Search

Despite the constant improvement of complex computer algorithms for the calculation of gas chromatographic retention indices (RIs), the simplest methods for their evaluation based on linear correlations between the indices of structural analogs from different taxonomic groups, RI.sup.1 [almost equal to] aRI.sup.2 + b, remain important. It is shown that symbatic variations of the first numerical differences of retention indices, [Formula omitted] = RI.sub.n.sub.+ 1 - RI.sub.n (equivalent to the first derivatives of the retention indices with respect to the structural parameter varied in the group), are the conditions of correctness for such correlations in the simplest groups (substituted methanes). A monotonic variation of [Formula omitted] in one of the groups with the presence of extrema in the other group is an unequivocal sign of the absence of a linear correlation between retention indices. If the values of [Formula omitted] in one of the groups increase and decrease in the other, the ranking order of compounds in any one of them should be reversed. It is shown that the simplest relationship RI.sup.1 [almost equal to] aRI.sup.2 + b is also applicable to more complex taxonomic groups (substituted ethanes, benzenes, and naphthalenes), and it allows one not only to estimate the RIs of compounds not yet characterized but also to refine known reference data., Author(s): I. G. Zenkevich [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. Petersburg, Russia The capabilities of identifying trace [...]

Unsubstituted hydrazones RR'C=NNH.sub.2 are unstable during gas chromatographic separation. Testing of their resistance to hydrolysis under reversed-phase HPLC showed that aromatic ketone hydrazones are stable. In contrast, aldehyde hydrazones are only stable in neutral methanol-water systems (in the absence of acidic modifiers). In acetonitrile-water systems containing 0.1% of formic acid, only aromatic ketone hydrazones are stable, while aldehyde derivatives are completely hydrolyzed. This difference in stability must be taken into account in determining other compounds of these classes. To detect the hydrolysis of analytes, we compared the retention indices of the initial carbonyl compounds and hydrazones at different volume ratios of organic modifiers and aqueous phases and different pH values of the eluent, the relative absorbance values of the characterized components A(254/220) = A(254)/A(220), and the chromatography-mass spectrometric data., Author(s): A. Deruish [sup.1] , G. V. Karakashev [sup.2] , A. I. Ukolov [sup.2] , I. G. Zenkevich [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, [...]

Gas chromatographic analysis of a long-stored sample of tetraethoxysilane (tetraethyl orthosilicate) showed that it lacked the main component due to the hydrolysis of this compound by traces of water. Instead, ethanol and three other components with retention indices (RIs) of 537 ± 2 (no. 1), 608 ± 1 (no. 2, the most abundant component), and 727 ± 3 (no. 3) on a column with a HP-5 stationary phase were detected. These components are unstable, and they cannot be isolated preparatively; as a result, they have not been characterized previously. To identify them, the chemical properties of this sample were characterized, and the recurrent approximations of the RIs of the detected components and their correlation with the retention indices of structural analogs were considered. It was established that they were congeners of the initial tetraethoxysilane, namely, exotic products of its partial hydrolysis-triethoxysilanol (C.sub.2H.sub.5O).sub.3SiOH, diethoxysilanediol (C.sub.2H.sub.5O).sub.2Si(OH).sub.2, and ethoxysilanetriol (C.sub.2H.sub.5O)Si(OH).sub.3. In accordance with published data, some silanediols R.sub.2Si(OH).sub.2 and silanetriols RSi(OH).sub.3 are stable, especially, compounds containing substituents capable of conjugation with vacant d orbitals of silicon atoms. Among them are phenyl- ([pi]-d conjugation systems) and alkoxy-substituted (p-d systems) silanediols and silanetriols. The identified products of partial hydrolysis of tetraethoxysilane belong to the latter type., Author(s): I. G. Zenkevich [sup.1] , D. A. Baranov [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. Petersburg, Russia [...]

We studied the regularities of gas-chromatographic retention indices of aliphatic enyne alcohols containing С=С and С[identical to]С bonds in the molecule (14 secondary and two tertiary homologues) on standard nonpolar polydimethylsiloxane stationary phases, which have not been characterized in sufficient detail earlier. Using homologous increments of retention indices i.sub.RI = RI - 100x, where Ñ = int(M/14), Ð is the molecular weight number, and 14 is the mass number of the CH.sub.2 homological difference, one can compare data for structural analogs, including alkanols, alkenols, and alkynols. The features of comparing i.sub.RI values for compounds belonging to different homologous groups y [identical to] M(mod14), for example, C.sub.nH.sub.2n+2O (y = 4), C.sub.nH.sub.2nO (y = 2), C.sub.nH.sub.2n-2O (y = 0), and C.sub.nH.sub.2n-4O (y = 12), are discussed. The main feature is that when the boundary value of y = 0 is passed, the relation between the parameter x and the number of carbon atoms in the molecule changes due to an increase in the formal unsaturation (the condition n = x - 1 is replaced by the condition n = x). Therefore, in the latter case, 100 should be subtracted from the i.sub.RI homologous increments to compare them with the data for structural analogs of other series. Taking this correction into account, we found that the i.sub.RI values for secondary enyne alcohols (12 ± 16) and previously characterized sec-alkynols (23 ± 21) virtually coincide, and for tertiary enyne alcohols, they are consistent with the data for all considered groups of structural analogues (alkanols, alkenols, and alkynols)., Author(s): I. A. Savina [sup.1] , D. M. Gusev [sup.1] , T. V. Zimina [sup.1] , A. A. Golovanov [sup.1] , I. G. Zenkevich [sup.2] Author Affiliations: (1) grid.446242.2, 0000 [...]

