Determination of the total content of similar substances using integrated indices or multivariate calibrations at a high intragroup selectivity of signals

   The total content (c_s ) of similar organic compounds is usually determined without their separation by measuring their generalized spectral signals at one or more analytical wavelengths (AW). The resulted es­ timates of c_s are approximately adequate if all the sensitivity coefficients of the substances (components of the desired group) are the values of the same order of magnitude. The possibility of a correct assessment of c_s with a strongly pronounced intragroup selectivity of signals has not been previously tested. Model hexane solutions of the known composition simultaneously containing mono-, bi-, and tricyclic arenes at с_s value about 10^-4 mol/dm³ were analyzed to verify this variant of the group analysis. At a fixed wavelength, the values of molar absorptivity of arenes with different numbers of rings differ by 2 - 3 orders of magni­tude. Two variants of group analysis were compared: 1) measurement of the generalized signal of arenes at 260 nm with subsequent calculation of the result using univariate calibration in terms of naphthalene or anthracene; 2) measurement of generalized signals for several m wavelengths in the spectral region of 250 - 290 nm with subsequent calculation of the result using the inverted multivariate calibration. The first method (calculation of the integrated index) led to large systematic errors, sometimes exceeding 100 % rel. (in modulus) which appeared insensitive both to the wavelength and to the nature of the stan­dard substance. The second method provided more correct results and even at m = 11 the errors of group analysis did not exceed 10 %. Nevertheless, the errors in the estimation of c_s dramatically increased if the sample contains components (arenes) of the desired group that were not taken into account when con­structing the inverted calibration. It is shown that with a strongly pronounced intragroup selectivity of signals, group analysis can and should be carried out using inverted multivariate calibrations.

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