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Anomalies in the chromatographic retention of sorbates in reversed phase HPLC are often attributed to variations in their mechanisms of retention. However, an equally important reason seems to be a change in the chemical nature of sorbates due to interaction with components of the eluent. Chromatographic properties of several oximes of aromatic carbonyl compounds in high-performance reversed phase liquid chromatography are characterized, including their retention indices and results from recurrent approximation of the dependences of sorbate retention on the concentration of the organic component of the eluent. Such approximation allows us to identify considerably more anomalies in the retention times than other means. Chromatographic information is supplemented by spectral parameters, specifically relative optical densities Аrel = А(λ1)/А(λ2). Compounds that are stable under conditions of separation are found in the series of oximes, along with examples of reversible hydration (oximes of 2-methoxy- and 3,4-dimethoxybenzaldehydes) and irreversible hydrolysis (oximes of 2- and 4-hydroxybenzaldehydes, acetophenone) with the formation of the corresponding aldehydes. It is shown that coefficients of the dependence of retention indices on the concentration of the organic component of the eluent for aldehydes mainly satisfy inequality dRI/dC > 0, and they are usually negative for their oximes. The differences between indices of retention of retention ΔRI = RI(oxime) − RI(aldehyde) in high-performance reversed phase (RP) liquid chromatography are consequently not constant. Instead, they fall as the concentration of methanol in the eluent rises. [ABSTRACT FROM AUTHOR]

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]

Anomalies observed earlier in the gas chromatographic indices of retention (RIs) of polar compounds (e.g., dimethylformamide) in nonpolar capillary columns are confirmed for packed columns. The anomalies consist of minima in the temperature dependences of retention indices, RI(T), or, in other words, a change in the sign of the dRI/dT derivatives. Such anomalies, especially at a low contents of the stationary phase and temperatures of separation, are additionally confirmed via the recurrent approximation of the retention volumes, VN(Т + 10 K) = aVN(T) + b, where Т is the column temperature. Two consistent ways of interpreting the revealed effects are used. One is based on considering the reversible increase in the polarity of the nonpolar stationary phase in the chromatographic zone, caused by the polar adsorbate in it. The other includes assessing the contributions from adsorption of the adsorbates at the phase boundaries. Both explain the anomalies in RIs, mainly at low temperatures of separation and contents of the stationary phase. A third possible reason (dimerization of the polar adsorbate) is excluded, due to the negligible difference between the contributions from the adsorption of polar dimethylformamide and nonpolar n-heptane. The role of adsorption at the phase boundaries in packed columns is confirmed by estimating the corresponding contributions from adsorption and the increase in the adsorbate retention indices at low contents of the phase on a solid support. [ABSTRACT FROM AUTHOR]

Recurrent approximation of retention times in reversed-phase high performance liquid chromatography (RP-HPLC), tR(C + ΔC) = atR(C) + b, where C is the acetonitrile concentration in the eluent, and ΔC is the constant "step" of its variation, for six specially synthesized N-substituted p-toluenesulfonamides confirmed the presence of anomalies previously revealed for some complex polyfunctional organic compounds. The reason for these anomalies is the presence of sulfonamide –SO2–NH fragments in the molecules, which leads to hydration of sorbates in aqueous solutions, or, more precisely, to a change in the ratio of their non-hydrated and hydrated forms because of a shift in the equilibrium Х + Н2О Х·Н2О (*) as a result of a change in the eluent composition. The same effect is indicated by the strong antibatic dependence of the retention indices RI(C) of all sulfonamides under study; the coefficients dRI/dC vary from –1.9 to –4.0, these values being much higher in magnitude than for compounds that do not form hydrates. Further independent evidence in favor of the transformation of sorbates due to variation of the eluent composition is the dependence of the relative absorbance Arel = A(254)/A(220) on the acetonitrile content in the eluent. This suggests changes in the chemical nature of chromophores in sulfonamide molecules depending on the equilibrium state (*). [ABSTRACT FROM AUTHOR]

