Igor Georgievich Zenkevich

An original algorithm is proposed for estimating the normal boiling points of organic compounds based on the values of their gas chromatographic retention indices (RIs) on the standard nonpolar polydimethylsiloxane stationary phases. The algorithm is characterized by simplicity of calculations without encoding molecular structures and using any increments of their structural fragments. The algorithm involves the stage of comparing the experimentally measured RI values with the molecular mass I ( M ) and molar refraction I (MR _D ) indices of the compounds using several logical criteria. The main criterion is as follows: if any two RIs, I ( M ), and I (MR _D ) (or all three of them) are close to each other, the formula for evaluating the T _b based on RI is applicable; otherwise, the calculations are forbidden. Criteria for forbidding such estimations for polar and perfluorinated organic compounds are formulated.

The free-radical chlorination of three cycloarenes (indane, tetralin, and indene) in heterophase systems upon chlorine generation in an aqueous solution through sequential additions of potassium permanganate to hydrochloric acid is considered. This method enables the controlled formation of chlorination products containing the minimal number of chlorine atoms in a molecule compared to reactions in homogeneous systems, and thus improves the identification of these products and more highly chlorinated ones. This feature also improves the reliability of the identification of chlorocycloarenes by mass spectra and gas-chromatographic retention indices when reference data are limited. Chlorination of indene predominantly yields dimeric products. Oxidation of cycloarenes by permanganic acid HMnO₄ proceeds in parallel with the free-radical chlorination and predominantly yields phthalonic (ortho-(HO₂C)−C₆H₄−COCO₂H) and phthalic acids. These products do not interfere with the determination of chlorinated derivatives because they do not present in the organic phase and are accumulated mainly in the aqueous phase.

The capabilities of an algorithm for comparing experimental and reference values of gas chromatographic retention indices (RIs) are discussed. This algorithm was designed to eliminate significant elements of uncertainty inherent in most modern recommendations, namely, fixed values of their permissible deviations ΔRI. The proposed algorithm is oriented toward the use of variable values of deviations ΔRI = (RI_ref – RI_expt). It is applicable to multicomponent mixtures in which individual components with known reference values of retention indices can be reliably identified. This makes it possible to calculate the parameters of the regression equations ΔRI = (RI_ref – RI_expt) = aRI_expt + b with the subsequent use of these equations to recalculate the experimental RI values into the corrected values RI_corr = RI_expt + ΔRI. For example, this algorithm allows one to use reference RI values for semistandard stationary phases containing 5% phenyl groups to identify compounds by indices determined on standard nonpolar polydimethylsiloxane stationary phases and vice versa. It is applicable to both statistically processed data (presented in the format mean value ± permissible deviation) and to the results of single determinations.

As a result of the assessment of the thermal stability of components of essential oils of Mentha piperita, Lavandula angustifolia, and a linalool sample, it is shown that most of them are sufficiently stable under the conditions of gas-chromatographic analysis at injector temperatures up to 300°C. The assessments were performed using an algorithm involving sigmoidal approximation (otherwise, logistic regression) of the dependences of the relative peak areas on the injector temperature. A slight decrease in amount was detected only for the γ-terpineol impurity in the linalool preparation. In addition, an increase in the γ-terpinene content was recorded as a result of the thermal decomposition of other components. Unexpectedly interesting results were obtained for the essential oil of Lavandula angustifolia, in which significant amounts of a low-volatile matrix—low-molecular polyethylene glycols—were found. This proves useful in revealing an important feature of the algorithm itself for characterizing the thermal stability of analytes. It was shown that an increase in the relative areas of chromatographic peaks with increasing injector temperature can be not only due to the formation of thermal decomposition products of unstable precursors, but also due to the presence of non-volatile or low-volatile matrix components in the samples. A sign of such an anomaly is provided by the ascending dependences of the peak areas of all components in the absence of analytes with descending dependences.

