally determined without their separation by measuring their generalized spectral signals at one or several analytical wavelengths. The possibility of the correct c_Σ estimation with the strongly pronounced intragroup selectivity of signals has not been previously tested. To verify this group analysis option, model hexane solutions of the known composition containing mono-, bi-, and tricyclic arenes at c_Σ ~ 10^–4 mol/dm³ simultaneously have been analyzed. At a fixed wavelength, the molar absorptivities of arenes with different numbers of rings differ by two to three orders of magnitude. Two options of group analysis have been compared: (i) measuring the generalized signal of arenes at 260 nm with the subsequent calculation of the result using the univariate calibration in terms of naphthalene or anthracene and (ii) measuring the generalized signals at m wavelengths in the spectral range of 250–290 nm with the subsequent calculation of the result using the inverted multivariate calibration. The first method (the calculation of the total index) has led to large systematic errors, sometimes over 100% in the absolute value, which do not decrease upon variation in the wavelength and nature of a standard substance. The second method has ensured more correct results, and already at m = 11, the errors of group analysis have been no more than 10%. Thus, at the strongly pronounced intragroup selectivity of the signals, the group analysis can and should be carried out using inverted multivariate calibrations. It has been established that the errors of c_Σ estimation sharply grow if a sample contains components of a desired group that were not taken into account when constructing the inverted calibration.