Some analytical problems, which are often considered incorrect for objective reasons, are considered. The main of these reasons is an anomalously large scatter of the initial data. It can be due to either the low reproducibility of the characteristics of substances, their quantities, analytical signal intensities, process conditions, etc., or variability due to differences in the nature of the objects themselves. In the latter case, the nature of data interpretation is influenced by analytical hypotheses adopted for their consideration. The tasks considered include variations in the component composition of developers for black-and-white negative photographic materials, comparison of temperature parameters of the gas-chromatographic separation of various organic compounds, toxicity characterization (LD.sub.50) of homologues using C.sub.3-C.sub.12 1-alkanols as an example, and the possibility of predicting sample preparation operations in the determination of drugs in blood plasma based on their physicochemical characteristics. The main features of data interpretation characterized by a high degree of uncertainty are revealed. It is noted that important conclusions can be drawn based on the facts of low reproducibility (one-dimensional arrays) or poor correlation of variables (two-dimensional arrays)., Author(s): I. G. Zenkevich [sup.1] , D. A. Nikitina [sup.1] , A. S. Kushakova [sup.2] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, [...]

The formation of hydrates of inorganic compounds is well known and is widely discussed in the contemporary literature. The reasons for the insufficiently detailed characterization of numerous well-known hydrates of organic compounds are analyzed, the main of which is their instability. The CAS numbers assigned to many of these hydrates confirm interest in these hydrates. We found that the reversible formation of the hydrates of various organic compounds can be a reason for anomalous dependences of their retention parameters (t.sub.R) on the concentration of an organic eluent modifier (c) in reversed-phase high-performance liquid chromatography. The recurrent approximation of the retention parameters t.sub.R(c + [DELTA]c) = at.sub.R(c) + b (*), where [DELTA]c = const is a constant step of c, is the most effective way to detect these anomalies. Deviations of dependences like (*) from linearity are observed with compounds in which variations in c and, consequently, in the water content of the eluent (1 - c) affect the equilibrium of the reaction [Formula omitted], Author(s): I. G. Zenkevich [sup.1], D. A. Nikitina [sup.1], A. Deruish [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. [...]

Until recently, one of the reasons for the unsatisfactory characterization of alicyclic ketals of aliphatic and alkylaromatic ketones was the absence of any mention of the presence of such compounds in natural samples. Another reason is in the low yields of such ketals on the interaction of ketones and diols under the conditions of conventional acid catalysis. To optimize the synthesis of trace amounts of such ketals, we used cobalt(II) bromide and dimethyl glyoxime as catalysts. The simplest chromatographic parameters, including the difference between the retention indices of the products and the starting compounds (difference in retention indices, DRI), are effective for identifying the target ketals in reaction mixtures. Such combined chromatography-mass spectrometric characteristics as homologous increments of retention indices (i.sub.RI) are informative not only for the group identification of analytes (assignment to the corresponding homologous series) and the assessment of their molecular weights (at low-intense signals of molecular ions). They are also useful for the determination of the total number of branches of the sp.sup.3-carbon skeleton of molecules (N) using the regression of N [almost equal to] ai.sub.RI + b., Author(s): E. V. Eliseenkov [sup.1], I. G. Zenkevich [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. Petersburg, Russia INTRODUCTION [...]

The series of the interaction products of substituted benzaldehydes XC.sub.6H.sub.4CHO with several aliphatic diols (ethanediol, 1,2-propanediol, 1,3-propanediol, 1,3-butanediol, and racemic 2,3-butanediol) were characterized for the first time by analytical parameters required for their chromatography-mass spectrometric identification: electron ionization mass spectra and gas-chromatographic retention indices (RIs) on a standard nonpolar polydimethylsiloxane stationary phase. It was found that it is expedient to combine this characterization of new compounds with controlling the correctness of the RIs by checking their correlations with data for one or several series of simpler and, therefore, characterized in more detail structural analogs. The proposed algorithm made it possible not only to confirm the correctness of all of the newly determined retention indices but also to reveal an erroneous reference value of the retention index of 4-ethoxyphenol. An appropriate selection of structural analogs provides an opportunity to use this method for controlling the retention indices of compounds in whose molecules steric interactions of structural fragments are manifested., Author(s): I. G. Zenkevich [sup.1], E. V. Eliseenkov [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. Petersburg, Russia The [...]