Possible anomalies of gas chromatographic retention indices (RIs) for homologs are difficult to determine as they are masked by the "normal" homologous dependence (~100 index units per CH2 homologous difference). A comparison of the RIs of homologs in the form of their homologous increments iRI = RI – 100x (х = int(M/14), where int is the integer part of the number equivalent to M = 14x + yМ, where уМ is the number of the homologous group of the compound; yМ ≡ M()) revealed two anomalies of chromatographic retention, which were not discussed previously. One of these is abnormal decrease in iRI of di-n-alkyl phthalates at increased number of carbon atoms in alkyl fragments, which is not associated with variations in their polarity or polarizability. The most probable reason is the position of the conformational equilibria of the alkyl fragments of molecules (predominantly "folded" into Gaussian coils) in the molecules of ortho-disubstituted aromatic compounds. Among the numerous theoretically possible structures of this type, the largest number of homologs with different numbers of carbon atoms, which makes it possible to trace the anomalies of their gas chromatographic properties, was characterized only in the series of dialkyl phthalates. The second anomaly relates to polymethyl-substituted aromatic compounds containing substituents at ortho-positions relative to each other. The presence of this structural fragment leads to an anomalous increase in the iRI values of homologs, which is also not related to their polarity or polarizability, but can be explained by steric hindrances to the intramolecular rotation of substituents relative to С(sp2)–C(sp3) ordinary bonds ("gear effect"). Both effects are compared with the so-called "propyl group effect" discussed in the 1980s, whose value does not exceed 10 index units for aliphatic dienes; in the given cases, the RI anomalies reach 160 units for dialkyl phthalates and 150 index units for methylbenzenes and methylnaphthalenes. [ABSTRACT FROM AUTHOR]

A study is performed of the effectiveness and advantages of approximating data on the temperature dependence of the solubility of inorganic salts in water (С) using first-order recurrent relations of type С(T) = aС(T + ΔT) + b, where ΔT is a constant increment of the temperature variations (e.g., 10 or 20°C) and a and b are coefficients calculated according to least squares. The linearity of recurrent dependences with coefficients of correlation R > 0.999 confirms there are no transformations of the dissolved substances in solutions in the considered ranges of temperature. It is noted that the emergence of readily visual anomalies observed in the graphs of such dependences should be considered a sign of the formation of hydrates, their destruction, or a change in composition. Grouping the points in the graphs of recurrent dependences along a number of lines (usually two) confirms the existence of different hydrate forms of the same salt in aqueous solutions. It is concluded that coefficients a and b of recurrent dependences are not the same for different salts because they are formed by different cations and anions that are not interrelated by any taxonomic signs. An interesting dependence is nevertheless found by considering data for 84 salts in the 0 to 100°C range of temperatures: coefficients b of recurrent relations for salts that form or exist as hydrates in aqueous solutions satisfy inequality b < 0. This simple rule ensures correct prediction of the formation of hydrates by a dissolved salt in around 91% of all cases. An interpretation of this pattern is proposed. [ABSTRACT FROM AUTHOR]

A way of controlling, adjusting, and (if needed) recovering values of the physicochemical properties of organic compounds is considered using the example of normal boiling and melting points of perfluoroalkylcarboxylic acids . Its relevance is due to the values of most properties from different sources of information often failing to correspond to one another, and the discrepancies can be substantial. At the first stage of data verification, the algorithm can include rejecting the greatest abnormalities using polynomial approximation of the dependence of physicochemical properties А on number n of carbon atoms in a molecule. The most reliable results are provided by a subsequent approximation of the data using recurrent relations A(n + Δn) = aA(n) + b. For boiling points, Δn = 1; for melting points, Δn = 2. This means verifying reference data to correspond to those for remaining homologs of the same series. For most homologs, we can select the data that corresponds best to others from several alternative values of different properties. Less often, it is necessary to adjust or exclude data for individual homologs. It should be noted that the considered procedure does not substantially distort the initial arrays of physicochemical characteristics, since the required adjustings do not generally exceed the uncertainties of many modern ways of calculating them. [ABSTRACT FROM AUTHOR]