The possibilities of GC–MS identification of the products of free-radical chlorination of cyclic ethers, namely, tetrahydrofuran, 2H-tetrahydropyran, and 1,4-dioxane directly in reaction mixtures (without isolation) are considered. The number of chlorine atoms in these products can be determined unambiguously using electron ionization mass spectra. The complexity of this problem is explained by the large number of possible congeners and isomers (e.g., in the case of 2H-tetrahydropyran, their number is 234), as well as insignificant informational maintenance. The absence of gas chromatographic retention indices (RIs) on standard nonpolar polydimethylsiloxane stationary phases even for simplest chloro derivatives of mentioned cyclic ethers makes impossible the calculation of increments, which allow interpretation of data for more chlorinated products. As a result, the data for compounds of other series (including alicyclic ones, having different conformations) should be used as a rough approximation. Nevertheless, by this way, some simplest products of tetrahydrofuran and 1,4-dioxane chlorination were identified. Besides that, the joint interpretation of mass spectrometric and gas chromatographic data allowed us to identify three from four isomeric hexachloro-1,4-dioxanes.

The potential of a new algorithm for comparing experimental and reference values of gas chromatographic retention indices (RIs) is discussed. This algorithm is designed to minimize significant elements of uncertainty typical of numerous contemporary recommendations, primarily, the fixed limiting values of permissible deviations between experimental and reference RI-values, ΔRI = (RI_ref − RI_exp). The algorithm proposed implies the calculation of deviations, ΔRI, for the most reliably identified constituents of multicomponent mixtures in different parts of chromatograms with known reference RI values, followed by calculation of coefficients of regression equations ΔRI = (RI_ref − RI_exp) = aRI_exp + b for both of the reduced sets of analytes. This equation allows for the recalculation of experimentally determined RIs into corrected values RI_corr = RI_exp + ΔRI, which means replacing the fixed “global” limits with data-dependent adaptive thresholds for different constituents of multicomponent samples. Such an algorithm makes it possible to use reference RI values for semi-standard nonpolar polydimethylsiloxane phases (with 5% phenyl groups and others) for the comparison with data determined with standard nonpolar polydimethylsiloxanes and vice versa, as well as to minimize the influence of possible erroneous reference RI data. It is applicable both to statistically processed reference data and to results of single measurements. Both of these kinds of reference data are known and presented in contemporary RI databases, e.g., in the NIST RI database.

The possibilities and features of using the double internal standard method are considered on the example of quantitative determination of 1-heptanol in a solution. Two homologues of the target analyte, 1-hexanol and 1-octanol, were selected as the standards. The advantage of this method, in difference from the “common” version of the internal standard technique, is the absence of the necessity of preliminary determination of the calibration coefficients, since the possible differences in the sensitivity of detection of different homologues are mutually compensated. Analysis of the results allowed revealing the main reasons of systematic errors and eliminating two principal of them. The first source of errors is the features of dozing microamounts of standards to samples, which can be accompanied by poorly quantifiable losses. The second source of the error is storage of hydrophilic aliphatic alcohols in contact with air, which can be accompanied by absorption of water vapors resulting in decreasing real content of standards in the samples and overestimation of analysis results. Third factor resulting in a slight underestimation of the results is a slight increase of the volume of samples after standards addition. Besides that, as it was proposed by the author of this method (Mark S. Vigdergauz, 1986), final result of analysis is presented as the geometric mean of results calculated for each standard. However, besides geometric means, calculation of average harmonic values is possible, thus slightly reducing overestimation of the results but complicating the calculations.

The potential of the new algorithm for comparing experimental and reference values of gas chromatographic retention indices (RI) is discussed. This algorithm is designed to eliminate significant elements of uncertainty typical of numerous contemporary recommendations, primarily the fixed limiting values of permissible deviations, D RI = (RI_ref – RI_exp). The algorithm proposed implies the calculation of deviations D RI for selected most reliably identified constituents of multicomponent mixtures with known reference RI values, followed by calculation of coefficients of regression equations D RI = (RI_ref – RI_exp) = aRI_exp + b for total sets of analytes. These equations allow recalculating the experimentally determined RIs into the corrected values RI_corr = RI_exp + D RI. Such algorithm makes it possible to use reference RI values for semi-standard nonpolar polydimethylsiloxane phases (with 5% phenyl groups and others) for the comparison with data determined with standard nonpolar polydimethylsiloxanes and vice versa. It is applicable both to statistically processed reference data and to results of single measurements.