A similarity of electron ionization mass spectra of various alkyl esters of benzene-1,2-dicarboxylic (phthalic) acid places special demands on the reliability of the identification of monoalkyl phthalates, often recognized as dialkyl phthalates and vice versa. An original mass spectrometric criterion is proposed for the group identification of monoalkyl phthalates (assignment to a subgroup of homologues), which assumes the calculation of the total relative intensity of ion peaks at m/z > 170. In the mass spectra of monoalkyl esters, this value does not exceed 1%, whereas for dialkyl ethers, it varies from 1 to 172%. Within the set subgroups of isomers, it becomes effective to use the homologous increments of the retention indices, i.sub.RI = RI - 100x, where x = int (M/14), to refine the identification results. Using predefined i.sub.RI values and the ratio Ð [almost equal to] 0.14(RI - i.sub.RI) + y, we can estimate the molecular weights of compounds in the mass spectra of which the molecular ion signals are not detected or are of low intensity (including monoalkyl phthalates). In addition, i.sub.RI values are in many cases informative for estimating the number of branches of the sp.sup.3 carbon skeleton of molecules (Z) based on regression relations of the form of i.sub.RI = aZ + b. The examination of homologous increments of the retention indices of dialkyl phthalates revealed an anomalous dependence of their values on the number of carbon atoms in the alkyl fragments of the molecules: from +125 (СÐ.sub.3) to -33 (С.sub.10Ð.sub.21). In the subgroup of monoalkyl phthalates, such anomalies were not observed., Author(s): I. G. Zenkevich [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. Petersburg, Russia The basis of the effectiveness [...]

Gas chromatographic analysis of samples in high-boiling polar organic solvents is characterized by several features. First, the resolution of the components depends on the injector temperature (T.sub.inj), which should not be lower than the boiling point (T.sub.bp) of the solvent. Second, the retention indices (RIs) of the components eluted before the polar solvent zone are somewhat overestimated compared to the retention indices of the components eluted after this zone. And, third, the so-called 'discrimination effects ' are observed for these samples, first of all, the dependence of the absolute peak areas of the components from T.sub.inj. It is confirmed that such dependence does not affect the results of quantitative analysis by the method of internal normalization. Nevertheless, the possibility of using application of this and other methods is recommended, and the condition of T.sub.inj [greater than or equal to] T.sub.bp should be satisfied to verify the thermal stability and mutual compatibility of the components of the test samples. These features are set in the analysis of both a solution of model analytes in 1,3-butanediol (T.sub.bp = 207°C) and a sample of the food flavoring, which is a solution of a complex mixture of components in dipropylene glycol (T.sub.bp 222-236°C). The reason for these effects is a slowdown in the process of sample evaporation and dynamic modification of the stationary phase of the chromatographic column by high-boiling polar solvents., Author(s): I. G. Zenkevich [sup.1], E. Leleev [sup.1] Author Affiliations: (1) grid.15447.33, 0000 0001 2289 6897, Institute of Chemistry, St. Petersburg State University, , 198504, St. Petersburg, Russia The most [...]

Characterizing the dependence of the retention indices (RIs) of sorbates of different chemical origins on the content (С) of organic solvents (methanol, acetonitrile) in eluents used in reversed-phase HPLC shows that the coefficients dRI/dC of the same compounds for eluents of different compositions differ considerably. The range of variations in coefficients dRI/dC for water–methanol eluents (–5.6 to +4.6) is approximately twice that of water–acetonitrile eluents (−4.5 to +0.8). It is also found that for nonpolar compounds when using water–acetonitrile eluents, the dRI/dC values are typically lower than those for water–methanol eluents. The opposite is observed for polar compounds. The signs and absolute values of coefficients dRI/dC are largely determined by the polarity of the sorbates. dRI/dC > 0 for nonpolar sorbates, and dRI/dC < 0 for sorbates of higher polarity. Determination even of the sign of this coefficient therefore provides important information about the chemical nature of sorbates. It is shown that coefficients dRI/dC do not correlate with either the RI values or hydrophobicity . However, they do correlate with homologous increments of hydrophobicity ilogP and the homologous increments of RI iRI for eluents of any composition. Quantities ilogP and iRI characterize the polarity of homologous series of sorbates, rather than the polarity of the particular sorbates. [ABSTRACT FROM AUTHOR]