Spectrometric Determination of the Total Concentration of Similar Analytes Using Inverted Multivariate Calibrations.

Inverted multivariate calibration methods are used for the spectrometric determination of the total concentration (cΣ) of substances of the same type in environmental samples and food products. They represent сΣ as a sum of generalized signals (Aj), measured at different wavelengths (λj) and multiplied by corresponding regression coefficients (kj). The values of kj are found using chemometric algorithms based on the absorption spectra of mixtures of the known composition included in the training set. At the additive absorbance of mixtures, this method of group analysis gives rather accurate results. The ability of determining the total concentration of analytes with nonadditive absorbance in this way is unobvious; this case has not been studied. The aim of this study was to test the applicability of inverted calibrations for the nonadditive absorbance of analytes. Mixtures of complex compounds of Cu, Co, Ni, Zn, and Pb with the photometric reagent 4-(2-pyridylazo)resorcinol (PAR) were used as model samples. Aqueous solutions of these complexes at pH 10 have rather similar absorption spectra with maxima at 490–530 nm. In the absorption spectra of many solutions containing ions of these metals and an excess of PAR, we observed statistically significant (up to 30 rel %) positive deviations from additivity. To construct an inverted calibration, 14 mixtures with a known total metal concentration (сΣ from 8 to 25 µmol/L) were used; their absorbance was measured at six wavelengths. The least-squares estimates of regression coefficients were found using the Microsoft Excel software. Based on the resulting calibration, the cΣ values were calculated for mixtures of test samples. Relative errors in determining сΣ did not exceed 8 rel % at RSD < 4%, and the systematic errors were statistically insignificant. A further increase in the number of analytical wavelengths and the size of the training set did not decrease the errors significantly. Thus, inverted calibrations are applicable to the determination of the total concentrations of similar substances with both additive and nonadditive absorbances of the sample. [ABSTRACT FROM AUTHOR]