Five antitumor drugs are selected as examples of polyfunctional organic compounds to test the possibilities of approximating their retention parameters in reversed-phase high-performance liquid chromatography using recurrent relationships of the form of tR(C + ΔС) = atR(C) + b, where C is the concentration of organic modifier in the composition of the eluent (acetonitrile), ΔC = const is a constant step of its change in concentration, and a and b are coefficients calculated using the least squares method. Examples of both ideally linear dependences in all ranges of acetonitrile concentrations and deviations from linearity in different ranges of its concentrations are revealed. Discussion of the possible reasons for such deviations (both structural factors and physicochemical properties) suggests that acid–base equilibria in the eluent are not the main ones, the positions of which depend on the concentration of the organic component. The reversible formation of hydrated forms typical of compounds whose molecules contain such polar fragments as amide ‒CO–NH– and sulfonamide –SO2–NH groups, seems more likely. Recurrent approximation of the retention parameters can be considered the simplest and most obvious way to reveal the specific interactions of the test compounds with components of the eluent. [ABSTRACT FROM AUTHOR]

Characteristic features of the gas chromatographic separation of the keto and enol tautomeric forms of ethyl acetoacetate on a capillary column with a BPX-1 standard nonpolar polydimethylsiloxane stationary phase are considered. It is confirmed that the chromatograms of a mixture of tautomers have a specific profile, i.e., a plateau between the peaks of the tautomers, which corresponds to the reversible keto enol transformation during separation. It is shown that the enol and keto forms of ethyl acetoacetate have different coefficients of the temperature dependence of the gas chromatographic retention indices (0.19 ± 0.03 and 0.02 ± 0.02, respectively). It is found there is no dependence of the relative peak areas of the tautomers on the nature of the solvent (polar ethyl alcohol and nonpolar hexane) at different temperatures; i.e., such ratios predominantly reflect the position of the keto enol equilibrium in the vapor phase of the injector of the chromatograph. It is concluded that this results in similar values of the thermodynamic parameters (standard enthalpy and entropy) of the tautomeric equilibrium determined upon the dosing of samples in different solvents. Possible distortions of the results due to the effects of discriminating between the compositions of the samples injected into capillary columns with gas flow splitting are discussed. An impurity in a sample of ethyl acetoacetate after long-term storage is identified as ethyl 2-hydroxy-3-oxobutanoate, the product of oxidation by dissolved atmospheric oxygen. [ABSTRACT FROM AUTHOR]

It is shown that the gas chromatographic separation of enantiomers on columns with achiral nonpolar stationary phases is principally possible as a result of the dynamic modification of stationary phases by sorbates under analysis. It is found that a number of key characteristic features is intrinsic to such separation: it can be only partial, it does not occur for all chromatographic columns, and it is observed only for some compounds and only within narrow ranges of quantities of sorbates that are close to the limits of mass overload of chromatographic systems. These characteristic features are illustrated by the examples of separating (1 R,5 R)-(+)- and (1 S,5 S)-(−)-α-pinenes on a WCOT column with an RTX-5 phase. The main characteristic feature of the separation of enantiomers as a result of the dynamic modification of stationary phases is the nonconformity of peaks in chromatograms with two individual enantiomers, compared to other ways and means for their separation; the first eluting peak belongs to the enantiomer that predominates in a mixture irrespective of its configuration, while the second peak corresponds to the racemic mixture of enantiomers; i.e., the ratio of peak areas in chromatograms does not correspond to the actual ratio of enantiomers in samples under analysis and is strongly distorted as a result of their incomplete separation. It is concluded that the separation of racemic mixtures in achiral systems is fundamentally impossible under any conditions, and this is one of the key criteria of the validity of the considered concept as a whole. [ABSTRACT FROM AUTHOR]

The character of the temperature dependences of the retention indices RI( Т) of polar sorbates on nonpolar stationary phases was found to depend on the dosed amounts of sorbates, but not on column overloading. A physicochemical model was suggested to explain the observed anomalies in RI( Т). [ABSTRACT FROM AUTHOR]

It is noted that the p K values of organic acids can be calculated using the unique recurrence relation p K( n + 1) = ap K( n) + b from the p K values of other (usually the simplest and, consequently, better characterized) homologues of the same series. It is shown that this relation is valid within two taxonomic groups: insertion homologues of the ω-substituted acids X(CH)COH ( n ≥ 1) and isomers that differ in the position of substituents X in their alkyl fragments, k-X(CH)COH ( n ≥ 1, 1 ≤ k ≤ n + 1). It is concluded that this algorithm is a consequence of the unique mathematical properties of recurrence relations. [ABSTRACT FROM AUTHOR]

It is shown through reversed phase high performance liquid chromatography that a characteristic feature of such abundant natural flavonoids as flavon-3-ols is an anomalously strong antibate dependence of their retention indices ( RI) on the organic solvent concentration ( C) in the eluent, dRI/ dC < 0. In order to interpret this anomaly, the specific optical rotation values [α] of natural (+)-(2 R,3 R)-dihydroquercetin in different solvents are compared, confirming the reverse formation of hydrated flavonoids in aqueous solutions. [ABSTRACT FROM AUTHOR]

A general gas-chromatographic method is proposed for determining mutual solubility of partially miscible organic solvents. The method involves gradual increase of the concentration of one component in the other component in the region below their mutual solubility limit, determining parameters of the linear regression c = aS + b ( c is the concentration, and S is the peak area), analysis of saturated solutions of components of binary systems in each other (at S ≈ const), and calculating c values from the obtained equation. The method was tested by known data on mutual solubility of components of the hexane-acetonitrile and hexane-nitromethane binary systems. Data for the hexane-2,2,2-trifluoroethanol and perfluorodecalin-acetonitrile binary systems are obtained for the first time; in the latter case, abnormally low mutual solubility of components is revealed. [ABSTRACT FROM AUTHOR]

Recurrent equations A( x + k) = aA(x) + b were shown to be applicable to the approximation not only of virtually arbitrary properties of organic compounds (A) in homologous series ( A = n C, k = 1 or 2) but also of the dependences of chromatographic retention parameters on the number of carbon atoms in homologue molecules ( A = t R). The same equations described the temperature dependences of retention times of arbitrary compounds under isothermal separation conditions in gas chromatography ( x = T, k = Δ T = const) and the dependences of retention times on the concentration of an organic solvent as an eluent component ( x = C, k = Δ C = const) under isocratic separation conditions in high performance liquid chromatography. [ABSTRACT FROM AUTHOR]

In addition to the earlier revealed physicochemical constants of homologues whose changes in arbitrary series obey the simplest linear recurrent relations A ( n + k) = aA( n) + b, such equations are shown to be applicable to the approximation of the solubility of organic compounds in water ( k = 1), temperature dependences of the solubility of organic and inorganic compounds in water ( k = Δ T), and nematic-isotropic phase transition temperatures for liquid crystals ( k = 2). The a and b coefficients of linear recurrent relations are only determined by the nature of the homologous difference, and, if the homologous difference is the same, they are close for different series. This enables various properties of virtually arbitrary organic compounds to be described by unified recurrent equations, which is equivalent to the existence of a general method for their calculation. For continuous properties (for the example of the temperature dependence of solubility), a method for solving recurrent equations with nonintegral or nonequidistant argument values is suggested. [ABSTRACT FROM AUTHOR]